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ROYAL SOCIETY OF CANADA 



Nitric acid — three volumes of acid to one of water. This is boiled 

 to expel oxides of nitrogen. This, as also the above solutions, should 

 be kept in the dark. 



The indicator is made by dissolving ferrous sulphate in water and 

 completely oxidizing by nitric acid on the water bath. It is then 

 evaporated to dryness repeatedly with a little concentrated sulphuric 

 acid to expel the excess of nitric acid, and ferric' carbonate added to 

 neutralize the sulphuric acid. The solution is then filtered and is 

 ready for use. 



Modus operandi. — 100 c.c. of the water to be examined are poured 

 into a beaker and 5 c.c to 10 c.c. of the nitric acid added. Then, from 

 a 2 c.c. pipette, graduated to 1/50 c.c. a slight excess of silver nitrate 

 solution is added. After a few minutes the precipitated chloride is 

 filtered out and well washed, the filtrate being received in a porcelain 

 basin. (The filter pump may frequently be used to advantage.) To 

 this is now added 1 c.c. of the indicator and the potassium sulphocyanide 

 solution run in (from a similar pipette to that used with the silver 

 nitrate) until a faint red colour is permanently produced. The end 

 point is sharp and easily noted. The difference between the number of 

 cubic centimetres of the silver nitrate and of the pot, sulphocyanide 

 solutions used represents the number of cubic centimetres of the former 

 required by the chlorine in 100 c.c. of the water. 



To ascertain the relative accuracy of the chromate and Volhard 

 methods, a series of determinations were made on certain dilute solu- 

 tions of chemically pure sodium chloride, with the results given in the 

 following table: 



Chlorine: In Parts per Million. 



