AMINO-ACID NITROGEN 55 



this they advise the addition of 20 c.c. of glacial acetic acid for 

 each 300 c.c. of urine employed, the acid being added before the 

 first precipitation. 



AMINO-ACID NITROGEN 



Henriques-Sorensen Formol Titration Method 1 



Principle. A solution containing ammo-acids is nearly neutral 

 in reaction. If formaldehyde be added, however, the following 

 reaction takes place with the formation of methylene deriva- 

 tives which are more strongly acid in reaction due to the destruc- 

 tion of the basic properties of the amino groups. The carboxyl 

 groups may then be titrated, using phenolphthalein as an indicator. 



R.CH.NH 2 



+CH 2 O = R-CH-N : CH 2 +H 2 O. 

 COOH | 



COOH 



The acidity as shown by the titration is a measure of the amount 

 of amino-acid nitrogen present. Ammonia likewise reacts with 

 formaldehyde in a similar manner as is shown in the following 

 equation : 



4NH 4 C1+6CH 2 = N 4 (CH 2 ) 6 +6H 6 0+4HC1. 



Hence the formol titration in the presence of ammonia gives 

 results which include both amino-acid and ammonia nitrogen. 

 Ammonia may be determined and a correction applied, or 

 the ammonia may be removed by means of phosphotungstic 

 acid. Phosphates also interfere by obscuring the end-point and 

 are removed by the addition of barium salts. 



It must be borne in mind that polypeptides and still more 

 complex protein derivatives likewise react with formol to a cer- 

 tain degree so that the results do not strictly represent "amino- 

 acid nitrogen." 



The method is, with some modifications involving the 

 preparation of the solution to be titrated, applicable in the deter- 

 mination of amino-acids in any medium, e.g., urine, protein digests, 



1 Henriques and Sorenson: Zeit. physiol. Chem., 1909, 64, 120. 



