ELECTRIFICATION OF WATER AND OSMOTIC PRESSURE

1. Amphoteric electrolytes form salts with both acids and alkalies. It is shown for two amphoteric electrolytes, Al(OH)₃ and gelatin, that in the presence of an acid salt water diffuses through a collodion membrane into a solution of these substances as if its particles were negatively charged, while water diffuses into solutions of these electrolytes, when they exist as monovalent or bivalent metal salts, as if the particles of water were positively charged. The turning point for the sign of the electrification of water seems to be near or to coincide with the isoelectric point of these two ampholytes which is a hydrogen ion concentration of about 2 x 10^–5 N for gelatin and about 10^–7 for Al(OH)₃. 2. In conformity with the rules given in a preceding paper the apparently positively charged water diffuses with less rapidity through a collodion membrane into a solution of Ca and Ba gelatinate than into a solution of Li, Na, K, or NH₄ gelatinate of the same concentration of gelatin and of hydrogen ions. Apparently negatively charged water diffuses also with less rapidity through a collodion membrane into a solution of gelatin sulfate than into a solution of gelatin chloride or nitrate of the same concentration of gelatin and of hydrogen ions. 3. If we define osmotic pressure as that additional pressure upon the solution required to cause as many molecules of water to diffuse from solution to the pure water as diffuse simultaneously in the opposite direction through the membrane, it follows that the osmotic pressure cannot depend only on the concentration of the solute but must depend also on the electrostatic effects of the ions present and that the influence of ions on the osmotic pressure must be the same as that on the initial velocity of diffusion. This assumption was put to a test in experiments with gelatin salts for which a collodion membrane is strictly semipermeable and the tests confirmed the expectation.     

INFLUENCE OF A SLIGHT MODIFICATION OF THE COLLODION MEMBRANE ON THE SIGN OF THE ELECTRIFICATION OF WATER

1. It is shown that collodion membranes which have received one treatment with a 1 per cent gelatin solution show for a long time (if not permanently) afterwards a different osmotic behavior from collodion membranes not treated with gelatin. This difference shows itself only towards solutions of those electrolytes which have a tendency to induce a negative electrification of the water particles diffusing through the membrane, namely solutions of acids, acid salts, and of salts with trivalent and tetravalent cations; while the osmotic behavior of the two types of membranes towards solutions of salts and alkalies, which induce a positive electrification of the water particles diffusing through the membrane, is the same. 2. When we separate solutions of salts with trivalent cation, e.g. LaCl₃ or AlCl₃, from pure water by a collodion membrane treated with gelatin, water diffuses rapidly into the solution; while no water diffuses into the solution when the collodion membrane has received no gelatin treatment. 3. When we separate solutions of acid from pure water by a membrane previously treated with gelatin, negative osmosis occurs; i.e., practically no water can diffuse into the solution, while the molecules of solution and some water diffuse out. When we separate solutions of acid from pure water by collodion membranes not treated with gelatin, positive osmosis will occur; i.e., water will diffuse rapidly into the solution and the more rapidly the higher the valency of the anion. 4. These differences occur only in that range of concentrations of electrolytes inside of which the forces determining the rate of diffusion of water through the membrane are predominantly electrical; i.e., in concentrations from 0 to about M/16. For higher concentrations of the same electrolytes, where the forces determining the rate of diffusion are molecular, the osmotic behavior of the two types of membranes is essentially the same. 5. The differences in the osmotic behavior of the two types of membranes are not due to differences in the permeability of the membranes for solutes since it is shown that acids diffuse with the same rate through both kinds of membranes. 6. It is shown that the differences in the osmotic behavior of the two types of collodion membranes towards solutions of acids and of salts with trivalent cation are due to the fact that in the presence of these electrolytes water diffuses in the form of negatively charged particles through the membranes previously treated with gelatin, and in the form of positively charged particles through collodion membranes not treated with gelatin. 7. A treatment of the collodion membranes with casein, egg albumin, blood albumin, or edestin affects the behavior of the membrane towards salts with trivalent or tetravalent cations and towards acids in the same way as does a treatment with gelatin; while a treatment of the membranes with peptone prepared from egg albumin, with alanine, or with starch has no such effect.     

THE ORIGIN OF THE ELECTRICAL CHARGES OF COLLOIDAL PARTICLES AND OF LIVING TISSUES

1. When a solution of a salt of gelatin or crystalline egg albumin is separated by a collodion membrane from a watery solution (free from protein) a potential difference is set up across the membrane in which the protein is positively charged in the case of protein-acid salts and in which the protein is negatively charged in the case of metal proteinates. The turning point is the isoelectric point of the protein. 2. Measurements of the pH of the (inside) protein solution and of the outside watery solution show that when equilibrium is established the value pH inside minus pH outside is positive in the case of protein-acid salts and negative in the case of metal proteinates. This is to be expected when the P.D. is caused by the establishment of a Donnan equilibrium, since in that case the pH should be lower outside than inside in the case of a protein-acid salt and should be higher outside than inside in the case of a metal proteinate. 3. At the isoelectric point where the electrical charge is zero the value of pH inside minus pH outside becomes also zero. 4. It is shown that a P.D. is established between suspended particles of powdered gelatin and the surrounding watery solution and that the sign of charge of the particles is positive when they contain gelatin-acid salts, while it is negative when the powdered particles contain metal gelatinate. At the isoelectric point the charge is zero. 5. Measurements of the pH inside the powdered particles and of the pH in the outside watery solution show that when equilibrium is established the value pH inside minus pH outside is positive when the powdered particles contain a gelatin-acid salt, while the value pH inside minus pH outside is negative when the powdered particles contain Na gelatinate. At the isoelectric point the value pH inside minus pH outside is zero. 6. The addition of neutral salts depresses the electrical charge of the powdered particles of protein-acid salts. It is shown that the addition of salts to a suspension of powdered particles of gelatin chloride also diminishes the value of pH inside minus pH outside. 7. The agreement between the values 58 (pH inside minus pH outside) and the P. D. observed by the Compton electrometer is not only qualitative but quantitative. This proves that the difference in the concentration of acid (or alkali, as the case may be) in the two phases is the only cause for the observed P.D. 8. The Donnan theory demands that the P.D. of a gelatin chloride solution should be 1½ times as great as the P.D. of a gelatin sulfate solution of the same pH and the same concentration (1 per cent) of originally isoelectric gelatin. This is found to be correct and it is also shown that the values of pH inside minus pH outside for the two solutions possess the ratio of 3:2. 9. All these measurements prove that the electrical charges of suspended particles of protein are determined exclusively by the Donnan equilibrium.     

