Noteworthy Method for Direct Determination of Sb^III and Total Inorganic Antimony in Natural Waters

Abstract

A procedure was developed for the direct determination of Sb^III and total inorganic Sb in water samples by hydride generation atomic absorption spectrometry (HG AAS), without pre-reduction of Sb^V. The NaBH₄ concentration was maintained at 0.4% (m/v) to perform the selective generation of stibine from Sb^III and online stibine generation was carried out with 5.0% (m/v) NaBH₄ to obtain the total antimony concentration. Sb^V was calculated as the difference between the values obtained for total Sb and Sb^III. In both cases, the procedure did not appear to be adversely affected by potential interferents such as Sb^V, As^III, As^V, Ca^II, Na^I, chloride and phosphate at a concentration of 0.5 mg L^-1. For the proposed procedure, the detection limits were 0.2 µg L^-1 for Sb^III and 0.3 µg L^-1 for total Sb, while the relative standard deviations were 0.1% for 10.0 µg L^-1 Sb^III and 0.4% for 10.0 µg L^-1 total Sb. A sampling rate of approximately 30 determinations perhour was achieved, requiring 300 mL of NaBH₄, and the amount of waste generated was in order of 960 mL, indicating that this analytical procedure can be considered as green. The method was shown to be satisfactory for the determination of trace levels of antimony in water samples. The result obtained for the assaying of certified drinking water samples was 9.7 ± 0.1 µg L^-1 (certified value 10.0 ± 0.5 µg L^-1).

antimony speciation; selective generation; HG AAS; water analysis

Introduction

The toxicity of antimony and its compounds is a matter of concern worldwide.¹ The World Health Organization² and United States Environmental Protection Agency³ have established maximum admissible concentrations for antimony in drinking water of 20 and 6 µg L⁻¹, respectively. In general, inorganic antimony is more toxic than organic antimony and, moreover, the trivalent species is 10 times more toxic than the pentavalent species. This is because Sb^III shows a high affinity for red blood cells and sulfhydryl groups in cell constituents, while erythrocytes are almost impermeable to Sb^V.⁴

Nowadays, this metalloid is widely employed in several applications including in brake linings and as an additive in the rubber vulcanization process.^5-8 Also, organic antimony compounds are used for the preparation of flame retardants. In the biomedical field, antimony compounds are used as therapeutic agents to treat parasitic diseases such as Leishmaniasis and Bilharziasis.⁹ Antimony is found in nature mainly as sulfide stibnite (Sb₂S₃). Considering the widespread industrial use of the various forms of Sb and their differences in toxicity, it is essential to develop analytical methods for their quantification.

Some previous papers describe experimental procedures for the determination of Sb^III and Sb^V, where Sb^V was determined after pre-reduction to Sb^III.^10,11 The reducing agents typically applied to convert Sb^V to Sb^III are potassium iodide and L-cysteine, and the former is the most widely used even though it is associated with several problems.¹² Potassium iodide requires relatively strong acidic conditions for both the sample and the reducing medium and the high concentrations applied can lead to the generation of considerable levels of iodine, which is especially disadvantageous in cases where the quantification of the analyte is carried out via hydride generation (HG) and atomic absorption spectrometry (AAS). In addition, the use of L-cysteine as a pre-reductant is advantageous because it allows the reduction to proceed under mildly acidic conditions.^13-15

HG combined with AAS as the detection technique has been frequently used for the direct determination of Sb in several different types of samples.^16-23 Some authors have demonstrated that the response obtained for Sb^V is around 50-80% lower than that obtained for Sb^III and it has been suggested that in the process of stibine formation from Sb^V, the Sb^V is firstly reduced to Sb^III with the subsequent formation of SbH₃.²⁴

The aim of this study was to develop a method for the direct determination of trace amounts of Sb^III and total Sb by hydride generation atomic absorption spectrometry (HG AAS) without an Sb^V pre-reduction step. The determination of inorganic antimony species in water matrices using the proposed method is recommended, based on the fact that the Sb^III species shows significantly higher signal intensity at low sodium tetrahydroborate (NaBH₄) concentrations than the Sb^V species. The sodium tetrahydroborate concentration used for the determination of Sb^III was 0.4% (m/v) and notable interference was not observed at a concentration of 0.5 mg L^-1 Sb^V. Since a flow system was employed, it is possible to perform the online generation of stibine with 5.0% (m/v) NaBH₄ in the same system used to obtain the total antimony concentration. The Sb^V is determined as the difference between the values obtained for the total antimony and Sb^III. The optimal conditions for the hydride generation and atomization of Sb^III were investigated. The method is suitable for application in routine analysis and monitoring studies.

