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ANALYTICAL CHEMISTRY, VOL. 51, NO. 8, JULY 1979
AIDS FOR ANALYTICAL CHEMISTS Determination of Chlorine in Concentrated Sulfuric Acid Solution with a Residual Chlorine Electrode Kenny D. Brown"' Coulfon Chemical Corporation, Oregon, Ohio 436 16
Gordon A. Parker Depariment of Chemistry, University of Toledo, Toledo, Ohio 43606
One of the many uses of sulfuric acid in industry is as a drying agent. In the production of chlorine, for example, wet chlorine gas is dried with sulfuric acid ( I ) . During the drying process, some chlorine dissolves in the acid and chlorine concentrations of 1000 pg of C12 per mL of acid, or more, may result (2). Handling of this contaminated sulfuric acid, during transit and by prospective users, requires special precautions. Vapor concentrations as low as 50 ppm C1, can be dangerous (3). Manufacturers, as well as users of this spent sulfuric acid, find it essential to know the chlorine content. Traditional methods for determination of chlorine in concentrated sulfuric acid are either slow or difficult to perform. T h e reduction of chlorine to chloride ion and subsequent precipitation with Ag(1) ion is time consuming ( 4 ) . T h e best alternate method is t o titrate iodine. formed by oxidation of iodide ion added t o the chlorine-containing sample, with standard thiosulfate solution using starch indicator ( 4 ) . This titration is unsatisfactory, however, if the acid sample contains impurities which can obscure t h e color change from blue-black to colorless. Frequently a brown color is present in these samples making end-point location difficult. Sample handling, too, during the titration, can result in loss of volatile chlorine, and the hazards of chlorine vapors and/or sulfuric acid spills are well known. With the use of a residual chlorine specific ion electrode, analysis of chlorine in sulfuric acid is simplified. With the older iodine method, titration of liberated iodine must be performed immediately upon each sample. When the proposed specific ion electrode measurement is made, a number of samples can be prepared and analyzed one after another in a short time. Most important, color and turbidity do not interfere and t h e need for titration is eliminated. T h e residual chlorine electrode is a relatively new specific ion electrode developed to measure residual chlorine in municipal water supplies. It is a solid-state electrode containing both indicating and reference portions in a single probe a n d responding to the presence of free iodine. T h e exact composition of the crystal sensor is proprietary information; however, for its use in water analysis, it responds to iodine formed when excess potassium iodide is added to a buffered, p H approximately 4.5, chlorine-containing sample. Hypochlorite ion, hydrochlorous acid, chloramines, and other species capable of oxidizing iodide to iodine also cause a response with this electrode. In this application, iodine equivalent to the chlorine present in sulfuric acid solution is measured. Reaction of chlorine with iodide in the strongly acid solution is not a problem and, as shown in this report, results using the electrode compare favorably with those from the titration procedure. EXPERIMENTAL Reagents. All chemicals were reagent grade. Distilled deionized water was used throughout the experiment. A standard solution of sodium thiosulfate, approximately 0.1 Tu', was prepared Present address: 7339 Starlawn Road, Perrysburg, Ohio 43551. 0003-2700/79/0351-1332$01 .OO/O
Table I. Determination of Chlorine Equivalent from Prepared Iodate-Iodide Standards in 70% ( w l w ) Sulfuric Acid A. Concentration of CI,, pg/mL, from thiosulfate titration Sample run 1 2 3 average
1
2
25.2 26.3 25.2 25.6
43.2 43.2 43.1 43.2
3
4
5
6
7
9 6 . 1 96.1 323 437 877 98.9 95.4 219 435 872 96.4 95.4 219 435 872 97.1 95.6 220 436 873 B. Concentration of CI,, pg/mL, from specific ion measurementa
run 1 2 3 average
27.5 45.0 100 27.3 44.5 1 0 5 28.0 45.0 100 27.6 44.8 1 0 2 C. Difference 2.0
1.6
4.9
230 450 225 450 225 440 227 447 in average values 100 100 100 100
4.4
7
9
940 930 920 930 57
D. Percent relative difference based on value from A a
7.8 3.7 5.0 4.6 3.2 2.1 Values are read directly from specific ion meter.
6.5 -
Table 11. Potential Readings for Determination of Chlorine Equivalent from Prepared Iodate-Iodide Standards in 70% ( w / w ) Sulfuric Acid Potential reading from specific ion meter, mV CI2 concn, d m L 20 80 100
200 500 800 1000
Run ~ _
_
1
2
3
-70 -53 -50 -4 2 -29 -23 -20
-69 -52 -50 -42 -29 -23 -20
-70 -53 -50 -41 -30 -24 -20
_
and standardized against an iodine solution using starch indicator. A weighed portion of potassium iodate was dissolved in 70% (w/w) sulfuric acid and its concentration verified by thiosulfate titration of liberated iodine. A 5% (w/v) aqueous potassium iodide solution was used as the source of iodine. A working standard equivalent to 100 fig of Clp per mL, or other known value, was prepared by appropriate dilution of the potassium iodate solution with additional 70% (w/w) sulfuric acid. Apparatus. An Orion Residual Chlorine specific ion electrode, Model 97-70, was used with an Orion Model 407A/L specific ion expanded scale meter. Procedure. Since chlorine solutions are unstable, known amounts of potassium iodate in 70% (w/w) sulfuric acid were used G 1979 American Chemical Society
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Table 111. Determination of Chlorine from Actual Samples of Spent Sulfuric Acid ( - 7 0 % w/w) Sample concn C1; Mg/mLa : concn ~ 1 pg/mLb difference in values relative difference, 3'% based on titration value a
From thiosulfate titration.
