Polarographic Determination of Bromite Ion Alan F. Krivis Department of Cketnistrj,, The Unicersity of Akron, Akron, Ohio 44304
G . R . Supp Olin Mathieson Clieniical Corp., New Huoen, Conn.
SODIUMBROMITE has assumed commercial importance recently, with the finding that it efficiently removes starch sizing from textiles (/-3). Stable sodium bromite solutions are prepared ( I , 3) by reacting bromine with caustic t o form hypobromite and then adjusting the p H to permit conversion of hypobromite to bromite. Residual hypobromite can then be destroyed by several alternative routes including reaction with acetone. During the synthesis, four oxidation states of bromine can be present in solution, Br-, BrO-, Br02-, and Br03-; of these, the most important is BrOe-. A number of volumetric methods have been proposed for the determination of BrO?- in the presence of BrO- and Br03-. All of them are based on reaction of the bromine species with arsenite. One general approach involves a determination of the combined BrO- and BrO?- with arsenite and a second titration with arsenite after destroying hypobromite with phenol (4-6) or ammonia (7, 8). Neither one of the latter reagents is truly selective in removing hypobromite when the bromite concentration is appreciable; under these conditions, both phenol and ammonia react with significant amounts of bromite also and vitiate the analysis. Andersen and Madsen ( 9 ) proposed a consecutive two-stage titration to avoid the difficulties encountered with selective destruction of hypobromite. The mixture first is titrated in a bicarbonate-buffered solution to determine hypobromite alone, then the p H is raised and osmium tetroxide is added for the titration of bromite. The volumetric methods cited d o not lend themselves t o rapid and automated operation, and a specific method useful for these purposes was sought. It was felt that a polarographic procedure might permit such a specific analysis and, therefore, a study of the hypobromite, bromite, and bromate system was undertaken. The polarographic behavior of the hypohalites and halates has been reported (10). The hypohalites undergo reduction at the potential for the anodic dissolution of mercury. Bromate is reduced irreversibly via a 6-electron reaction to bromide. In caustic solution, the half-wave potential for the bromate reduction is cu. - 1.7 V. No reports of the polarographic behavior of bromite could be located. EXPERIhlENTAL
Reagents. Solid sodium bromite (SociCtC d’Etudes Chimiques pour 1’Industrie et I’Agriculture, Paris) was dissolved in 0.1M N a O H solution. The stock solutions were re(1) R. Kircher and R . Periat, U. S. Potent 3,095,267, June 25, 1963. (2) J. Leclerc. U. S. Parent 3,083.072, June 25, 1963. (3) J. Maybeck, R. Kircher and J. Leclerc, U. S. Porent 3,085,854, April 16, 1963.
(4) J. Breis, P1i.D. Thesis, University of Strasbourg, 1959. (5) R. Chapin, J . Amer. Clie177.SOC.,56, 2211 (1934). 19,662 (1947). (6) L. Farkas and M. Lewin, ANAL.CHEM., (7) J. Clarens. Compt. Rend., 157, 216 (1913). (8) M. H. Hashmi and A. A. Ayaz, ANAL.CHEM., 35, 908 (1963). (9) T. Aiiderseii and H. E. L. Madsen, ibid., 37, 49 (1965). (10) I. M. Kolthoff and J. J. Lingane, “Polarography,” 2nd Ed., Interscience Publishers, New York, N. Y . , 1954.
PH Figure 1. Half-wave potentials for the reduction of bromite ion as a function of p H frigerated at 0 “C. and assayed ( 9 ) immediately before use. Additional solutions of sodium bromite were prepared as indicated in the patent literature (I, 3) and also assayed before use. All other chemicals used in this study were reagent grade. Apparatus. A Sargent Model XV Polarograph and a Metrohm Polarecord E-261R were used in conjunction with a thermostated (25 i 0.01 “C) H-cell containing a saturated calomel reference electrode ( I I ) to obtain the polarographic data. No damping was used with the Sargent Polarograph and a damping position of 5 was used with the Metrohm Polarecord. The current and potential values were determined graphically using the average of the recorder traces. Capillaries with constants of 1.95 and 2.16 mg2/3t1/6were used. Procedure. Accurately weigh a sample containing ca. 1 mg of sodium bromite into a 10-ml volumetric flask, add 5 ml of 1M NaOH, and swirl until homogeneous. Add 0.1 ml of 0.5% gelatin solution and dilute to volume with distilled water. Mix thoroughly, transfer the solution to the H-cell, and deoxygenate with nitrogen for 10 minutes. Insert the indicating electrode into the solution and obtain a polarogram. (Measurements should be made as rapidly as possible since mercury may react with bromite and/or hypobromite.) Measure the current for the wave a t cu. -0.9 V and compare to a previously prepared calibration curve to determine the bromite content. (11) J. D. Kornyathy, F. Malloy, and P. J. Elving, ANAL.CHEM., 24, 431 (1952). VOL. 40, NO. 13, NOVEMBER 1968
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Table I. Polarographic Values for Sodium Bromite in 0.5M Sodium Hydroxide NaBrO2, mg -Ep?,o V 0.18 0.88 0.18 0.86 0.35 0.92 0.45 0.87 0.53 0.84 0.71 0.89 0.88 0.86 0.90 0.86 1 .OG 0.90 1.41 0.89 1.41 0.89 1.80 0.87 2.12 0.91 a Mean value was -0.88 V. b Least squares value was 5.84 pLA/mg.
