The Determination of Chlorine Dioxide in Treated Surface Waters MILDRED A. POST and W. ALLAN MOORE Robert A. Toft Sanitary Engineering Center, Bureau of State Services, Public Health Service, Department of Health, Education, and Welfare, Cincinnati, Ohio ,This investigation was undertaken as no specific sensitive method existed for the determination of chlorine dioxide in low concentrations. Chlorine dioxide (0.05 to 1.5 mg./liter) in a treated water supply is reacted with l-amino8-naphthol-3,6-disulfonic acid, and the bluish-pink color produced is measured spectrophotometrically at 525 mp. The reaction is carried out in an acetate-buffered solution at a pH of 4.1 to 4.3. Chlorine interference can be eliminated by means of malonic acid. The effect of 18 diverse ions is discussed. A procedure is also given for the determination of chlorine dioxide in water containing suspended matter. Excellent recovery results for chlorine dioxide added to various types of waters, made demand-free, indicate that the procedure is a practical method well-adapted to different types of waters. Chlorine dioxide is used for disinfection purposes in many water treatment plants both in the United States and Canada.
C
(I, 2, 6, 8 ) for the determination of chlorine dioxide either do not make a distinction between this compound and other forms of chlorine, or are not sufficiently sensitive to detect concentrations used in water treatment practice. This investigation was undertaken to find a specific sensitive method for the* determination of chlorine dioxide in low concentrations. Of the 30 different compounds checked in an effort to find a suitable indicator, 1-amino-8-naphthol-3,6 disultonic acid proved to be the most satisfactory. URRENT METHODS
APPARATUS A N D REAGENTS
Spectrophotometer, Beckman Model B (5-cm. cells). Beckman pH meter. Assembly for generating chlorine dioxide (Figure 1). Chlorine Dioxide Solution. Ten grams of C.P. sodium chlorite are dissolved in 750 ml.of distilled water contained in a two-neck, round-bottomed flask.which is then connected to the receiver containing 1500 ml. of water. I n 5-ml. increments and at 5-minute intervals, 20 ml. of 10% sulfuric acid 1872
ANALYTICAL CHEMISTRY
are added to the graduated cylinder fitted into the side neck of the flask. Air agitation is started prior to the first addition, and the cylinder is raised slightly a t each addition. Air is bubbled through the solution for a p proximately one-half hour after the last acid addition or until a deep yellow chlorine dioxide solution is obtained in the receiver. This solution should t o stored in dark bottles away from light and prepared fresh weekly. To determine whether a chlorine-free solution was generated by this procedure, another apparatus was set up in which the chlorine dioxide was dried by ptlasing over calcium chloride and then over C.P. sodium chlorite to remove any chlorine that might possibly have been formed. Comparison tests showed that chlorine dioxide generated by the first procedure was chlorine-free. Standard Chlorine Dioxide Solution. This should be prepared fresh daily by diluting a suitable aliquot of the stock solution and determining the concentration of chlorine dioxide by means of the usual thiosulfate titration. Mg./liter C10, =
ml. 0.025N/Naz&08 X 0.3375 X loo0 vol. of sample This calculation is based on the reaction between chlorine dioxide and potassium iodide as given by Mellor (4). A 250-ml. aliquot requires 5 to 8 ml. of 0.025N sodium thiosulfate. If the solution is stronger than this, chlorine dioxide is easily lost. In preparing a series of readings, no more than seven aliquots should be taken from one solution before redetermining its strength as chlorine dioxide is very volatile. Stopper the flask after each withdrawal and keep in the dark when not in use. Chlorine Solution. A stock solution was prepared by bubbling chlorine gas slowly into distilled water. It was necessary to prepare this solution daily because an oily film formed on the surface with longer standing. From this stock solution, a standard solution is prepared and standardized by the usual method. Purification
of
1-Amino-8-naphA good tech-
thol-3,6-disulfonic Acid.
nical grade of this acid (trade name, H acid) should be light gray in color. Dissolve 100 grams of the dry powder in 750 ml. of distilled water and add enough sodium carbonate to give a slight test to pink litmus. This requires ap roximately 22 grams. Add 5 grams of %arc0 and 5 grams of Celite. Stir the mixture for 10 minutes and filter by suction. Precipitate the H acid from solution with approximately 40 ml. of C.P. hydrochloric acid to a definite blue test on Congo Red paper. Filter the mixture by suction using sharkskin filter paper and wash the filter cake with 100 d.of distilled water. Press the cake dry and repurify as above. H acid, with a single purification, gives a high blank so it is best to recrystallize twice. Dry the filter cake overnight in an oven a t 60" C., and store it in a dark bottle. The yield of dry powder is about-72 grams. Distilled Water. The distilled water used in making up all reagents and dilutions must be both chlorine and chlorine dioxide demand-free. Such water was prepared according to standard methods (8).
