228 



I>. A yellow prcH-iiiitate iir(/\('s tlio jireseiice of potassium in the solu- 

 tion (.-mnnoninni cuniponnds nnist be absent). 



1. To the remainder of tiltrate I add an excess of perchloric acid-. 

 A white crystalline precipitate of potassium perchlorate is formed. Fil- 

 ter (II) and test a few drops of filtrate II with the sodium cobaltic ni- 

 trite. If a precipitate is formed, add to the tiltrate II more perchloric 

 acid, tilter again and test as above. When the sodium cobaltic nitrite 

 shows that all potassium has been removed by the ijerchloric acid, proceed 

 us directed in B, 2, for the detection of sodium. 



2. To the filtrate from B, 1, add a few drops of hydrofluosilicic acid, 

 H^SiF". A cloudy 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. 



This method has been tried in this laboratory with excellent results 

 Some of the advantages are: 



1. Magnesium does not interfere and need not be removed. Mag- 

 nesium perchlorate is very soluble. Magnesium flnosilicate is soluble and 

 only precipitated, even in the alcohol solution, when large amounts are 

 present. 



2. The test for sodium is delicate but traces of sodium which are 

 present in so many reagents are not detected. This is an advantage over 

 the spectrum test where all substances show sodium. 



3. The tests are simple and easily understood and followed by the 

 students. 



4. The tests are decisive and the student has confidence in his work. 



5. Only a short time is reiiniriMl to make a test. 



2 The perchloric acid must bo froo from sodium but the presence of potassium 

 does no harm bocavise potassium is dctcn-ted previously, by the use of sodium co- 

 baltic nitrite, and any polassiuin present is precipitated l)y the alcoholic perchloric 

 acid solution. 



Indiana University, 

 Blooniin(jt()>i. I iiditnta. 



