Preparing Buffer Solutions for pH meter Calibration
Buffer solutions are used for the calibration of pH meters as described in the post entitled "Calibrating a pH meter using buffers" . In most pH measurements, a single glass electrode-reference electrode probe assembly is transferred between two solutions. Ex is the measured potential of the solution tested and Es is the measured potential of a standard pH buffer solution.
By definition the pH of a solution x is given by:
pHx = pHs +( Ex - Es)/k
where pHs is the pH of the standard pH buffer solution, pHx the pH of the tested solution, Es and Ex the corresponding potentials and k is a constant at a specific temperature.
The instrument if it is properly calibrated must be capable of reproducing pH values to 0.02 pH units.
To prepare the standard pH buffer solutions (recommended by NIST), the indicated weights of the followig pure materials should be dissolved in water of specific conductivity not greater than 5 micromhos:
- Potassium Tetraoxalate (0.05M) - Dissolve 12.61 g of KH3(C2O4)2.2H2O in water to make 1000 ml
- Potassium Biphthalate (0.05M) - Dissolve 10.12 g of KHC8H4O4 previously dried at 110 C for 1 hour, in water to make 1000 ml
- Equimolar Phosphate (0.05M) - Dissolve 3.53 g of Na2HPO4 and 3.39 g of KH2PO4, each previously dried at 120 C for 2 hours, in water to make 1000 ml
- Sodium Tetraborate (0.01M) -Dissolve 3.80 g of Na2B4O7.10H2O in water to make 1000 ml. Protect from atmospheric carbon dioxide.
- Calcium Hydroxide, saturated at 25 C - Dissolve an excess of calcium hydroxide with water, and decant at 25 C before use. Protect from atmospheric carbon dioxide.
The above solutions must be stored in hard glass or polyethylene bottles fitted with a tight closure or a carbon dioxide-absorbing tube (soda lime). Fresh solutions should be prepared at intervals not to exceed 3 months using carbon dioxide-free water.
Carbon dioxide-free water is distilled water that has been boiled vigorously for at least 5 minutes and allowed to cool without contact with the atmosphere.
Table 1 indicates the pH of the buffer solutions as a function of temperature.
| Table 1: pH Values of Buffer Solutions for Standardization |
|---|
Temperature(°C) | Potassium Tetraoxalate 0.05 M | Potassium Biphthalate 0.05 M | Equimolar Phosphate 0.05M | Sodium Tetraborate 0.01M | Calcium Hydroxide saturd. 25 °C |
| 0 | 1.666 | 4.003 | 6.984 | 9.464 | 13.423 |
|---|---|---|---|---|---|
| 5 | 1.668 | 3.999 | 6.951 | 9.395 | 13.207 |
| 10 | 1.670 | 3.998 | 6.923 | 9.332 | 13.003 |
| 15 | 1.672 | 3.999 | 6.900 | 9.276 | 12.810 |
| 20 | 1.675 | 4.002 | 6.881 | 9.225 | 12.627 |
| 25 | 1.679 | 4.008 | 6.865 | 9.180 | 12.454 |
| 30 | 1.683 | 4.015 | 6.853 | 9.139 | 12.289 |
| 35 | 1.688 | 4.024 | 6.844 | 9.102 | 12.133 |
| 38 | 1.691 | 4.030 | 6.840 | 9.081 | 12.043 |
| 40 | 1.694 | 4.035 | 6.838 | 9.068 | 11.984 |
| 45 | 1.700 | 4.047 | 6.834 | 9.038 | 11.841 |
| 50 | 1.707 | 4.060 | 6.833 | 9.011 | 11.705 |
| 55 | 1.715 | 4.075 | 6.834 | 8.985 | 11.574 |
| 60 | 1.723 | 4.091 | 6.836 | 8.962 | 11.449 |
| 70 | 1.743 | 4.126 | 6.845 | 8.921 | |
| 80 | 1.766 | 4.164 | 6.859 | 8.885 | |
| 90 | 1.792 | 4.205 | 6.877 | 8.850 | |
| 95 | 1.806 | 4.227 | 6.886 | 8.833 |
References
- CRC Handbook of Chemistry and Physics, 52nd edition, The Chemical Rubber Co., (1971)
- U.S. Pharmacopeia, USP 37
- R. G. Bates, J. Res. Natl. Bur. Stand. (U.S.), 66A:179 (1962)
- David W. Oxtoby, H.P. Gillis, Alan Campion, “Principles of Modern Chemistry”, Sixth Edition, Thomson Brooks/Cole, 2008
- Steven S. Zumdahl, “Chemical Principles” 6th Edition, Houghton Mifflin Company, 2009
Key Terms
pH buffer solutions, calibrating a pH meter, buffers, buffer solutions,
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