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ANALYTICAL CHEMISTRY, VOL. 50, NO. 2, FEBRUARY 1978
(30) L. Meites and S. A. Moros, Anal. Chern., 31, 23 (1959). (31) Safeguards Analytical Laboratory Evaluation, U . S . Energy Res. Dev. Administration Rep ., NBL-282 (1976). (32) Safeguards Analytical Laboratory Evaluation, U.S.Energy Res. Dev. Adm%istration Rep., NBL-283 (1977). (33) J. R. Weiss, C. E. Pietri, A. W. Wenzel, and L. c. Nelson, Jr., u s At. Energy Cornrn. Rep., NBL-265 (1972).
(34) "1976 Annual Book of ASTM Standards", American Society for Testing and Materials, Easton, Md., (1976), Part 45, Nuclear Standards, pp 21 1-214.
RECEIVED
for review August 10, 1977. Accepted November
3, 1977.
Chromatographic Determination of Chromium(V1) with Coulometric Detection Based on the Electrocatalysis by Adsorbed Iodine of the Reduction at Platinum Electrodes in Acidic Solutions J. H. Larochelle' and D. C. Johnson" Department of Chemistry,
Iowa
State University, Ames,
Iowa 500 7 7
Adsorbed iodine at a Pt electrode is demonstrated to be an electrocatalyst for the irreversible electrochemical reduction of Cr(V1) in HCI and H2S04solutions. Determination of Cr(V1) in a coulometric flow-through electrode is possible in the range 0.4 ng to 80 pg. No interference by dissolved 0, is observed. Application of the coulometrlc detector for chromatographic determinations of Cr(V1) Is presented.
T h e maximum concentration of Cr(V1) presently permitted in drinking water by the Safe Drinking Water Act ( 1 ) is 0.05 pg/mL (50 ppb). Cr(II1) is of less concern and, consequently, analytical techniques must be specific for Cr(V1). We have been interested in the development of a general methodology for determining ionic species a t trace levels in complex aqueous samples based on liquid chromatographic separation with in-stream amperometric and coulometric detection (2-5). Results of this work have convinced us that determinations of many species by this technique can be made with detection limits at the part-per-billion level or lower (3, 5 ) . We report here the determination of Cr(V1) without interference from a large number of inorganic species including oxygen by liquid chromatography with coulometric detection. Iodine is rapidly adsorbed at Pt electrodes in acidic solutions of iodide salts (6-8). A maximum of approximately 1.5 X lo-' mol/cm2 is adsorbed from solutions containing as little as 2 pM NaI. T h e adsorption is irreversible and desorption does not readily occur even if the electrode surface is bathed with pure water. Lane and Hubbard (9) determined t h a t the Ifrom solution is adsorbed as a neutral species, whereas F-, C1-, and Br- are adsorbed as anions. Adsorbed I is removed at large positive potential ( E > 1.2 V vs. SCE in 1.0 M H,SO,) with concurrent oxidation to IO3- (7). The surface of the Pt electrode is oxidized simultaneously. No adsorption of I occurs a t a Pt surface covered with oxide (6-8). Electrocatalysis of several irreversible reactions by halogens adsorbed a t Pt electrodes has been reported. Anson (10) determined that adsorbed Br- catalyzed the oxidation of the EDTA-Co(I1) anion. Hubbard and co-workers (11-14) reported the accelerating influence of free halide ions on the interconversion of Pt(I1) and Pt(1V) complexes at Pt elec'Present address, Department of Chemistry, St. John's University, Collegeville, Minn. 56321. 0003-2700/78/0350-0240$01.00/0
trodes. Davenport and Johnson ( 1 5 ) reported the electrocatalysis of the irreversible oxidation of Sb(II1) by adsorbed I and Taylor and Johnson (3) applied the electrocatalyzed reaction for determination of Sb(II1) by liquid chromatography with coulometric detection. Lane and Hubbard (16) reported the electrocatalysis of the oxidation of ascorbic acid by adsorbed I a t a Pt electrode. Recently, Johnson and Resnick (17)reported that the reduction of Fe(II1) in HC104 solutions is irreversible in the absence of traces of dissolved halides but is dramatically catalyzed by addition of halide salts, including I-, a t micromolar levels. T h e Cr20?'-Cr3+ system is well known to be irreversible a t Pt electrodes in acidic solutions (18). T h e cathodic reduction of Cr20T2-was reported by Kabasakaloglu and Uneri (19)to occur a t the same potential as for reduction of Pt oxide. These factors are undoubtedly responsible for the observed deviations of the potentiometric titration curve from theory for the titration of Fez' by Cr20i2-. The deviation occurs when the Cr20T2--Cr3+couple is predominately responsible for establishing the electrode potential, Le., beyond the equivalence point (20). Liteanu and Haiduc (21)studied the effects of various electrode pretreatment and observed t h a t the titration curve was closer to theory when the Pt electrode was pretreated by oxidation with aqua regia followed by reduction of the surface oxide with an iodide solution. They noted that iodide is adsorbed a t Pt electrodes.
