268 TITRIMETRIC METHODS 



POTASSIUM 



Norberg (1937) developed a method for the determination of 

 less than 1 X 10"^ milliequivalent of potassium ( < 3.91 fig.) with 

 a precision of about 1 to 2 X 10 *" milliequivalent in samples of 

 biological material containing between 1 X 10"^ and 3.5 X 10"'* 

 milliequivalent. The method is based on precipitation, as the 

 chloroplatinate, of the potassium extracted from incinerated bio- 

 logical material, isolation of the precipitate by centrifugation, con- 

 version of the chloroplatinate to the iodoplatinate, and titration of 

 the latter with thiosulfate. Tests revealed that the presence of 

 sodium in concentrations 150 times that of the potassium had no 

 influence. 



A method described by Cunningham, Kirk, and Brooks (1941) 

 is suited to the analysis of biological material when the ratio of 

 sodium to potassium does not exceed twenty. For quantities of 

 potassium over 2 ^g. the error does not exceed 0.5%, and over 0.7 //.g. 

 it does not exceed 3%. As in the Norberg method, the potassium 

 extracted from the incinerated sample is precipitated as the chloro- 

 platinate. Cunningham et al. collect the precipitate on a filter stick, 

 dissolve the substance, reduce it with sodium formate, and titrate 

 the chloride thus produced electrometrically. 



Only the Norberg method will be described, since it has the 

 advantage of employing a simple indicator titration rather than 

 the electrometric procedure, and the separation of the chloroplatinate 

 by centrifugation would also appear to be a little simpler than 

 filtration on a specially constructed filter stick. 



Norberg Method for Potassium 



SPECIAL APPARATUS 



Apparatus jor Ashing the Sample. That employed in the combined 



sodium and potassium method (page 266) of Linderstr0m-Lang 



is used. 

 Glass Precipitation Tubes. These may be either of the forms shown 



in Figure 36 (page 167). 

 Equi'pment lor Removal of Supernatant Fluid over the Precipitate. 



Illustrated in Figure 37 (page 167). Suction is applied at A 



