Stripping potentiometry of indium in aqueous chloride solutions

Microchemical Journal 69 Ž2001. 13᎐19

Stripping potentiometry of indium in aqueous chloride solutions 夽 Megan M. Wilson, Howard D. DewaldU Department of Chemistry and Biochemistry, Ohio Uni¨ ersity, Athens, OH 45701, USA Received 22 August 2000; received in revised form 2 October 2000; accepted 3 October 2000

Abstract Stripping potentiometry ŽSP. has been used effectively for the determination of trace levels of several metal species in solution. Indium ion, which can be readily reduced and has a high solubility in mercury, can be determined directly using SP. The reduction potential of indium is located between those of lead and cadmium, necessitating optimization of several experimental parameters for successful quantitative analysis. A method for the SP of InŽIII. is presented which utilizes constant current stripping analysis ŽCCSA. at a mercury-film glassy carbon electrode in KCl solutions in the presence of PbŽII. and CdŽII.. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Indium; Stripping potentiometry; Constant current

1. Introduction Indium has a variety of uses. One common use is in the manufacturing of alloys. One indium alloy that is produced is a dental alloy, which is utilized because of its relatively low toxicity w1x. There are several bearing alloys produced for use

夽 Abstracted in part from Megan S. McGowan, M.S. Thesis, Ohio University, 1999. Presented in part at the 31st Central Regional American Chemical Society Meeting, Columbus, Ohio, June 1999. U Corresponding author. Tel.: q1-740-593-1755; fax: q1740-593-0148. E-mail address: [email protected] ŽH.D. Dewald..

in nuclear reactor control rods, such as Ag᎐In᎐Cd alloy w1x. Low melting alloys are employed in germanium transistors, thermistors, and photoconductors w2x. Another field in which indium is utilized is in semiconductor research, where several thin films containing indium have been studied w1,3,4x. Some examples of these films include In 2 Se 3 w1,3,5x and InSb w1,6x. Indium is also used as a label in medical research, where the indium species bind to proteins w7᎐9x. These proteins could then be analyzed by immunoassays and quantified by stripping voltammetry ŽSV. of the indium w8,9x. Another use for indium in medical research is as an internal standard, because of the low concentrations existing normally w10x. An ex-

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M.M. Wilson, H.D. Dewald r Microchemical Journal 69 (2001) 13᎐19

ample of this is in the determination of Pb in blood, where indium is added to the sample to aid the researcher in determining the presence of Pb w10x. Indium has also been used in aluminum battery systems, where Al᎐In alloy was found to increase anodic dissolution current densities w11x. Many spectrometric methods utilizing indium as an internal standard are employed, such as inductively-coupled plasma ŽICP. ᎐mass spectrometry ŽMS. and ICP᎐atomic emission spectrometry w12,13x. Graphite furnace atomic absorption spectrometry ŽGFAAS. has been used to analyze geochemical and sediment samples for the presence of indium w14x. An extractive separation of indium and iron used di-2-ethylhexyl phosphoric acid as the solvent was analyzed by X-ray spectrometry w15x. Trioctylamine was used in the solvent extraction of indium and thallium in a chloride medium with measurement of the metal concentrations by liquid scintillation counting w16x. Indium has also been studied using a variety of electrochemical techniques. The most common technique employed has been polarography at the dropping mercury electrode ŽDME. w17x. As already mentioned w8᎐10x stripping voltammetry ŽSV. has also been used for trace analysis of samples containing indium. Other SV reports include simultaneous determination of Sn and In in alloys w18x, determination in marine samples w19x and natural waters w20x, development of an expert system for determination of several trace metals w21x and determination as a dopant in samples of thermoelectric bismuth᎐telluride material w22x. An adsorptive stripping voltammetry ŽAdSV. report detailed the use of morin as an organic complexing agent in determining the indium concentration in zinc refinement residue w23x. Stripping potentiometry ŽSP. is a derivative of SV. Both techniques utilize a preconcentration step, usually by deposition onto the surface of an electrode. Following the deposition, in SV the deposit is determined by application of a potential that dissolves the deposit and the resulting current is proportional to the analyte concentration in the sample. Whereas, after deposition in SP, the potential is disconnected and the deposit is dissolved, ‘stripped’, non-electrolytically w24,25x

or by application of a small constant current w25,26x. The length of time required for the stripping is proportional to the analyte concentration in the sample. SP has several advantages over SV. No purging or deaeration is necessary since dissolved oxygen is often used as a chemical oxidant. Background charging currents do not arise in SP. No potential waveform generator is required, thus the equipment is simpler. In this work, a SP method has been developed for the determination of In in the presence of Pb and Cd. Such a method might be applied as an internal standard in the determination of Pb in blood w10,27x or in an immunoassay w8,9,28x.

