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Perfluorosulfonic acid membranes MF-4SC with a uniform and gradient distribution of multiwalled carbon nanotubes (CNTs) along the length of the samples have been synthesized. The effect of the presence and concentration of CNTs in the membranes on the equilibrium and transport properties of materials has been studied. The cross sensitivity of Donnan potential (DP) sensors to nicotinic acid, sodium, and hydroxonium ions for membranes of different compositions has been determined. The incorporation of 0.5-1.0 wt. % CNTs into a membrane has led to a decrease in the detection limits of nicotinic acid ions in aqueous solutions by a factor of 2-3. An array of cross-sensitive DP-sensor based on MF-4SK and MF-4SC + 1.0 wt. % CNT membranes has been used to determine the active and auxiliary substances in a pharmaceutical based on nicotinic acid. The relative error of determination is 1.9%.