Spectrophotometric Determination of Sulfate Ion with Barium Iodate and the Linear Starch Iodine System Willie L. Hinze’ and Ray E. Humphrey2 Department of Chemistry, Sam Houston State University, Huntsville, Texas 77340
Sulfate ion has been determined spectrophotometrically by procedures involving an ion exchange reaction with a slightly soluble barium salt to form less soluble barium sulfate and release an absorbing anion into the solution. Probably the most widely used method for sulfate employing this type of reaction is the procedure in which barium chloranilate is used and the visible ( I ) or ultraviolet absorption (2) of the released chloranilate ion measured. A recent report on the spectrophotometric determination of sulfate in coal ash using barium chloranilate (3) summarized many of the applications of the method to a variety of materials. The exchange of sulfate ion for iodate ion by interaction of the dissolved sulfate with relatively insoluble barium iodate has been used for the titrimetric determination of sulfate. The iodate ion released is reduced with iodide ion in acid solution and the iodine titrated with thiosulfate (4-6). The solubility of barium iodate in water is high enough to result in a rather large blank ( 6 ) so that the method is not applicable to very low levels. Microgram amounts of sulfate can be determined if 65-80% acetonitrile is used to lower the solubility of the barium iodate (7). This is an example of an “amplification” reaction (8) since each sulfate releases two iodate ions which yield twelve iodine atoms. Although similar ion exchange reactions involving the release of iodate ion have been used for the spectrophotometric measurement of anions by measuring the absorption of the starch-iodine blue complex formed on adding iodide and starch, sulfate apparently has not been determined in this way. Chloride has been determined spectrophotometrically with silver iodate (9, 10) and bromide, chloride, cyanide, iodide, thiocyanate, sulfide, sulfite, and thiosulfate have been measured with mercuric iodate (1 1 ) . This paper reports the spectrophotometric determination of sulfate ion in the low parts per million range employing barium iodate and the linear starch-iodide reagent.
EXPERIMENTAL Apparatus. Absorption measurements were made with Beckman ACTA I11 and D K - 2 A spectrophotometers. An International clinical centrifuge and a n International Model SBV centrifuge were used to separate the excess barium iodate from reaction solutions. Reagents. Barium iodate was prepared by precipitation by adding an aqueous solution of barium nitrate to an aqueous solu’Present address, Department of Chemistry, Texas A & M University, College Station, Texas 77843. ’Author to whom correspondence should b e addressed.
(4) (5) (6) (7) (8) (9) (10) (11)
R. J, Bertolacini and J. E. Barney 1 1 , Anal. Chem.. 29, 281 (1957). R. J . Bertolacini and J. E. Barney 1 1 , Anal. Chem.. 30, 202 (1958). H . N. S.Schafer, Anal. Chem.. 39, 1719 (1967). D. A. Webb, J. Exp. Biol.. 16,438 (1939). L. Erdeyand E. Banyai. 2. Anal. Chem.. 161, 16 (1958). J. L. Lambert and D. J . Manzo, Anal. Chim. Acta. 54, 530 (1971). H . Weisz and U. Fritsche. Mikrochim. Acta. 1970, 638. R. Belcher, Taianta. 15,357 (1968). J . Sendroy, J r . , J. Bioi. Chem.. 120,419 (1937). J. L. Lambert and S. Y. Yasuda, Anal. Chem , 27, 444 (1955) W. L. Hinze and R. E. Humphrey, Anal. Chem.. 45, 385 (1973).
a i 4 e ANALYTICAL CHEMISTRY, VOL. 45, NO. 4, APRIL is73
tion of potassium iodate. The precipitate was washed several times with distilled water, once with ethanol, filtered, and dried. Sodium sulfate, used to prepare solutions containing known amounts of sulfate ion, was a J. T. Baker Chemical Co. “Baker Analyzed” reagent. Stock solutions were made up in 3 : 1 ethanolwater. The linear starch-iodide reagent was prepared according to the procedure of Lambert and Zitomer 112). The reagent contained 11 grams of “Baker Analyzed” cadmium iodide and 20 grams of Fisher Scientific Co. potato starch in 1 liter. This reagent preparation was stable for 2-3 months when stored in an amber bottle. A mixed solvent containing three volumes of ethanol and one volume of water was used for the ion exchange reactions. The solubility of barium iodate is decreased by the ethanol so that the absorbance of the blank is lowered to a reasonable value. Procedure. Two dilution procedures were used in order to cover a larger range of concentrations. For the higher concentrations, a measured volume of the stock sulfate solution was diluted to 10 ml with 3 : l ethanol-water, about 25-35 mg of barium iodate added, and the solutions were allowed to stand for 30 minutes with occasional shaking. After centrifuging for 5 minutes, a 5-ml aliquot was withdrawn and placed in a 100-ml volumetric flask containing about 80 ml of water. Two ml of 2.5M HC1 solution and 3 ml of the starch reagent were added and the solution was diluted to volume with distilled water. The absorbance was measured at 625 nm after ten minutes. The procedure for lower concentrations involved diluting measured volumes of a standard sulfate solution to a final volume of 25 ml with 3 : l ethanol-water. These solutions were in contact with 25-35 mg of barium iodate with occasional shaking for 30-40 minutes. The excess solid was then removed by centrifugation and filtration. A 24-ml aliquot was then placed in a 100-ml volumetric flask containing about 70 ml of water. After 2 ml of 2.5M HC1 and 3 ml of the starch reagent were added, the solution was diluted to volume with distilled water. The absorbance was measured a t 587 nm after ten minutes.
