Determination of Residual Chlorine in Sewage HESRY C. MARKS AND ROBERT R. JOINER Wullace & Tiernun Company, Inc., Bellecille, A'. J . The o-tolidine method gi,es much lower values of residual chlorine in sewage than a recently dew eloped amperometric titration. T w o possible reasons for the error are the decomposition of organic chloramines on addition of the acid o-tolidine solution and the slowness of reaction between the latter and some of the organic chloramines. These effects are demonstrated on solutions of pure amino compounds. They can be separated b) studFing the effect of acidification on the residual chlorine in sewage. Further information is gained by comparing the results given bw the o-tolidine method with those of the titration upon partial dechlorination of the sample.
T
HE shortcomings of the usual methods for determiriation of residual chlorine in senage prompted a search for a more satisfactory method, which led to the development of a new titration procedure (4j. The sample is first treated with a measured excess of a standard solution of phenylarsene oxide, folloivcd immediately by potassium iodide. The p H is then decreased to 2.5 or less by the addition of hydrochloric acid. Finally, the unreacted phenylarsene oxide is titrated with standard iodine using starch or, preferably, a polarized electrode as the end-point indicator. The iodine liberated by the chlorine reacts immediately with the phenylarsene oxide, so that none is lost by reaction with the sewage. By carrying out the backtitration a t low pH, the iodine is rendered unreactive toward the sewage, and a permanent end point is possible. The results obtained with this method n-ere found to agree satisfactorily with those obtained by the p-aminodimethylaniline method using potassium iodide (6). The values obtained were also reasonable when compared to bacteriological results, always showing residual chlorine xvhen there was a decrease in bacterial numbers. The o-tolidine test frequently fails to indicate a residual a t low dosage ranycs. rven though sonic degret, of sterilization is obtained (5'). On sewage the nen- method showed c niethod even larger than had been suspected, and it was concluded that much of the residual chlorine is present as organic chloramines not measured b>-this test. TKOpossible reasons for this were put forth: (1) The cahlorainine could he sufficiently inert to leave certain types of chlorine demand unsatisfied. .It tht>low pH of the o-tolidine test, it would become sufficiently active to react with the unsatisfied demand and be dissipated. The amino acid, cystine, is an example of a structure that will not reduce the chlorine of a chloramine group until after acidification. (2) The chloramine could react so slorvly with o-tolidine that fading would intervene to prevent complete color development. An cmmple of a chloramine of this t>-pewill be found in the chloramine formed from piperidine. The present work demonstrates on model systrnis that hoth mcchanisms are possible and attempts to estimate their relative importance in causing the errnrs in the o-tolidiiic method as applied to sen-age. EXPERIMENTAL DETAILS
Residual chlorine determinations were made by the o-tolidine
tion M ab eoiiiplete. The !ratel used for making solutions and for dilution purposes Tvas zero demand water made in the usual Ivay. EFFECT OF LOWERIXG pH ON A IIODEL SYSTEM
Solutions containing the amino acid, I-cystine, illustrate the case where lowering the p H causes disappearance of residual chlorine. Cystine is found in natule and is present in selvage ( 2 ) . Results in Table I obtained with a dose of 15 p.p.m. of chlorine and 25 p.p,m. of cystine show that the residual chlorine in such a solution acts in the same n-ay as in sewage xith respect to the three test methods. The o-tolidine method always indicates a small fraction of that measuled by the amperometric titration or the p-aminodimethylaniline method.
Table I.
Determination of Residual Chlorine in Solutions of Cvstine
(25 p.p.in. I-cystine; p H 7.0; room temperature: chlorine dose 15 p.p.m.1 Residual Chlorine, P.P.M. Contact .%mperometric p-dminodiTime, I I i n . +Tolidine titration methylaniline 5 15
30 60
The connection b e b e e n pII lowering arid loss of available chlorine was demonstrated with a solution containing 50 p.p.m. of cystine treated x i t h 20 p.p.m. of chlorine. After various contact periods, the residual chloiine waz determined by the o-tolidine te-t and by two amperoinetric titration procedures. I n one, the sample was brought t o pH 1.5 with hydrochloric acid, held for 30 seconds, then treated with phenvlarsene oxide solution and potassium iodide. I n the other, the phenylarsene oxide and potassium iodide vere added a3 usual before changing to pH 1.5. Both sample. \rere finally backtitrated with standard iodine solution. I n the first procedure, the chlorine ivas given an opportunity to react at the l o x pH just as in the o-tolidine test. The second tltratiorl procedure vias the same in every reqpect, except that the chlorine reacted r i t h the reagent before the pH was lowered. Table I1 shon s that the o-tohdine method and the first titration method are about equally affected by the low pH. The failure to obtain the full o-tolidine color in this case is due not to the presence of lew reactive chlorine but to the loss caused by acidification.
test, bj- amprrometric titration carried out according to the
method previously described (41, and by the p-aminodimethylaniline test according to Paliri ( 5 ) . The o-tolidine test was made with permanent glass standards, and both this and the p-aminodimethylaniline test were compensated for turbidity-. Sufficient acid Fas added in the o-tolidine test to lower the pH to the proper point (f), and the maximum color developed was read. All pH measurements were made with t'he glass electrode. 111 all cases where chlorine \vas added to dolutions of pure amino compounds, sufficient time was allowed, and the proper tests were made to make certain that combina-
Table 11.
