SOLUTIONS AND STAINS 33 



the precipitated calcium carbonate granular and promote 

 its settling. It is then allowed to stand quietly and after half 

 an hour the clear liquid is siphoned off from the precipitate 

 and filtered through paper or a cotton plug. 



A 10 c.c. portion is rapidly titrated with boric acid solu- 

 tion (31 grams per liter), using powdered phenolphthalein as 

 indicator (the usual alcoholic solution of phenolphthalein 

 will not serve) in order to determine the amount of boric acid 

 to be added to the rest of the filtrate. The end-point is the 

 disappearance of the pink color. Each cubic centimeter of 

 boric acid required for the 10 c.c. sample calls for the 

 a'ddition of 3 grams boric acid per liter of filtrate. An excess 

 of boric acid should be avoided, as it favors the liberation of 

 hypochlorous acid and renders the solution less stable. It is 

 best to add slightly less than the calculated amount. The con- 

 centrated solution thus prepared contains about 4 per cent, of 

 sodium hypochlorite, and before use should be diluted with 

 about 7 parts of water and titrated with ^ thiosulphate to 

 determine its precise hypochlorite concentration. It is then 

 accurately diluted to the required strength (usually 0.5 to 

 0.45 per cent.). 



PREPARATION FROM CHLORINE AND SODIUM CARBONATE. 

 Chlorine may be obtained as- the compressed gas in steel 

 cylinders and is easily measured by a chlorine meter manu- 

 factured for the purpose. This is a stable, economical and 

 convenient source of chlorine. A solution is prepared con- 

 taining 15 grams of dry sodium carbonate per liter (= 17,6 

 grams mono hydrate or 40 grams of washing soda.) A 

 measured quantity, 4.8 grams (or about 1600 c.c.) of chlorine 

 gas is allowed to run into the solution for each liter. Ten c.c. 

 of the solution is then titrated. If the solution is too weak 

 more chlorine is introduced. If the solution is too strong it 

 should be diluted to a concentration of 0.5 per cent. NaOCl 

 with 1.5 per cent, sodium carbonate solution, which at the 

 same time serves to correct the unduly diminished alkalinity 



