A new method is developed, allowing control of chemical equilibrium in such a way that a criterion depending on the chemical composition is constant with temperature and pressure variations. The application of the implicit function theorem and Kuhn-Tucker conditions allows calculating the sensitivity of chemical composition with respect to internal parameters. The equations are derived that allow calculation of the dependence between temperature and pressure under a fixed value of a linear or non-linear function of chemical composition. Numerical calculations are conducted that confirm the theoretical results.