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Ion‐selective Electrode Determinations of Exchangeable Potassium in Soils
Author(s) -
Farrell R. E.,
Scott A. D.
Publication year - 1987
Publication title -
soil science society of america journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.836
H-Index - 168
eISSN - 1435-0661
pISSN - 0361-5995
DOI - 10.2136/sssaj1987.03615995005100030008x
Subject(s) - chemistry , potentiometric titration , potassium , atomic absorption spectroscopy , glass electrode , analytical chemistry (journal) , ion selective electrode , electrode , electroanalytical method , ionic strength , valinomycin , reference electrode , electrochemistry , inorganic chemistry , nuclear chemistry , chromatography , aqueous solution , selectivity , biochemistry , physics , organic chemistry , quantum mechanics , catalysis
Procedures using ion‐selective electrodes (ISEs) for the accurate determination of NH + 4 ‐and Ba 2+ ‐exchangeable K + in soil extracts are described. The exchangeable K + in 30 soil samples was extracted with neutral solutions of 1.0 M NH 4 OAc and 0.5 M BaCl 2 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 Ba 2+ ‐ 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 BaCl 2 extracts, where it yielded exchangeable K + values that were only poorly correlated ( r = 0.655) with those obtained by AAS. However, the Ba 2+ ‐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 NH 4 OAc 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.