Experimental

Instrumentation

For the antimony determination, a flame atomic absorption spectrophotometer (Varian, SpectrAA-220, Victoria, Australia) equipped with a continuous flow Varian VGA 77 vapor generation accessory was used. The VGA system consists of a three-channel peristaltic pump equipped with Tygon and polyethylene tubes that carry the sample through a reaction coil, where it is acidified and mixed with a suitable reductant. The stibine vapor formed is transferred to an absorption cell for determination by the spectrometer. The antimony hollow cathode lamp (Varian, Victoria, Australia) was operated at 7.0 mA, the wavelength was set at 217.6 nm and the spectral band pass was set at 0.1 nm. The analysis was carried out with a burner height of 17 mm, acetylene flow rate of 2.0 L min⁻¹ and air flow rate of 13.5 L min⁻¹. The sample, HCl, NaBH₄, and nitrogen gas were introduced at flow rates of 4.0, 7.0, 5.0 and 100 mL min^-1, respectively. The instrumental parameters used for the VGA 77 were those recommended by the manufacturer. The reaction time obtained after optimization was 60 s.

Reagents and solutions

All chemicals and reagents used were of analytical grade or higher purity and deionized water obtained from a water purification system (Gehaka, São Paulo, Brazil) was used throughout. Before use, laboratory glassware was kept overnight in 10% (v/v) nitric acid (aqueous solution) followed by ultra-sonication for 1 h and final rinsing with deionized water. Working standard solutions of Sb^III and Sb^V were all prepared daily through the appropriate dilution of 1000 mg L⁻¹ SbCl₃ (Merck, Darmstadt, Germany) and 1000 mg L⁻¹ SbCl₅ (Merck, Darmstadt, Germany) stock solutions, respectively. These solutions were stored in polyethylene bottles in a refrigerator at 4 °C. Sodium tetrahydroborate solutions were prepared daily from 97% NaBH₄ (Fluka, Switzerland) in 0.4% (m/v) NaOH (Merck, Darmstadt, Germany) solution to give stability since NaBH₄ decomposes easily. A solution of 5% (v/v) HCl was prepared through dilution of concentrated hydrochloric acid in water (Merck, Darmstadt, Germany).

Study of the experimental variables

Since NaBH₄ is used directly in the HG system for stibine generation and high concentrations of reductant are able to reduce total Sb to Sb^III and, in turn, to stibine, a study was conducted to compare the analytical signal obtained from Sb^V and Sb^III solutions, separately. In order to evaluate the effect of sodium tetrahydroborate concentration on the analytical signal for solutions containing 10 µg L^-1 Sb^III or Sb^V, the sodium tetrahydroborate concentration was evaluated in the range of 0.1 to 10.0% (m/v). The sodium tetrahydroborate flow rate was maintained at 5.0 mL min^-1 and the HCl concentration at 5.0% (v/v). The optimization of the parameters affecting the antimony determination by HG AAS was performed using a univariate method. Other variables studied were the sample and sodium tetrahydroborate flow rates and the HCl concentration.

Study on interfering ions

In order to investigate the selectivity of the proposed method, the determination of Sb^III in the presence of possible interferences was investigated. For this purpose, experiments were conducted using a 2^7-3 fractional factorial design and the results analyzed by calculating the interference factor. The influence of potentially interfering ions (Sb^V, As^III, As^V, Ca^II, Na^I, Cl^-, PO₄^3-) was verified by comparing the analytical signals of a solution containing only Sb^III and a solution containing Sb^III ions together with potentially interfering ions. In this study, a specie was considered as an interferent in the Sb^III determination by the proposed procedure when the difference between the analytical signal for the solution containing only Sb^III ions and that also containing the possible interfering ions was higher than 10%. Thus, the interference factor, IF, is defined by equation 1:

where A' is the analytical signal for the solution containing the analyte in the presence of possible interferents, and A is the analytical signal for the solution containing Sb^III in the absence of possible interferents. An interference factor of 1.00 indicates that there is no interference, while factors greater than 1.10 or smaller than 0.90 indicate an increase or a decrease in the analytical signal due to interference. Other ions present in the certified sample (APS 1075), such as Ni, Cu and Zn, showed no interference in amounts 6, 2 and 7-fold higher than the total Sb value, respectively.

