Potentiometric Determination of Potassium Q/I Relationships
Methods of determining potassium quantity-intensity (Q/I) relationships with ion-selective electrodes (ISEs) were developed and evaluated with three Iowa soils. In doing so, soil suspensions and their filtrates were characterized with single-ISE [ISE(S)] and dual-ISE [ISE(D)] procedures, the ISEs were calibrated in terms of ion concentrations to obtain Q/I relationships based on concentration ratios (CR), and the results were compared with those obtained by conventional analyses of the filtrates by atomic absorption spectrometry (AAS). By using ISEs with filtrates from soil suspensions that were equilibrated for 18 h, it was established that the ISE(S) and ISE(D) methods yield essentially the same Q/I curves and related parameters as the reference AAS method. The potentiometrically determined CR values agreed particularly well with the AAS values that were based on Ca2+ plus Mg2+ because of the response of the Ca-ISE to Mg2+. Subsequent experiments on the feasibility of using the ISE(D) procedure with the soil suspensions (to eliminate the filtration step) also yielded results that were in excellent agreement (no significant difference at α = 0.05) with those obtained by analyzing the filtrates. The time required for Q/I determinations was further reduced by using the ISEs with soil suspensions to which successive increments of KCl were added. This procedure, however, altered the shape and critical parameters of the Q/I curves. Clearly more information about the relevance of the different Q/I results is needed. The ISE methods provide a simple and rapid means of obtaining the necessary data.
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