Errors in pH measurements in dilute or sulfhydryl-containing buffers

Large errors in pH measurements were found with most types of reference electrodes when used in buffers containing sulfhydryl reagents (either 1–5 mm 2-mercaptoethanol or 5–10 mm dithiothreitol), or when electrodes were used in dilute buffers (≤25 mm) without sulfhydryl reagents. These errors which are confined to the reference electrodes depend on its history, although new electrodes exhibit errors in sulfhydryl solutions. The errors appear independent of pH and buffer ions. Although high ionic strengths reduce dilution errors, sulfhydryl-induced errors persist. Numerous tests have been unsuccessful in clarifying the molecular sources of these errors. Sulfhydryl effects are usually discernible by the downward pH drift, whereas errors in dilute buffer are frequently stable. Due to the prevalence and magnitude of these errors, increased precautions in pH measurements are needed, as suggested.