17. Determination of the Oxygen Consumed.

Solutions Required.

(a) Solution of Potassium Permanganate.-Dissolve 0.395 gramme of potassium permanganate in 1 litre of freshly distilled water just tinted pink by adding a few drops of a solution of permanganate of potash. Label: PERMANGANATE SOLUTION (10 c.c. = 0.001 gramme

of oxygen).

(b) Sodium Thiosulphate Solution.-Dissolve 1 gramme of pure crystallized thiosulphate in 1 litre of distilled water. For the incubator test it is more convenient to use a solution of double this strength (see Table IV.)..

(c) Dilute Sulphuric Acid 1 in 4.-Add gradually 100 c.c. of pure sulphuric acid to 300 c.c. of distilled water.

(d) Potassium Iodide.-Dissolve 1 part of the pure recrystallized salt in 10 parts of distilled water.

(e) Starch Solution.-Take about a gramme of rice starch and work into a cream with cold water. Pour into 100 c.c. of boiling water and continue boiling for two minutes. When cold, decant off the clear liquid for use. The solution does not keep, and must be freshly prepared each time.

In this process it is necessary to make a control determination with recently boiled distilled water, as the hyposulphite solution is unstable. The time during which the process is continued, namely, one, three, or four hours, and the temperature, 80° F., or boilingpoint, is somewhat a matter of fashion. Thresh recommends boiling for fifteen minutes; Tidy recommended one hour and three hours at 80° F.; at the present time, four hours at 80° F. is the most usual time. It is obvious that unless the time and temperature are fixed, the results of this process can never be generally adopted.

My usual procedure is to estimate at the end of 3 minutes, and then compare with a 3 minutes' estimation after incubating at 80°F. for 7 days (vide infra).

Cleanse two 12-ounce stoppered bottles by rinsing round with strong sulphuric acid, followed by washing with clean water. Into 1 place 100 c.c. of pure distilled water, and in the other, which we will call 2, pour 100 c.c. of the effluent or sewage to be examined; into each pipette 10 c.c. of the dilute sulphuric acid, and then 10 c.c. of the permanganate solution; allow them to stand at the temperature decided upon, and as often as the permanganate solution in the bottle 2 fades add another 10 c.c. of standard permanganate.

At the expiration of the time fixed upon add 2 c.c. of the iodide solution. The pink colour of the permanganate is thereby changed to yellow by the oxygen of the permanganate liberating an equivalent amount of iodine.

Now add from a burette the thiosulphate solution, until the yellow turns a faint straw colour; then add 1 c.c. of starch solution:

a deep blue colour appears; add the thiosulphate gradually until the blue colour absolutely disappears. Make a note of the thiosulphate used, and proceed in the same way with the other bottle.

With the distilled water the 10 c.c. of the permanganate should remain unchanged, so that if x c.c. of thiosulphate are used, these xc.c. correspond to 10 c.c. of the permanganate solution—that is, absorb 1 milligram of oxygen. If the effluent, being analyzed, takes only y c.c. of the thiosulphate, the oxygen consumed by the oxidizable matter in it is 1-2 milligrammes. If a second 10 c.c. of permanganate had been required by the water in question, the oxygen consumed is 2 - milligrammes.

У

x

x

For convenience in my own laboratory practice, I drew up the following table, Table III., which gives the amount of oxygen consumed equivalent to so many c.c.s of thiosulphate used. For instance, suppose 12 c.c. of thiosulphate solution were used and the standard or control amount with distilled water was 15 c.c., then by turning up the figure opposite the 12 c.c. in the 15 c.c. column, it is found that this is equivalent to 0-2 c.c. of oxygen absorbed.

18. Incubator Test.

This gives an indication of the amount of putrescible organic matter in the effluent.

A 4-oz. glass stoppered bottle which holds just over 100 c.c. is completely filled with the sample to be examined, tightly stoppered and put in the incubator at 80° F. for 7 days. At the end of that time it is removed and the amount of oxygen consumed by 100 c.c. at the end of three minutes is estimated. This is compared with the amount obtained before incubation. If it is the same amount, then no putrescible matter is present, but if the oxygen consumed is greater after incubation than before, the inference is that putrescible matter is present which has consumed more of the oxygen. The excess of the oxygen consumed after 7 days' incubation over the oxygen consumed before incubation is the measure of the putrescibility of the sample.

19. To Immediately Estimate the Oxidation Effected by Sewage Filters.

The quickest method of estimating the oxidation effected by any scheme is to take advantage of the fact that the ultimate products of the oxidation of organic matter are acids, nitrogenous matter being oxidized into nitrous and nitric acids, and carbonaceous organic matter into oxalic, tartaric, and other organic acids. These organic acids, rich in oxygen, combine with the ammonia, and also decompose the carbonates of lime and soda present, and the degree to which they are formed may be measured by the degree in which the alkalinity is diminished.

This process is of little use in comparing one scheme with another, but when once the constants are known for any scheme, the oxidation effected by bacteria beds or filters at any moment can instantly be measured. The author strongly recommends this process to the attention of all sewage farm managers.

The process consists in estimating the loss of alkalinity-the index of the production of highly oxidized compounds (such as nitric acid) by means of a decinormal solution of sulphuric acid.

100 c.c. of the tank effluent and the filtrate are taken. To each 3 or 4 drops of a one per cent. solution of methyl orange are added. The decinormal sulphuric acid is then run in from a 10 c.c. burette, a drop at a time, until the yellow colour begins to turn to red.

Each c.c. used is equal to 5 parts per 100,000 of carbonate of lime, and the reduction in the alkalinity, the measure of the oxidation effected by the filter, is read in terms of carbonate of lime.

When it is known what reduction in the alkalinity is the result of oxidation to produce a given quantity of nitrate and effect a given reduction in the organic matter, this test is of the greatest value, as it can be made in the course of a few minutes.