THE INFLUENCE OF ELECTROLYTES ON THE ELECTRIFICATION AND THE RATE OF DIFFUSION OF WATER THROUGH COLLODION MEMBRANES

1. When pure water is separated by a collodion membrane from a watery solution of an electrolyte the rate of diffusion of water is influenced not only by the forces of gas pressure but also by electrical forces. 2. Water is in this case attracted by the solute as if the molecules of water were charged electrically, the sign of the charge of the water particles as well as the strength of the attractive force finding expression in the following two rules, (a) Solutions of neutral salts possessing a univalent or bivalent cation influence the rate of diffusion of water through a collodion membrane, as if the water particles were charged positively and were attracted by the anion and repelled by the cation of the electrolyte; the attractive and repulsive action increasing with the number of charges of the ion and diminishing inversely with a quantity which we will designate arbitrarily as the "radius" of the ion. The same rule applies to solutions of alkalies. (b) Solutions of neutral or acid salts possessing a trivalent or tetravalent cation influence the rate of diffusion of water through a collodion membrane as if the particles of water were charged negatively and were attracted by the cation and repelled by the anion of the electrolyte. Solutions of acids obey the same rule, the high electrostatic effect of the hydrogen ion being probably due to its small "ionic radius." 3. The correctness of the assumption made in these rules concerning the sign of the charge of the water particles is proved by experiments on electrical osmose. 4. A method is given by which the strength of the attractive electric force of electrolytes on the molecules of water can be roughly estimated and the results of these measurements are in agreement with the two rules. 5. The electric attraction of water caused by the electrolyte increases with an increase in the concentration of the electrolyte, but at low concentrations more rapidly than at high concentrations. A tentative explanation for this phenomenon is offered. 6. The rate of diffusion of an electrolyte from a solution to pure solvent through a collodion membrane seems to obey largely the kinetic theory inasmuch as the number of molecules of solute diffusing through the unit of area of the membrane in unit time is (as long as the concentration is not too low) approximately proportional to the concentration of the electrolyte and is the same for the same concentrations of LiCl, NaCl, MgCl₂, and CaCl₂.     

ON THE LOCATION OF THE FORCES WHICH DETERMINE THE ELECTRICAL DOUBLE LAYER BETWEEN COLLODION PARTICLES AND WATER

1. The cataphoretic P.D. of suspended particles is assumed to be due to an excess in the concentration of one kind of a pair of oppositely charged ions in the film of water enveloping the particles and this excess is generally ascribed to a preferential adsorption of this kind of ions by the particle. The term adsorption fails, however, to distinguish between the two kinds of forces which can bring about such an unequal distribution of ions between the enveloping film and the opposite film of the electrical double layer, namely, forces inherent in the water itself and forces inherent in the particle (e.g. chemical attraction between particle and adsorbed ions). 2. It had been shown in a preceding paper that collodion particles suspended in an aqueous solution of an ordinary electrolyte like NaCl, Na₂SO₄, Na₄Fe(CN)₆, CaCl₂, HCl, H₂SO₄, or NaOH are always negatively charged, and that the addition of these electrolytes increases the negative charge as long as their concentration is below M/1,000 until a certain maximal P.D. is reached. Hence no matter whether acid, alkali, or a neutral salt is added, the concentration of anions must always be greater in the film enveloping the collodion particles than in the opposite film of the electrical double layer, and the reverse is true for the concentration of cations. This might suggest that the collodion particles, on account of their chemical constitution, attract anions with a greater force than cations, but such an assumption is rendered difficult in view of the following facts. 3. Experiments with dyes show that at pH 5.8 collodion particles are stained by basic dyes (i.e. dye cations) but not by acid dyes (i.e. dye anions), and that solutions of basic dyes are at pH 5.8 more readily decolorized by particles of collodion than acid dyes. It is also shown in this paper that crystalline egg albumin, gelatin, and Witte''s peptone form durable films on collodion only when the protein exists in the form of a cation or when it is isoelectric, but not when it exists in the form of an anion (i.e. on the alkaline side of its isoelectric point). Hence if any ions of dyes or proteins are permanently bound at the surface of collodion particles through forces inherent in the collodion they are cations but not anions. The fact that isoelectric proteins form durable films on collodion particles suggests, that the forces responsible for this combination are not ionic. 4. It is shown that salts of dyes or proteins, the cations of which are capable of forming durable films on the surface of the collodion, influence the cataphoretic P.D. of the collodion particles in a way entirely different from that of any other salts inasmuch as surprisingly low concentrations of salts, the cation of which is a dye or a protein, render the negatively charged collodion particles positive. Crystalline egg albumin and gelatin have such an effect even in concentrations of 1/130,000 or 1/65,000 of 1 per cent, i.e. in a probable molar concentration of about 10^–9. 5. Salts in which the dye or protein is an anion have no such effect but act like salts of the type of NaCl or Na₂SO₄ on the cataphoretic P.D. of collodion particles. 6. Amino-acids do not form durable films on the surface of collodion particles at any pH and the salts of amino-acids influence their cataphoretic P.D. in the same way as NaCl but not in the same way as proteins or dyes, regardless of whether the amino-acid ion is a cation or an anion. 7. Ordinary salts like LaCl₃ also fail to form a durable film on the surface of collodion particles. 8. Until evidence to the contrary is furnished, these facts seem to suggest that the increase of the negative charge of the collodion particles caused by the addition of low concentrations of ordinary electrolytes is chiefly if not entirely due to forces inherent in the aqueous solution but to a less extent, if at all, due to an attraction of the anions of the electrolyte by forces inherent in the collodion particles.     

