PURE SODIUM CHLORIDE 25 



PURE SODIUM CHLORIDE. 



NICHOLAS KNIGHT. 



It is often convenient and necessary to have on hand a supply 

 of pure sodium chloride for the preparation of standard solutions. 

 It is quite easy to obtain the substance with a "C. P." label, but 

 it does not always follow that the article is as pure as the label 

 may indicate. 



It seems to be a difficult matter to obtain common salt that is 

 entirely free from potassium chloride. We have prepared sodium 

 chloride by different methods. We have tested these and also the 

 various samples we had on hand in the laboratory stock room, 

 and have usually found a small quantity of potassium chloride 

 present. 



1. We prepared a saturated solution of ordinary common 

 salt, into which we passed hydrogen chloride, made by heating 

 pure concentrated hydrochloric acid. We filtered off the salt 

 crystals using a pump and after drying we obtained 0.42 per 

 cent of potassium chloride. 



2. We dissolved 100 grams of ordinary common salt in 300cc 

 water and filtered into an evaporating dish. This was heated to 

 boiling, and milk of lime added in small excess. The precipitate 

 was filtered off, and the excess of calcium and barium precipitated 

 with sodium carbonate. It was again filtered and the excess of 

 sodium carbonate was changed to sodium chloride with pure 

 dilute hydrochloric acid. After drying we found in the specimen 

 0.32 per cent of potassium chloride. 



3. We made a solution of caustic soda by dissolving metallic 

 sodium in distilled water, and we neutralized this with pure hy- 

 drochloric acid. The analysis of the dried salt showed 0.27 per 

 cent of potassium chloride. The sodium must have contained a 

 small quantity of metallic potassium. 



4. The foregoing experiment was repeated using the purest 

 caustic soda in the laboratory that had not been purified by 

 alcohol. The resulting sodium chloride showed 0.48 per cent po- 

 tassium chloride. 



