208 CAPILLARY TUBE COLORIMETRY 



PHOSPHATE 



A colorimetric capillary tube method for inorganic phosphate was 

 developed by Walker ( 1933) as an adaptation of the Kuttner ( 1927, 

 1930) modification of the Bell-Doisy phosphomolybdic acid method. 

 Walker's procedure enables analysis of as little as 0.08 fA. of liquid 

 containing less than 1 m^tig. phosphate phosphorus with a mean error 

 of about +0.1% and a mean deviation of about ±2.5% for solu- 

 tions of known concentration. A recent discussion by Sumner (1944) 

 of phosphomolybdic acid methods should be consulted. For other 

 methods see pages 124, 226, and 280. 



Walker Method for Phosphate 



SPECIAL REAGENTS 



Standard Phosphate Solution. Prepare standards in the range of 

 1.5 to 7.0 milligram per cent phosphorus differing from one an- 

 other by 0.5 milligram per cent and below 1.5 milligram per cent 

 by 0.1 or 0.2 milligram per cent. 



Molybdic-Sulfuric Acid Reagent. To 1 vol. of 10 A^ sulfuric acid 

 (282 ml. cone, acid, 95%, sp.gr. 1.84, to 1 1.) add 2 vol. distilled 

 water and 1 vol. 7.5% sodium molybdate soln. Store in a brown 

 glass-stoppered bottle. 



Stannous Chloride Stock Solution. Prepare a 40% soln. in cone, 

 hydrochloric acid and store in a brown glass-stoppered bottle. Do 

 not use longer than one week. 



Stannous Chloride Working Solution. Dilute the stock soln. 1:100 

 and prepare fresh each day. 



PROCEDURE 



1. Introduce a column of about 0.2 /xl. of the trichloroacetic acid 

 supernatant (page 199) or other phosphate soln. into a capillary tube 

 (0.35 mm. inside diameter) followed by an equal vol. of the molyb- 

 dic-sulfuric acid reagent and withdraw the two columns 3 cm. from 

 the end of the tube. 



2. Introduce a third equal column of the stannous chloride work- 

 ing soln. and seal the ends of the portion of the tube containing the 

 liquids. 



3. In a similar manner prepare tubes with the standard phos- 

 phate solns. 