AMPHOTERIC COLLOIDS : I. CHEMICAL INFLUENCE OF THE HYDROGEN ION CONCENTRATION.

1. It has been shown in this paper that while non-ionized gelatin may exist in gelatin solutions on both sides of the isoelectric point (which lies for gelatin at a hydrogen ion concentration of C_H = 2.10^–5 or pH = 4.7), gelatin, when it ionizes, can only exist as an anion on the less acid side of its isoelectric point (pH > 4.7), as a cation only on the more acid side of its isoelectric point (pH < 4.7). At the isoelectric point gelatin can dissociate practically neither as anion nor as cation. 2. When gelatin has been transformed into sodium gelatinate by treating it for some time with M/32 NaOH, and when it is subsequently treated with HCl, the gelatin shows on the more acid side of the isoelectric point effects of the acid treatment only; while the effects of the alkali treatment disappear completely, showing that the negative gelatin ions formed by the previous treatment with alkali can no longer exist in a solution with a pH < 4.7. When gelatin is first treated with acid and afterwards with alkali on the alkaline side of the isoelectric point only the effects of the alkali treatment are noticeable. 3. On the acid side of the isoelectric point amphoteric electrolytes can only combine with the anions of neutral salts, on the less acid side of their isoelectric point only with cations; and at the isoelectric point neither with the anion nor cation of a neutral salt. This harmonizes with the statement made in the first paragraph, and the experimental results on the effect of neutral salts on gelatin published in the writer''s previous papers. 4. The reason for this influence of the hydrogen ion concentration on the stability of the two forms of ionization possible for an amphoteric electrolyte is at present unknown. We might think of the possibility of changes in the configuration or constitution of the gelatin molecule whereby ionized gelatin can exist only as an anion on the alkaline side and as a cation on the acid side of its isoelectric point. 5. The literature of colloid chemistry contains numerous statements which if true would mean that the anions of neutral salts act on gelatin on the alkaline side of the isoelectric point, e.g. the alleged effect of the Hofmeister series of anions on the swelling and osmotic pressure of common gelatin in neutral solutions, and the statement that both ions of a neutral salt influence a protein simultaneously. The writer has shown in previous publications that these statements are contrary to fact and based on erroneous methods of work. Our present paper shows that these claims of colloid chemists are also theoretically impossible. 6. In addition to other physical properties the conductivity of gelatin previously treated with acids has been investigated and plotted, and it was found that this conductivity is a minimum in the region of the isoelectric point, thus confirming the conclusion that gelatin can apparently not exist in ionized condition at that point. The conductivity rises on either side of the isoelectric point, but not symmetrically for reasons given in the paper. It is shown that the curves for osmotic pressure, viscosity, swelling, and alcohol number run parallel to the curve of the conductivity of gelatin when the gelatin has been treated with acid, supporting the view that these physical properties are in this case mainly or exclusively a function of the degree of ionization of the gelatin or gelatin salt formed. It is pointed out, however, that certain constitutional factors, e.g. the valency of the ion in combination with the gelatin, may alter the physical properties of the gelatin (osmotic pressure, etc.) without apparently altering its conductivity. This point is still under investigation and will be further discussed in a following publication. 7. It is shown that the isoelectric point of an amphoteric electrolyte is not only a point where the physical properties of an ampholyte experience a sharp drop and become a minimum, but that it is also a turning point for the mode of chemical reactions of the ampholyte. It may turn out that this chemical influence of the isoelectric point upon life phenomena overshadows its physical influence. 8. These experiments suggest that the theory of amphoteric colloids is in its general features identical with the theory of inorganic hydroxides (e.g. aluminum hydroxide), whose behavior is adequately understood on the basis of the laws of general chemistry.     

VALENCY RULE AND ALLEGED HOFMEISTER SERIES IN THE COLLOIDAL BEHAVIOR OF PROTEINS : III. THE INFLUENCE OF SALTS ON OSMOTIC PRESSURE,MEMBRANE POTENTIALS,AND SWELLING OF SODIUM GELATINATE.

A detailed study was made on the influence of salts on those physicochemical properties of sodium gelatinate which are regulated by Donnan''s law of membrane equilibria; namely, osmotic pressure, membrane potentials, and swelling. It was found that the influence of salts on these properties in the case of sodium gelatinate obeys the same rules of valency as in the case of the influence of salts on gelatin chloride as discussed in a previous publication. The rules state that when a salt is added to an ionized protein, without causing a change in the hydrogen ion concentration of the protein, the general effect is a depression of the mentioned properties. The degree of depression depends not only on the concentration of the salt but on the electrical properties of the ions constituting the salt. Of the two or more oppositely charged ions of which a salt consists, only the valency of those ions which carry charges opposite to those carried by the protein ions affects the degree of depression which increases with the valency of the ions. It was also found that the phenomenon of swelling of gelatin becomes modified by solubility of the gelatin when salts are added in concentrations higher than N/4. Emphasis is laid on the point that the valency rule holds perfectly also in relation to swelling as long as the phenomenon is pure swelling which is the case when salt solutions of concentrations lower than N/4 are added to gelatin.     