Samples

Mineral water samples were obtained from local store. For the recovery tests, the samples were spiked with Sb^III and Sb^V at a concentration of 10.0 µg L^-1. The accuracy was also determined using samples of certified drinking water (APS-1075). This certified reference material was obtained from Alpha Resources Inc. (Stevensville, Michigan, USA). The proposed procedure was also applied to sea water samples from Itaguá Beach (Ubatuba, SP, Brazil), Lamberto Beach (Ubatuba, SP, Brazil) and Fortaleza Beach (Ubatuba, SP, Brazil) and river water samples of two different locations along the Uberabinha River, which runs through an urban zone in the city of Uberlândia (Minas Gerais, Brazil). In addition, the proposed procedure was also applied to sediment samples collected from Itaguá Beach (Ubatuba-SP). These samples were subjected to the Bureau Commune de Reference (BCR) sequential extraction method (4^th extraction stage) proposed by the European Community Bureau of Reference.²⁵

Results and Discussion

Evaluation of antimony hydride generation conditions

The conditions for HG were optimized to obtain maximum sensitivity and reproducibility of the method. Figure 1 shows the results for the system.

Figure 1
Optimization of the following HG conditions: (a) HCl flow rate, (b) NaBH₄ flow rate and (c) sample flow rate.

The efficiency of the antimony hydride generation was investigated using 6.0% (v/v) HCl at different flow rates from 1.0 to 9.0 mL min^-1. The results indicate an increase in the absorbance signal with an increase in the flow rate up to 7.0 mL min^-1 (Figure 1a). At higher flow rates, the signal decreased since the sample and the reagent did not have sufficient time to mix. Lower flow rates were not sufficient to generate atomic hydrogen in adequate quantities to react with BH₄^-efficiently,²⁶ while at higher flow rate, excessive formation of H₂ occurs interfering in the analytical signal.^26-28 Thus, the most appropriate hydrochloric acid flow to ensure the formation of a sufficient amount of stibine was 7.0 mL min^-1.

Sodium tetrahydroborate flow rates ranging from 1.0 to 12.0 mL min^-1 were evaluated. The analytical signal was relatively low when the sodium tetrahydroborate flow rate was less than 5.0 mL min^-1. For flow rates above 5.0 mL min^-1 there was a slight decrease in the analytical signal, and above 7.0 mL min^-1 a sharp decrease was observed (Figure 1b). This can be explained by the formation of H₂ gas in excess, resulting in dilution of the stibine formed, allowing the hydrides to reach the atomization cell. Thus, the flow of sodium tetrahydroborate solution was maintained at 5.0 mL min^-1.

Another important factor to be studied is the sample flow rate. Although a high flow rate is preferable in order to achieve higher sampling frequency, the sample flow rate is limited by the reaction in the hydride generation system and the capacity of the pumping system to maintain a stable flow under high pressures. The sample flow rate was studied in the range of 1 to 10 mL min^-1. The analytical signal increased until a sample flow rate of 4.0 mL min^-1 and from then on, it remained almost constant up to 8.0 mL min^-1, after which it decreased (Figure 1c). This decrease is related to the greater amount of Sb^III introduced into the gas-liquid separator when high sample flow rates are used, and the NaBH₄ is not sufficient to reduce the whole amount of sample present and generate stibine. Thus, the sample flow was set at 4.0 mL min^-1.

The experimental conditions for the determination of Sb^III and Sb^V were established for the selective generation of stibine from Sb^III and online stibine generation to obtain the total antimony concentration. The hydride generation parameters evaluated were the NaBH₄and HCl concentrations. Each parameter was evaluated for each antimony species using 10 µg L^-1 Sb^III and Sb^V solutions.

The NaBH₄ concentration is an important parameter for the generation of stibine, because it is formed in the presence of hydrogen generated by NaBH₄ in an acidic medium. In this case, H₂ acts as a stripping gas, besides acting in the formation of stibine. Since the amount of H₂ present is dependent on the NaBH₄ and HCl flow rates and concentrations, all of these parameters need to be studied. Figure 1 shows that the generation of hydride from Sb^Vleads to slower stibine generation compared with Sb^III, making it necessary to increase the NaBH₄ concentration in order to compare the results obtained for the two species. Results show that the absorbance increases with an increase in the NaBH₄ concentration in the determination of both inorganic antimony forms (Figure 2). However, the response obtained for Sb^V is lower than that obtained for Sb^III. The results of previous studies suggest that in the process of stibine formation from Sb^V, there is firstly the reduction of Sb^V to Sb^III with the subsequent formation of SbH₃.²⁴ A NaBH₄concentration higher than 0.4% (m/v) is required to obtain complete antimony hydride generation from both Sb^III and Sb^V, while a NaBH₄concentration higher than 5.0% (m/v) is needed for stibine hydride generation from Sb^V. These results demonstrate that the determination of Sb^III and Sb^V is possible under these conditions. The Sb^III was determined under conditions where there was no hydride generation from Sb^V. Thus, a NaBH₄ concentration of 0.4% (m/v) was selected in order to obtain the best analytical performance. The total antimony concentration was determined in the same flow system using a NaBH₄concentration of 5.0% (m/v).

