CANADIAN FISHERIES EXPEDITION, lOl'rlo 



353 



most commonly used is Mohr's titrimetric method, wliich is both rapid and accurate:^ 

 to a certain quantity of the water sample a solution of nitrate of silver is added; the 

 silver chloride being precipitated as a white flocculent deposit: — 



Xa CI + Ag N Oz = Ag CI + Na N Oa. 

 An indicator is applied to show when all the chlorine has been precipitated, and 

 the ;i)nount of nitrate of silver added is measured. As an indicator, yellow potassic 

 chromate (K2 Cr O4) in nearly saturated solution is used. It combines with the 



Fig. 4.— Knudsen's 

 automatic pipette 



Fig. 5. — A bulb-burette for 

 titration. 



nitrate of silver (Ag2 Or O4) ; but this reaction does not take place until there is 

 no chlorine left. When a few drops of the potassic chromate is added to the water- 

 sample the latter turns yellow, but when all the chlorine has been precipitated by the 

 addition of nitrate of silver the sample turns red, and the titration is completed. 



iThe method is thoroughly described in B. HeUand-Hcitisen, The Ocean waters. International 

 Revue der gasamten Hydrobiologie und Hydrobidogie und Hydrographie Leipzic, Hydr. Suppl. 

 Sen. IH, 2 p, 34 af. 



