Ion‐selective Electrode Determinations of Exchangeable Potassium in Soils

Abstract

AbstractProcedures using ion‐selective electrodes (ISEs) for the accurate determination of NH+4 ‐and Ba2+‐exchangeable K+ in soil extracts are described. The exchangeable K+ in 30 soil samples was extracted with neutral solutions of 1.0 M NH4OAc and 0.5 M BaCl2 and determined by atomic absorption spectrometry (AAS) to establish the K+ levels sought with the electrodes. Potentiometric measurements were then made with electrochemical cells consisting of either a valinomycin‐based K+‐selective electrode (VKE) or a cationic glass‐membrane electrode (CGE) and a double‐junction sleeve‐type reference electrode assembly with a saturated LiOAc salt bridge. The electrochemically analyzed solutions (1:5 extract/water dilutions) had a common ionic strength and a pH near 7 or a LiOH‐adjusted pH of 11.6. A comparison of the AAS values for the Ba2+‐ and NH+4‐exchangeable K+ demonstrated the feasibility of using either extractant for an evaluation of ISE methods. Interferences from NH+4 and Li+ limited the use of the CGE to the neutral BaCl2 extracts, where it yielded exchangeable K+ values that were only poorly correlated (r = 0.655) with those obtained by AAS. However, the Ba2+‐exchangeable K+ values obtained with the VKE were highly correlated (r = 0.983) with and not significantly different (α = 0.05) from those obtained by AAS with the neutral extracts and even more so with solutions at pH 11.6 (r = 0.998; α = 0.01). Accurate direct‐potentiometric determinations of the exchangeable K+ in the NH4OAc extracts were not feasible, even at pH 11.6. But the NH+4‐exchangeable K+ values obtained with the VKE and a modified standard additions technique were highly correlated (r = 0.987) with and not significantly different (α = 0.05) from those obtained by AAS.