REAGENTS AND SOLUTIONS 599 



to the blue end point. The iodide solution is then diluted to such a 

 degree that 10 c.c. are exactly equivalent to 5 c.c. of the silver solution. 

 (c) A solution used as an indicator, to regulate acidity, and provide 

 an oxidizing agent. 



Sodium citrate (NasCeHsOy+S^ H 2 O) .................... 446.0 grams. 



Sodium nitrite .......................................... 20.0 grams. 



Soluble starch .......................................... 2.5 grams. 



Water to ............................................... 1000.0 c.c. . 



The starch is first dissolved with the aid of heat in about 500 c.c. of the 

 water. The citrate and nitrite are then added, and the mixture is heated 

 until all is dissolved. The solution while still hot is filtered through 

 cotton, the filter washed with hot water, the nitrate allowed to cool, 

 and made up to 1000 c.c. The solution keeps indefinitely. 



Combined Hydrochloric Acid (Protein Salt). To prepare so-called 

 combined hydrochloric acid simply add a soluble protein such as Witte's 

 peptone to free hydrochloric acid of the desired strength until it no 

 longer responds to free acid tests (see chapter on Gastric Digestion). 

 For example, if 0.2 per cent combined acid is required the protein would 

 be added to 0.2 per cent free hydrochloric acid. 



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 Colorimetric 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. 



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 



1 Test for free acid, p. 15 3; 



2 Determination of creatinine, p. 506. 

 8 Ehrlich's diazo reaction, p. 454. 



