Criteria for using approximative formulas were derived for simple protolytic equilibria in the case of strong acids and bases as well as for weak acids aqueous solutions. The approximative formulas can be used if the analytical concentration of given protolyt is higher than the value of limiting (critical) concentration calculated from criterial equation derived in this paper. For strong acids and bases the limiting concentration under which the equlibrium concentration of hydrogen ions can not be calculated according the approximative equation is under micromolar range. For extremely weak acids the cubic equation should be fully aproximated with quadratic equation without linear term. If the weak acid has high value of the dissociation constant using the quadratic equation for calculating the equilibruim concentration of hydrogen ions is a must.