Figure 2
Behavior of the analytical signal for Sb^III and total Sb as a function of the total variation in the concentration of NaBH₄.

The effect of varying the HCl concentration on the analytical signal for Sb^III and Sb^V was investigated from 0.05 to 10.0% (v/v) HCl. The signals related to Sb^III and Sb^V increased up to a concentration of 6.0% (v/v) HCl. The sensitivity was not significantly different on comparing 6.0 and 10.0% (v/v) HCl and, thus, 6.0% (v/v) HCl was chosen for further measurements.

Comparative tests for efficiency of the reduction of Sb^V

Since the use of NaBH₄ at high concentrations proved to be effective in reducing Sb^V and the total antimony could be determined without a pre-reduction step, a study was conducted to evaluate the effectiveness of this reagent in comparison with L-cysteine. For this, two calibration curves with concentrations ranging from 0 to 10 µg L^-1 Sb^V were constructed. The calibration curve obtained when the reduction was performed online using 5.0% (m/v) NaBH₄ was compared with that obtained for Sb^Vsolutions subjected to a pre-reduction step with L-cysteine. It was observed that the analytical signals obtained for the Sb^V determination using the proposed method are similar to those obtained when performing the pre-reduction step.

Studies of interfering ions

The effects of representative potential interferents that are known to affect the hydride process were determined, i.e., Sb^V, As^III, As^V, Ca^II, Na^I, Cl^- and PO₄^3-. To investigate the extent of their effect on the determination of antimony, 10 µg L^-1 of Sb^III was spiked with increasing levels of these potential interferents and values for the interference factor were calculated for each experiment (data not shown). A considerable signal depression was observed for all interfering ions when present in a fifty-fold excess. Sb^III can be tolerated without interference up to a 25-fold excess.

Analytical figures of merit and application

The equations for the calibration curves obtained for aqueous standard solutions of Sb^III and Sb^V were: A = 0.023 + 0.056[Sb^III] and A = 0.001 + 0.050[Sb^V], where A is the absorbance and the Sb concentration is expressed as µg L^-1. The correlation coefficients for the calibration curves were 0.9984 and 0.9997, respectively. For the Sb^IIIdetermination, 0.4% (m/v) NaBH₄ was used and for the Sb^Vdetermination, 5.0% (m/v) NaBH₄ was used. The calibration curves were linear up to a concentration range of 0.7-50.0 µg L^-1 for Sb^III and 0.7-50.0 µg L^-1 for Sb^V. The detection limits based on 3σ/m, where σ is the standard deviation of 10 measurements of a blank and m the slope of the calibration curve, were 0.2 µg L^-1 for Sb^III and 0.3 µg L^-1for total Sb. Based on ten measurements (n = 10), relative standard deviations of 0.1% for 10 µg L^-1 Sb^III and 0.4% for 10 µg L^-1 total antimony were obtained, demonstrating excellent repeatability. The analytical frequency was 30 samples per hour. The NaBH₄ consumption was around 300 mL and waste generation in the order of 960 mL h^-1. Thus, the determination of Sb in water samples by HG AAS represents an acceptable green analysis procedure. Its rank on the Eco-Scale (71) results from the minimal amounts of reagents used and waste generated.²⁸

The proposed method was applied to the quantification of antimony in mineral, sea and river water samples (Table 1). Sb^III was not detected in these samples and Sb^V was detected in sea and river water samples. To validate the method for the analysis of water, two samples were spiked with Sb^III and Sb^V to give a final concentration of 10.0 µg L^-1 of Sb^III and Sb^V. As can be seen in Table 1, recovery percentages of 98.8 to 103.7% were obtained. The results for sediment samples collected at Itaguá Beach (Ubatuba-SP) were in the range of 2.8 ± 0.5 µg L^-1 to 3.8 ± 0.5 µg L^-1 total Sb. Sb^III was not detected in these samples.