CATAPHORETIC CHARGES OF COLLODION PARTICLES AND ANOMALOUS OSMOSIS THROUGH COLLODION MEMBRANES FREE FROM PROTEIN

1. It had been shown in previous papers that when a salt solution is separated from pure water by a collodion membrane, water diffuses through the membrane as if it were positively charged and as if it were attracted by the anion of the salt in solution and repelled by the cation with a force increasing with the valency. In this paper, measurements of the P.D. across the membrane (E) are given, showing that when an electrical effect is added to the purely osmotic effect of the salt solution in the transport of water from the side of pure water to the solution, the latter possesses a considerable negative charge which increases with increasing valency of the anion of the salt and diminishes with increasing valency of the cation. It is also shown that a similar valency effect exists in the diffusion potentials between salt solutions and pure water without the interposition of a membrane. 2. This makes it probable that the driving force for the electrical transport of water from the side of pure water into solution is primarily a diffusion potential. 3. It is shown that the hydrogen ion concentration of the solution affects the transport curves and the diffusion potentials in a similar way. 4. It is shown, however, that the diffusion potential without interposition of the membrane differs in a definite sense from the P.D. across the membrane and that therefore the P.D. across the membrane (E) is a modified diffusion potential. 5. Measurements of the P.D. between collodion particles and aqueous solutions (ε) were made by the method of cataphoresis, which prove that water in contact with collodion particles free from protein practically always assumes a positive charge (except in the presence of salts with trivalent and probably tetravalent cations of a sufficiently high concentration). 6. It is shown that an electrical transport of water from the side of water into the solution is always superposed upon the osmotic transport when the sign of charge of the solution in the potential across the membrane (E) is opposite to that of the water in the P.D. between collodion particle and water (ε); supporting the theoretical deductions made by Bartell. 7. It is shown that the product of the P.D. across the membrane (E) into the cataphoretic P.D. between collodion particles and aqueous solution (ε) accounts in general semiquantitatively for that part of the transport of water into the solution which is due to the electrical forces responsible for anomalous osmosis.     

DONNAN EQUILIBRIUM AND THE PHYSICAL PROPERTIES OF PROTEINS : I. MEMBRANE POTENTIALS.

1. It is shown that a neutral salt depresses the potential difference which exists at the point of equilibrium between a gelatin chloride solution contained in a collodion bag and an outside aqueous solution (without gelatin). The depressing effect of a neutral salt on the P.D. is similar to the depression of the osmotic pressure of the gelatin chloride solution by the same salt. 2. It is shown that this depression of the P.D. by the salt can be calculated with a fair degree of accuracy on the basis of Nernst''s logarithmic formula on the assumption that the P.D. which exists at the point of equilibrium is due to the difference of the hydrogen ion concentration on the opposite sides of the membrane. 3. Since this difference of hydrogen ion concentration on both sides of the membrane is due to Donnan''s membrane equilibrium this latter equilibrium must be the cause of the P.D. 4. A definite P.D. exists also between a solid block of gelatin chloride and the surrounding aqueous solution at the point of equilibrium and this P.D. is depressed in a similar way as the swelling of the gelatin chloride by the addition of neutral salts. It is shown that the P.D. can be calculated from the difference in the hydrogen ion concentration inside and outside the block of gelatin at equilibrium. 5. The influence of the hydrogen ion concentration on the P.D. of a gelatin chloride solution is similar to that of the hydrogen ion concentration on the osmotic pressure, swelling, and viscosity of gelatin solutions, and the same is true for the influence of the valency of the anion with which the gelatin is in combination. It is shown that in all these cases the P.D. which exists at equilibrium can be calculated with a fair degree of accuracy from the difference of the pH inside and outside the gelatin solution on the basis of Nernst''s logarithmic formula by assuming that the difference in the concentration of hydrogen ions on both sides of the membrane determines the P.D. 6. The P.D. which exists at the boundary of a gelatin chloride solution and water at the point of equilibrium can also be calculated with a fair degree of accuracy by Nernst''s logarithmic formula from the value pCl outside minus pCl inside. This proves that the equation x² = y ( y + z) is the correct expression for the Donnan membrane equilibrium when solutions of protein-acid salts with monovalent anion are separated by a collodion membrane from water. In this equation x is the concentration of the H ion (and the monovalent anion) in the water, y the concentration of the H ion and the monovalent anion of the free acid in the gelatin solution, and z the concentration of the anion in combination with the protein. 7. The similarity between the variation of P.D. and the variation of the osmotic pressure, swelling, and viscosity of gelatin, and the fact that the Donnan equilibrium determines the variation in P.D. raise the question whether or not the variations of the osmotic pressure, swelling, and viscosity are also determined by the Donnan equilibrium.     

AMPHOTERIC COLLOIDS : IV. THE INFLUENCE OF THE VALENCY OF CATIONS UPON THE PHYSICAL PROPERTIES OF GELATIN.

1. A method is given by which the amount of equivalents of metal in combination with 1 gm. of a 1 per cent gelatin solution previously treated with an alkali can be ascertained when the excess of alkali is washed away and the pH is determined. The curves of metal equivalent in combination with 1 gm. of gelatin previously treated with different concentrations of LiOH, NaOH, KOH, NH₄OH, Ca(OH)₂, and Ba(OH)₂ were ascertained and plotted as ordinates, with the pH of the solution as abscissæ, and were found to be identical. This proves that twice as many univalent as bivalent cations combine with the same mass of gelatin, as was to be expected. 2. The osmotic pressure of 1 per cent solutions of metal gelatinates with univalent and bivalent cation was measured. The curves for the osmotic pressure of 1 per cent solution of gelatin salts of Li, Na, K, and NH₄ were found to be identical when plotted for pH as abscissæ, tending towards the same maximum of a pressure of about 325 mm. of the gelatin solution (for pH about 7.9). The corresponding curves for Ca and Ba gelatinate were also found to be identical but different from the preceding ones, tending towards a maximum pressure of about 125 mm. for pH about 7.0 or above. The ratio of maxi mal osmotic pressure for the two groups of gelatin salts is therefore about as 1:3 after the necessary corrections have been made. 3. When the conductivities of these solutions are plotted as ordinates against the pH as abscissæ, the curves for the conductivities of Li, Na, Ca, and Ba gelatinate are almost identical (for the same pH), while the curves for the conductivities of K and NH₄ gelatinate are only little higher. 4. The curves for the viscosity and swelling of Ba (or Ca) and Na gelatinate are approximately parallel to those for osmotic pressure. 5. The practical identity or close proximity of the conductivities of metal gelatinates with univalent and bivalent metal excludes the possibility that the differences observed in the osmotic pressure, viscosity, and swelling between metal gelatinates with univalent and bivalent metal are determined by differences in the degree of ionization (and a possible hydratation of the protein ions). 6. Another, as yet tentative, explanation is suggested.     

