1 80 HOUGHTON : THE DETERMINATION OF [Vol. 75 The Determination of Residual Chlorine in Water BY G. U. HOUGHTON (Read at the meeting of the Society on Wednesday, November 2nd, 1949) SuNoPsis-The introduction of break-point chlorination created a need for tests which would distinguish between free and combined residual chlorine. A satisfactory method for the determination of residual chlorine must be highly sensitive, tolerably free from interference from traces of other oxidants and able to give sharp differentiation between free chlorine and chloramines . A review is given of the methods a,vailable, including several methods, both colorimetric and amperometric, based on the original acid tolidine method of Ellms and Hauser, and methods using P-amino-dimethylaniline, methyl orange or neutral o-tolidine, and an iodimetric method for higher chlorine con tents.THE large-scale chlorination of water has now been practised for over 40 years, so that the measurement of small amounts (under 1 p.p.m.) of residual chlorine has long been a matter of considerable technical importance. It is therefore rather remarkable that prior to the second world war very few methods of measuring residual chlorine had been put forward and, indeed, the well-known o-tolidine method has, until quite recently, been unchallenged. However, in 1939 the whole subject received considerable impetus from the discovery, or rather the re-discovery, of the break-point effect in the chlorination process.* The recognition of the importance of this effect by Faber3 and a little later by Griffin4 and Calvert5 created a need for simple control tests which would distinguish between free and combined residual chlorine.It also directed attention to the reactions which take place between hypochlorite and ammonia at high dilution. By this time too, it had become widely appreciated that chloramines are far less potent germicides than free hypochlorous acid. The ideal require- ment nowadays is therefore a method which is at once sensitive, free from interference by traces of other oxidants and able to give sharp differentiation between free chlorine and chloramine; it should also preferably be sufficiently simple to serve as a waterworks control test. The more important of the tests that have been used for residual chlorine and some of the recent advances that have been made in the methods for its determination are as follows. THE O-TOLIDINE COLORIMETRIC METHOD- This method was devised by Ellms and Hauser6 as far back as 1913 and has the great merit of simplicity combined with high sensitivity : unfortunately, however, other oxidants interfere, notably ferric iron, nitrite and oxidised manganese.Owing to the difficulty of preparing reliable standards from chlorine water or hypochlorite it has always been the custom to match the o-tolidine-chlorine colours against permanent standards prepared from dichromate or against colour-glasses. The standards employed have been those originally given by Ellms and Hauser, but it has long been realised that they are not entirely satisfactory, the main difficulty being that they are only strictly applicable at a cell depth of 300mm.The Ellms and Hauser method has been studied exhaustively in the past and various procedures have been proposed whereby the effect of interfering oxidants may be overcome. Nevertheless, in view of the great importance of the o-tolidine method it has recently been reviewed by the Control of Chlorination Committee of the American Waterworks Association, whose recommendations were incorporated in the 1946 edition of “Standard Methods for the Examination of Water and Sewage” (p. 92). The method recommended by the American Committee was based mainly on the thorough studies of the o-tolidine method made earlier by Chamberlain and Glass.’ The method put * The Dutch workers, Holwerda in 1928’ and I?ays as early as 1914a had demonstrated the phenomenon, but i t is fair to say that it was the intensive studies of the American workers in 1939 and 1940 that opened up a new chapter in chlorination practice.April, 19501 RESIDUAL CHLORINE IN WATER 181 forward by these workers, which was subsequently adopted, differs from the earlier American standard method in the following important particulars- (1) The use of a greater proportion of tolidine in the final colour mixture.(2) The use of a more acid reagent, so that the final pH of the solution lies between 0.3 and 1.3 and there is consequently less interference from iron and nitrite and also more rapid colour-formation. (3) The addition of the sample to the reagent instead of vice versa. (4) The permanent standards proposed are a modification of those worked out by Scott and are buffered chromate solutions, without any.addition of copper sulphate. They have the great merit that for chlorine contents up to 1.0 p.p.m. the cell depth employed is immaterial. In addition to the advantages mentioned above, Chamberlain and Glass state that the procedure gives increased stability of colour and that Beer’s law is obeyed up to 1.5 p.p.m. of chlorine and deviations therefrom are at a uniform rate between 1-5 and 10p.p.m. Manganese, however, still interferes. It must also be noted that with the more strongly acid reagent and quicker colour