ION EXCHANGE 395 



ill HCl, evaporating to dryness, redissolving, and passing through a 

 hydrogen-ion exchanger. The filtrate, containing only IICl and H3PO4, 

 was adjusted to pH 4.63 and titrated with NaOH to a pH of 8.98 to give 

 a measure of the phosphorus present. 



In the gravimetric determination of sulfate as the barium salt, it has 

 been found advantageous to eliminate, by exchange, such cations as 

 sodium, aluminum, and iron which tend to coprecipitate (5). Conversion 

 of a metal to an anion and subsequent separation are illustrated by studies 

 with potassium and vanadium in which the latter was converted to van- 

 adate. After passage through a cation exchanger saturated with ammo- 

 nium ions, the potassium only was retained, thus effecting the separa- 

 tion (2). 



Ions from Nonelectrolytes. An application of this principle has been 

 the development of a convenient method for the estimation of ammonia 

 in urine. The urine is treated with zeolite previously saturated with 

 sodium ion, which then retains ammonium ion (5). After the exchanger 

 is washed, it is treated with XaOH, which serves to liberate the ammo- 

 nium ion and convert it to ammonia, which can then be estimated by the 

 Nessler method. This principle has also been employed to retain the 

 ammonia from urine so as to permit determination of urea in the filtrate 

 by the standard method of hydrolysis with urease and subsequent estima- 

 tion of the liberated ammonia (5). 



An especially important application has been in the determination of 

 thiamine (8). The standard fluorimetric method is based on the oxida- 

 tion of thiamine to thiochrome and the measurement of its fluorescence in 

 the absence of other fluorescing substances. The thiamine can be freed 

 from interfering substances by treatment with an exchanger that retains 

 thiamine but not the impurities. Elution generally provides an extract 

 satisfactory for analysis. In a typical procedure the sample is heated 

 with 0.1 N HCl to extract the vitamin, and the extract is treated with an 

 acetate-buffered phosphatase (Takadiastase or Clorase) to hydrolyze the 

 thiamine esters. The mixture is filtered, and the filtrate passed through 

 a column of Decalso which has been previously saturated with potassium 

 ion. After the column has been washed with water, the thiamine is dis- 

 placed with an acid solution of KCl. An aliquot of the eluate is treated 

 with sodium hydroxide, potassium ferricj^anide, and isobutyl alcohol, and 

 the fluorescence measured in comparison with standards. 



Substances of Different Acidic or Basic Strength. An important appli- 

 cation of separations based on differences in acidic or basic strength has 

 been in studies with amino acids. For purposes of discussion, the amino 

 acids resulting from the hydrolysis of most proteins may be classified as 

 follows : 



Basic aynino acids: arginine, histidine, lysine, hydroxyglycine, ornithine. 



Dicarboxylic amino acids: glutamic acid, aspartic acid. 



