Determination of Microquantities of Iodine in Water Solution by Amperometric Titrations H. P. KRAMER, W. ALLAN MOORE, AND DWIGHT G. BALLINGER Public Health Service, Environmental Health Center, Cincinnati, Ohio The amperometric titration method for determining micro quantities of iodine has the distinct advantage of ability to determine the concentration of the germicidal entities actually present at pH levels within the practical range of water treatment practice. The effect of ammonia on iodine residuals under the conditions of this investigation was found to be negligible. The oxidation state of iodine in water solution is a function of time and pH. At a given pH, free iodine residuals diminish as a function of time. This loss in free iodine concentration is caused by the oxidation-reduction reaction of hypo-
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H E effectiveness of free chlorine and chloramine as water disinfecting agents has been evaluated for different pH values and time intervals (1). Neither iodine nor bromine has been investigated as extensively as has chlorine and chloramines. Both iodine and bromine have been recommended as germicidal agents for swimming pools (5). The objective of this study was the selection or development of a residual iodine test in water solution suitable for the concentration range 0.2 to 2.0 p.p.m. The method was to find immediate application in an investigation carried out a t the Environmental Health Center on the effectiveness of iodine as a germicide in water solution. In the study of the bactericidal properties of chlorine and chloramines] Chang ( 8 ) has shown how the chemical form of chlorine changes with variations in pH. Marks and Strandskov ( 4 ) have evaluated the sporicidal efficiencies of chlorine, bromine, and iodine with respect to pH and, based upon equilibrium considerations, have shown how the chemical composition of the water solution of each of the halogens varies with pH. With chlorine, the predominant compound present over the range of pH 2 to 8 is hypochlorous acid; there is no free chlorine above p H 2. With iodine the molecular entity predominates below a p H of 8 a t which level significant hypoiodous acid formation occurs. These authors show that the molecular iodine has the highest sporicidal efficiency. Other investigators have shown that iodine and hypoiodous acid have germicidal properties in water solution ( 7 ) . Iodine undergoes certain fundamental reactions in water solution, as shown by Wyss and Strandskov ( 7 ) . In these reactions pH is the dominating factor in determining the chemical entity present. The oxidation-reduction reaction in which both iodic acid and iodide ion are formed has been shown ( 7 )to be catalyzed much more strongly by phosphate buffers than by certain other buffers. In this study phosphate, phthalate, glycocoll, borate, and citrate buffers were investigated. Of these buffers it was found that borate and citrate had the least effect on the stability of hypoiodous acid. Theoretically, in a water-iodine system, the relative and absolute concentrations of iodine, hypoiodous acid, iodide ion, and iodate ion determine the germicidal efficiency. A trial run was made to evaluate the germicidal effect of the iodide and iodate, and the results indicate that in the range of concentrations under investigation, these two entities were not germicidal. ilccord-
iodous acid, which results in the formation of iodate and iodide ions. The selection of a system of buffers which minimized the catalytic action encountered with certain buffers and the use of chlorine-demandfree water resulted in negligible loss of free iodine in a 30-minute contact period. In order to secure precise results when utilizing the amperometric titration method, the electrodes should be sensitized by an appropriate procedure. The determination of free iodine by the amperometric method was found to be accurate to 10.01 p.p.m. in concentrations varying from 0.2 to 2.0 p.p.m.
ingly, analytical methods were sought which would quantitatively measure iodine and hypoiodous acid (here called free iodine) and which would include no iodide and iodate. The method selected must be efficient in the p H range 6.5 to 9.15. Various colorimetric methods were tried in an attempt t o develop a suitable procedure. Fluorescein gave poor color differentiation with small changes in iodine concentration. Both p-aminodimethylaniline and tetramethyl benzidine gave excellent color development with a sensitivity of 0.05 p.p.m. within the range of 0.05 to 0.2 p.p.m. The best color development and the least color production in the blank were obtained a t p H 2 for both reagents. The requirement of p H adjustment nullified the usefulness of these methods for this study. Amperometric determination of the free iodine was investigated using a Wallace and Tiernan amperometer with phenylarsene oxide as the standard titrating solution. Marks and Glass (3) claim an accuracy of 0.01 p.p.m. for chlorine by this method, which is equivalent to approximately 0.04 p.p.m. of iodine. It was found that free iodine concentrations could be determined by this method without pH adjustment over the range investigated-namely] 6.5 to 9.15. Thus the free iodine could be determined at the p H actually employed in the bactericidal tests.
