To standardize the pH meter, select two Buffer Solutions for Standardization
whose difference in pH does not exceed 4 units and such that the expected pH of the material under test falls between them. Fill the cell with one of the Buffer Solutions for Standardization
at the temperature at which the test material is to be measured. Set the temperature control at the temperature of the solution, and adjust the calibration control to make the observed pH value identical with that tabulated. Rinse the electrodes and cell with several portions of the second Buffer Solution for Standardization,
then fill the cell with it, at the same temperature as the material to be measured. The pH of the second buffer solution is within ±0.07 pH unit of the tabulated value. If a larger deviation is noted, examine the electrodes and, if they are faulty, replace them. Adjust the slope or temperature control to make the observed pH value identical with that tabulated. Repeat the standardization until both Buffer Solutions for Standardization
give observed pH values within 0.02 pH unit of the tabulated value without further adjustment of the controls. When the system is functioning satisfactorily, rinse the electrodes and cell several times with a few portions of the test material, fill the cell with the test material, and read the pH value. Use carbon dioxide-free water (see Water
in the section Reagents, Indicators, and Solutions
) for solution or dilution of test material in pH determinations. In all pH measurements, allow a sufficient time for stabilization.