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1.5 BUFFER SOLUTIONS

          The successful completion of many Pharmacopoeial tests and assays requires adjustment to, or maintenance of, a specified pH by the addition of buffer solutions. The following text sets out the compositions of various buffer solutions required. For convenience, however, the preparations of several single-purpose solutions are described in individual appendices.

          Standard solutions of definite pH are readily available in buffer solutions prepared from the appropriate reagents. In addition, buffer solutions, buffer tablets, and buffer solids may be obtained from commercial sources in convenient prepackaged form. Such preparations are available for the entire working range in pharmaceutical analysis, but are not recommended for pH meter standardization.

          The required reagents are described in Appendix 1.1. All crystalline reagents, except boric acid, should previously be dried at 110 to 120º for 1 hour. Buffer solutions should be prepared using carbon dioxide-free water and stored in bottles made of alkali-free glass, and must not be used later than 3 months after preparation.

Ammonia Buffer pH 10.0 Dissolve 5.4 g of ammonium chloride in 70 ml of 5 M ammonia and dilute with water to 100 ml.

Acetate Buffer pH 3.0 Dissolve 12 g of sodium acetate in water, add 6 ml of glacial acetic acid and dilute with sufficient water to 100 ml.

Acetate Buffer pH 3.5 Dissolve 25.0 g of ammonium acetate in 25 ml of water and add 38.0 ml of 7 M hydrochloric acid. Adjust the pH to 3.5 with either 2 M hydrochloric acid or 5 M ammonia and dilute with water to 100 ml.)

Acetate Buffer pH 4.4 Dissolve 136 g of sodium acetate and 77 g of ammonium acetate in water and dilute to 1000 ml with water. Add 250 ml of glacial acetic acid and mix.

Acetate Buffer pH 4.6 Dissolve 5.4 g of sodium acetate in 50 ml of water, add 2.4 g of glacial acetic acid and dilute to 100 ml with water. Adjust the pH if necessary.

Acetate Buffer pH 6.0 Dissolve 100 g of ammonium acetate in 300 ml of water, add 4.1 ml of glacial acetic acid, adjust the pH, if necessary, using 10 M ammonia or 5 M acetic acid and dilute to 500 ml with water.

Acetic-Ammonia Buffer pH 3.7, Ethanolic To 15 ml of 5 M acetic acid add 60 ml of ethanol and 24 ml of water. Adjust the pH of the solution to 3.7 with 10 M ammonia and dilute with water to 100 ml.

Barbital Buffer pH 8.6, Mixed Dissolve 1.38 g of barbital, 8.76 g of barbital sodium and 380 mg of calcium lactate in sufficient water to produce 1000 ml.

Borate Buffer pH 9.0 To 50 ml of a solution containing 618.9 mg of boric acid and 745.6 mg of potassium chloride, add 21.30 ml of 0.20 M sodium hydroxide and dilute with water to 200.0 ml. At 20º, the solution may be used as a solution of standard pH.

Imidazole Buffer Solution pH 7.3 Dissolve 340 mg of imidazole and 580 mg of sodium chloride in sufficient water, add 18.6 ml of 0.1 M hydrochloric acid and dilute to 100 ml with water. Adjust the pH if necessary.

Phosphate Buffers Solutions from pH 5.8 to pH 8.0 may be prepared by mixing 50 ml of a solution of 0.2 M potassium dihydrogenphosphate with the quantities of 0.20 M sodium hydroxide specified in the following table, and diluting with water to 200.0 ml. At 20º, the solutions may be used as solutions of standard pH.

Phosphate Buffer pH 6.8, Mixed Dissolve 2.880 g of disodium hydrogenphosphate and 1.145 g of potassium dihydrogenphosphate in sufficient water to produce 100 ml.

Phosphate Buffer pH 7.0, Mixed, 0.067 M Dissolve 908 mg of potassium dihydrogenphosphate in sufficient water to produce 100 ml (Solution A). Dissolve 2.38 g of disodium hydrogenphosphate in sufficient water to produce 100 ml (Solution B). Mix 38.9 ml of solution A with 61.1 ml of solution B.

Saline pH 7.4, Phosphate-buffered Dissolve 2.38 g of disodium hydrogenphosphate, 0.19 g of potassium dihydrogenphosphate and 8.0 g of sodium chloride in sufficient water to produce 1000 ml and adjust the pH if necessary.

Tris-chloride Buffer pH 7.5 Dissolve 727 mg of tris (hydroxymethyl)methylamine and 527 mg of sodium chloride in 95 ml of water, adjust the pH to 7.5 with 2 M hydrochloric acid and dilute to 100 ml with water.

Tris-EDTA BSA Buffer Solution pH 8.4 Dissolve 6.1 g of tris(hydroxymethyl)methylamine, 2.8 g of disodium edetate, 10.2 g of sodium chloride and 10 g of bovine serum albumin in water, adjust to pH 8.4 using 1 M hydrochloric acid and dilute to 1000 ml with water.