§ 141. WATER. 273 



carbonate that is nearly always present, add water till 

 the original volume is about restored, and proceed to 

 titrate the solution for the organic matter with potassic 

 permanganate (§ 91). 



A good drinking Avater should not contain more than 

 3-4 parts of organic matter in 100,000 parts. 



If, on adding to 100 c. c. of the water 2 or 3 drops of 

 concentrated sulphuric acid and a little of a freshly pre- 

 pared mixture of potassic iodide and boiled starch, the 

 water is colored blue, nitrous acid is present, and a correc- 

 tion must be made in the determination of the organic 

 matter. For this purpose add 10 c.c. of the dilute sul- 

 phuric acid to 100 c.c. of the water, and then add per- 

 manganic solution till the first trace of a red color appears. 

 The aniount of the standard solution required for this 

 must be subtracted from the total amount required in the 

 first trial. 



The presence of nitrous acid in drinking water should, 

 however, be regarded with suspicion, for it indicates that 

 the water, perhaps, contained nitrogenous organic matter, 

 from which it may not yet be entirely freed ; such organic 

 matter is far more harmful than that containing no nitrogen. 



Other prominent methods of making this important 

 determination of organic matter, and particularly nitrog- 

 enous matter, in water, have not yet been sufficiently 

 tested to justify their insertion here, although, without 

 doubt, good methods will soon be worked up out of the 

 material now in hand. Many good chemists still allow 

 some value to the indications that are furnished by the 

 permanganic test relative to the badness of the water in 

 sanitary respects, while others have no confidence in it. 

 Other substances besides nitrous acid may increase the 

 amount of permanganic solution that is reduced, and so 

 give too large an amount of organic matter, as ferrous sul- 

 phate, for instance. 



f. Lime. — To determine this base, which, in one form or 

 12* 



