The methyl red stock solution was added to sample solution and the absorbance values of this solution were measured at the maximum absorption wavelengths of the acidic form, the basic form, and the isosbestic point of the indicator. From the measurements of absorbance, the pH of this solution was calculated. The range of absorbance at the isosbestic point was maintained within 0.1 ${sim}$ 0.3. The error of pH measurement was within ${pm}0.08$ pH unit in the pH range of sample solution of $pK_I{pm}1$.