REAGENTS AND SOLUTIONS 641 



Picramic Acid. Permanent Standard for Lewis-Benedict Blood Sugar 

 Method* A solution of picramic acid makes a very satisfactory 

 permanent standard. . The color is identical in quality with that formed 

 in the method and its solution keeps perfectly. The formula of the 

 permanent standard is: 



Picramic acid o . 064 gram. 



Sodium carbonate (anhydrous) o. 100 gram. 



Water to make 1000.0 c.c. 



Dissolve the picramic acid with the aid of heat in 25-50 c.c. of distilled 

 water which has been made alkaline with sodium carbonate. Cool and 

 dilute to i liter. This solution has the same intensity of color as that 

 obtained by the proposed method with 0.64 mg. of sugar when the 

 final volume of the reaction fluid is made 10 c.c. The solution should 

 be standardized against pure glucose. A satisfactory preparation .of 

 picramic acid may be obtained from the J. T. Baker Chemical Co., 

 Phillipsburg, N. J. 



Picric Acid, Pure. 2 As purchased picric acid contains 10 per cent 

 of added water. By exposure to the air between large filter papers 

 (best in a warm place) the water will disappear by evaporation. Dry 

 picric acid may be prepared in this way. If the alkaline picrate formed 

 from this acid gives too deep a color, it may be purified as follows: 



(a) Method of Folin and Doisy. Transfer about 600 gm. of wet 

 picric acid, or about a pound of dry picric acid, to a large beaker 

 (capacity not less than 4 liters). Pour on boiling water until the 

 beaker is nearly full and add 200 c.c. of saturated (50 per "cent) sodium 

 hydroxide solution. Stir, and if necessary heat again until all the 

 picric acid has been dissolved, yielding a deep red picrate solution. 

 To the hot solution add rather slowly, with stirring, 200 gm. of sodium 

 chloride. Cool in running water to about 3OC., with occasional 

 stirring. Filter on a large Buchner funnel and wash a few times with 

 5 per cent sodium chloride solution. Transfer the picrate to a large 

 beaker, fill with boiling water, and when the picrate is dissolved add, 

 with stirring, first 50 c.c. of 10 per cent sodium hydroxide solution, and 

 then 100 gm. of sodium chloride. Cool to 3oC., with stirring, filter, 

 and wash with sodium chloride solution, as before. Repeat the solu- 

 tion and precipitation of the sodium picrate once more, but for the 

 last washing of the last precipitated picrate use distilled water instead 

 of the sodium chloride solution. 



Dissolve the purified picrate in the same large beaker with boiling 

 distilled water, and filter while hot on a large folded filter, collecting 



1 Determination of sugar in blood, p. 287. 



2 Determination of creatinine p. 280, and calcium p. 293, in blood. 



