CHEMICAL ANALYSIS 49 



differenf parts of England 7*2, 7*33, 7*24, and similar numbers 

 of c.c. of dissolved oxygen per litre at ordinary temperature. 

 Useful data to record are — 



7 c.c. of oxygen per litre = i part by weight in 100,000. ^ 

 I cubic foot of = 40*6 grammes, 

 Or, I gallon of = 5*52 grammes. 



This process is interfered with by the presence of large 

 quantities of organic substances, which absorb the liberated 

 iodine, and by the nitrites, which occur in many effluents, them- 

 selves setting free iodine when acidified. Such interference can 

 be to a great extent prevented by working very rapidly, running 

 in the thiosulphate till the brown colour of iodine has just dis- 

 appeared, not using starch, and can be completely eliminated 

 by Rideal and Stewart's modification (Analyst, June, 1901), 

 which also has the advantage of giving a determination of the 

 immediate oxygen consumed. Fifty or 100 c.c. of the original 

 liquid, acidified with i c.c. of H2SO4, are first titrated with 

 decinormal permanganate, till a faint pink colour persists after 

 ten minutes. The c.c. required is calculated to the volume of 

 the bottle in which the free oxygen determination is to be 

 done. This amount is then placed in the bottle, together with 

 I c.c. of sulphuric acid (if more than 10 c.c. of permanganate 

 are required, 2 c.c. of acid must be added), the bottle is filled 

 with the liquid under examination, mixed by rotation, and 

 allowed to stand five or ten minutes. It is advisable to add 

 about 0*1 c.c. of permanganate in excess, so that the liquid 

 remains faintly pink. If this addition should be overdone, 

 the bottle is opened, h c.c. of a 2 per cent, solution of neutral 

 potassium oxalate added with a pipette, the neck filled up with 

 the water under examination, the stopper inserted, and the 

 bottle rotated as before. The colour quickly disappears, and 

 the solution is ready for the manganous chloride, etc., as in 

 the ordinary determination. To balance the sulphuric acid, a 

 rather larger proportion of soda is required. When working 

 with sewages and effluents we use a 50 per cent, soda +10 per 

 cent. KI ; after oxidizing with permanganate, 3 c.c. of this, and 

 afterwards 5 c.c. of HCl, are added. 



The volume correction for the reagents is small, and can 



^ This enables us to easily translate the ordinary figures of c.c, of O per litre 

 into parts by weight per 100,000 ; the latter mode of recording is adopted com- 

 monly in America, and has the advantage of uniformity with the other analytical 

 figures. 



