URINE 523 



~ . , . 20 (reading of standard) . __ . 



Calculation : _ , , r- = me. of ammonia N in amount of urine 



R (reading of unknown) 



used. 



Calculate the per cent of ammonia and the 24 hour output. 



Interpretation. See page 520. 



Amino-acid Nitrogen 



i. 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 derivatives which are more strongly acid 

 in reaction due to the destruction of the basic properties of the ammo 

 groups. The carboxyl groups may then be titrated using phenol- 

 phthalein as an indicator. 



R.CH.NH 2 



+ CH 2 = 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 formalde- 

 hyde in a similar manner as is shown in the following equation: 



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



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 inter- 

 fere 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 certain 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 determination of amino- 

 acids in any medium, e.g., urine, protein digests, etc. When poorly 

 dissociated acids, e.g., some fatty acids, are present, these will in part 

 be included in the result and lead to values which are too high. Certain 

 of the amino-acids when present in large amounts will give erroneous 

 results, but in the ordinary urine or digest these errors are either 

 negligible or compensate each other. In the titration of colored solu- 

 tions the control' solution which is necessary in this method must be 

 colored to correspond with the color of the unknown solution. 



1 Henriques and Sorensen: Zeit. physiol. chem., 64, 120, 1909 