Table I.
Effect of Time on Free Iodine Concentration
pH
Initial Free Iodine Concn., P.P.M.
6.5
0.25 0.60
9.15
0.78 2.59 0.25 0.53 0.97
Free Iodine Concentrations, P.P.M. Elapse of time, minutes 30 60 90 120 0.23 0.21 0.19 0.54 0.58 0 . 7 6 0172 0 . 7 1 2.55 2.47 2.41
0.18 0.52 0.62 2.41
0.25 0.22 0.18 0.18 0.52 0.49 0.47 0.46 0 . 9 1 0 . 8 6 0.81 0 . 7 1
Free Iodine Concn., P.P.M. after Adjustment 0.23 0.69 0.75 2.52 0.24 0.60 0.88
The possibility of any iodine ammonia-nitrogen reaction was of interest in this study, since such a reaction would exert an iodine demand. T o determine this factor, ammonia nitrogen solutions in unbuffered chlorine-demand-free water in concentrations of 0.1 and 0.2 p.p.m. were prepared prior to the addition of definite amounts of iodine. After a 2-hour interval the iodine residuals
1892
1893
V O L U M E 2 4 , NO. 1 2 , D E C E M B E R 1 9 5 2 were determined amperometrically. Under the conditions of this experiment, no significant iodamine formation was evident. This fact does not preclude such formation under other conditions. It was assumed that chlorine-demand-free water would also be iodine-demand-free because of the greater chemical activity of chlorine. This assumption appears to be justified based upon the results shon-n under experimental conditions. All water used in these determinations r a s made chlorine-demand-free by a slight modification of the method of Megregian (6). Sulfite addition was stopped when the chlorine residual dropped below 0.005 p.p.m. and boiling n-as substituted t o remove the last traces
Table 11.
Stability of Iodine Residuals
(30 minutes a t room temperature in buffered waters)
No. of Detns.
pH
6 43 14 24
6.5 7.5 8.5 9.15
0.22 0.21 0.20 0.21
0.19 0.21 0.19 0.19
0.03 0.00 0.01 0.02
0.05 0.04 0.05 0.05
7 49 14 24
6.5 7.5 8.5 9.15
0.44 0.42 0.42 0.44
0.40 0.41 0.41 0.42
0.04 0.01 0.01 0.02
0.07 0.05 0.04 0.06
11
51 14 25
6.5 7.5 8.5 9.15
0.61 0.62 0.61 0.62
0.60 0.60 0.60 0.58
0.01 0 02 0.01 0.04
0.04 0.06 0.06 0.07
12 46 14 26
6.5 7.5 8.5 9.15
0.81 0.81 0.80 0.81
0.79 0.78 0.78 0.78
0.02 0.03 0.02 0.03
0.05 0.07 0.06 0.08
7 33 6 30
6.5 7.5 8.5 9.15
1.01 1.00 1.01 1.01
1.00 0.98 0.98 0.97
0.01 0.02 0.03 0.04
0.03 0.07 0.05 0.08
7 8 10 21
6.5
1.61 1.62 1.43 1.60
1.60 1.58 1.43 1.56
0.03 0.06 0.02 0.09
4 16 8 22
6.5 7.5
2.02 2.03 1.81 2.04
2.02 1.99 1.81 1.98
0.01 0.04 0.00 0.04 0.00 0.04 0.00 0.06
7.5 8.5 9.15
8.5
9.15
-4verage P.P.31. Initial Final Loss
Maximum Loss Shown by Any Sample in This Series
0.01 0.14 0.01 0.13
Extending the amperometric method to measurements of low concentrations of iodine with the apparatus specified, led to difficulties in obtaining reproducible results. The necessity for sensitizing the electrode to iodine was observed. Either a preliminary 0.5-hour soaking of the electrode in an approximately 2 p.p.m. iodine solution or making three to five titrations on an iodine solution before the actual titration of the sample proved successful. Furthermore, the sensitivity of the electrode could be maintained by removing the end products of the titration as soon as possible.
