The assay of free potassium in biological buffers with a K+-selective glass electrode

Abstract

The performance of the Kent K⁺-selective glass electrode in several biological buffers at neutral pH was evaluated in terms of Nernstian response, repeatability, response time and selectivity. The electrode exhibited a linear response between 2 times 10^?5 to 5 times 10^?4 and 10^?2 M K⁺, with a slope of 54.9–63.1 mV per decade change in K⁺ activity. In successive calibrations in the range of 10^?5 to 10^?2 M K⁺, the coefficient of variation of the potential in a given K⁺ concentration decreased with increasing K⁺ concentration, and was lower than 5%, indicating that in this range of concentrations, the electrode exhibited good repeatability. The response time for a sudden tenfold increase in K⁺ concentration was 1.3–3.6 min for 10^?5 M, and 0.5–1 min for 10^?4 M K⁺. The influence of Ca²⁺ and Mg²⁺ on electrode, potential was very small, but Na⁺ and H⁺ strongly interfered with electrode response. The selectivity coefficient K⁺/Na⁺ was 0.11 and K⁺/H⁺ 3.8. The results suggested that in several biological buffers containing no Na⁺ and with neutral pH, the K⁺-selective glass electrode can be used to assay with accuracy and rapidity free potassium in the range of 10^?5 to 10^?2 M, being therefore an alternative to valinomycin-based electrodes.