development, the “flash” and “arsenite - tolidine” methods, vide infya, become far more difficult to apply. THE 0-TOLIDINE “FLASH” TEST- It has long been known that chloramines produce colour with acid o-tolidine reagent much more slowly than does free chlorine.In 1940, Laux8 used this fact as the basis of a qualitative test for free chlorine. The sample is rapidly mixed with o-tolidine reagent and the rate of colour development observed. Any delay in colour development is taken to indicate the presence of chloramine. The test is very rough, but on account of its simplicity has been much used, particularly in swimming-bath control, and it is accepted as standard in the United States. Oxidised manganese interferes by giving a “flash” colour with tolidine and the rate of colour development is dependent on temperature. 0-TOLIDINE - ARSENITE METHOD- This rather ingenious method, which was first put forward by Hallinan9 in 1944, is a means of distinguishing between free and combined residual chlorine, but it also permits a correction to be made for interfering substances.Use is made of the fact that under the prescribed experimental conditions arsenite can reduce both chlorine and chloramines but not nitrite or oxidised iron or manganese. Free chlorine is therefore measured by adding arsenite immediately after the tolidine, so as to reduce any chloramine before it has time to produce colour. Simultaneously, colorimetric evaluations are made for total residual chlorine and interfering oxidants (viz., without arsenite) and for interfering oxidants only (viz., by adding arsenite before tolidine). The amount of total, free and combined chlorine can then be ascertained by difference.Although this method does not give very clear-cut differentiation of the free chlorine and has certain pitfalls, it has proved very useful. It is seriously affected by temperature, and traces of bromide cause difficulty by accelerating the production of colour by the chloramine. AMPEROMETRIC TITRATION METHOD- This review would not be complete without brief reference to the amperometric method of determining residual chlorine first described by Marks and Glasslo in 1942. As far as is known this method has not been used in this country except for research purposes, but it is of great promise. The procedure consists in titrating the chlorine with sodium arsenite in neutral solution, the end-point being observed amperometrically using a gold or platinum cathode and a silver anode in 2 M potassium chloride. Under these conditions the iron, manganese and nitrite do not oxidise the arsenite and do not interfere.A first titration is made which gives the free chlorine: potassium iodide is then added to “activate” the chloramine which may then be determined by a second titration. The outstanding merits of this method are its freedom from interference by manganic compounds and high precision: it was claimed by Marks and Glass that amounts of chlorine up to 10 p.p.m. could be deter- mined to within 0.01 p.p.m. $-AMINO-DIMETHYLANILINE (P.A.D.A.) METHOD- for available chlorine, with which it gives a red coloration. . A number of workersr1J2J3J4 have investigated the use of this substance as a reagent Moore16 found that at pH 6.0182 HOUGHTON : THE DISTERMINATION OF [Vol. 75 the reaction was obtained with free chlorine only and that it was necessary to lower the pH to 4.0 to obtain a coloration with chloramines: on this fact he based a qualitative and semi- quantitative method.Palin16 placed the reactions on a more rigidly quantitative basis by matching the red colour against that produced by running a standard iodine solution into a control tube containing buffer and reagent ; reaction of the chloramine was obtained by subsequent addition of potassium iodide. Manganese interferes somewhat in the amino-dimethylaniline method but, as when using o-tolidine, the interfering colour may be allowed for by its measurement after reduction of the chlorine with arsenite. Copper also interferes but may be inhibited by using hexa- metaphosphate in the buffer solution.A comparator fitted with colour standards from 0.1 to 2.0 p.p.m. of chlorine has now been developed for use with this reagent in the control of swimming-pool chlorination. METHYL ORANGE METHOD FOR FREE CHLORINE- In hydrochloric acid solution (pH 3), free chlorine bleaches methyl orange but chloramine is without immediate effect. This was the basis of a volumetric method for the determination of free chlorine advanced by Holwerdal in 1928, Quite recently Taras has again investigated this method and has put forward a colorimetric17 and a micro-titrationl* procedure using methyl orange. These methods are of considerable interest and appear to be worthy of further study and trial. NEUTRAL 0-TOLIDINE METHOD- If in the o-tolidine test the reagent is of low acid concentration (q., 5 ml.of 20 per cent. v/v hydrochloric acid per litre) only free chlorine will produce a colour, the pH of the test solution being too high to effect hydrolysis of the chloramines. The “neutral” tolidine test, which depended on this fact, was proposed by Laux and Nickelfg in 1942. In their test the colour produced was blue with samples of pH below about 7.9 but yellow or orange at higher pH values. This colour