632 PHYSIOLOGICAL CHEMISTRY 



Strictly speaking there is no such thing as "combined" acid in this 

 sense. When the protein is added a protein salt of the acid is formed 

 which ionizes differently from the free acid. 



Congo Red. 1 Dissolve 0.5 gram of Congo red in 90 c.c. of water 

 and add 10 c.c. of 95 per cent alcohol. 



Congo Red-Fibrin. This may be prepared by placing fibrin in 

 faintly alkaline Congo red solution and heating to 80 C. The fibrin is 

 then washed and preserved under glycerol. 



Creatinine, Standard Solution for Colorirnetric Method. 2 Dissolve 

 i gram of pure creatinine in 1000 c.c. of N/io HC1. The solution con- 

 tains i mg. of creatinine per cubic centimeter. For blood analysis 

 transfer 6 c.c. of this solution to a liter flask, add 10 c.c. of normal 

 HC1, dilute to mark with water and mix. 



Cross and Bevan's Reagent. Combine two parts of concentrated 

 hydrochloric acid and one part of zinc chloride by weight. 



Ehrlich's Diazo Reagent. 3 Two separate solutions should be pre- 

 pared and mixed in definite proportions when needed for use. 



(a) Five grams of sodium nitrite dissolved in i liter of distilled water. 



(b) Five grams of sulphanilic acid and 50 c.c. of hydrochloric acid in 

 i liter of distilled water. 



Solutions (a) and (b) should be preserved in well-stoppered vessels 

 and mixed in the proportion i : 50 when required. Green asserts that 

 greater delicacy is "secured by mixing the solutions in the proportion 

 1:100. The sodium nitrite deteriorates upon standing and becomes 

 unfit for use in the course of a few weeks. 



Esbach's Reagent. 4 Dissolve 10 grams of picric acid and 20 grams 

 of citric acid in i liter of water. 



Fehling's Solution. 5 Fehling's solution is composed of two definite 

 solutions a copper sulphate solution and an alkaline tartrate solution, 

 which may be prepared as follows : 



Copper sulphate solution = 34.65 grams of copper sulphate dissolved 

 in water and made up to 500 c.c. 



Alkaline tartrate solution = 125 grams of potassium hydroxide and 

 173 grams of Rochelle salt dissolved in water and made up to 500 c.c. 



These solutions should be preserved separately in rubber-stoppered 

 bottles and mixed in equal volumes when needed for use. This is done 

 to prevent deterioration. 



Ferric Alurn Solution. 6 A cold saturated solution. 



1 Test for free acid, p. 156. 



2 Determination of creatinine, pp. 580 and 528. 



3 Ehrlich's diazo reaction, p. 469. 



4 Esbach's method, p. 551. 



6 Fehling's method, p. 541. Fehling's test, pp. 25 and 433. 

 6 Volhard- Arnold method, p. 572. 