acid, then raised to approximately 6 with 0.1 N sodium hydroxide. One milliliter of citrate buffer was then added to hold the p H a t 6.5. The result of the titration then performed was called free iodine concentration in p,p.m. after adjustment. Table I shows an increasing loss of free iodine with increasing time, Solutions of greater concentration exhibited the greater loss. The reaction is shown to be reversible since the free iodine after the p H adjustment is approximately equal to the initial concentration. One of the conditions required for the germicidal testing of iodine residuals was a stable p H for the period of exposure of the test organism. It was, therefore, desirable to buffer the solutions a t the required p H levels with a buffer which Lyould be nontoxic and I\-ould not catalyze the oxidation-reduction reaction of iodine in solution, Borate and citrate buffers most nearly satisfied these requirements. In the procedure, as outlined below, Palitzch borate buffer was used at 9.15, 8.5, and 7.5, using 10 ml. per liter. For p H 6.5 Sorensen's citrate buffer was used, 10 ml. being added per liter of sample. With the citrate buffer it was found necessary t o make the addition subsequent to preparation of the chlorine-demand-free water to avoid the decomposition of the citrate in the presence of chlorine a t high temperature. ( A Beckman line-operated pH meter was used in making all pH measurements. ) PROCEDURE
Make all water used in the following procedure chlorinedemand-free by the previously mentioned method. Sensitize the electrode immediately before use by soaking in an approximately 2 p.p.m. iodine solution for about 30 minutes or by making three to five preliminary titrations on an iodine solution (1 to 2 p.p.ni.). 1. Stock solution: Dissolve 0.50 gram of iodine in 300 ml. of ethyl alcohol (95%). Dilute this solution to 1000 ml. with water. Allow to stand for a t least 24 hours before using to satisfy any halogen demand of the water and glassware. 2. Buffer solution, use 10 ml. per liter: Sorensen citrate buffer for pH 6.5; Palitzch borate buffer for pH 7.5, 8.5, 9.15. 3. Dilute the stock solution 1 to 500 with buffered water. 4. Pipet three 100-ml. aliquots of the diluted stock solution into titration cells and dilute each to 200 ml. with water. 5 . Titrate amperometrically with a n approximately 0.003 N phenylarsene oxide solution. (For convenience a 1 to I dilution of the solution furnished by Wallace and Tiernan may be used. .V = 0.00282; 1ml. = 0.358 mg. of iodine.) 6. Dilute and titrate the sample in the same manner as in 4 and 5. 7. Report mean result as the free iodine concentration.
Table 111. Precision of Amperometric Titrations Iodine Concn., P.P.M.
No. of Replicates
Range
Standaid Deviation
EFFECT OF TIhlE ON FREE IODINE CONCENTRATION
In order to demonstrate that the oxidation-reduction reaction exhibited by hypoiodous acid is a function of time, the authors decided to reverse this reaction after a suitable period by the addition of hydrogen ion. Results obtained from one determination, run in triplicate, at each of the iodine concentrations and pH values indicated are given in Table I. The following procedure was used: Iodine stock solution was added to 2 liters of buffered chlorinedemand-free water. Three 100-ml. aliquots were diluted to 200 ml. with water and titrated. The average of these results was reported as initial free iodine concentration. The amperometric determinations of free iodine were made a t 30,60,90, and 120 minutes on the iodine solution prepared. After the 120-minute period, 10 ml. of a 770 solution of potassium iodide were added to another aliquot. The p H was then lowered to between 2 and 3 with 1% sulfuric
STABILITY STUDY
Sctual results obtained in the bacteriological experiments on free iodine concentrations of 0.2 to 2.0 p.p.m. a t pH 6.5, 7.5, 8.5, and 9.15 are shown in Table 11. Each determination is the mean of three replicate titrations on a sample. The means of concentrations and losses for the indicated number of determinations of each concentration and a t each pH level are shown. Generally the tendency exists for the largest Iosseq to appear at the highest pH levels. The maximum loss exhihited by an individual sample was found a t the highest concentration used. The greatest mean loss was shown by the samplt- of highest p H and highest concentration. The stability of the
ANALYTICAL CHEMISTRY
1894 iodine solutions under the conditions specified was suitable for the bacteriological studies. PRECISION OF METHOD
To establish the precision of the amperometric titration method for the measurement of iodine in the concentration range of interest in this study, unbuffered solutions were titrated in replicate. The pH values in all cases were between 6.6 and 6.8. Table I11 gives the range and standard deviation for each series. These values indicate the feasibility of the amperometric method in measuring low iodine residuals.
