Buffer Solutions for Standardization
are to be prepared as directed in the
Buffer salts of requisite purity can be obtained from the National Institute of Science and Technology. Solutions may be stored in hard glass or polyethylene bottles fitted with a tight closure or carbon dioxide-absorbing tube (soda lime). Fresh solutions should be prepared at intervals not to exceed 3 months using carbon dioxide-free water. The table indicates the pH of the buffer solutions as a function of temperature. The instructions presented here are for the preparation of solutions having the designated molal (m)
concentrations. For convenience, and to facilitate their preparation, however, instructions are given in terms of dilution to a 1000-mL volume rather than specifying the use of 1000 g of solvent, which is the basis of the molality system of solution concentration. The indicated quantities cannot be computed simply without additional information.
Equimolal Phosphate, 0.05 m
Dissolve 3.53 g of Na2
and 3.39 g of KH2
each previously dried at 120
for 2 hours, in water to make 1000 mL.
Calcium Hydroxide, saturated at 25
Shake an excess of calcium hydroxide with water, and decant at 25
before use. Protect from absorption of carbon dioxide.
Because of variations in the nature and operation of the available pH meters, it is not practicable to give universally applicable directions for the potentiometric determinations of pH. The general principles to be followed in carrying out the instructions provided for each instrument by its manufacturer are set forth in the following paragraphs. Examine the electrodes and, if present, the salt bridge prior to use. If necessary, replenish the salt bridge solution, and observe other precautions indicated by the instrument or electrode manufacturer.
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.
Where approximate pH values suffice, indicators and test papers (see Indicators and Indicator Test Papers, in the section Reagents, Indicators, and Solutions) may be suitable.
For a discussion of buffers, and for the composition of standard buffer solutions called for in compendial tests and assays, see Buffer Solutions in the section Reagents, Indicators, and Solutions.