A general calculation programme for the methods KNOWN ADDITION, KNOWN SUBTRACTION, SAMPLE ADDITION, and SAMPLE SUBTRACTION has been worked out. It is derived from Gran's method of potentiometric titrations. The preliminary knowledge of slope S [mV/pX] in the studied medium is not needed as it is found by iteration. On the contrary, the experiment is concluded as erroneous if the recorded data are unsuitable. The ratio of the determined ion in a sample to that in the introduced standard [mol/mol] is followed. Linearity of experimental data is checked graphically. A hardware supplement has been developed for the application ISEPRO in the form of an additional card for PC (A/D converter). The procedure is demonstrated in the record of potentiometric determination of cyanide by CN-ISE.