5O2 PHYSIOLOGICAL CHEMISTRY 



3. Formol Titration Method (Malfatti). 1 Principle. This method 

 is based on the reaction taking place when formalin solution is 

 added to a solution containing ammonium salts (see Amino-acid 

 Nitrogen, below). An acid reaction is produced in the mixture, 

 which is then titrated with standard alkali using phenolphthalein as 

 an indicator. Amino-acids give the same reaction so that the result 

 of the titration represents ammonia + amino-acid nitrogen. This 

 method may be used for the rapid clinical estimation of these forms of 

 nitrogen as a substitute for an ammonia determination, but the results 

 do not represent ammonia as is sometimes stated. 



Procedure. To 25 c.c. of urine in a 200 c.c. Erlenmeyer flask add 15-20 

 grams of finely pulverized potassium oxalate, a few drops of phenolphthalein, 

 and titrate to a faint but permanent pink color with N/io NaOH. (The urine 

 mixture just after neutralization in the urinary acidity determination (see page 

 479) may be used.) Then add 10 c.c. of neutral formalin solution (see amino- 

 acid nitrogen), mix well and titrate with N/io sodium hydroxide to a permanent 

 pink color. 



Calculation. One c.c. of N/io sodium hydroxide is equivalent to 1.7 

 mg. of ammonia. Multiply the number of cubic centimeters of N/io alkali 

 used by 1.7 and by 4 to get the number of milligrams of ammonia + amino- 

 acid nitrogen (expressed as ammonia) in 100 c.c. of the urine examined. 



Amino-Acid Nitrogen 



i. Henriques-Sorensen Formol Titration Method. 2 Principle 

 A solution containing amino-acids is nearly neutral in reaction. If 

 formaldehyde be added, however, the following reaction takes place 

 with the formation of methylene derivatives which an more strongly acid 

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

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

 olphthalein as an indicator. 



R.CH.NH 2 



+ CH 2 = R CH N: CH 2 + H 2 0. 

 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: 



Hence the formol titration in the presence of ammonia gives results 

 which include both ' amino-acid and ammonia nitrogen. Ammonia 



1 Malfatti: Z. anal. Chem., 47, 273, 1908. 



2 Henriques and Sorensen: Zeit. physiol. chem., 64, 120, IQOQ. 