LITERATURE CITED Butterfield, C. T., U . S. Pub. Health Repts., 63, 9 3 4 4 0 (1948). C h a n g , S. L., J . -4m. W a t e r W o r k s Assoc., 36, 1192 (1944). M a r k s , H. C., a n d Glass, J. R., Ibid., 34, 1227 (1942). M a r k s , H. C., a n d Strandskov, F. B., Ann. S. Y . A c a d . Sci., 53, 163 (1950). (5) M a r k s , H. C., a n d Ytrandskov, F. B . , L-. S. P a t e n t 2,443,429 (1948). (6) Megregian, Stephen, C. S. Pub. Health Repts., 63, 1 3 7 4 1 (1948). (7) Wyss, O., a n d Strandskov, F. B., Arch. Biochem., 6 , 261-8 (1945). RECEIVED for review May 28, 1952. Accepted July 24, 1952.
Determination of Copper Ion by a Modified Sodium Diethyldithiocarbamate Procedure Iron, Zinc, a n d Lead Interference Eliminated C. A. NOLL AND L. D. BETZ W . H . & L . D . Betz, Gillingham and Worth Sts., Philadelphia 24, Pa. The American Public Health Association’s method for the determination of copper ion in water involves the use of sodium diethyldithiocarbamate, to produce a yellow color suitable for colorimetric estimation. Iron interferes, and zinc and lead produce interfering turbidity. By reducing the concentration of the sodium diethyldithiocarbamate to 1% of the strength previously used and diluting the ammonium hydroxide reagent, it was found possible to eliminate these interferences. With modified procedure small amounts of copper can be determined accurately by a simple, rapid procedure, without solvent extraction.
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HE present method for the determination of copper ion in water (1), involves the use of sodium diethyldithiocarbamate to preduce a yellow color suitable for colorimetric estimation. Iron interferes with this method, and an extraction procedure with a solvent such as carbon tetrachloride is necessary when iron is present. Zinc and lead produce an interfering turbidity, and extraction with carbon tetrachloride is required in the pre-ence of these ions. Blanks must be prepared by a similar extraction procedure.
Table I.
Concentrations Tolerable without Interference
(At copper concentration = 6.0 p.p.m.) P.P.M. NarSOa 1000 KzCrOa NaaSOa 1000 Quebracho tannin NaCl 1000 Chestnut tannin NaNOi 1000 Pu’asPsOlo KNOI 1000 Fe+++ CaCla as CaCOa 1000 Zn MgSOl as CaCOs 1000 Pb ++ + +
P.P.M. 500 50 50 5 10 105 2“
0 Data supplied by Edward Chow, assistant public health chemist, California Department of Public Health.
Table 11. Effect of 10 P.P.M. of Fe+f+ at Low Copper Concentrations Present Copper as Cu, ‘P.P.M. 0.0
0.1 0.25 0.50
Found, Copper as Cu, P.P.M. Analyst A Analyst B Analyst C 0.00 0.09 0.26 0.50
0.02 0.07 0.23 0.48
0.00 0 .10 0.23 0.52
It was thought that by reducing the concentration of the reagents employed in this test, i t might be possible t o reduce or eliminate the interference of iron in particular. Accordingly, tests were conducted with successive dilutions of the reagents until jt was found that iron interference was eliminated. The concentration of the sodium diethyldithiocarbamate solution was reduced t o 1% of the strength previously used, and the ammonium hydroxide reagent was also diluted. By these measures i t was found possible t o eliminate the interference due to iron, zinc, and lead. K i t h the modified procedure, small amounts of copper can be determined accurately in the presence of these potentially interfering ions by a simple, rapid procedure, without solvent extraction. APPARATUS
Both a Leitz-Rouy photometer and a Klett-Summerson photometer have been satisfactorily employed for this determination. Both instruments used were equipped with a logarithmic scale. Yessler tubes can also be employed with visual comparison. REAGENTS
Ammonium hydroxide, 0.25 N . Sodium diethyldithiocarbamate, 0.1 gram per liter. PROCEDURE
K i t h the Leitz-Rouy photometer, the 415-mw color filter and the 20 X 20 mm. absorption cell are employed. A ‘Lzero”reference blank is prepared by adding t o a beaker 10 ml. of sample, 10 ml. of 0.25 N ammonium hydroxide, and 10 ml. of distilled water. This blank sample is used t o set the meter pointer a t zero immediately prior t o test.
