§ 91. QUANTITATIVE METHODS. 151 



carefully dried, gently ignited, and weighed, and the 

 operation must be repeated as long as there is any gain in 

 weight, in order to be sure that there is at least as much 

 carbonic acid in the substance after ignition as before. 

 Then determine carbonic acid in the ash, or a portion of 

 it, and in a portion of the original substance ; the excess 

 in the ash over what was in the quantity of substance 

 taken is to be subtracted from the weight of the ash. 



e. Small quantities of organic matter, as in water, may 

 be determined by the following volumetric j^rocess {Kuhel^ 

 Fresenius's Zeitschrift^ 6, 252). 



Dissolve about 0.4 grm. of crystallized potassic per- 

 manganate in 1 litre of water, and also 0.398 grm. of j^ure 

 oxalic acid in 1 litre of water. 



Put 100 c.c. of distilled water and 10 c.c. of a dilute 

 sulphuric acid, containing 30 grms. of concentrated acid 

 in 100 c.c, in a flask of about 300 c.c. capacity, heat the 

 mixture to boiling, add 3-4 c.c. of the j^ermanganate so- 

 lution, boil the red liquid 5 minutes, remove the lamp, and 

 add 10 c.c. of the solution of oxalic acid ; potassic per- 

 manganate is then cautiously added from a burette or pi- 

 pette, with constant stirring, until a faint red color ap- 

 pears throughout the liquid. The total amount of 

 permanganate added, corresponding to the 10 c.c. of the 

 oxalic acid solution, = 2 milligrammes. 



Now, to make a determination of organic matter in a 

 sample of drinking water, for instance, boil 100 c.c. of the 

 water in a flask of 400 or 500 c.c. capacity, down to "" \^ its 

 initial volume, to decompose ammoniacal compounds that 

 are very liable to be present in such a water, by means of 

 the calcic carbonate that is also nearly always present ; 

 add distilled water until the original volume is nearly re- 

 stored, and 10 c.c. of the dilute sulphuric acid; heat to 

 boiling, add 5 or 6 c.c. of the permanganate solution, and 

 boil 5 minutes, whereby the red color should not be de- 

 stroyed; then add 10 c.c. of the oxahc acid, and restore 