difference was a serious drawback, as also was the fact that the colour was somewhat affected by the o-tolidinelchlorine ratio, besides fading rapidly. Palin20 has recently re-examined the neutral o-tolidine test and used the reagent in a novel method of chlorine determination.In this new method the tolidine is used at pH 5 to 6, in the presence of metaphosphate and wxth an o-tolidinelchlorine ratio of 6/1. Under these conditions free chlorine produces a pure, stable, blue colour which, provided the residuum does not exceed 4 p.p.m., can be matched against standard glasses; the higher residual chlorine may be titrated with ferrous ammonium sulphate. Further, after addition of potassium iodide, monochloramine may be made to give a blue colour with the reagent and likewise be titrated or matched. Chlorine testing kits (one of them photo-electric) which make use of the Palin neutral o-tolidine method are on the market. In studies using this method, Palin observed that with certain chlorinated waters the results obtained for total residual chlorine were decidedly lower than the corresponding iodimetric figures. Moreover, it appeared that in these waters a chloramine was present which could not react with iodide to give the colour reaction unless the solution was first acidified and then brought back to neutrality with bicarbonate.This discovery was in line with an observation made many years previously by Harold21 that unless acid was present in the iodimetric titration, all the chloramine would not react with iodide. This phenomenon was attributed by Harold to the presence of dichloramine. From detailed investigations, Palin has likewise concluded that in his neutral tolidine method the first fraction of the chloramine, activated by iodide, is monochloramine, while the second, which is not so activated unless previously hydrolysed, is dichloramine.Nitrogen trichloride may also be produced during break-point chlorination and by a further extension of the Pdlin method, it too may be determined. For this purpose the solution is tested again after prior extraction with carbon tetrachloride, the nitrogen trichloride present being calculated from the difference between the “free chlorine” values before and after extraction. It is thus possible to draw up a balance sheet showing the proportions of free chlorine and mono-, di- and tri-chloramines in the total residual content. Unfortunately, any oxidised manganese present interferes, even under the practically neutral conditions, but may be allowed for by its measurement after adding arsenite to reduce the chlorine. Iron and nitrite are without effect.It would seem that these studies by Palin are most important. Not only are they valuable from the point of view of analytical control but, applied in this way, the neutralApril, 19501 RESIDUAL CHLORINE IN WATER 183 o-tolidine method should be useful for research purposes, e.g., for the study of possible differences in the germicidal efficiency of the three chloramines. Incidentally, they also show that it is not safe to regard as chloramine only that part of the chlorine residuum which reacts after the so-called “activation” by iodide. Such an assumption was made by Marks and Glass and, indeed, by Palin himself in his earlier work using amino-dimethylaniline. The mechanism by which iodide induces the reaction of monochloramine with tolidine is uncertain; it may depend on intermediate liberation of iodine and this is a point which might repay investigation.In an analogous way bromide promotes the reaction of tolidine and chloramine in acid solution. In the Palin (neutral tolidine) method any nitrogen trichloride reacts as free chlorine. The formation of this compound during break-point chlorination has been recognised by several workers (e.g., Holwerdal and Marks and Glasslo), but there is apparently little informa- tion as to its germicidal value. A feature of Palin’s results is that they give a simple method of measuring the content of trichloride and they show that surprisingly large amounts may be present. The possibility that nitrogen trichloride would react as free chlorine in the amino- dimethylaniline and arsenite - tolidine tests must also be borne in mind. IODIMETRIC METHOD- In general, this method is useful for chlorine residua over 1 p.p.m., provided nitrite and manganese are absent and ferric iron does not exceed 2p.p.m.The thiosulphate used is conveniently 0.0025 N and a 500-ml. or 1000-ml. sample may be used, but in the past the advisability of standardising the t hiosulphate under similar conditions has usually been ignored. The iodimetric method suggested by the Joint British Water Analysis Committee therefore requires the thiosulphate to be standardised against iodine liberated from iodate a t high dilution. I t should also be stressed that since, as mentioned earlier, dichloramine does not appear to react with iodide in neutral solution, the sample should always be acidified with sulphuric acid before iodimetric titration.L4PPROVED BRITISH METHODS FOR THE DETERMINATION OF RESIDUAL CHLORINE- The British Committee that has been considering methods of water analysis soon found that the prescription of an approved method for residual chlorine would be one of its most difficult