The surface charge density, sigma, on spherical silica particles was measured in 0.05, 0.01, and 0.005 M aqueous KCl solutions by potentiometric titration. Two different methods for a linear transformation to evaluate the parameters describing the sigma-pH plots including a surface potential for spherical particles were applied; each one has advantages and shortcomings. The equations derived by C. P. Huang and W. Stumm [J. Colloid Interface Sci. 43, 409 (1972)] give a higher correlation coefficient for the linear plot. An excellent agreement between the recalculated sigma-pH plots and the experimental data is found on application of the method of J. S. Noh and J. A. Schwarz [J. Colloid Interface Sci. 139, 139 (1990)] using the data of three concentrations in one regression analysis.