POTASSIUM 269 



through a rubber tube. The low-power microscope aids in the 

 careful removal of the liquid. 



SPECIAL REAGENTS 



Ashing Reagent. 1.2% barium chloride (BaCl2.2H20) soln, con- 

 taining 3.2% barium hydroxide (Ba(OH)2.8HoO). 



Precipitatioji Reagent. Prepare 0.02 M chloroplatinic acid by dis- 

 solving 0.1 g. of the substance (H2PtCl6.4H20) in distilled water 

 and making up to 10 ml. The soln. is stable for at least a month 

 if stored in a refrigerator. 



Pure Absolute Alcohol. 



Phosphate Buffer (pH 6.98). Combine 4 ml. M/15 primary potas- 

 sium phosphate, 6 ml. M/15 secondary sodium phosphate, and 

 90 ml, distilled water. 



2 N Potassium Iodide. Dissolve 3.32 g. potassium iodide in water 

 and make up to 10 ml. Filter if necessary. The soln. must contain 

 no free iodine. 



0.02 N Sodium Thiosulfate (0.496% NaoS203.5H20). Standardize 

 preferably by treating 7-8 fA. 0.01 N potassium chloride in the 

 same fashion as the soln. of the unknown. 



Green-Light Filter Solution. Add 4 ml. M/15 phosphate buffer, 



pH 6.36 (7.5 ml. primary potassium phosphate -f 2.5 ml. secondary 

 sodium phosphate) to 6 ml. 0.04% bromothymol blue. Do not use 

 the soln. longer than a week, since the color eventually changes. 



PROCEDURE 



1. Ash the sample as described in the combined sodium and 

 potassium method of Linderstr0m-Lang (page 267) and extract the 

 ash with 100 /xl. water in the same manner. 



2. Pipette an aliquot of about 100 /A. of the centrifuged clear 

 extract containing 1 X 10"^ to 3.5 X 10"^ milliequivalent potassium 

 into a precipitation tube and add 10 ix\. precipitation reagent with 

 a hand pipette. 



3. Evaporate the water at 90-95°; it does not matter if the 

 heating continues for 1-2 hr. after the water has been driven off. 



4. Wash the residue with 100 /A. absolute alcohol, added with a 

 hand pipette. Use a platinum needle 0.8 mm. thick to stir up the 

 solid and rinse the needle with 10 /xl. absolute alcohol before remov- 

 ing it from the tube. 