Effect of Acidification on Residual Chlorine in Cystine Solution
(50 p p.m. I-cystine: pH 7.0; room temperature; chlorine dose 20 p.p.m.) Residual Chlorine, P.P.11. Contact Acid Surm Time, 3Iin. o-Tolidine titration" titration6 5 0.17 0.80 0.05 15 0.07 0.00 0.02 30 a Sample acidified before adding phenylarsene oxide b Sample acidified after adding phenylarsene oxide.
1197
ANALYTICAL CHEMISTRY
1198 Table 111. Determination of Residual Chlorine in Solutions of Piperidine (61 p.p.m. piperidine; p H 7.0: room temperature; chlorine dose 6 p.p.m.) Residual Chlorine, P.P.hf. Contact Acid Norm Time, hfin. o-Tolidine titrationa titration6 1 1.1 5.75 5.65 15 0.5 5.75 5.65 30 0.5 5.60 5.70 60 0.5 5.70 5.65 120 0.5 5.55 5.40 " Sample uvidified before sdding phenylarsene oxide. t .Saii.ple ncidiiied ;frer ndding phenylnr3ene oxide.
TEST ON SLOWLY ACTING CHLORAMINE
A chlorinated solution of piperidine, an amine found in nature, affords an example of an unreactive chloramine not quantitatively measured by o-tolidine. Table I11 gives the results obtained with a solution containing 61 p.p.m. of piperidine treated with 6 p.p.m. of chlorine. At various time intervals, samples were analyzed for residual chlorine by the three methods described. Comparison of the two titration results shows that here acidification does not cause a loss of available chlorine. Comparison of the titration results with the o-tolidine result shows that the latter registers less than one tenth of the available chlorine actually present. This error is evidently due to the slowness of the reaction between the chloramine and the o-tolidine. Not all amino compounds that may be found in nature form chloro derivatives that react so slowly with o-tolidine. Solutions of hippuric acid, glycine, tyrosine, and methyl urea, for example, form S-chloro compounds which can be quantitatively determined by the o-tolidine method. If the method could be shown to indicate consistently only the more active fraction of the residual chlorine, it would still be very useful. There is no indication that this is the case, and experimental results given beloiv provide some evidence to the contrary.
just sufficient to neutralize the chlorine shown by it in the first place. An experiment with a chlorinated piperidine solution of the composition given above showed a very different result. Samples were withdrawn a t various times after addition of chlorine, and residual chlorine was determined by the o-tolidine test and by amperometric titration. At the same time, an additional sample was partially dechlorinated with fresh sodium sulfite solution, the remaining available chlorine being determined on this sample by both methods. The results given in Table IV show that the residual chlorine according to titration was in every case decreased by almost exactly the theoretical amount. Although in every case there was more than enough sulfite to reduce all the residual chlorine shown by the o-tolidine test, the latter was decreased by a smaller percentage than the total. In fact, the o-tolidine method gave practically the same reading over a wide range of total residual chlorine. The actual percentage of the total chlorine shown by o-tolidine is seen to vary from 3.3 to 23% and is highest in the lowest range of residual chlorine. It is concluded that o-tolidine reacts with S-chloropiperidine a t a certain slow rate which is independent of concentration over a considerable range, and that, in this case, the color developed is not a measure of reactivity. The application of this procedure to chlorinated sewage aids in the interpretation of the results given by the o-tolidine method in this case. Samples of raw sewage and of sewage effluent were treated with chlorine, and after 15 minutes' contact, residual chlorine was determined by the two methods. Then, to an aliquot was added sufficient fresh sodium sulfite solution t o remove LL fraction of the available chlorine. Residual chlorine was then immediately determined on the treated sample and again on the untreated original bv both methods. This last result for the untreated fraction was used as a basis for dechlorinating a further aliquot by a different amount and repeating thc cycle. This was continued until a series of samples had been tested.
