226 ANALYSIS OF WATER 



219. Chlorides. 



The determination of chlorides is of importance in 

 water analysis, as their presence frequently indicates 

 sewage pollution, besides being harmful for many 

 industrial purposes. If much magnesium chloride is 

 present in water used for boilers, hydrochloric acid is 

 liberated by the superheated steam and frequently 

 causes considerable trouble. 



Chlorides are estimated by titration with standard 



silver nitrate solution, using potassium chromate as 



N 

 indicator (p. io8). The silver solution used is — and 

 ^^ ^ lOO 



I c.c. =0-000355 gram CI. 



One hundred c.c. of the water are measured into a 



porcelain basin, or into a beaker placed upon a white 



tile, and the silver nitrate is run in with constant stirring 



until a faint red tint, due to silver chromate, is observed. 



The volume of solution used is read off and the 



equivalent amount of chlorine calculated from it. A 



blank should be carried out with distilled water at the 



same time. 



Water containing more than 2 parts of chlorine per 100,000 is 

 distinctly suspicious, but its presence may, of course, be explained 

 in some other way than by sewage pollution, and in many clay 

 soils the uncontaminated water supply contains considerably 

 greater proportions of chlorides. 



Acidity in Water. 



Most waters are slightly alkaline towards methyl orange 

 and sometimes also to phenolphthalein. Occasionally, however, 

 a water will be acid to phenolphthalein, but react neutral or 

 alkaline to methyl orange, the acidity in this case being due to 

 dissolved carbon dioxide, towards which methyl orange is not 

 sensitive. Many waters derived from peaty soils, are, however, 

 acid to both indicators, the acidity being due to humic acids, 

 etc., obtained from decaying vegetable matter. As the presence 