tasks, The shortcomings of the Ellms and Hauser o-tolidine method were realised, but the Committee had to bear in mind the effect of any changes on the validity of the residual chlorine testing kits which are in use at waterworks and swimming-baths throughout the country. I t was recognised that any change should only be undertaken after full discussion with all parties concerned.It accordingly recommended that the existing Ellms and Hauser method and standards be approved pro tern. but that a new and wider Committee should be convened to go into the whole question. The position was complicated by the fact that while the Committee was sitting, the new (1946) American standard methods for residual chlorine were laid down. Much time and trouble have obviously been devoted to the new American methods, but it was agreed that further study was necessary before they could be recommended forthwith as the basis on which British testing kits could be manufactured. The Committee were also impressed by the fact that the whole subject of residual chlorine determination was in a somewhat fluid condition and that new and apparently improved methods were still being introduced.Dr. A. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. In the preparation of this survey I have received a number of useful suggestions from T. Palin, and to him my grateful acknowledgment is due. REFERENCES Holwerda, K., Medeelingen van den Dienst der Volksgezondheid in Nederlandsch-Indae, 1928, 17, Ruys, J. D., “Drinkwaterreiniging met Hypochlorieten,” 1914 (vide Lea, C., and Mills, A. J., Faber, H. A., Water Works and Sewage, 1939, 86, 337. Griffin, A. E., J . Amer. Water Works Assoc., 1939, 31, 2121. Calvert, C. K., Water Works a d Sewage, 1940, 87, 299. Ellms, J. W., and Hauser, S. J., Ind. Eng. Chem., 1913, 5 , 916, 1030. Chamberlain, N. S., and Glass, J. R., J . Amer. Water Works Assoc., 1943, 35, 1066, 1206. Laux, P. C., Ibid., 1940, 32, 1027.Hallinan, F. J., Ibid., 1944, 36, 296. Marks, H. C., and Glass, J. R., Ibid., 1942, 34, 1227. 251 (Part 1 ) ; 1930, 19, 325 (Part 2). Annual Conference of the National Association of Bath Superintendents, 1949).184 HOUGHTON : RESIDUAL CHLORINE IN WATER [Vol. 75 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. Kolthoff, I. M., Chem. Weekblad, 1926, 23, 203. Alfthan, K., and Jarvis, A. C., J . Amer. Water Works Assoc., 1928, 20, 407. Haase, L. W., and Gad, G., 2. anal. Chem., 1936, 107, 1. Byers, D. H., and Mellon, M. G., Ind. Eng. Chem., Anal. Ed., 1939, 11, 202. Moore, W. A., J . Amer. Water Works Assoc., 1.943, 35, 427. Palin, A. T., Analyst, 1945, 70, 203. Taras, M., J . Amer. Water Works Assoc., 1946, 38, 1146. -, Ind. Eng. Chem., Anal. Ed., 1947, 19, 342. Laux, P.C., and Nickel, J. B., J . Amer. Watev Works Assoc., 1942, 34, 1785. Palin, A. T., J. Inst. Water Eng., 1949, 2, 100. Harold, C. H. H., 29th Ann. Rept. of Director of Water Exam. Met. Water Bd., 1934. SOUTH ESSEX WATERWORKS Co. LANGHAM, COLCHESTER LANGHAM VALLEY WORKS DISCUSSION MR. R. F. MILTON drew attention to a completely new method for the estimation of residual chlorine. He said that this was the first time that a direct method for the determination of chlorine had been put forward, all the existing methods being based on oxidation potential and not directly on the presence of chlorine, A short description of this method had been published’; briefly, it depended on the fact that when free chlorine was brought into contact with the cyanide ion, cyanogen chloride was produced quantitatively.The cyanogen chloride was then allowed to react with pyridine to form a quarternary derivative which, when coupled with a.n aromatic amhe, produced an intense colour which could be made the basis of a quantitative estimation. This method would also detect free bromine, but otherwise was entirely specific for free chlorine. Mr. Milton felt that the method was worth further investigation. The high concentration of pyridine, 6 per cent., was a disadvantage, but it might be possible to use other organic compounds such as nicotinic acid. DR. HOUGHTON replied that he had made a few tria.ls of the method described by Mr. Milton, working on 6-ml. quantities of water. He could confirm the high sensitivity of the method, but he had found difficulty with the benzidine precipitating. He thought the method showed promise, however, and was of considerable interest. DR, H. LIEBMANN said that he had tried Mr. Milton’s recently published method, and could also confirm its high sensitivity. However, this sensitivity appeared to vary in accordance with the concentration of the residual free chlorine, decreasing very markedly a t higher concentrations as determined in distilled water by amperometric titration. Above about 4 p.p.m. of free chlorine, further increase produced only a very slight increase in colour density. He had also obtained some evidence that a t least a part of the chloramine is estimated in addition to free, available chlorine by Milton’s method. These experiments, however, had been carried out on highly polluted water, with which all methods for the determination of free chlorine became rather uncertain. For this reason they were not specific. REFERENCE TO :DISCUSSION 1. Milton, R. F., Natuvs, 1949, 164, 440.