Determination of Antimony Using Forced-Flow Liquid Chromatography with a Coulometric Detector Larry R . Taylor1 and Dennis C.
Johnson2
Department of Chemistry, lowa State University, Ames, lowa 50070
The highly irreversible electrochemical oxidation of Sb(lll) at a platinum electrode in dilute HCI is electrocatalyzed by I- or 12 specifically adsorbed at the electrode surface. This phenomenon was applied for the determination of antimony in several standard alloys and in human hair by a highly sensitive and selective method using forced-flow liquid chromatography with a platinum coulometric detector. Because the electrolytic efficiency of the detector was loo%, the quantity of Sb(lll) in the samples was calculated directly from the integral of the current-time peak for Sb(lll) using Faraday's law. Use of a calibration curve was unnecessary.
Johnson and Larochelle recently described a simple design for a tubular electrode ( I ) . When packed with chips of platinum, the electrode can be operated with 100% electrolytic efficiency for a large range of fluid flow rates. The electrode was successfully applied as a detector for forced-flow liquid chromatography and the determination of Cu and Fe in several standard NBS alloys. As a result of the 100% efficiency, the time integral, Q, of the electrical current for the elution and electrolysis of an electroactive species is related to the number of gram-equivalents by the Faraday equation
I? = F (g-equiv)
(1)
where F is the Faraday constant (96,480 coulombs/gequiv). The goal of extensive research in our laboratory has been the development of a coulometric procedure suitable for the determination of Sb(II1) in the effluent stream of the liquid chromatograph. Coulometric procedures for the determination of antimony using solid electrodes have previously been based on the oxidation of Sb(II1) to Sb(V) by Br2 ( 2 ) , IZ (3, 41, and C r 2 0 ~ -( ~5 ) electrogenerated a t constant current. The general irreversibility of the Sb(II1)-Sb(V) couple has prevented development of controlled-potential techniques based on their direct electrooxidation or reduction. Controlled-potential deposition of Sbo a t noble metal electrodes is accompanied by simultaneous evolution of H2 and is not useful for coulometry or amperometry. Taylor, Davenport, and Johnson (6) investigated the catalytic enhancement by Sb(II1) of the anodic wave for Br- a t a rotating platinum disk electrode in acidic solutions of Br-. The enhancement current is proportional to the analytical concentration of Sb(III), provided the concentration of Sb(II1) is less than that of Br-. A coulometPresent address, Standard Oil of Ohio, Research, Cleveland, Ohio 44128. * Person to whom correspondence should be addressed.
ric electrode was tested for its applicability to the determination of Sb(II1) by this scheme. Samples of Sb(II1) in 1.00mM NaBr-1.OM HzSO4 were injected into a stream of 1.00mM NaBr-1.OM HzS04. A current peak for oxidation of the Sb(II1) was observed on the base line for oxidation of Br-. The relative error for the determination of peak area was 1.5 Fg. A large number of chemical species interfere in the determination. More recently, Davenport and Johnson (7) discovered that the highly irreversible electrooxidation of Sb(II1) a t a platinum electrode in dilute HC1 is electrocatalyzed by I- specifically adsorbed a t the electrode surface. The choice of supporting electrolyte is important; the anodic process is not electrocatalyzed in HzSO4 or HC104. In 1-2M HCI, the predominant antimony species is SbC14- and a mechanism was proposed according to which adsorbed I- functions as an electron-transfer bridge. The limiting current for the anodic process a t a rotating disk electrode was determined to be controlled by convective-diffusional processes of mass transport over a large range of rotational velocity and concentration. Hubbard, Osteryoung, and Anson (8) found that the adsorption of I- a t platinum electrodes is irreversible and desorption does not occur even when the electrode surface is washed in an I - -free solution. We report here the application of liquid chromatography and the electrocatalyzed oxidation of Sb(II1) in a platinum coulometric detector for the determination of S b in several standard alloys and in human hair.
