Potentiometric Determination of Water Using Spent Imidazole-Buffered Karl Fischer Reagents

A new approach to the use of the Karl Fischer (KF) reagent is presented. The method is based on direct potentiometry (zero current) using spent KF reagents and a platinum wire as indicating electrode. A large number of imidazole-buffered methanolic reagents were investigated; the most promising one was based on a 3:1 mixture of chloroform:methanol containing 2 M imidazole/0.2 M sulfur dioxide/0.1−0.2 M iodide/0.5 M trichloroacetic acid. For this reagent, the slope of the calibration curve, −log [H 2 O] versus the redox potential, was found to be strictly linear in the interval 10 -3 −1 M water, with a typical regression coefficient of 0.9999. For calibrations carried out in the interval 10 -3 −1 M, the standard deviation of the slope values, for three additions of water for each calibration curve, obtained at eight different occasions (i.e., n = 24), using two different types of cells, was 1.5 mV, with a mean slope value of 37.5 mV. A lower working range, 10 -4 −10 -3 M water, could also be used for analytical purposes, although the slope of the calibration curve in this interval was only about 30 mV/decade. The response time, i.e., the time required to reach the drift level, was, for this reagent, relatively short (2−20 s). The most rapid response was obtained for larger concentrations of water. The drift in the potential readings was typically about 0.1 mV min -1 for the reagent mixture described above. Since the method operates at extremely low iodine concentrations, 10 - 8 −10 - 13 M, the possibilities of achieving discrimination between the water reaction and some iodine-consuming interferences such as sodium tetraborate, cyclohexanone and a penicillin were investigated. A procedure is described which shows that the large positive error caused by sodium tetraborate under normal KF-conditions could be almost completely eliminated by means of a simple graphic evaluation of the potentiometric response curve obtained in the interval 10 - 3 M to 10 - 2 M of water.