2. Experimental 2.1. Apparatus A TraceLab Model PSU20 potentiostat and Model SAM20 sample station was connected to a personal computer Ž710 Acer, 10 MHz. with the TAP2 Trace Talk software. A three-electrode system was used which consisted of a 3-mm-diameter glassy carbon electrode ŽGCE., a saturated calomel electrode ŽSCE. and a platinum auxiliary electrode. Nitric acid washed 50-ml polypropylene sample cells were employed. A motorized propeller, controlled by the software, was used to stir the solution. All equipment was supplied by Radiometer ŽWestlake, OH, USA.. The propeller stir rates were measured in water with a Type 631-BL Strobotac ŽGeneral Radio, MA, USA.. Stir position 5 Žthe usual setting. corresponded to 570 Ž"10. rev.rmin. 2.2. Reagents and supplies Doubly distilled and de-ionized water prepared by passing the laboratory water through a resin cartridge ŽBarnstead D8902. connected to a glass still ŽBarnstead Fi-streem2. was used for preparation of all solutions. Mercury stock solutions were prepared volumetrically from certified ACS reagent grade mercuryŽII. nitrate wHgŽNO 3 . 2 ⭈ H 2 Ox from Fisher Scientific ŽPittsburgh, PA, USA.. pH adjustment was made with either

M.M. Wilson, H.D. Dewald r Microchemical Journal 69 (2001) 13᎐19

reagent grade nitric acid ŽFisher. or reagent grade hydrochloric acid ŽFisher.. Potassium nitrate, 99.99% ŽAldrich Chemical Co., Milwaukee, WI, USA. or potassium chloride, certified ACS reagent grade ŽFisher. was used in supporting electrolyte preparation. Atomic absorption standards ŽFisher. were used to prepare standard solutions of cadmium, indium, and lead. 2.3. Procedures The GCE was polished on a Microcloth felt pad with a 0.05-␮m ␥-alumina slurry ŽBuehler Ltd, Lake Bluff, IL, USA.. After polishing, the electrode was rinsed with water, dilute nitric acid, and again with water. A mercury film was deposited in situ onto the GCE. The film was prepared from a plating solution consisting of 20 ml of KNO 3 or KCl electrolyte solution containing 80 ppm HgŽII. by applying a potential of y0.900 V while stirring the solution for 4 min. The electrode was then conditioned for 15 s at a potential of y0.100 V with stirring. The film deposition and conditioning steps were repeated three times in order to obtain a uniformly thick Hg film. Prior to analysis of a sample, the Hg film was held at y0.100 V for 120 s. After formation of the Hg film, a 50-␮l addition of analyte sample standard solution was made to the 20-ml of plating solution. The sample standard compositions were 10 ppm CdŽII., 10 ppm InŽIII., and 10 ppm PbŽII.. The SP preconcentration step was performed at a deposition potential of y0.900 V with stirring for 45 s, unless indicated otherwise. The stripping step was performed by removing the deposition potential and stopping the stirring. The amalgamated analytes were reoxidized by diffusion of the chemical oxidants present in solution, namely HgŽII. and dissolved oxygen. The process was enhanced by applying a 15-␮A oxidizing current in the constant current stripping analysis ŽCCSA. mode. The stripping signal is presented in counts ŽsrV. vs. potential ŽmV.. The defined peak potential ŽPOTn. and maximum integrated peak width ŽWIDn. are given in Table 1.

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Table 1 Stripping potential and maximum integration peak width parameters Žas defined by the POTn and WIDn software commands. used for SP of 25 ppb CdŽII., InŽIII., and PbŽII. in 0.25 M KCl Ion

Stripping potential ŽmV.

Integration peak width ŽmV.