EXPERIMENTAL Apparatus. The tubular electrode was constructed by Pine Instrument Co. of Grove City, Pa., and was packed with platinum chips as described in Ref. ( I ) . The reference electrode was a Beckman Model 39270 Calomel Electrode filled with a saturated solution of NaC1. Electrode potentials were measured with respect to the reference using a Model 260 Digital Voltmeter from Data Technology Corp. The counter electrode was a coil of 20gauge platinum wire wound around the tip of the reference electrode. The three-electrode potentiostat was constructed with operational amplifiers and is described in Ref. (9). Current-potential ( I - E ) curves were recorded on a Model 815 X-Y Recorder from Bolt, Beranek, and Newman, Inc. Current-time ( I - t ) curves were recorded with a Model XL 860 Stripchart Recorder from Leeds & Northrup or a Model SRG from Sargent Welch Co. The I-t peaks obtained for analysis of samples were integrated by a Keuffel and Esser Compensating Planimeter. The I-t peaks obtained for the characterization of the coulometric detector were integrated electronically by an analog integrator constructed from a Zeltex ZA801-MZ operational amplifier. The integrator was calibrated by integration of the electrical current passing through a standard 10-K resistor connected to a 0.250-V signal for a known time period. The liquid chromatograph was constructed from a design by Seymour, Sickafoose, and Fritz ( I O ) . The chromatograph is de-
( 1 ) D. C. Johnson and J . H . Larochelle, Talanfa,20, 959 (1973). (2) R. A . Brown and E. H . Swift. J. Amer. Chem. Soc.. 71, 2717 ( 1 949). (3) J . J . Lingane and A . J . Bard, Anal. Chim. Acta, 16, 271 (1957) (4) W. M . Wise and J . P. Williams, Anal. Chem., 36, 1863 (1964). (5) A . J . Kostromin and A. H . Akhmentov, Zh. Anal. Chim., 24, 503 (1969). (6) L. R. Taylor, R. J. Davenport, and D. C. Johnson, Talanta, 20, 947 (1973).
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R. J . Davenport and D. C. Johnson, Anal. Chem., 45, 1755 (1973). A. T. Hubbard, R. A. Osteryoung, and F. C. Anson, Anal. Chem., 38, 692 ( 1966), L. R. Taylor, P h . D . Thesis, lowa State University, Arnes, lowa, 1973. M. D. Seymour, J. P. Sickafoose, and J . S. Fritz, Anal. Chem., 43, 1734 (1963).
scribed in Ref. ( I ) . Two different sample loops were used. The volume of each was standardized according to the procedure described in Ref. ( 1 ) . The values were determined to be 0.5065 and 2.017 ml. Eluents were contained in polyethylene or glass bottles. It was determined that extended contact of the acids with polyethylene ( > 6 months) resulted in contamination of the acids. Results reported here were obtained with uncontaminated solutions in new bottles. Eluent flow was maintained by pressurizing the bottles with compressed He. A mixing chamber, Model 2MC constructed by Pine Instrument Co., was used for injecting a solution of NaI into the effluent stream prior to its passage through the detector. This injection was part of the pretreatment procedure for the electrode. Reagents. A l.OmM stock solution of Sb(II1) was prepared by dissolving Analytical Reagent SbzO3 from Baker Chemical Co. in 30 ml of concentrated HCI. The solution was diluted with a mixture of HCI and H z 0 and the final acid concentration was 2M HCI. A second solution of 2.0mM Sb(II1) was prepared by dissolving Sbz03 in hot, concentrated HzS04. The acidity after dilution was 1M HzSO4. The stock solutions of Sb(II1) were standardized using constant-current coulometry with electrogeneration of Br2. The procedure is described in Ref. (6). Dilutions of the standard solution of Sb(II1) were made using a Gilmont micrometer buret and calibrated volumetric flasks. Two standard alloys from National Bureau of Standards were analyzed. Their certificate values were as follows: NBS-53 contained 78.87% Pb, 10.91% Sn, 10.09% Sb, and
