Lerrs anp Braxe—Estimation of Oxygen in Solution. 457 
eubie centimetre of which is equivalent to 1 cubic centimetre of 
oxygen at N. T. P., but nothing is gained by preparing a solution 
of exactly this strength. The solution becomes turbid after some 
days, but it is easily cleared by filtration previous to a series of 
determinations, so that the same solution may be employed for 
months, and has been thus kept and used in the authors’ experi- 
ments. 
(2) Standard Permanganate.—The most convenient solution is 
of such strength that 1 cubic centimetre is equivalent to 1 cubic 
centimetre of oxygen at N. T. P.; and such a solution is obtained 
by dissolving 5°65388 grammes of the pure crystallized potassium 
salt in 1 litre of distilled water.’ 
For the estimation of the dissolved oxygen in distilled water 
itself, or in ordinary water, a solution of one-tenth this strength 
may be used; but for all practical purposes the stronger solution 
will be found to give sufficiently accurate results, especially if a 
narrow burette of 10 ¢.c. capacity is employed with which readings 
may be made accurately to 0°025c.c. The stronger solution keeps 
well (in the dark), but the weaker one should be prepared freshly 
from the former for each series of experiments. 
(3) Standard Bichromate, of a strength corresponding with 
that of the permanganate, may be employed instead of the latter; 
and in the case of sewage effluents and in that of sea water, is 
undoubtedly to be preferred, as will be shown by the results to be 
mentioned presently. Of course, in using bichromate, potassium 
ferricyanide must be employed as indicator; and as the ferrous 
sulphate solution is highly diluted by the water under examination, 
the strong bichromate can alone be employed with advantage. This 
is prepared by dissolving 8°7906 grammes of the pure crystallized 
potassium salt in 1 litre of distilled water, each cubic centimetre 
of such a solution corresponding with 1 cubic centimetre of oxygen 
AGEN. <0. BP: 
1 This amount is calculated on the basis of (1) Ostwald’s atomic weights, O= 16, 
and KMn0O, = 158-226, and (2), the mean of the determination of the density of 
oxygen by Regnault, Leduc, Rayleigh, and Jolly (1 litre at N.T.P.=1°4293914 
grammes). 
A decinormal solution of the permanganate (3:164 grammes per litre) is also of suit- 
able strength for the titrations, 1 c.c. of this solution corresponding with 0°56 c.c. of 
oxygen at N.T.P. 