THE ORIGIN OF THE POTENTIAL DIFFERENCES RESPONSIBLE FOR ANOMALOUS OSMOSIS

1. Collodion bags coated with gelatin on the inside were filled with a M/256 solution of neutral salt (e.g., NaCl, CaCl₂, CeCl₃, or Na₂SO₄) made up in various concentrations of HNO₃ (varying from N/50,000 to N/100). Each collodion bag was put into an HNO₃ solution of the same concentration as that inside the bag but containing no salt. In this case water diffuses from the outside solution (containing no salt) into the inside solution (containing the salt) with a relative initial velocity which can be expressed by the following rules: (a) Water diffuses into the salt solution as if the particles of water were negatively charged and as if they were attracted by the cation and repelled by the anion of the salt with a force increasing with the valency of the ion. (b) The initial rate of the diffusion of water is a minimum at the hydrogen ion concentration of about N/50,000 HCl (pH 4.7, which is the point at which gelatin is not ionized), rises with increasing hydrogen ion concentration until it reaches a maximum and then diminishes again with a further rise in the initial hydrogen ion concentration. 2. The potential differences between the salt solution and the outside solution (originally free from salt) were measured after the diffusion had been going on for 1 hour; and when these values were plotted as ordinates over the original pH as abscissae, the curves obtained were found to be similar to the osmotic rate curves. This confirms the view expressed by Girard) Bernstein, Bartell, and Freundlich that these cases of anomalous osmosis are in reality cases of electrical endosmose where the driving force is a P.D. between the opposite sides of the membrane. 3. The question arose as to the origin of these P. D. and it was found that the P.D. has apparently a double origin. Certain features of the P.D. curve, such as the rise and fall with varying pH, seem to be the consequence of a Donnan equilibrium which leads to some of the free HNO₃ being forced from the solution containing salt into the outside solution containing no (or less) salt. This difference of the concentration of HNO₃, on the opposite sides of the membrane leads to a P.D. which in conformity with Nernst''s theory of concentration cells should be equal to 58 x (pH inside minus pH outside) millivolts at 18°C. The curves of the values of (pH inside minus pH outside) when plotted as ordinates over the original pH as abscissae lead to curves resembling those for the P. D. in regard to location of minimum and maximum. 4. A second source of the P.D. seems to be diffusion potentials, which exist even if no membranes are present and which seem to be responsible for the fact that the rate of diffusion of negatively charged water into the salt solution increases with the valency of the cation and diminishes with the valency of the anion of the salt. 5. The experiments suggest the possibility that the establishment of a Donnan equilibrium between membrane and solution is one of the factors determining the Helmholtzian electrical double layer, at least in the conditions of our experiments.     

THE REVERSAL OF THE SIGN OF THE CHARGE OF MEMBRANES BY HYDROGEN IONS

1. It had been shown in previous papers that when a collodion membrane has been treated with a protein the membrane assumes a positive charge when the hydrogen ion concentration of the solution with which it is in contact exceeds a certain limit. It is pointed out in this paper that by treating the collodion membrane with a protein (e.g. oxyhemoglobin) a thin film of protein adheres to the membrane and that the positive charge of the membrane must therefore be localized in this protein film. 2. It is further shown in this paper that the hydrogen ion concentration, at which the reversal in the sign of the charge of a collodion membrane treated with a protein occurs, varies in the same sense as the isoelectric point of the protein, with which the membrane has been treated, and is always slightly higher than that of the isoelectric point of the protein used. 3. The critical hydrogen ion concentration required for the reversal seems to be, therefore, that concentration where enough of the protein lining of the membrane is converted into a protein-acid salt (e.g. gelatin nitrate) capable of ionizing into a positive protein ion (e.g. gelatin) and the anion of the acid used (e.g. NO₃).     

ON THE CAUSE OF THE INFLUENCE OF IONS ON THE RATE OF DIFFUSION OF WATER THROUGH COLLODION MEMBRANES. I

1. In three previous publications it had been shown that electrolytes influence the rate of diffusion of pure water through a collodion membrane into a solution in three different ways, which can be understood on the assumption of an electrification of the water or the watery phase at the boundary of the membrane; namely, (a) While the watery phase in contact with collodion is generally positively electrified, it happens that, when the membrane has received a treatment with a protein, the presence of hydrogen ions and of simple cations with a valency of three or above (beyond a certain concentration) causes the watery phase of the double layer at the boundary of membrane and solution to be negatively charged. (b) When pure water is separated from a solution by a collodion membrane, the initial rate of diffusion of water into a solution is accelerated by the ion with the opposite sign of charge and retarded by the ion with the same sign of charge as that of the water, both effects increasing with the valency of the ion and a second constitutional quantity of the ion which is still to be defined. (c) The relative influence of the oppositely charged ions, mentioned in (b), is not the same for all concentrations of electrolytes. For lower concentrations the influence of that ion usually prevails which has the opposite sign of charge from that of the watery phase of the double layer; while in higher concentrations the influence of that ion begins to prevail which has the same sign of charge as that of the watery phase of the double layer. For a number of solutions the turning point lies at a molecular concentration of about M/256 or M/512. In concentrations of M/8 or above the influence of the electrical charges of ions mentioned in (b) or (c) seems to become less noticeable or to disappear entirely. 2. It is shown in this paper that in electrical endosmose through a collodion membrane the influence of electrolytes on the rate of transport of liquids is the same as in free osmosis. Since the influence of electrolytes on the rate of transport in electrical endosmose must be ascribed to their influence on the quantity of electrical charge on the unit area of the membrane, we must conclude that the same explanation holds for the influence of electrolytes on the rate of transport of water into a solution through a collodion membrane in the case of free osmosis. 3. We may, therefore, conclude, that when pure water is separated from a solution of an electrolyte by a collodion membrane, the rate of diffusion of water into the solution by free osmosis is accelerated by the ion with the opposite sign of charge as that of the watery phase of the double layer, because this ion increases the quantity of charge on the unit area on the solution side of the membrane; and that the rate of diffusion of water is retarded by the ion with the same sign of charge as that of the watery phase for the reason that this ion diminishes the charge on the solution side of the membrane. When, therefore, the ions of an electrolyte raise the charge on the unit area of the membrane on the solution side above that on the side of pure water, a flow of the oppositely charged liquid must occur through the interstices of the membrane from the side of the water to the side of the solution (positive osmosis). When, however, the ions of an electrolyte lower the charge on the unit area of the solution side of the membrane below that on the pure water side of the membrane, liquid will diffuse from the solution into the pure water (negative osmosis). 4. We must, furthermore, conclude that in lower concentrations of many electrolytes the density of electrification of the double layer increases with an increase in concentration, while in higher concentrations of the same electrolytes it decreases with an increase in concentration. The turning point lies for a number of electrolytes at a molecular concentration of about M/512 or M/256. This explains why in lower concentrations of electrolytes the rate of diffusion of water through a collodion membrane from pure water into solution rises at first rapidly with an increase in concentration while beyond a certain concentration (which in a number of electrolytes is M/512 or M/256) the rate of diffusion of water diminishes with a further increase in concentration.     