H Acid Solution. Dissolve 0.85 gram (0.0025 mole) of the purified dry powder in 50 ml. of ethylene glycol (boiling point 195-7 " C.) by heating slightly. When the powder is in solution, add 50 ml. of water. A pale green solution is obtained which should be stable for 1 week a t room temperature. If kept in a dark bottle a t 20" C. or lower, its stability can be extended to 2 weeks. Sodium Acetate Buffer. Dissolve 243 grams of C.P. sodium acetate (CHr COONa.3Hz0) and 360 ml. of glacial acetic acid in water and dilute to 1 liter. The p H should be 4.1 to 4.3. Malonic Acid Solution. Dissolve 1.0 gram of malonic acid (practical grade) in water and dilute to 1 liter. Femc Chloride Solution. Dissolve 0.0484 gram of ferric chloride (FeCIJ.6H20) in water and dilute to 1 liter. One milliliter contains 0.01 mg. of iron. Zinc Sulfate Solution. Dissolve 130 grams of zinc sulfate (ZnSO1.7 H 2 0 ) in water and dilute to 1 liter. Sodium Hvdroxide. A 0.5N solution is used. Sulfuric Acid. A 10% solution bv volume is used. With-this reageit demand-free water is not necessary.
Figure 1.
Chlorine dioxide generator
285 5-
I
I
I
510
515
520
I
I
I
I
525
530
535
540
.-L IO
5S5
WAVE LENGTH rnn
Figure 2.
Absorption curve
PROCEDURE
Preparation of Standard Curve for Chlorine Dioxide. Preparation of ita
standard curve as well as all chlorine dioxide determinations should be carried out at 20' t o 25' C. To a 100-ml. volumetric flask, approximately half-filled with chlorine dioxide demand-free water, pipet the required amount of chlorine dioxide standard solution below the surface of the liquid so that no loss will occur. Immediately close the flask with a glass stopper and keep closed until ready to complete the determination. A series of such solutions is prepared in the concentration range of 0.05 to 1.5 mg. per liter of chlorine dioxide. Then add from a pipet 5 ml. of the acetate buffer. In adding reagents, avoid pipets with very small tip openings. The reagents should be added as rapidly as possible in order to avoid loss of chlorine dioxide. Then add 1.0 d.of ferric chloride solution and dilute to the mark with chlorine dioxide demand-free water. Following dilution, 0.4 ml. of the H acid reagent is added and the mixture shaken. Keep it in the dark until ready to read. Twenty to 25 minutes after shaking, determine the absorbance a t 525 mp against distilled water in the comparison cell (5-cm. cells are preferable). If desired, the time element can be longer but it should be consistent for all the samples and the same as that employed in making the standard curve. A daily blank is run using all of the reagents in the absence of chlorine dioxide. A sufficient interval should be allowed between completion of each solution t o avoid any difficulty in making the photometer readings on time. Beer's law is followed for chlorine dioxide concentnitions between 0 to 1.5 mg. per liter and for chlorine between 0 and 1.0 mg. per liter. The readings made a t 525 mp are based on the absorption curve shown in Figure 2. Determination of Chlorine Dioxide. If chlorine or chloramines are known to be absent, the following procedure is used. T o a 100-ml. volumetric flask, add 90 ml. of sample or, depending upon the concentration of chlorine
dioxide, a n aliquot diluted to approximately 90 ml. Then add 5 ml. of acetate b d e r and 1.0 ml. of ferric chloride solution and dilute to the mark with demand-free water. Following dilution, add 0.4 ml. of H acid solution, mix well, and keep in the dark until the reading is taken. Run the blank and subtract it from the reading. Net reading X 100/90 equals the reading which would be obtained for an undiluted sample. The concentration of chlorine dioxide is read from a previously prepared standard curve. Chlorine Dioxide in the Presence of Chlorine. T o 90 ml. of sample in a 100-ml. volumetric flask, add 5 ml. of the acetate buffer and mix; add 2 ml. of 0.1% malonic acid and mix again. Stopper the flask tightly and let it stand in the dark for 20 minutes. Then add 1.0 ml. of ferric chloride solution and dilute the contents to the mark with demand-free water. Add 0.4 ml. of H acid solution, shake the flask well, and keep it in the dark until the reading is taken. The concentration of chlorine dioxide is read from the standard curve. Chlorine. To obtain the concentration of chlorine present, repeat the above procedure omitting malonic acid. The absorbance reading obtained represents chlorine and chlorine dioxide. Subtract the chlorine dioxide reading obtained in the malonic acid procedure to obtain the value for chlorine. Chlorine reading X 100/90 equals the reading for the undiluted sample. Read the concentration of chlorine from a standard curve which has been previously prepared. Table I represents a check on this method. Chlorine Dioxide in Water Containing Suspended Matter. To 90 ml. of sample in a graduated separatory funnel, add 1.0 ml. of zinc sulfate solution and 1.7 ml. of 0.5N sodium hydroxide. Dilute the mixture to the 100-ml. mark with demand-free water. Close the funnel with a glass stopper, mix, and allow the floc to settle. Draw the liquid off to the 80-ml. mark and
