Ion-selective Electrode Determinations of Exchangeable Potassium in Soils1
- R. E. Farrell and
- A. D. Scott2
Procedures 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.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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