ELECTRICAL CHARGES OF COLLOIDAL PARTICLES AND ANOMALOUS OSMOSIS

1. It has been shown in previous publications that when solutions of different concentrations of salts are separated by collodion-gelatin membranes from water, electrical forces participate in addition to osmotic forces in the transport of water from the side of the water to that of the solution. When the hydrogen ion concentration of the salt solution and of the water on the other side of the membrane is the same and if both are on the acid side of the isoelectric point of gelatin (e.g. pH 3.0), the electrical transport of water increases with the valency of the cation and inversely with the valency of the anion of the salt in solution. Moreover, the electrical transport of water increases at first with increasing concentration of the solution until a maximum is reached at a concentration of about M/32, when upon further increase of the concentration of the salt solution the transport diminishes until a concentration of about M/4 is reached, when a second rise begins, which is exclusively or preeminently the expression of osmotic forces and therefore needs no further discussion. 2. It is shown that the increase in the height of the transport curves with increase in the valency of the cation and inversely with the increase in the valency of the anion is due to the influence of the salt on the P.D. (E) across the membrane, the positive charge of the solution increasing in the same way with the valency of the ions mentioned. This effect on the P.D. increases with increasing concentration of the solution and is partly, if not essentially, the result of diffusion potentials. 3. The drop in the transport curves is, however, due to the influence of the salts on the P.D. (ε) between the liquid inside the pores of the gelatin membrane and the gelatin walls of the pores. According to the Donnan equilibrium the liquid inside the pores must be negatively charged at pH 3.0 and this charge is diminished the higher the concentration of the salt. Since the electrical transport is in proportion to the product of E x ε and since the augmenting action of the salt on E begins at lower concentrations than the depressing action on ε, it follows that the electrical transport of water must at first rise with increasing concentration of the salt and then drop. 4. If the Donnan equilibrium is the sole cause for the P.D. (ε) between solid gelatin and watery solution the transport of water through collodion-gelatin membranes from water to salt solution should be determined purely by osmotic forces when water, gelatin, and salt solution have the hydrogen ion concentration of the isoelectric point of gelatin (pH = 4.7). It is shown that this is practically the case when solutions of LiCl, NaCl, KCl, MgCl₂, CaCl₂, BaCl₂, Na₂SO₄, MgSO₄ are separated by collodion-gelatin membranes from water; that, however, when the salt has a trivalent (or tetravalent?) cation or a tetravalent anion a P.D. between solid isoelectric gelatin and water is produced in which the wall assumes the sign of charge of the polyvalent ion. 5. It is suggested that the salts with trivalent cation, e.g. Ce(NO₃)₃, form loose compounds with isoelectric gelatin which dissociate electrolytically into positively charged complex gelatin-Ce ions and negatively charged NO₃ ions, and that the salts of Na₄Fe(CN)₆ form loose compounds with isoelectric gelatin which dissociate electrolytically into negatively charged complex gelatin-Fe(CN)₆ ions and positively charged Na ions. The Donnan equilibrium resulting from this ionization would in that case be the cause of the charge of the membrane.     

DONNAN EQUILIBRIUM AND THE PHYSICAL PROPERTIES OF PROTEINS : II. OSMOTIC PRESSURE.

1. It had been shown in previous publications that the osmotic pressure of a 1 per cent solution of a protein-acid salt varies in a characteristic way with the hydrogen ion concentration of the solution, the osmotic pressure having a minimum at the isoelectric point, rising steeply with a decrease in pH until a maximum is reached at pH of 3.4 or 3.5 (in the case of gelatin and crystalline egg albumin), this maximum being followed by a steep drop in the osmotic pressure with a further decrease in the pH of the gelatin or albumin solution. In this paper it is shown that (aside from two minor discrepancies) we can calculate this effect of the pH on the osmotic pressure of a protein-acid salt by assuming that the pH effect is due to that unequal distribution of crystalloidal ions (in particular free acid) on both sides of the membrane which Donnan''s theory of membrane equilibrium demands. 2. It had been shown in preceding papers that only the valency but not the nature of the ion (aside from its valency) with which a protein is in combination has any effect upon the osmotic pressure of the solution of the protein; and that the osmotic pressure of a gelatin-acid salt with a monovalent anion (e.g. Cl, NO₃, acetate, H₂PO₄, HC₂O₄, etc.) is about twice or perhaps a trifle more than twice as high as the osmotic pressure of gelatin sulfate where the anion is bivalent; assuming that the pH and gelatin concentrations of all the solutions are the same. It is shown in this paper that we can calculate with a fair degree of accuracy this valency effect on the assumption that it is due to the influence of the valency of the anion of a gelatin-acid salt on that relative distribution of the free acid on both sides of the membrane which Donnan''s theory of membrane equilibrium demands. 3. The curves of the observed values of the osmotic pressure show two constant minor deviations from the curves of the calculated osmotic pressure. One of these deviations consists in the fact that the values of the ascending branch of the calculated curves are lower than the corresponding values in the curves for the observed osmotic pressure, and the other deviation consists in the fact that the drop in the curves of calculated values occurs at a lower pH than the drop in the curves of the observed values.     