1 380 379 -1
0.3
2 470 473 +3 0.2
3 580 579 -1 0.2
4 585 579 -6 1.0
5 747 740 -7 0.9
6 780 773 -7 0.9
From specific ion electrode measurement.
as standards for comparing measurements of liberated iodine by thiosulfate titration and specific ion electrode reading. One mL of the diluted standard potassium iodate solution was added to 10 mL of deionized water followed by addition of 1 mL of 5% (w/v) potassium iodide solution. The sample was swirled gently for 2 min to assure complete reaction and then diluted to exactly 100 mL with deionized water. For specific ion measurement, the electrode was inserted directly into this solution and, allowing a minute or two for equilibration, a value read from the specific ion meter using the specific ion scale. Occasionally a millivolt value was noted. For titration, the entire sample was titrated with standard thiosulfate titrant.
RESULTS AND DISCUSSION A series of prepared standards in 70% (w/w) sulfuric acid was analyzed for their iodine contents by thiosulfate titration and by specific ion measurement. Results are shown in Table I. One will note t h a t the proposed method gives values for residual chlorine consistently higher than those obtained by the titration method. T h e differences are, however, tolerable for routine determination of residual chlorine in spent sulfuric acid and the ease and speed of measurement strongly favor this approach for all but t h e most exacting requirements. Potential response of t h e residual chlorine electrode to liberated iodine in 707' (w/w) sulfuric acid is linear with increasing iodine concentration. Table I1 indicates t h e potential values observed in a set of measurements. Plots of these data on semilogarithmic paper are linear, within experimental error, and t h e mean value of t h e slopes of these plots, from the data of Table 11, is 29.2 mV per decade change by a least-square treatment. For a system obeying the Nernst equation, a value of 29.6 mV per decade change in concentration is expected. Deviations from linearity for these measurements become greater as one measures samples outside the range equivalent to 0.2 to 20.0 wg of Cla per mL. This is in accord with the electrode manufacturer's statement for this electrode based upon measurements in aqueous
systems. An appropriate dilution will, of course, be necessary when analyzing actual samples to achieve a final solution that falls within this range. Table I11 lists the results of residual chlorine measurements upon actual samples of spent sulfuric acid by both t h e traditional and proposed procedures. One notes that with real samples the values obtained by specific ion measurements are, generally, slightly lower than those obtained from the titration procedure. Agreement between values by t h e two methods is better than obtained when synthetic standards were studied. Not only could other constituents in the actual samples affect the electrode response but the validity of the titration results, because of the difficult end-point location in t h e colored samples, is suspect. In either case, agreement between t h e results by t h e two procedures is 1% or less. Other strong oxidizing agents, when present, will,in addition to chlorine, convert iodide ion to iodine. Modification of the conditions necessary to achieve selective, quantitative oxidation by, e.g., bromine, manganese dioxide, copper(I1) ion etc., in concentrated sulfuric acid is currently in progress and will be reported in future publications. At present, in this report, a successful rapid alternate procedure to the traditional titration method is proposed for quantitative determination of residual chlorine in concentrated sulfuric acid.
LITERATURE CITED (1) Shreve, R. N.; Brink, J. A. "Chemical Process Industries", 4th ed.; McGraw-Hill: New York, 1977. (2) Fasullo, 0. T. "Sulfuric Acid Use and Handling"; McGraw-Hill: New York, 1955. (3) Sax, N. I. "Dangerous Properties of Industrial Materials", 4th ed.; Van Nostrand Reinhold: New York, 1975. (4) Furman, N. H., Ed. "Standard Methods of Chemical Analysis", 6th ed.; D. Van Nostrand: New York, 1962: Vol. 1.
RECEIVED for review December 18, 1978. Accepted March 5 , 1979.
Determination of Sulfate in Organically Colored Water Samples Christopher S. Cronan Department of Biological Sciences, Dartmouth College, Hanover, New Hampshire 03755
One of the major difficulties faced in biogeochemical studies of soil solutions and surface waters containing organic color is t h e accurate determination of sulfate (I). T h e analytical chemistry literature applying to the measurement of dissolved sulfate has recently been reviewed by Fishman and Erdmann (21, and appears to lack a simple and rapid means of compensating for organic interferences in the measurement of this anion. Currently, one of t h e principal automated methods for sulfate determination (Technicon method 226-72W) involves the use of a methylthymol blue color reaction (3-5). It has been found that colored soluble organics may produce a background interference in the methylthymol blue test that cannot be properly compensated for with a normal blanking procedure or by the method of additons. Similar interferences 0003-2700/79/035 1-1333$0 1.OO/O
may plague other sulfate methods. After conducting tests aimed a t selectively removing either sulfate or organics from t h e colored samples, i t has been concluded t h a t ultraviolet oxidation methods ( 6 ) ,in combination with a modified automated sulfate procedure, may provide one of the most efficient means of performing accurate sulfate determinations on colored natural water samples. This potentially serious analytical problem became apparent when untreated and UV-treated water samples containing organic color were compared with the methylthymol blue test. After correction for sample blank absorbance, the untreated colored water samples showed higher sulfate estimates than paired samples t h a t had been UV-irradiated prior to sulfate measurement. Meanwhile, sulfate standards showed no 1979 American Chemical Society