Table 1. Determination of Chlorine Dioxide and Chlorine in a Mixed Solution
Added, Mg./L. ClOn c1*
Found, Mg./L. Cl02 c12
0.05 0.10
1.00 0.10
0.05
0.10 0.50
0.50
0.10
0.10
0.49
0.10
1.00 0.12 0.52
0.11
then run the clear supernatant into a 100-ml. volumetric flask. Add 5 *ml. of the acetate buffer, mix, and eliminate the chlorine present by the addition of 2.0 ml. of 0.1% malonic acid; mix and allow to stand 20 minutes. If chlorine is known to be absent, this step can be omitted. Then add 1 ml. of ferric chloride solution and, following dilution to 100 ml., add 0.4 ml. of H acid reagent. Shake the flask well and let it stand in the dark for 20 minutes; determine the absorbance a t 525 mp. X/lOOO X 80 = Y ml.
=
actual volume of sample used lOO/Y X net reading = absorbance for undiluted sample X = volume of sample taken
Some loss of chlorine dioxide will occur with this method. There is absorption by the floc and loss of chlorine dioxide when the supernatant liquid is transferred from the separatory funnel to the flask. A solution containing 0.3 mg. per liter chlorine dioxide when treated in this manner gave a final concentration of 0.25 mg. per liter. Chlorine dioxide may be determined visually by preparing a series of standards in the range of 0.05 to 1.0 mg. per liter. With higher concentrations of chlorine dioxide, the color is too intense to be read visually. DISCUSSION
Factors Affecting Determination.
EFFECTOF HEAT. As the strengths of chlorine and chlorine dioxide soluVOL. 31, NO. 1 1 , NOVEMBER 1959
m
1873
tions are easily affected by heat, they should be stored at 20”to 25” C. EFFECT OF REAGENT CONCENTRATION. No increase in color could be obtained by thc addition of more than 0.4 ml. of H acid reagent. EFFECTOF PH. A p H of 4.1 to 4.3 was the best workable range for determining chlorine dioxide, as in this p H range maximum absorption can be obtained using an acetate buffer. This buffer has the added advantage of easily controlling the p H of hard water. Potassium acid phthalate buffers were found to be unsatisfactory in bhe presence of iron. Chloramines are unstable at p H 4.1 to 4.3 and the chlorine, formed from their decomposition, can be destroyed with malonic acid. FUNCTIOS OF FERRIC CHLORIDE.The purpose of a.dding ferric chloride is to increase the sensitivity of the reaction. In addition to deepening the color, especially for the low concentrations of chlorine dioxide, it has a stabilizing influence; it is much easier to obtain replicate re:idings when it is used. In dyestuff chemistry (9) some metallized dyes are superior in stability and fastness to light in comparison with dyes of thtt same composition where the metal is omitted. The same type of stability may be the factor involved here. INTERFERENCES. The presence of iron and copper to 0.5 mg per liter and of manganese up to 0.1 mg. per liter has no effect on the determination of chlorine dioxide. Because chlorine dioxide converts these metals to their insoluble oxides (3, 6, 7), it would seem unlikely that concentrations greater than those mentioned would be found in a treitted water supplJ-. Chlorine dioxide h w been found to be superior to chlorine in the oxidation of manganese (7’). Table I1 shows the influence of diverse ions on the determination of chlorine dioxide. The concentrations given for most of the ions are reasonably close to those occurring in treated hraters. Nitrites in concentrations greater than 0.5 p.p.m. will show a positive interference. Kitrites in higher concentrations can be destroyed in 10 minutes by adding 0.45 ml. of 10% sulfuric acid and 1 ml. of 1% sulfamic acid to the neutralized samples. Of all the ions studied, polyphosphates show the greatest interference, but the exact mechanism of this interference or the elimination of it has not been determined. Water treatment plants which use polyphosphates routinely in the treatment process will have to make the standard curve in the presence of a known concentration of polyphosphates.
1874
ANALYTICAL CHEMISTRY
Table II. Effect of Diverse ions on the Determination of CIO,”
Concn. of Ion Added, Mg./L.