Table V shows that the titration in this case also agrees very well with the residual chlorine calculated from the amount of dechlorinating agent added. And again the o-tolidine reading was decreased by only a fraction, even in cases where there were If a solution containing several forms of available chlorine added several times the amount of sodium sulfite theoretically of various degrees of reactivity is partially dechlorinated, the required to reduce it to zero. The results vrith sewage differ more reactive fraction of the chlorine should be removed. If the from those with piperidine in that the percentage reduction in o-tolidine method is to any extent selective, it should show zero residual chlorine shown by o-tolidine is greater than in the total. residual after the addition of an amount of dechlorinating agent This is evidence that the available chlorine shown by o-tolidine is slightly more reactive than a t Table IV. Relation between Degree of Dechlorination and Residual Chlorine least part of the chlorine not (61 p.p.m. piperidine; 6 p.p.m. C1z dose; room temperature: P H 7) Dechlorinated Samples shown. The available chlorine Untreated Portion Theoretical Contact Residual Chlorine, P.P.M. Residual Chlorine, P.P.M. Chlorine Time, lost by acidification may be, and Sfin. o-Tolidine Titration Removed, P.P.11. o-Tolidine Titration probably is, more active than the 15 0.20 5.60 0.56 0.17 5.20 5,60 1.68 0.18 4.00 fraction making the greatest con30 0.20 5.60 2.80 0.15 2.60 45 0.18 tribution to the o-tolidine color. 5 . 6 0 3 . 9 2 0 . 1 5 1 . 6 5 60 0.20 STEPWISE DECHLORIm 4TION 4ND RESIDUAL CHLORINE DETERRIINATION
8:
Table V.
0.18
j.45
5.04
0 07
0.30
Relation between Degree of Dechlorination and Residual Chlorine
Initial Residual Chlorine, P.P.M. o-Tolidine Titration
(Room temperature, p H 7) Untreated Portion Final Residual C12, P.P.M. Chlorine o-Tolidine Titration Removed P.P.M.
Dechlorinated Sample Residual Cl?, P.P.hI. o-Tolidine Titration
1.50 1.50 1.90 1.75 1.75 1.50
6.65 6.85 8.15 7.75 7.25 7 .OO
R a w sewage, original chlorine dose 17.5 p.p.m. 2.00 6.50 1.50 2.74 6.65 1.50 4.08 7.75 1.76 4.65 7.25 1.75 5,08 1.50 7.00 5.60 6.85 1.50
0.45 0.35 0.20 0.10 0.07 0.00
4.40 3.85 3.85 2.45 1.80 1.26
1 .oo 1 .oo 1.25 2.00 1.75 1.50
4.20 4.65 4.75 5.90 5.35 5.15
Sewnge effluent, original chlorine dove 10.5 p.p.m. 0.84 0.90 3.95 1.40 1 .oo 4.20 1.90 1.00 4.65 2.95 1.75 5.35 3.20 5.15 1.50 3.60 4.75 1.25
0.50 0.40 0.18 0.18 0.08 Trace
3.15 2.75 2.26 2.56 2.00 1.25
RELATIVE IMPORTANCE OF SOURCES OF ERROR
It is evident that the relative importance of the two sources of error may be estimated for sewage by comparing the results of the o-tolidine test, the normal amperometric titration, and the amperometric titration after acidification. The difference between the last two methods gives an estimate of the acidification effect, while that between the first and third is due to the Jowness of the reaction Tvith o-tolidine. A series of tests on
1199
V O L U M E 20, NO. 12, D E C E M B E R 1 9 4 8 SUMMARY AND CONCLUSIONS
Table VI.
Contact Time, Min.
Effect of Acidification on Residual Chlorine in Sewage ( p H 7, room temperature) Residual Chlorine, P.P.M. Acid o-Tolidine titration
Piorm titration
R a w sewage, chlorine dose 12 p.p.m 15 30 60 120
0.10 0.02 0.00 0.00
0.68 0.34 0.00 0.00
4.75 3.87 2.69 0.83
Sewage effluent, chlorine dose 6 . 5 p.p.m 1
15 30 60 120
0.75 0.60 0.40 0.18 0.05
5.46 4.11
3.56 2.07 0.68 0.35 0.00
2.56
1,86 0 85
typical samples of raw sei5age and of effluent will serve as an illustration. Samples of each were chlorinated in the laboratory. At various contact times aliquots were removed, each of which was analyzed for residual chlorine by the three methods. The results are given in Table VI. With this particular sample of chlorinated raw sewage, dissipation of available chlorine on acidification causes the error in the o-tolidine test. Previous acidification causes the titration to give practically as lon a result as the o-tolidine method. The titration of the effluent sample after acidification is significantly lower than the normal titration but higher than the o-tolidine result. Both sources of error are about equally important for this particular effluent sample.
