ANALYSIS OF POTABLE WATERS. 
61 
ia a flask with 30 grains of undiluted hydrochloric acid, or 50 grains of diluted sulphuric 
acid (1 of acid to 3 of water); either acid may he used with equal advantage. The per¬ 
manganate may then be added either in successive small doses till an excess is reached, 
or an excess may be added at once and the excess determined at the end of the experi¬ 
ment ; either plan will answer. If the method by gradual addition be preferred, to 
each flask must be added 20 grains of the permanganate solution, the flasks being ar¬ 
ranged on a white ground side by side in front of a window : no artificial heat must be 
employed. 1 * At intervals of fifteen minutes the flasks are examined, and successive quan¬ 
tities of 10 or 5 grains of the permanganate solution are added, in proportion to the 
rapidity and completeness with which the colour disappears. These successive additions 
of the permanganate must be made until the last addition remains sensibly unaltered 
after the lapse of half an hour, which generally occurs between two and three hours 
from the commencement of the experiment. The number of water-grains of permanga¬ 
nate consumed in each case is then ascertained, deducting the last portion, which, it is 
estimated, remains unaltered in the flask. A little uncertainty occurs in estimating the 
amount of residual tint by the eye, and this occasions a variation of 2 or 3 divisions in 
different experiments. On multiplying by 20 the number of water-grains of perman¬ 
ganate solution consumed in each flask, we have the quantity of oxygen in ten-thou¬ 
sandths of a grain consumed in oxidizing the organic matter in one gallon of each sample 
of water. 
The following table comprises the results of a series of trials with the same solution 
of permanganate, under varying circumstances, upon six different samples of acidulated 
water, and may serve as a specimen of the effects which such variations exert upon the 
amount of permanganate destroyed.—Temp, from 59° to 68° F. (from 15° to 20° C.) 
Permanganate added gradually. 
A. B. C. 
1st water .... 
.. 41 
41 
40 
2nd „ .... 
.. 47 
42 
— 
3rd „ .... 
.. 35 
35 
— 
Acid used in A, dilute sulphuric 
„ „ B, hydrochloric 
„ „ C, sulphuric 
Permanganate added in excess. 
E. 
F. 
G. 
H. 
I. 
K. 
4th water.... 
.. 16 
15 
15 
14 
13 
— 
5th ., . 
.. 44 
48 
46 
49 
50 
51 
6th „ . 
45 
47 
43 
47 
43 
Duration of experiment 2f hours. 
This process may be simplified by adding at once an excess of the solution of perman¬ 
ganate, allowing the whole to stand for three hours, and then destroying the permanga¬ 
nate in excess by the addition of a reducing agent. Various reducing agents were at 
first tried unsuccessfully: amongst them were sulphurous acid, sulphite of soda, hypo¬ 
sulphite of soda, protochloride of tin, and arsenious acid; but the results of repeated trials 
with the same water varied so much, that it was obvious this plan could not be relied 
on, parti} 7 owing to the action of the varying quantity of dissolved oxygen in the waters 
upon the reducing agent employed, and partly owing to the difficulty of removing a 
certain residual brown tint, due to a compound of one of the intermediate oxides of 
manganese, which is commonly produced in rraters rich in organic matter. When the 
water was originally mixed with the permanganate in excess, this brown tint disappears 
slowly, and only by using an excess of the reducing agent. 
This difficulty, however, may be obviated, as suggested by Mr. V. Harcourt, by adding 
to the water, at the termination of the oxidizing action, a small quantity of a solution of 
iodide of potassium and alittle starch-paste; the excess of permanganate or of manganic salt 
is by this means at once reduced to a manganous salt, and the amount of iodine liberated 
may be determined by the addition of a standard solution of hyposulphite of soda, which 
may be graduated by means of the solution of permanganate itself. For this purpose 
10 grains of the crystallized hyposulphite when dissolved in 10,000 grains of water, or 1 
gramme of the salt in a litre of water, will furnish a solution of convenient strength ^ 
* Two hundred and fifty cubic centimetres of the water with 2 c.c. of hydrochloric acid may 
be employed, and successive amounts of 0'5 c.c. of the permanganate solution may be used : on 
multiplying by 4 the number of cubic centimetres of permanganate employed, it will give the 
amount of oxygen in tenths of a milligramme required to oxidize the organic matter in each 
litre of water. 
The solution of potassic permanganate, as well as the solution of dichromate, should be pre¬ 
served in bottles, made of glass, free from lead. 