i,* pa
0.84 0.86 1.86 1.83 2.44 3.3 4.8 4.8 6.2 7.5 8.0 9.6 12.4
RESULTS AND DISCUSSJON
Bromite ion is stable in solution at very high p H levels ( I , 3). For that reason, 0.5M NaOH was chosen as a suitable supporting electrolyte for analytical purposes; the sample would suffer minimum decomposition during analysis. Polarograms of sodium bromite solutions disclosed a reduction wave for BrOa- at about -0.88 V, which was well separated from the hypobromite (0.0 V) and bromate (-1.7 V) waves and could be easily analyzed. The diffusion current varied linearly with bromite concentration to about 2 mg/lO ml. A reaction between sodium bromite and mercury was noted in that a pool of mercury allowed t o remain in the bottom of the cell for an extended period of time became coated with a solid.
To optimize precision and accuracy, therefore, the dropping mercury electrode should be inserted into the cell only immediately before the measurements are to be made and the polarograms should be run as rapidly as possible. As a means of evaluating the method, samples of sodium bromite solutions containing various sodium bromite concentrations were analyzed polarographically and titrimetrically ( 9 ) over a period of about a year; these samples contained varying amounts of the other bromine species in addition t o bromite. Table I lists the data. Treatment of these accumulated data by a least squares method (12) gave a regression coefficient of 5.84 pajrng with a standard error of 0.20. This value corresponds to a diffusion current constant, I, of 3.65. pH effects. The effect of p H o n the reduction was studied briefly also. In essence, a fixed amount of pure sodium bron i t e was polarographed in inorganic buffers at several pH values above 7. Attempts to extend the study to lower pH levels were unsuccessful. Visible decomposition, including bubble formation and a color change, was noted after adding the bromite to buffers below pH 7 . A marked cathodic shift in half wave potential was observed over the range of p H 7 to 10.4. The slope of a plot of Eli2 c‘s. p H (Figure 1) corresponded to 232 mV/pH unit. which is in good agreement with the theoretical value of 236 mV/pK unit for a reduction involving four protons. Therefore, the electrode reaction, over the pH range 7-10, appears to be: Br0,-
+ 4H+ + 4e-
+
Br-
+ 2H20
RECEIVED for review June 13, 1968. Accepted July 31, 1968. (12) 0. L. Davies, Ed., “Statistical Methods in Research and Production.” Oliver and Boyd, London, 1961.
Cationic Glass Electrode Response to Alkali Metal Ions in Nonaqueous Solvents James E. McClure and Thomas B. Reddy Research Seraice Department and Chemical Department, American Cyanamid Co., I937 West Main Street, Stangord, Conn.
THEPROPERTIES of cationic glass electrodes in aqueous solvents are well known (1-3). Recent work has shown that the electrodes also respond to cations in partially aquated solvents ( 2 , 4 ) and in nonaqueous systems such as methanol ( 4 ) . Very little is known, however, about the performance of such electrodes in aprotic organic solvents. Solutions of alkali metal ion salts in aprotic solvents areof current interest because of their utility as electrolyte systems in high energy density battery systems. Boden (5) has reported the response of the cationic glass electrode to Li+ over a limited concentration range in the presence of Mg*+, K+, NH4+ and (C2H&N+ in propylene carbonate. This paper presents the results of some preliminary experiments which were carried out to determine (1) G. A. Rechnitz, Chem. Eng. News, 45 (2% 146 (1967). (2) G. A. Rechnitz, Rec. Chem. Progr., 26 (4), 242 (1965). (3) G. Eisenman, “Advances in Analytical Chemistry and Instrumentation,” Vol. 4, C. N. Reilley, Ed., Interscience, New York, N. Y.,1965. (4) Ibid., p 295. ( 5 ) D. Boden, presented at the Electrochemical Society Meeting, May 7-12, 1967, Dallas, Texas.
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ANALYTICAL CHEMISTRY
the behavior of the cationic glass electrode in propylene carbonate, acetonitrile, and dimethylformamide. The electrodes were evaluated in terms of response, response time, selectivity, and durability. Ions investigated included Li”, Na+, and K+. Response was tested over a concentration range of lO-“o 10-2M. EXPERIMENTAL
Reagents. Stock solutions (0.5 or 1M) of NaSCN, KSCN in propylene carbonate, acetonitrile, and dimethylformamide were prepared from reagent grade salts which had been dried to less than 0.04z HsO. Li+ solutions were prepared from G . F. Smith LiClO, which had been dried at 120 “C t o 0.2% HzO. Acetonitrile and propylene carbonate were purified according to a published procedure (6). After purification, these solvents contained less than 40 ppm H 2 0 . Baker reagent grade dimethylformamide containing 200 ppm HzO was
(6) J. E. McClure and D. L. Maricle, ANAL.CHEM.,39, 236 (1967).