CdŽII. InŽIII. PbŽII.

y680 y580 y480

"40 "50 "100

3. Results and discussion The SP with the PSU20 and TAP2 Trace Talk software is based on method building made up of several procedures. The procedures consist of command lines that define a particular step or operation in the method. Additionally, several other parameters were studied to obtain the best response. 3.1. Optimization of measurement parameters 3.1.1. Electrolyte composition KNO 3 and KCl solutions were studied in consideration of their common use in voltammetric techniques, relative abundance, inexpensiveness, and inertness. KNO 3 is often used as an electrolyte because it exhibits non-complexing character in the presence of most metallic ion species. Concentrations of 0.5᎐1.3 M KNO 3 were studied. Fig. 1 shows a representative stripping potentiogram for a mixture of 25 ppb CdŽII., InŽIII., and PbŽII. in 1.3 M KNO 3 . At all electrolyte concentrations studied, peak resolution was a problem. The first peak observed at y644 mV is a combined response for Cd and In. The peak had an area that was the result of both metals being stripped. The peak at y488 mV is that for Pb oxidation. Thus KNO 3 was not suitable as an electrolyte. KCl is just as common as KNO 3 , but it has a tendency to form complex ions with metallic ions, including indium w2,29᎐31x. Formation of these complexes leads to shifts in potentials required to oxidize the amalgamated metallic species, and

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M.M. Wilson, H.D. Dewald r Microchemical Journal 69 (2001) 13᎐19 Table 2 Comparison of stripping potentials observed for 25 ppb CdŽII., InŽIII., and PbŽII. as a function of KCl concentration a KCl concentration ŽM.

Avg. peak potential ŽmV. Cd

In

Pb

0.050 0.250 0.500

y636 Ž1.5. y672 Ž1.8. y691 Ž1.3.

y567 Ž1.7. y578 Ž3.5. y600 Ž4.0.

y443 Ž8.4. y479 Ž1.4. y492 Ž2.9.

a

Average of four replicate scans and standard deviation.

Fig. 1. SP-CCSA of 25 ppb CdŽII., InŽIII., and PbŽII. in 1.3 M KNO 3 . Constant current s 15 ␮A. Edep s y0.9 V; tdep s 45 s; stir rate positions 5 Ž570 rev.rmin.. ECdrIn s y644 mV; EPb s y488 mV.

can lead to improved selectivity of one analyte species over another species w32x. Concentrations of 0.0005᎐1.3 M KCl were studied. The peak shapes, degree of peak resolution, and value of the stripping potentials at the various KCl concentrations were compared. In low concentrations of KCl Ž0.0005᎐0.040 M. the Pb was resolved from In, but In and Cd were only partially resolved. In higher concentrations of KCl Ž0.050᎐1.3 M., all three peaks were resolved. Table 2 gives values of potential shifts observed as a function of KCl concentration studied over the range 0.050᎐0.5 M. The values shown are the average of four scans. The In had the smallest potential shift, while Cd has the largest. The potential shifts can be related to the stability of the chloride complexes. The formation constants Žlog K 1 . are: 1.42 for InŽIII., 1.62 for PbŽII., and 1.95 for CdŽII. w33x. As the electrolyte concentration increased, the capacitance background signal also increased. Large background signals require baseline corrections to be performed. Thus 0.25 M KCl was selected as the electrolyte, as it allowed for potential resolution but with less background signal ŽFig. 2.. 3.1.2. Deposition time and stir rate During the deposition, the two parameters that are under control are time and diffusion layer thickness. Either increasing the deposition time or decreasing the diffusion layer thickness by

increasing the stir rate can increase the stripping signal. However, there are practical limitations. Deposition times should be no longer than required to obtain a reproducible signal. Long deposition times can deplete the metal ion in the bulk solution Žespecially in small sample volumes., may result in intermetallic compound formation, or saturate the mercury film with metal-amalgam. Deposition times ranging from 20 s to 5 min were studied ŽFig. 3.. After a 20-s deposition only a small amount of analyte was reduced. For longer deposition times of 3᎐5 min, the amounts of analyte concentrated into the mercury film were large, causing the peak widths to broaden. The deposition time of 45 s to 1 min was selected as the most efficient; the stripping peaks observed were relatively sharp and well defined. Stirring with the TraceLab instrument is performed with a motorized propeller and controlled

Fig. 2. SP-CCSA of 25 ppb CdŽII., InŽIII., and PbŽII. in 0.25 M KCl. Constant current s 15 ␮A. Edep s y0.9 V; tdep s 45 s; stir rate positions 5 Ž570 rev.rmin.. ECd s y678 mV; EIn s y592 mV; EPb s y478 mV.

