STANDARDIZATION OF pH BUFFERS

       1.0 PROCEDURE

          Note: Use Carbon dioxide free water.

          Reagents:

(1)   Potassium Tetra oxalate, 0.05 M (pH 1.68 at 25°C):  Dissolve 3.1525 g of KH₃(C₂O₄)₂. 2H₂O in purified water to make 250 mL.                                                                                           

(2)   Potassium  Biphthalate,  0.05 M (pH 4.01 at 25°C):  Dissolve  2.5300 g of  KHC₈H₄O₄,  previously dried at  110°C for 1 hour, in purified water to make 250 mL.                                                             

(3)   Equimolal Phosphate, 0.05 M (pH 6.86 at 25°C): Dissolve 0.8825 g of anhydrous disodium hydrogen phosphate  (Na₂HPO₄) and 0.8475 g of potassium dihydrogen phosphate  (KH₂PO₄), each previously dried at 120°C for 2 hours, in purified water to make 250 mL.                                                              

(4)   Sodium Tetraborate, 0.01M (pH 9.18 at 25°C): Dissolve 0.9500 g of Na₂B₄O₇.10H₂O  in  purified water to make 250 mL. Protect from absorption of carbon dioxide.                                                     

(5)   Calcium Hydroxide, saturated (pH 12.45 at 25°C) : Shake the excess of calcium hydroxide with  purified  water  and  decant at  25°C before  use.  Protect from absorption of carbon dioxide.

Procedure :

(1)   Operate and calibrate the pH meter and electrode system according to the SOP for the pH meter.  

(2)   All measurements should be made at the same temperature of 25°C ± 2°C unless otherwise stated in the testing procedure in the monograph.                                                                                              

Note : 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 sample under  test  falls  between  them.

         pH Values of Buffer Solutions for Standardization

Standardization, general testing procedure, potassium dihydrogen