Storage Liquor. This is tested by any of the above methods. It has been proposed to determine the strength of the bleach solution by the use of a hydrometer but the results are not sufficiently accurate and the method cannot be recommended.
If bleach is properly broken up and thoroughly agitated in the mixing tank at least 95 per cent of the available chlorine should be extracted. The efficiency of the extraction process is checked by comparing the tests of the storage liquor with those of the dry bleach and each batch of liquor should be tested daily. It is sometimes advisable to take two samples from each tank, one soon after a tank has been put into operation, and a second sample at the end of the run. Considerable differences are occasionally found between these samples and are due, either to inadequate agitation of the liquor in the storage tank, or inefficient mixing in the mixing tank. If the results are irregular the former is the more probable cause but if the second sample is invariably stronger the mixing tank operations should be investigated. The increased concentration of the second sample is due to unextracted bleach passing out of the mixing tank and gradually becoming leached as the tank contents are run off. If the bleach is lumpy and is not subsequently broken up, losses are almost inevitable.
Hale[3] found that during the period when the New York City supply was being treated with bleach it was necessary to constantly check the operations of the labourers by frequent samples. “During one week about 95 per cent of the chlorine added was actually applied, the second week it dropped to 85 per cent. and the third week to 75 per cent. Whenever a poor run is called to the attention of the labourers, results improve.”
By taking two samples daily from each tank discharged the author has been able to obtain an average annual efficiency on the Ottawa plant of 94 per cent., i.e. the solutions contained 94 per cent. of the available chlorine contained in the bleach. In making such checks it is necessary to keep a careful account of the stock of bleach to prevent labourers adding a few extra pounds of bleach to compensate for losses.
Sludge forms an appreciable but unavoidable source of loss of material. When the sludge reaches the outlet of the hypochlorite pipe the sludge must be run to waste; otherwise it will pass over and tend to choke the dosage control apparatus. If the sludge is run into the same body of water that forms the source of supply, it must be discharged very slowly to prevent a possibility of over dosage and damage to fish life. With proper control, sludge losses can easily be kept under 2 per cent. and often under 1 per cent.
The greatest source of unavoidable loss in hypochlorite plants is from deterioration of the bleach during storage; in warm climates this loss may exceed 10 per cent. In Ottawa where high temperatures are only experienced during the summer months the loss from this cause has averaged from 7-8 per cent. on the bleach stored during that period.
Detection and Estimation of Free Chlorine. The oldest and probably the best known test for free chlorine in water is the Wagner test, made by adding a few drops of potassium iodide and starch; the presence of chlorine is indicated by a deep rich blue colouration that is proportional in intensity to the quantity of chlorine present. When this test is used as a colorimetric method for the estimation of chlorine several difficulties are encountered; the intensity of the colour produced by the majority of treated waters gradually diminishes and the loss is usually more rapid than in the standards made up with distilled water; a different result is obtained if the solutions are acidified and the results vary with different acids, acetic acid yielding a much lower result than a mineral acid such as hydrochloric acid; in the presence of acid the colouration usually intensifies on standing, whereas the standard intensifies but little. The difference caused by the addition of acid is imperfectly understood but it is obvious that the chlorine set free by the acid cannot be present in the “free” state; it is probably in a semi-labile condition loosely attached to organic compounds. Whether this semi-labile chlorine is available for germicidal action is at present not definitely known but it has been noted by several observers that the germicidal action proceeds after the “free” chlorine reaction has disappeared.
The method used by the author for the estimation of free chlorine is as follows: place 500 c.cms. of the sample in a stoppered bottle, add 1 c.cm. of 5 per cent KI solution, 2 drops of conc. HCl and 1 c.cm. of starch solution and titrate with N/1000 sodium thiosulphate until colourless. The difficulty introduced by the opalescence of the liquid is overcome by pouring portions of the liquid into two Nessler tubes and adding a drop of thiosulphate solution to one and noting if any reduction of colour occurs on shaking; if the intensity of the colour is diminished, the contents of both tubes are poured back into the bottle and titrated until no further colour removal, as shown by the tubes, can be obtained. One c.cm. of N/1000 sodium thiosulphate = 0.07 p.p.m. of available chlorine when 500 c.cms. of water are used.
Adams[4] has employed the colorimetric method of estimating the colour obtained after the addition of dilute H2SO4, KI, and starch but used standard solutions of dyes for comparison. The standards were prepared from mixtures of Brilliant Mill Green “S” and Cardinal Red “J” and were made up weekly.
Phelps found that ortho-tolidine in acetic acid solution produced an intense yellow colouration with free chlorine and suggested the use of this reagent as a qualitative test for chlorine. Ellms and Hauser[5] developed this process into a quantitative one and substituted hydrochloric acid for acetic acid as a solvent. One c.cm. of the reagent (1 gram of pure o-tolidine dissolved in 1 litre of 10 per cent of hydrochloric acid) is added to 100 c.cms. of the sample in a Nessler tube and the colour compared after five minutes with permanent standards made up with mixtures of potassium bichromate and copper sulphate. This method was adopted as the official standard method of the American Public Health Association; the details are given in the Appendix ([p. 147]).