Table 1
Experimental recovery of antimony in water samples spiked with 10.0 μg L^-1 of Sb^III and Sb^V by HG AAS

Sample	SbIII added / (μg L–1)	SbIII found / (μg L–1)	Recovery / %	SbV added / (μg L–1)	SbV found / (μg L 1)	Recovery / %
A	–	N.D.	–	–	N.D.	–
	10	9.88	98.8	10	9.99	99.9
B	–	N.D.	–	–	N.D.	–
	10	9.98	99.8	10	10.37	103.7
C	–	N.D.	–	–	1.00	–
	10	9.99	99.9	10	9.82	88.2
D	–	N.D.	–	–	N.D	–
	10	9.78	97.8	10	10.18	101.8
E	–	N.D.	–	–	N.D	–
	10	8.86	88.6	10	8.95	89.5
F	–	N.D.	–	–	N.D	–
	10	8.56	85.6	10	10.47	104.7
G	–	N.D.	–	–	0.90	–
	10	9.42	94.2	10	9.98	90.8

• A and B: mineral water samples; C, D and E: sea water samples from Itagua Beach (Ubatuba, SP, Brazil), Lamberto Beach (Ubatuba, SP, Brazil) and Fortaleza Beach (Ubatuba, SP, Brazil), respectively. F and G: river water samples of two different locations along the Uberabinha River, which runs through an urban zone in the city of Uberlandia (Minas Gerais, Brazil). N.D.: not detectable.

The accuracy of the method was evaluated by analyzing drinking water reference material APS 1075 with a certified inorganic antimony concentration of 10.0 ± 0.5 µg L^-1. The result obtained for total Sb was 9.7 ± 0.1 µg L^-1 and Sb^III was not detected. These results are in good agreement with the certified values, confirming the reliability of the proposed method.

In this study, the proposed method was described and compared, according to the detection technique used. Important details related to previously published procedures for Sb^III and total Sb determination are given in Table 2. Most of these methods were applied to the analysis of antimony in environmental samples (water, sediment and particulate matter). To compare the method proposed herein, the limit of detection was taken into account. The lowest limit of detection reported for the previously published methods in Table 2 is 0.02 µg L^-1. However, these methods often require tedious sample pre-treatment procedures such as preconcentration and extraction steps.^29,30 It can be observed that the method described herein shows better or similar performance when compared with previously published procedures in terms of the limit of detection. Furthermore, the method does not require the use of a pre-reduction step and offers the advantage of selective determination.

Table 2
Comparison of methods for determination of Sb^III and Sb^V using hydride generation

Sample	Specie	Reduction step	Detection	LOD / (μg L–1)	Ref.
Soil, sediment	SbIII	L–cysteine	HG IAT F AAS	0.2	10
	SbV
Particulate matter	Sbtot	–	HG QTAAS	0.3	16
	SbV			0.2
Food packaging materials	SbIII	L–cysteine	CPE ET AAS	0.02	27
Water	SbIII	Selective sorption	GF AAS	0.18	28
	SbV			0.25
Water	SbIII/	NaBH4	HG F AAS	0.2	This study
	SbV			0.3

• HG IAT F AAS: hydride generation by integrated atom trap flame atomic absorption spectrometry, HG QT AAS: hydride generation quartz tube atomic absorption spectrometry, CPE ET AAS: cloud point extraction electrothermal atomic absorption spectrometry, GF AAS: graphite furnace atomic absorption spectrometry; LOD: limit of detection.

Conclusions

The use of the HG AAS system described herein was found to be effective for the determination of Sb^III and Sb^V. The use of HG AAS provides adequate sensitivity and accuracy for the direct determination of antimony (Sb^III and Sb^V) in water samples using the same flow system, thereby avoiding the tedious reduction procedures required for the determination of Sb^III and Sb^V by previously published methods. Since the pre-reduction step is eliminated, the need for sample handling and therefore the risk of contamination are reduced. The number of samples that can be analyzed per hour is 30. NaBH₄ consumption is around 300 mL and waste generation is in the order of 960 mL h^-1, indicating that this can be considered as a green analysis procedure. As a result, this is an attractive technique for the routine determination of trace amounts of antimony in water samples.

Acknowledgments

The authors are grateful for financial support from the Brazilian Federal Governamental and State funding agencies: Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Fundação de Amparo à Pesquisa do Estado de Minas Gerais (FAPEMIG) and Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES).

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Publication Dates

• Publication in this collection
  May 2015

History

• Received
  25 Nov 2014
• Accepted
  17 Mar 2015