M.M. Wilson, H.D. Dewald r Microchemical Journal 69 (2001) 13᎐19

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reduction parameter can be used. In utilizing these background correction procedures there was little or no effect on the stripping response. Thus no baseline correction was utilized. 3.2. Precision and calibration

Fig. 3. Effect of varying the deposition time from 20 s to 5 min on the In stripping peak area. 0.25 M KCl. Constant current s 15 ␮A; Edep s y0.9 V; stir rate positions 5 Ž570 rev.rmin.. Standard additions Žppb.: 25 Ž⽧.; 50 Ž䢇.; 75 Ž'.; 100 ŽB.; 125 Ž夹..

by the software. The settings range from 0 Žoff. to 10 Žhigh.. A comparative study examining the effect of the stir rate was made. With no stirring a small amount of the metal analyte present in solution was deposited by diffusion. From stir positions 1 and 2, the peak area increased. Settings 3᎐7 yielded a relatively flat response. At position 9, a decrease was observed. The results for position 9 were likely the result of solution turbulence and non-laminar flow. Stir position 5 was chosen.

With the Trace Talk software the standard addition method is used to determine the metal ion concentrations. A study of the repeatability of multiple scans was performed at five concentrations, ranging from 25 ppb to 500 ppb InŽIII.. Table 3 shows the results for 11 replicates at each concentration level. The largest percent relative standard deviation Ž%R.S.D.. was 13.2% at 25 ppb, which decreased to 4.15% at 500 ppb. These values lie well within expected levels w34x. When the average peak area vs. InŽIII. ppb concentration was subjected to least squares analysis, the resultant equation was y Žms. s 1.6 " 0.094 Žmsrppb. q 19.3" 28.8 ms and the correlation coefficient, r, equaled 0.9949. The standard addition method was performed on each metal ion analyte, Cd, In and Pb. The standard addition consisted of 50-␮l additions of a 10-ppm standard solution to 20 ml of 0.25 M KCl electrolyte spanning the concentration range of 25᎐250 ppb. Each addition was analyzed in quadruplicate. Least squares analysis was performed on the average peak area vs. metal ion concentration for each of the analytes and the results are presented in Table 4. The most sensitive response was Pb and the least sensitive was Table 3 Statistical results of precision of InŽIII. stripping peak areaa

3.1.3. Baseline correction Prior to sample analysis, in each SP experiment the PSU20 potentiostat measures the capacitance background and stores the data in memory. The background response can be recalled and subtracted from a sample stripping curve. The background curve, an uncorrected sample stripping curve, or the corrected sample stripping curve can be displayed. Alternatively, a ‘BASE’ digital data

InŽIII. concentration Žppb.

Mean Žms. High Žms. Low Žms. S.D. %R.S.D.

25

125

250

375

500

61.08 74.92 50.76 8.04 13.2

185.8 215.4 162.5 17.4 9.35

454.7 494.6 418.5 24.5 5.39

646.1 686.7 602.0 28.7 4.45

792.3 837.3 737.8 32.9 4.15

a Supporting electrolyte: 0.25 M KCl; Edep s y0.9 V; tdep s 45 s; stir positions 5 Ž570 rev.rmin.; N s 11.

M.M. Wilson, H.D. Dewald r Microchemical Journal 69 (2001) 13᎐19

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Table 4 Least-squares treatment from standard additiona Metal ion

Slope Žmsrppb.

Intercept Žms.

r

Cd In Pb

2.44" 0.07 1.78" 0.08 3.37" 0.10

52.3" 11.0 51.4" 12.8 178 " 16

0.9966 0.9915 0.9963

a Supporting electrolyte: 0.25 M KCl; Edep s y0.9 V; tdep s 45 s; stir positions 5 Ž570 rev.rmin.; N s 4.

In. Relatively good linearity was obtained, as indicated by the 0.99 correlation coefficients, but there was curvature in the line as the concentration of each ion increased.

4. Conclusions SP of 25᎐250 ppb concentrations of indium ion in a chloride electrolyte has been achieved in the presence of similar concentrations of CdŽII. and PbŽII.. This range covers previous indium uses as an internal standard in blood Pb analysis w10x. Indium used as a chemical label in immunoassays ranges from 1 to 10 ppb w8,9x and is a possible application. Extensions of the work will explore other non-complexing electrolytes, such as KClO4 or K 2 SO4 . Intermetallic compound formation of zinc᎐indium and other metallic ion species commonly associated with indium, such as Fe and Cu, could be investigated. The utility of other electrode substrates, such as gold or platinum, and recently developed iridium-based mercury electrodes w35x, iridium electrodes w36x or bismuthcoated carbon w37x could be explored. Finally, SP methods could be developed for other group 13 metals, such as Ga and Tl. Ga is widely used in the semiconductor industry and is often found in residue from coal burning. Tl was used previously in many insecticides and rodenticides. References w1x S. Budavari ŽEd.., The Merck Index: An Encyclopedia of Chemicals, Drugs, and Biologicals, 12th ed., Merck, Whitehouse Station, NJ, 1996, p. 850.

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