Tensammetric titration of potassium ion with tetraphenylborate

We had undertaken an investigation of tetraphenylborate (TFB) by alternating current polarography, aiming to evidence some capacitive signals with potential analytical application. It was found that TFB produces a characteristic tensammetric peak at - 0.95 V vs SCE in slightly acidic media (pH 4 - 5, acetate buffer) or NaCl 0.1 to 0.5 M. The electrocapillary curves recorded in the absence and respectively in the presence of the TFB ion clearly evidenced the adsorption properties of the TFB ion in the potential region 0 to 0.95 V. The influence of different experimental parameters, such as pH, ionic strength, temperature, TFB concentration on the tensammetric signal was investigated. The tensammetric signal depends linearly on the TFB concentration in a limited range (till about 0.4 mM), and at higher concentration the plot tend to attend a constant value. From the analytical point of view, TFB concentration should be measured on the basis of the tensammetric signal, but also by measuring the capacitive current at - 0.4 V. The tensammetric peak current was used as an analytical signal in the amperometric back-titration of the potassium ion in different samples of bottled mineral waters. The method was validated by comparison with the gravimetric method. Thus, the potassium content in a comercial sample of mineral water was 16.3 ± 1 mg/l according to the gravimetric method, while the tensammetric method indicated a content of 17.5 ± 0.7 mg/l. The method is fast and requires only moderateamounts of TFB.