﻿MATHERS AND LEE: POTASSIUM AND SODIUM 



181 



than the volume of the water present), and filter (I) through a 

 small paper but do not wash. Discard the residue. 



Transfer about one cubic centimeter of filtrate (1) to a test 

 tube and add one drop of sodium cobaltic nitrite, NasCo(N02)6. 



A. No precipitate is formed. Proceed as in B, 2, for the de- 

 tection of sodium. 



B. A yellow precipitate proves the presence of potassium in the 

 solution (ammonium compounds must be absent). 



1. To the remainder of filtrate (I) add an excess of perchloric 

 acid. 2 A white crystalhne precipitate of potassium perchlorate is 

 formed. Filter (II) and test a few drops of filtrate (II) with the 

 sodium cobaltic nitrite. If a precipitate is formed, add to filtrate 

 (II) more perchloric acid, filter again and test as above. When the 

 sodium cobaltic nitrite shows that all potassium has been removed, 

 proceed as directed in B, 2, for the detection of sodium. 



2. To the filtrate (II) add a few drops of hydrofluosi- 

 licic acid, HoSiFe. A flocculent precipitate indicates the presence of 

 sodium in the solution. (This precipitate is not very voluminous 

 and must be looked for carefully if only a little sodium is present. 

 Turn the test tube and examine the sides for adhering precipitate.) 



Experiments. Normal solutions of potassium chloride and 

 sodium chloride were used in testing the method. Very strong solu- 

 tions of sodium cobaltic nitrite ^ and hydrofluosihcic acid were used 

 so that only a few drops were required to give a test. In each case 

 in the tables given below the solution was diluted to about one cc. 



The test for potassium with sodium cobaltic nitrite is very deli- 

 cate, as shown by this table : 



Gm. KCl. 



Gm. KoO. 



Kind of Test. 



.012415 



.007833 



Very strong. 



.009932 



.006266 



Very strong. 



.007449 



.004699 



Very strong. 



.004966 



.003133 



Very strong. 



.002483 



.001566 



Very strong. 



2 The perchloric acid must be free from sodium, but the presence of potassium does no harm 

 because potassium was detected previously, by the use of sodium cobaltic nitrite, and any potassium 

 present is precipitated by the alcoholic perchloric acid solution. A very satisfactory method of making 

 perchloric acid is described in this bulletin, page 176. 



2 The sodium cobaltic nitrite is best prepared according to Erdmann, Anorganische Chemie, 642. 

 Dissolve 30 gms.of crystalline cobalt nitrate in 60 cc. of water and add to this a saturated solution of 

 sodium nitrite containing 50 gms. sodium nitrite, in 100 cc, and add to this solution 10 cc. of strong 

 acetic acid. 



