240 CUVETTE COLORIMETRY 



The uric acid method is based on precipitation of the suljstance 

 with zinc, solution of the precipitate in dilute acid and water, addi- 

 tion of cyanide followed by Benedict's arsenophosphotungstic acid 

 reagent, and development of the color by a procedure which yields a 

 clear solution. The method was designed for the analysis of 1 ml. of 

 sample having less than 1 milligram per cent uric acid, i.e., less then 

 10 /xg. For another method see page 213. 



The creatinine method involves absorption of the compound on 

 Lloyd reagent, removal of impurities, and liberation of the creatinine 

 by the same alkaline picrate in which the color is developed. Creatine 

 is determined by the increase in creatinine after conversion to the 

 latter. With the 3 ml. cuvettes which Borsook used, the smallest 

 quantity of creatinine which could be measured was 1 ju,g. The 

 absolute error with all concentrations was ±0.1 jug. For another 

 method see page 211. 



The allantoin method depends on the enzymatic conversion of 

 allantoin to allantoic acid (in the presence of cyanide to prevent the 

 formation of allantoin from uric acid), acid hydrolysis of the 

 allantoic acid to urea and glyoxylic acid, and colorimetric measure- 

 ment of the latter. With 2 ml. of sample, the least that can be 

 measured is 0.05 milligram per cent (1 /xg.) with an error of ±5%. 



Sure and Wilder (1941) employed the micro attachment on the 

 Evelyn photoelectric colorimeter for the measurement of creatine 

 and creatinine. Gold-plated plungers had to be used because the 

 ordinary cadmium-coated plungers were corroded by the saturated 

 picric acid. The error in the conversion of creatine to creatinine by 

 the procedure employed varied from —0.79 to +2.66% in test 

 experiments. 1 ml. blood filtrate was used by the authors for each 

 analysis. 



Borsook Method for Uric Acid 



SPECIAL REAGENTS 



2.5% Zinc Chloride. 



10% Sodium Carbonate. 



N/14 Hydrochloric Acid. 



5% Sodium Cyanide (containing 2 ml. cone, ammonium hydroxide 



per liter. Prepare fresh every 6—7 weeks. 

 Benedict's Arsenophosphotungstate Reagent. Dissolve 100 g. pure 



