HYDROLYSIS 13 



Some modification of the method is necessary for the titration of 

 amino acids when they are present in dark brown solutions such as 

 result from the hydrolysis of proteins. Sorensen and Jessen-Hansen 

 [1907] have given the following procedure: The reaction of the solu- 

 tion must be approximately decinormal ; 25 c.c. of such a solution is 

 used and placed in a 50 c.c. measuring flask. If the solution is not 

 of this concentration in acid, 20 c.c. are taken and treated with ^N 

 hydrochloric acid or sodium hydrate as required to produce the proper 

 acidity. In the case of a solid substance 1-3 grams are dissolved in 

 25 c.c. of -I N hydrochloric acid. Four c.c. of 2N barium chloride solu- 

 tion (244 grams per litre) are added and then gradually about 20 c.c. of 

 *33N silver nitrate solution (56*7 grams per litre), which is best dropped 

 into the solution from a small measuring cylinder. After each addition 

 of silver nitrate the mixture is frequently and thoroughly shaken. The 

 foam settles on standing for a short time and the flask is then filled with 

 water free from carbon dioxide ; if great accuracy is required four drops 

 more of water are put in to allow for the volume of the silver chloride. 

 The solution is filtered through an 1 1 cm. filter paper, taking care to 

 bring as much as possible of the precipitate upon the filter. The 

 filtrate, which is at first cloudy, is again poured carefully on the filter. 

 The filtrate is now quite clear, and 1 5 or 30 c.c. are titrated with forma- 

 lin using phenolphthalein as indicator. The reagents required are (i) 

 a solution of 0*5 gram of phenolphthalein in 50 c.c. alcohol + 50 c.c. 

 water ; (2) a neutral formalin solution which must be prepared fresh 

 for every series of experiments. It is prepared by adding I c.c. of 

 phenolphthalein solution to 50 c.c. of commercial formalin (30-40 per 

 cent.) and adding 4 2N baryta or sodium hydrate solution until it has a 

 faint pink colour. In order to be sure of the end point in the titration 

 a control colour is prepared in the same volume of liquid as is used in 

 the actual experiment. E.g., 10 c.c. of the formalin solution are added 

 to 20 c.c. of boiled distilled water : 5 c.c. of *2N baryta or sodium 

 hydrate are added and the solution titrated with -2N hydrochloric 

 acid, which is dropped in until the solution has a faint pink colour. 

 A drop of *2N baryta or soda is added to produce a distinct red 

 colour. Twenty c.c. of the solution l to be estimated are treated with 

 10 c.c. of the formalin solution and then with *2N baryta or soda until 

 there is a red colour ; a few more c.c. of alkali are added. The solu- 

 tion is titrated with -2N hydrochloric acid until the colour is paler 



1 Henriques and Gjaldbak [1910] neutralise the solution to litmus before titrating in the 

 presence of formalin. 



