Simultaneous adsorptive stripping voltammetric determination of molybdenum(VI), uranium(VI), vanadium(V), and antimony(III)

Abstract

The simultaneous adsorptive stripping voltammetric (AdSV) determination of the four elements molybdenum, antimony, vanadium, and uranium as their chloranilic acid (2,5-dichloro-4,6-dihydroxy-1,3-benzoquinone) complexes is described, the parameters of the procedure were varied to produce the best peak resolution with regard to the sensitivity. A pH of 2.3–3 (for sweet or sea water, respectively), a chloranilic acid concentration of 1×10⁻⁴ mol l⁻¹ and an accumulation potential of +50 mV were chosen for the analysis. Furthermore, the parameters for the differential pulse mode used were taken into consideration. A set of calibration curves was recorded. The 3σ-detection limits were calculated: 0.18 μg l⁻¹ for uranium, 0.14 μg l⁻¹ for vanadium, 0.27 μg l⁻¹ for antimony (III) and 0.07 μg l⁻¹ for molybdenum. The method was applied to standardized NASS 4 sea water, sea water from the North Fiji Basin, local potable water, and a sewage sample from a suspended uranium slag heap.

The batch-procedure was transferred to an automatic flow-through voltammetric device under parameter optimization.

Introduction

In previous publications the single adsorptive stripping voltammetric (AdSV) determinations for uranium, molybdenum, antimony, vanadium and tin using chloranilic acid as complexing reagent 1, 2, 3, 4, 5, 6 were described. Detection limits of 24 ng l⁻¹ for uranium, 23 ng l⁻¹ for tin, 21 ng l⁻¹ for molybdenum, 210 ng l⁻¹ for antimony(III), 560 ng l⁻¹ for antimony(V), and 21 ng l⁻¹ for vanadium were calculated using accumulation times of up to 5 min. The optimized conditions such as pH, supporting electrolyte solution, accumulation potentials, and the parameters of the differential pulse mode are different for all procedures. In Table 1 the parameters for the single AdSV determinations for all four elements are listed.

The trace analysis of these four elements is important for monitoring their concentrations in the environment. Uranium is present in surface and sea water in a concentration of about 1–3 μg l⁻¹ in the form of the amphoteric uranyl cation (UO²⁺₂). Several complexing reagents already have been applied to determine uranium by AdSV, such as pyrogallol 7, 8, oxine 9, 10, mordant blue 11, 12, cupferron [13], diisopropylmethylphosphate [14], triisobutylphosphate and tripropylphosphate 15, 16 and 4-(5-bromo-2-pyridylazo)-N,N-diethyl-3-hydroxylaniline [17]. The advantage of using chloranilic acid as complexing reagent has already been shown in previous publications 1, 2 as well as in 18, 19.

Vanadium is present in high concentrations in fossil fuels and brought into the environment by combustion of these sources at a rate of more than 100 000 tonnes per year. In waters the dissolved vanadium is normally present as vanadate. Since the solubility of vanadium (IV) salts are very low, these compounds are accumulated in sediments and particles. The German law suggests photometry [20] and inductively-coupled plasma optical emission spectroscopy (ICP-OES) [21] for quantification of vanadium in environmental samples. For aqueous samples, electrochemical methods such as voltammetry are useful. Thereby, usually two reduction steps are mentioned which are assigned from V(V) to V(IV) and from V(IV) to V(II). Different voltammetric techniques can be applied to receive these reduction responses. Acidified supporting electrolyte solutions containing rhodanide is known for the determination of vanadium in bitumen [22]. In petroleum vanadium is analysed by means of alternating current polarography [23]. A catalytic wave using direct current polarography is described in [24]. For lower concentrations of vanadium, the technique of adsorptive stripping voltammetry (AdSV) is suitable. As complexing reagents some compounds are already known in literature, such as pyrogallol [25], 8-hydroxyquinoline [26], catechol [27], antipyrylazo III [28] and cupferron [29] as well as chloranilic acid [4].

Antimony is usually present in the aqueous environment in the oxidation states (III) and (V). Trivalent antimonials are generally more toxic than pentavalent forms [30]. A simple method to distinguish between valence states is cathodic stripping voltammetry after accumulation as amalgam from hydrochloric acid solution [31]. The known AdSV methods using triphenylmethane dyes [32] and catechol [33] as complexing reagent are not selective for the speciation analysis of Sb(III) and Sb(V). A specific determination of the two oxidation states is described in [3]. The method already has been improved by Bond [34].

Molybdenum is ubiquitous in the environment and plays an important role in our ecosystem [30]. The predominant form in which molybdenum occurs at neutral pH in water is MoO²⁻₄[35].The polarographic determination of molybdenum uses the reduction of Mo(VI) to Mo(V) and from Mo(V) to Mo(III) [36]. In recent years several AdSV procedures have been developed using various molybdenum complexes such as with oxine [37], tropolone [38], chloranilic acid [6], Eriochrome Blue Black R [39], and 2-(benzylidene)aminobenzohydroxamic acid [40]. The AdSV determination of molybdenum with mandelic acid [41] and the polarographic determination (using a dropping mercury electrode) with cupferron [42] are based on catalytic reactions of the central atom. In conclusion of a comparison of different adsorptive stripping voltammetric techniques, which is submitted for publication [43] chloranilic acid was found to be the best complexing reagent for application in batch and flow through voltammetry beside oxine, tropolon and cupferron.

One main advantage of voltammetry is the ability to apply oligo element analysis. The anodic stripping voltammetry of zinc, cadmium, lead and copper in one run is used in routine analysis [44]. Some well-established AdSV procedures allow a simultaneous determination of several elements, such as cobalt and nickel with dimethylglyoxime [44] or nickel, cobalt, cadmium, lead and copper with 1-phenylpropane-1-pentylsulfonylhydrazone-2-oxime as chelating reagent [45], as well as titanium and molybdenum with mandelic acid and iron(III), vanadium (V) and manganese with solvochrome violet [46].

The application of wide band complex forming reagents is often of little use for application to real samples, because many elements produce signals which then interfere with one another. Chloranilic acid, in contrast, only forms electroactive and adsorbable complexes with a hand full of elements, as mentioned above.

In this work a procedure for the simultaneous determination of uranium, vanadium, antimony, and molybdenum is developed. All four elements are relevant for environmental analysis. The mean concentrations are in the sub-μg l⁻¹ range and, therewith, under the detection limits of most other analytical methods, like inductively-coupled plasma mass spectroscopy (ICP-MS) or atomic absorption spectroscopy (AAS). For voltammetry, however, low analyte concentrations can even be easily determined in samples with high salinity, such as sea water samples, without interference.