pH Buffer Solution Preparation

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. E_x is the measured potential of the solution tested and E_s is the measured potential of a standard pH buffer solution.

By definition the pH of a solution x is given by:

pH_x = pH_s +( E_x - E_s)/k

where pH_s is the pH of the standard pH buffer solution, pH_x the pH of the tested solution, E_s and E_x 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

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Relevant Posts

Measuring the pH of a Solution with a pH meter

Calibrating a pH meter using buffers

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References

1. CRC Handbook of Chemistry and Physics, 52nd edition, The Chemical Rubber Co., (1971)
2. U.S. Pharmacopeia, USP 37
3. R. G. Bates, J. Res. Natl. Bur. Stand. (U.S.), 66A:179 (1962)
4. David W. Oxtoby, H.P. Gillis, Alan Campion, “Principles of Modern Chemistry”, Sixth Edition, Thomson Brooks/Cole, 2008
5. Steven S. Zumdahl, “Chemical Principles” 6th Edition, Houghton Mifflin Company, 2009

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Key Terms

pH buffer solutions, calibrating a pH meter, buffers, buffer solutions,

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