On solutions of pure amino compounds low results in the o-tolidine method for residual chlorine can be caused either by loss of available chlorine on acidification or by slow rate of production of color. Both mechanisms may contribute to the errpr in the method as applied to sewage. When a sample of chlorinated sewage or amino compound containing a certain amount of residual chlorine according to amperometric titration is partially dechlorinated, the resulting rcsidual chlorine concentration found by amperometric titration agrees very well with that calculated from the quantity of dechlor used. Even though the residual chlorine indicated by the otolidine method on the same sample is a small fraction of the total and also of the amount removed by dechlorination, the mading with o-tolidine does not become zero. It is concluded that the residual chlorine measured by the o-tolidine test is not ncwxsarily the most reactive portion. Because there is more than one independent cause for the error in o-tolidine reading, this error will not be a constant one. LITERATURE CITED (1) rim. Public Health Assoc., New York, "Standard Methods for Examination of Water and Sewage," 9th ed., 1946. ( 2 ) Elder, A. L., and Buswell, -1.M.,Ind. Eng. Chem., 21, 560-2 (1929). (3) Lea, C., J . Soc. Chem. Ind., 52, 245-50T (1933). (4) Marks, H. C., Joiner, R. R., and Strandskov, F. B., Water R. Sewage W o r k s , 95, 175-8 (1948). (5) Palin, A. T., Analyst, 70, 203-7 (1945). RECEIVED l l a y 3, 1948. Presented before t h e Division of Water, Sewage. CHEMICAL a n d Sanitation Chemistry a t the 113th l l e e t i n g of the A M E R I C A N SOCIETY, Chicago, Ill.
Polarographic Determination of Folic Acid W. J . MADER
AMI
H. 1. FREDIANI, Merck & Co., Znc., R a h t c q , X . J .
Folic acid may be determined quantitatively and rapidly by polarographic means. At a pH of 9 to 9.Sy in 1% tetramethyl ammonium hydroxide E '/2 us. S.C.E. = 0.98 volt and the diffusion current constant = 1.72. The use of cadmium as an internal standard permits a precision of *29& irrespective of ordinary temperature (=!=lo"C.) and drop rate (0.75 second) changes. The method may be applied to folic acid tablets and tablets with Bg added. I t cannot he __ used in the presence of iron.
D
ESPITE numerous publications within recent years and widespread interest in folic acid (pteroylglutamic acid, vitamin Bc), a rapid method for the determination of this vitamin has been much desired. In this laboratory a polarographic method has proved sufficiently simple, precise, and specific for use on relatively pure folic acid as well as known mixtures of folic acid and the necessary constituents for the formation of various tablets. Although the authors have not been in position to compare their method with the only other nonmicrobial assay JO far suggested for this liver L. c a m factor ( I ) , it is offered as a viorkable, rapid assav method for chemists interested in folic acid. PRINCIPLES OF METHOD
The pure folic acid, or preparation containing it, is dissolved in tetramethyl ammonium hydroxide solution containing cadmium chloride as internal standard. Sufficient ammonium chloride is used to prevent precipitation of the cadmium from the alkaline solution. A polarogram of this solution yields two clearly defined waves, one a t 0.74 volt (against the saturated calomel electrode) for the cadmium and one a t 0.98 for the folic acid (Figure 1). With known folic acid concentrations (and fixed cadmium concentration) a straight line results in plotting the step-height ratios of cadmium-folic acid against folic acid concentration using log log coordinates (Figure 2).
RE4GENTS
Place 100 ml. of lOy0tetramethyl annnonium hydroxide (Eastman KO.1515) in a 1-liter volumetric flask, and add 11 grams of reagent ammonium chloride and 500 ml. of distilled water. When solution is complete add 115.0 grams of reagent cadmium chloride (CdC12 2.5 H20, carefully weighed) and shake until the solutio11 is clear. Add 10 ml. of 0.1% alcoholic methyl red as maximum suppressant and dilute t o the mark with distilled water. The concentration of ammonium chlorids used (approximately 0.2 X )is not critical; a 0.1 -11' solution provided sufficient buffering action to prevent precipitation of the cadmium. The concentration of tetramethyl ammonium hydroxide is also not a critical faotor, as the curves obtained with a standard folic acid sample were not affected by a 507, change in concentration (increase or decrease) of this constituent. It is, of course, necessary that the solution be definitely alkaline. The final concentration of methyl red used (0.001%) has been found in this laboratory sufficiently strong to eliminate maxima effect. Decrease of this concentration to 0.0005% permitted occasional maxima to be observed in the cadmium wave. The weight of cadmium chloride used was such as to provide a cadmium wave height comparable to that of the folic acid. In this way physical measurement er-