STABILITY OF SUSPENSIONS OF SOLID PARTICLES OF PROTEINS AND PROTECTIVE ACTION OF COLLOIDS

1. It is shown that the concentrations of different salts required to precipitate suspensions of gelatin-coated collodion particles in water are practically identical with the concentrations of the same salts required for the "salting out" of gelatin from aqueous solutions. Neither effect shows any relation to the electrical double layers surrounding the particles. 2. It is shown that at the isoelectric point of gelatin, suspensions of gelatin-coated collodion particles are not stable and it had been shown previously that gelatin is least soluble at the isoelectric point. The addition of salt increases both the solubility of gelatin in water as well as the stability of suspensions of gelatin-coated collodion particles in water, and both effects increase with the valency of one of the ions of the salt. 3. This latter effect is not due to any charges conferred on the gelatin particles by the salts, since the cataphoretic experiments show that salts like NaCl, Na₂SO₄, or CaCl₂, which at the isoelectric point of gelatin increase the solubility of gelatin as well as the stability of suspensions of gelatin-coated collodion particles, leave the particles practically uncharged in the concentrations in which the salts are efficient. 4. It follows from all these facts that the stability of suspensions of gelatin-coated particles in water depends on the solubility of gelatin in water; e.g., on the chemical affinity of certain groups of the gelatin molecule for water. 5. Though crystalline egg albumin is highly soluble in water, the stability of collodion particles coated with crystalline egg albumin does not depend upon the affinity of the albumin molecule for water, but depends practically alone on the electrical double layer surrounding each particle. As soon as the P.D. of this double layer falls below 13 millivolts, the suspension is no longer stable. 6. The critical potential for the stability of suspensions of collodion particles coated with genuine egg albumin is the same as that for particles of boiled (denatured) white of egg. Since through the process of heating, egg albumin loses its solubility in water, it is inferred that egg albumin undergoes the same change when it forms a film around a solid particle like collodion. 7. The influence of electrolytes on the stability of suspensions of collodion particles coated with casein or edestin was similar to that of collodion particles coated with egg albumin. The experiments are, however, complicated by the fact that near the isoelectric point CaCl₂ and even NaCl cause a suspension again at concentrations of about M/2 or 1 M, while still higher concentrations may cause a precipitation again. These latter effects have no connection with double layers, but belong probably in the category of solubility phenomena. 8. These experiments permit us to define more definitely the conditions for a general protective action of colloids. Protective colloids must be capable of forming a durable film on the surface of the suspended particles and the molecules constituting the film must have a higher attraction for the molecules of the solvent than for each other; in other words, they must possess true solubility. Only in this case can they prevent the precipitating action of low concentrations of electrolytes on particles which are kept in suspension solely by the high potentials of an electrical double layer. Thus gelatin films, in which the attraction of the molecules for water is preserved, have a general protective action, while crystalline egg albumin, casein, and edestin, which seem to lose their attraction for water when forming a film, have a protective action only under limited conditions stated in the paper.     

INFLUENCE OF THE CONCENTRATION OF ELECTROLYTES ON THE ELECTRIFICATION AND THE RATE OF DIFFUSION OF WATER THROUGH COLLODION MEMBRANES

1. When a watery solution is separated from pure water by a collodion membrane, the initial rate of diffusion of water into the solution is influenced in an entirely different way by solutions of electrolytes and of non-electrolytes. Solutions of non-electrolytes, e.g. sugars, influence the initial rate of diffusion of water through the membrane approximately in direct proportion to their concentration, and this. influence begins to show itself under the conditions of our experiments when the concentration of the sugar solution is above M/64 or M/32. We call this effect of the concentration of the solute on the initial rate of diffusion of water into the solution the gas pressure effect. 2. Solutions of electrolytes show the gas pressure effect upon the initial rate of diffusion also, but it commences at a somewhat higher concentration than M/64; namely, at M/16 or more (according to the nature of the electrolyte). 3. Solutions of electrolytes of a lower concentration than M/16 or M/8 have a specific influence on the initial rate of diffusion of water through a collodion membrane from pure solvent into solution which is not found in the case of the solutions of non-electrolytes and which is due to the fact that the particles of water diffuse in this case through the membrane in an electrified condition, the sign of the charge depending upon the nature of the electrolyte in solution, according to two rules given in a preceding paper. 4. In these lower concentrations the curves representing the influence of the concentration of the electrolyte on the initial rate of diffusion of water into the solution rise at first steeply with an increase in the concentration, until a maximum is reached at a concentration of M/256 or above. A further increase in concentration causes a drop-in the curve and this drop increases with a further increase of concentration until that concentration of the solute is reached in which the gas pressure effect begins to prevail; i.e., above M/16. Within a range of concentrations between M/256 and M/16 or more (according to the nature of the electrolyte) we notice the reverse of what we should expect on the basis of van''t Hoff''s law; namely, that the attraction of a solution of an electrolyte for water diminishes with an increase in concentration. 5. We wish to make no definite assumption concerning the origin of the electrification of water and concerning the mechanism whereby ions influence the rate of diffusion of water particles through collodion membranes from pure solvent to solution. It will facilitate, however, the presentation of our results if it be permitted to present them in terms of attraction and repulsion of the charged particles of water by the ions. With this reservation we may say that in the lowest concentrations attraction of the electrified water particles by the ions with the opposite charge prevails over the repulsion of the electrified water particles by the ions with the same sign of charge as that of the water; while beyond a certain critical concentration the repelling action of the ion with the same sign of charge as that of the water particles upon the latter increases more rapidly with increasing concentration of the solute than the attractive action of the ion with the opposite charge. 6. It is shown that negative osmosis, i.e. the diminution of the volume of the solution of acids and of alkalies when separated by collodion membranes from pure water, occurs in the same range of concentrations in which the drop in the curves of neutral salts occurs, and that it is due to the same cause; namely, the repulsion of the electrified particles of water by the ion with the same sign of charge as that of the water. This conclusion is supported by the fact that negative osmosis becomes pronounced when the ion with the same sign of charge as that of the electrified particles of water carries more than one charge.     