RESULTS AND DISCUSSION Sulfate ion was measured over the range of approximately 1-4 ppm using the lower dilution procedure. The effective molar absorptivity for sulfate is 31,000. For a higher concentration range from 4-14 ppm. a higher dilution was used and the molar absorptivity value found to be 9400. The molar absorptivity values are based on the concentrations of sulfate ion in the solutions to which the solid barium iodate was added. Beer’s law data for the different dilutions is presented in Table I. Maximum absorption occurs a t different wavelengths in the two procedures because of the different amounts of ethanol present. The color formed is very stable. Details concerning this absorbing system were summarized previously ( 1 1 ) . Reproducibility is reasonably good as shown by recovery data in Table I1 for the higher concentration range. The use of barium iodate for the spectrophotometric determination of sulfate compares favorably with the barium chloranilate method in which the absorption of the chloranilate ion is measured. The extent of exchange between barium iodate and sulfate has been found to be essentially complete by thiosulfate titration (5, 7). Comparison of absorption of a reaction solution with known amounts of iodate in this work indicate the reaction is (12) J. L.
Lambert and F. Zitomer, Anal. Chem.. 35, 405 (1963)
Table II. Recovery Data For Sulfate
Table I. Beer’s Law Data for Sulfate Low dilutionn
so4*-,PPm
A, 587 nmc
s042- present, ppm S 0 4 z - found, ppma
High dilutionb
S 0 4 2 - . ppm
A, 625 nma
0.40 0.31 4.1 1.2 8.1 0.84 2.4 0.82 11 1.02 3.0 1.10 1.40 1.53 14 4.2 a 24 ml diluted to 100 ml. 5 ml diluted to 100 ml. Effective molar absorptivity of 31,000, A = 0.15 for the blank. Effective molar absorptivity of 9,400, A = 0.10 for the blank.
80-90% complete. The molar absorptivity of the chloranilate ion is about 1000 in the visible region and close to 25,000 in the ultraviolet region a t the optium p H value ( 3 ) . The sensitivity for sulfate using barium iodate and measuring visible absorption should be as high as the chloranilate method measuring ultraviolet absorption. The major difference is that a dilution is required using barium iodate because of the solubility of the compound and the high molar absorptivity of the starch-iodine complex. Probably the most important consideration in comparing barium chloranilate and barium iodate for measuring sulfate would be the relative solubilities. This factor would be important as far as interferences are concerned since other anions which form insoluble barium salts would release chloranilate or iodate ions. The extent of the interference would depend on the relative solubilities of the barium compounds involved. The solubility of barimoles per um chloranilate is reported to be 5.2 X liter in 50% ethyl alcohol ( I ) . From this work, it is estimated that the solubility of barium iodate in 75% ethyl moles per liter. The alcohol is approximately 1-2 x solubility could be lowered by employing other solvents miscible with water, such as acetonitrile (7). It was found in this work that although the blank is reasonably low in 1:l ethanol-water the Beer’s law plot is linear only above
a
2.1 2.0 4.2 4.1 6.2 6.4 9.3 9.6 17 17 High dilution procedure. A measured at 625 nm.
Error,
YO
-3.8 -1.5 +2.6 4-3.0
-0.0
5 ppm sulfate and precision is poor. Results using 95% ethanol or methanol were satisfactory but sensitivities were lower than in 3: 1 ethanol-water. A rather extensive study of the interference of chloride, fluoride, and phosohate ions on the determination of sulfate with barium chloranilate has been reported ( 3 ) . Interferences using barium iodate might be expected to be somewhat similar although differences would arise because of differences in the solubility of barium chloranilate and barium iodate. A very limited study of interferences in this work showed that bicarbonate, borate, hydroxide, and sulfite ions could not be tolerated while bromide, chloride, and nitrate ions had no effect. The barium iodate procedure should be useful for the determination of sulfate using visible absorption a t a much lower level than is possible with barium chloranilate. The sulfate concentration range with barium iodate using the lower dilution is about the same as can be measured using the ultraviolet absorption of the chloranilate ion. Some flexibility exists as to the concentration range covered using barium iodate since different dilution factors can be used. Received for review September 25, 1972. Accepted November 30, 1972. This work was part of the M.A. thesis of Willie L. Hinze, Sam Houston State University. May 1972. The authors express their appreciation to the Robert A. Welch Foundation of Houston, Texas, for partial support of this research.
I CORRESPONDENCE lnterelement Effects in the Flame: Spectroscopic Determination of Aluminum, Molybdenum, and Vanadium in a Nitrous Oxide-Acetylene Flame Formed on a Circular Slot Burner Sir: In recent years renewed attention has been focused on interelement effects in flame spectroscopy as a result of repeated observations that a variety of concomitants enhanced the atomic absorption or emission signals of a number of elements in nitrous oxide-acetylene flames. These enhancements have been difficult to explain because they occur for systems that heretofore would have been expected to show depressions of the analyte signal. Among the most perplexing of these observations were those of Dagnall et al. ( I ) , who studied the effect of con-
(1) R M . Dagnall, G. F Kirkbright, T S West, and R Wood, Chem., 42, 1029 (1970).
Anal.
comitants on Al, Mo, V, Ti, and Zr emission and on Al, Mo, and V fluorescence in an inert gas separated nitrous oxide-acetylene flame formed on a circular slot burner. These authors reported that enhancements were, in general, significantly larger in thermal emission than in fluorescence, and that a few concomitants produced large depressions of the analyte signal. No hypotheses were proposed to account for either the effects observed or the differences between the emission and fluorescence results. We have recently reinvestigated a number of the more prominent enhancement effects observed in emission by Dagnall et al. Although our experimental conditions were similar to those used by Dagnall et al., we were unable to confirm the surprisingly large enhancements they reported. This brief communication summarizes our results. A N A L Y T I C A L CHEMISTRY. VOL. 45, NO. 4, APRIL 1973
815