EXPERIMENTAL Reagents. All chemicals were Baker Reagent Grade except as specified. Water was triply distilled with deionization in a mixed-bed ion-exchange column after the first distillation and the second distillation was from alkaline permanganate solution. The separation column was prepared from Fisher A-540 Adsorption Alumina which was ground wet with a porcelain mortar and pestle. The ground alumina was then sieved while being flooded with deionized water to facilitate the sieving process. A retainer of sintered glass was constructed in one end of the glass tube to be used for the chromatographic column by partial fusion of a plug of finely ground glass. The column was filled by transfer of a water slurry of the 180-250 mesh fraction of the ground alumina. Water was drawn out of the tube during packing by gentle suction applied to the tube end with the sintered plug. The column was installed in the chromatograph so that the sintered glass retainer was at the low pressure end of the packed column, thereby preventing loss of alumina due to eluent flow. Instrumentation. Preliminary voltammetric data were obtained with a rotating Pt-disk electrode (RPtDE) with an area of 0.47 cm2. The electrode was constructed by Pine Instrument 1978 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 50, NO. 2, FEBRUARY 1978
3: -
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1c
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Flgure 1. I d - f d curves for Cr(V1)at RRDE in 1.0 M H2S04. 2 X Cr2072-:2.0 V/min scan: 8, = fractional surface coverage of electrode by adsorbed I M
Co. of Grove City, Pa., and was rotated in a PIR rotator also from Pine Instrument Co. The liquid chromatograph was essentially that described in Reference 2. The volume of the sample loop was 0.504 mL and the glass chromatographic column had internal dimensions of 2 X 80 mm. The flow-through detector was the packed P t tubular electrode designated Detector C in Reference 2. Instrumentation for potentiostatic control of the detector potential, current measurement, and peak integration was constructed in our laboratory from operational amplifiers according to conventional designs. Some recorded chromatographic peaks were integrated by a Kueffel and Esser compensating planimeter. Procedures. Activation of the RPtDE was achieved by immersion into a solution of approximately 0.1 M NaI followed by rinsing with distilled water. Activation of the chromatographic detector was achieved by injecting several 0.5-mL samples of 0.1 M KI into the chromatographic system. Preliminary investigations with the flow-through electrode were performed without the presence of the alumina column in the liquid chromatograph. Aqueous solutions of 0.01668 M (0.1001 N) K2Cr207were injected into a water stream which was subsequently mixed with a reagent stream of 4.0 M HC1. The resultant stream passed through the detector. Study of the effects of fluid flow rate in the detector was made by controlling the sample stream at 0.14.2 mL/min and varying the reagent stream from 0.1-5 mL/min. The separation procedure ultimately determined t o provide the most satisfactory analytical results was as follows: The aqueous sample of Cr(V1) was injected into a stream of 0.18 M HCl flowing through the alumina column at 0.5 mI,/min. Seven minutes after sample injection, the eluent was changed to 1.8 M HCl to bring about elution of Cr(V1) from the column. The effluent was continuously mixed with the stream of 4.0 M HCl flowing at 3.0 mL/min prior to passage through the detector. Complete elution occurred within 5 min at which time the eluent was changed to 4.0 M HC1 to wash the column and ensure complete desorption of all sample components. After a 4-min washing, the eluent was changed back to 0.18 M HCl. Injection of a subsequent sample was delayed until 4 min after the final eluent change to ensure the complete removal of 4.0 M HC1 from the chromatograph. Water Sample. A water sample (No. 10334) was obtained from the Veterinary Diagnostic Laboratory at Iowa State University. The sample was obtained from the effluent stream of a tannery and had been acidified with 10% by volume of concentrated HC1. Dilution of the sample was necessary prior to analysis to decrease the HCl concentration. A 1.00-mL aliquot of the sample as received was pipetted into each of six 50-mL volummetric flasks. To the flasks were added sequentially 0.00, 0.25, 0.50, 1.00, 1.50, and 2.00-mL aliquots of a standard aqueous solution containing 1.73 ppm Cr(V1) was K2Cr207.The contents of each flask were diluted to volume, mixed, and analyzed.
RESULTS AND DISCUSSION Electrocatalysis at RPtDE. Current-potential (I-E) curves obtained a t the R P t D E for reduction of Cr(V1) are shown in Figure 1for a supporting electrolyte of 1.0 M H2S04 and in Figure 2 for 1.0 M HCl. T h e I-E curves shown were obtained a t four values of rotational velocity for the presence
Figure 2. I,,-fd curves for Cr(V1) at RRDE in 1.0 M HCI. 2 X M Cr,O7*-; 2.0 V/min scan: 8 , = fractional surface coverage of electrode by adsorbed I
Figure 3. I d - f d curves for O2at RPtDE in 1.0 M H2S04and 1.0 M HCI. 2 X M Cr20,'-; 6.0 V/min scan; (A) residual, 0 WMI-, (6) air saturated, 0 WMI-: (C) residual, 4 pM I-; (D) air saturated 4 pM I-
and absence of adsorbed I on the electrode surface. With the absence of adsorbed I, the cathodic waves are markedly irreversible. The current appears to approach a limiting value in the region of E < 0.1 \' and it may be possible that hydrogen adsorbed on the Pt electrode for E < 0.0 V electrocatalyzes the reduction process. The apparent reversibility of the reduction of Cr(V1) is dramatically improved by the presence of adsorbed I a t the electrode surface. In both solutions extensive potential ranges exist, as a result of the adsorbed I, for which the cathodic current is potential independent and appears limited by the rate of mass transport. A linear plot with zero intercept was obtained for the limiting current measured in the plateau region of the I-E curves for each electrolyte as a function of the square root of rotational velocity for velocities