THE INFLUENCE OF ELECTROLYTES ON THE CATAPHORETIC CHARGE OF COLLOIDAL PARTICLES AND THE STABILITY OF THEIR SUSPENSIONS : I. EXPERIMENTS WITH COLLODION PARTICLES.

1. When collodion particles suspended in water move in an electric field they are, as a rule, negatively charged. The maximal cataphoretic P.D. between collodion particles and water is about 70 millivolts. This is only slightly more than the cataphoretic P.D. found by McTaggart to exist between gas bubbles and water (55 millivolts). Since in the latter case the P.D. is entirely due to forces inherent in the water itself, resulting possibly in an excess of OH ions in the layer of water in contact and moving with the gas bubble, it is assumed that the negative charge of the collodion particles is also chiefly due to the same cause; the collodion particles being apparently only responsible for the slight difference in maximal P.D. of water-gas and water-collodion surfaces. 2. The cataphoretic charge of collodion particles seems to be a minimum in pure water, increasing as a rule with the addition of electrolytes, especially if the cation of the electrolyte is monovalent, until a maximal P.D. is reached. A further increase in the concentration of the electrolyte depresses the P.D. again. There is little difference in the action of HCl, NaOH, and NaCl or LiCl or KCl. 3. The increase in P.D. between collodion particles and water upon the addition of electrolyte is the more rapid the higher the valency of the anion. This suggests that this increase of negative charge of the collodion particle is due to the anions of the electrolyte gathering in excess in the layer of water nearest to the collodion particles, while the adjoining aqueous layer has an excess of cations. 4. In the case of chlorides and at a pH of about 5.0 the maximal P.D. between collodion particles and water is about 70 millivolts, when the cation of the electrolyte present is monovalent (H, Li, Na, K); when the cation of the electrolyte is bivalent (Mg, Ca), the maximal P.D. is about 35 to 40 millivolts; and when the cation is trivalent (La) the maximal P.D. is lower, probably little more than 20 millivolts. 5. A reversal in the sign of charge of the collodion particles could be brought about by LaCl₃ but not by acid. 6. These results on the influence of electrolytes on the cataphoretic P.D. between collodion particles and water are also of significance for the theory of electrical endosmose and anomalous osmosis through collodion membranes; since the cataphoretic P.D. is probably identical with the P.D. between water and collodion inside the pores of a collodion membrane through which the water diffuses. 7. The cataphoretic P.D. between collodion particles and water determines the stability of suspensions of collodion particles in water, since rapid precipitation occurs when this P.D. falls below a critical value of about 16 millivolts, regardless of the nature of the electrolyte by which the P.D. is depressed. No peptization effect of plurivalent anions was noticed.     

THE INFLUENCE OF SALTS ON THE RATE OF DIFFUSION OF ACID THROUGH COLLODION MEMBRANES

1. The writer had previously published the observation that if a salt solution made up in an acid solution (e.g. HCl) of a definite pH (e.g. 3.0) is separated by a collodion membrane from pure water containing the same acid of the same pH, acid is at first driven from the salt solution into the water, so that the pH of the latter becomes at first lower than that of the solution. 2. It is shown in this paper that this paradoxical phenomenon is not due to any peculiarity of the membrane but is a consequence of the well known fact that the diffusion constant of an acid is increased by a salt.     

SYNERESIS AND SWELLING OF GELATIN

1. When solid blocks of isoelectric gelatin are placed in cold distilled water or dilute buffer of pH 4.7, only those of a gelatin content of more than 10 per cent swell, while those of a lower gelatin content not only do not swell but actually lose water. 2. The final quantity of water lost by blocks of dilute gelatin is the same whether the block is immersed in a large volume of water or whether syneresis has been initiated in the gel through mechanical forces such as shaking, pressure, etc., even in the absence of any outside liquid, thus showing that syneresis is identical with the process of negative swelling of dilute gels when placed in cold water, and may be used as a convenient term for it. 3. Acid- or alkali-containing gels give rise to greater syneresis than isoelectric gels, after the acid or alkali has been removed by dialysis. 4. Salt-containing gels show greater syneresis than salt-free gels of the same pH, after the salt has been washed away. 5. The acid and alkali and also the salt effect on syneresis of gels disappears at a gelatin concentration above 8 per cent. 6. The striking similarity in the behavior of gels with respect to syneresis and of gelatin solutions with respect to viscosity suggests the probability that both are due to the same mechanism, namely the mechanism of hydration of the micellæ in gelatin by means of osmosis as brought about either by diffusible ions, as in the presence of acid or alkali, or by the soluble gelatin present in the micellæ. The greater the pressures that caused swelling of the micellæ while the gelatin was in the sol state, the greater is the loss of water from the gels when the pressures are removed. 7. A quantitative study of the loss of water by dilute gels of various gelatin content shows that the same laws which have been found by Northrop to hold for the swelling of gels of high concentrations apply also to the process of losing water by dilute gels, i.e. to the process of syneresis. The general behavior is well represented by the equations: See PDF for Equation and See PDF for Equation where P ₁ = osmotic pressure of the soluble gelatin in the gel, P ₂ = stress on the micellæ in the gelatin solution before setting, K_e = bulk modulus of elasticity, V_o = volume of water per gram of dry gelatin at setting and V_e = volume of water per gram of gelatin at equilibrium.