Ion Added NH, Na +
+
Ca +z Mg A1 +a Zn +,
+f
Pb
+f
e1-
ClOaPod-’ (ortho) PO4-’ ( yro) PO4-’ (!exameta) PO,-a (hexameta) s04C2
NO’FNO*-
Found, Mg./L.
0.2 500 100 100 0.1 200 0.05 300 5.0 0.1 1 .o 0.5 1.0
600 10 6 0.5 0.2
SiOJ-l 0
Concn.
of CIOl
0.20 0.20 0.20 0.20 0.20 0.20 0.19 0.20 0.20 0.20 0.15 0.17 0.13 0.20 0.20 0.20 0.22 0.19
0.20 mg. per liter C10~added.
Recovery of Chlorine Dioxide in Various Types of Waters. T o test the practicability of the proposed method for chlorine dioxide, known amounts of this compound were added to various types of waters made demandfree, and the concentrations were then determined. The deep-ne11 water used contained approximately 400 p.p.m. total hardness. The results (Table 111) show close correlation between the concentrations of chlorine dioxide added and found, and indicate that the procedure given for determining chlorine dioxide is a practical method welladapted to different types of waters.
gas into a sodium chlorite solution, the pH of this reaction mixture was 3, at which the chlorite would decompose to chlorine dioxide. Other Proposed Methods. The otolidene and the OTA methods ( 1 , 8) make no distinction between chlorine and chlorine dioxide. Numerous experiments were tried in which malonic acid was used to eliminate chlorine, but, because of lack of correlation between color derived from known concentrations of chlorine dioxide with o-tolidene and the permanent chlorine standards recommended by standard methods (8), these procedures could not be adapted. This same drawback is evident in the o-tolidene-arsenite-oxalic acid (OTO) test ( 5 ) ,in which oxalic acid is used to tie up the chlorine present. However, the use of oxalic acid causes the formation of precipitates in hard waters. Malonic acid could be used in place of oxalic acid for eliminating chlorine interference in the H acid test. The necessary concentration did not affect the absorbance reading of 0.05 mg. per liter of chlorine dioxide and did not produce a precipitate in hard water. The amperometric method ( 8 ) cmploying phenyl arsenoxide was tried for both chlorine and chlorine dioxide. The results obtained for chlorine checked the titration values very closely, but no positive results could be obtained for chlorine dioxide. The tyrosine method described by Hodgden and Ingols (2) as specific for chlorine dioxide was investigated, but W R S found not to be sensitive at low concentrations. LITERATURE CITED
Table I l l . Determination of ClO, in Various Types of Water
ClOl Added, Mg./L. 0.05 0.10 0.30 0.50 0.60 0.80 1 .oo
C102Found, Mg./L. River Tap Well water water water 0.047 0.10 0.30 0.50 0.59 0.79 0.99
0.047 0.10 0.31 0.52 0.61 0.82 1.00
0.047 0.10 0.29 0.49 0.58 0.78 0.98
This procedure does not distinguish between chlorites and chlorine dioxide. Chlorites at p H 4.2 will decompose with the formation of chlorine dioxide which will also be measured colorimetrically. If, however, only chlorine dioxide is applied in water treatment plants, there will be no problem of chlorite interference. At one water plant where chlorine dioxide is generated by passing chlorine
(1) Hallinan, F. J., J . Am. Water Works Assoc. 36, 296302 (1944). (2) Hodgden, A. W., Ingols, It. S.,, ~ N A L . CHEM.26, 1224-6 (1954). (3) McCarthy, J. A., J. New Engl. Wakr Wo&s Assoc. 59, 252 (1945). (4) Mellor, J. W., “A Comprehensive
Treatise on Inorganic and Theoretical Chemistry,” Vol. 2, p. 289, Longmans, Green, London, 1922. (5) Olin Mathieson Chem. Corp., Baltimore, Md., Ind. Chem. Division Booklet, “Treatment of Water Supplies with Chlorine Dioxide,” Longmans, Green, London, 1955. (6) Reidnour, B. F., Taste Odor Control J . 13,2 (1947). (7) Simmons, P. D., Water Works Eng. 100, 1258 (1947). (8) “Standard Methods for the Examination of Rater, Sewage, and Industrial
Wastes,” 10th ed., Am. Public Health Assoc., New York, 1955. (9) Venkataraman, K., “Synthetic Dyes,” Vol. 1 , p. 403, Academic Press, New York, 1952. RECEIVEDfor review June 5, 1957. Accepted July 22, 1959. Division of Water, Sewage, and Sanitation Chemistry, 131st Meeting, ACS, Miami, Fla., April 1957.
