15. Determination of substance A in the analyzed solution was carried out by direct potentiometry. First, standard solutions were prepared with various precisely known concentrations c(A) of the analyte. after that, the reference solutions were introduced one by one into an electrochemical cell, including an appropriate ion-selective indicator electrode and a silver chloride reference electrode, and the electromotive force E of the circuit was measured for each reference solution. Then, under the same conditions, the electromotive force of the circuit with the analyzed solution was measured.
According to the data below, construct a calibration graph in the coordinates E - pA, where pA \u003d -lg c (A), and determine the molar concentration of substance A in the analyzed solution.
Option Substance A Electromotive force E of the circuit, mV
Reference solutions with concentration с(А), mol/l Analyzed
solution
0.00001 0.0001 0.001 0.01 0.1
to K - -60 -7 46 100 60

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