For compendial purposes, pH is defined as the value given by a suitable, properly standardized, potentiometric instrument (pH meter) capable of reproducing pH values to 0.02 pH unit using an indicator electrode sensitive to hydrogen-ion activity, the glass electrode, and a suitable reference electrode. The instrument should be capable of sensing the potential across the electrode pair and, for pH standardization purposes, applying an adjustable potential to the circuit by manipulation of standardization, zero, asymmetry, or calibration control, and should be able to control the change in millivolts per unit change in pH reading through a temperature and/or slope control. Measurements are made at 25 ± 2, unless otherwise specified in the individual monograph or herein.
The pH scale is defined by the equation:
pH = pHs + (E ES) / k
in which E and ES are the measured potentials where the galvanic cell contains the solution under test, represented by pH, and the appropriate Buffer Solution for Standardization, represented by pHs, respectively. The value of k is the change in potential per unit change in pH and is theoretically [0.05916 + 0.000198(t 25)] volts at any temperature t.
It should be emphasized that the definitions of pH, the pH scale, and the values assigned to the Buffer Solutions for Standardization are for the purpose of establishing a practical, operational system so that results may be compared between laboratories. The pH values thus measured do not correspond exactly to those obtained by the definition, pH = log aH+. So long as the solution being measured is sufficiently similar in composition to the buffer used for standardization, the operational pH corresponds fairly closely to the theoretical pH. Although no claim is made with respect to the suitability of the system for measuring hydrogen-ion activity or concentration, the values obtained are closely related to the activity of the hydrogen-ion in aqueous solutions.
Where a pH meter is standardized by use of an aqueous buffer and then used to measure the pH of a nonaqueous solution or suspension, the ionization constant of the acid or base, the dielectric constant of the medium, the liquid-junction potential (which may give rise to errors of approximately 1 pH unit), and the hydrogen-ion response of the glass electrode are all changed. For these reasons, the values so obtained with solutions that are only partially aqueous in character can be regarded only as apparent pH values.
BUFFER SOLUTIONS FOR STANDARDIZATION OF THE pH METER
Buffer Solutions for Standardization are to be prepared as directed in the accompanying table.* 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.
pH Values of Buffer Solutions for Standardization
Potassium Tetraoxalate, 0.05 m Dissolve 12.61 g of KH3(C2O4)2·2H2O in water to make 1000 mL.
Potassium Biphthalate, 0.05 m Dissolve 10.12 g of KHC8H4O4, previously dried at 110 for 1 hour, in water to make 1000 mL.
Equimolal Phosphate, 0.05 m Dissolve 3.53 g of Na2HPO4 and 3.39 g of KH2PO4, each previously dried at 120 for 2 hours, in water to make 1000 mL.
Sodium Tetraborate, 0.01 m Dissolve 3.80 g of Na2B4O7·10H2O in water to make 1000 mL. Protect from absorption of carbon dioxide.
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.
* Commercially available buffer solutions for pH meter standardization, standardized by methods traceable to the National Institute of Standards and Technology (NIST), labeled with a pH value accurate to 0.01 pH unit may be used. For standardization solutions having a pH lower than 4, a labeled accuracy of 0.02 is acceptable. Solutions prepared from ACS reagent grade materials or other suitable materials, in the stated quantities, may be used provided the pH of the resultant solution is the same as that of the solution prepared from the NIST certified material.