Wuttig, Kathrin (2008) Electrochemical and spectrochemical investigations into the determination of Niobium and Zirconium complexes in seawater. (Diploma thesis), Christian-Albrechts-Universität, Kiel, 121 pp.

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Abstract

The focus of this diploma work is on an electrochemical and spectrochemical investigation of Niobium and Zirconium speciation in seawater. Previously all published investigations of Nb and Zr were done at significantly higher concentrations (μmol L-1) were the in situ speciation may be considerably different than that found in ambient seawater (nmol L-1 or pmol L-1). At concentrations in the μmol L-1 range solubility issues coupled with the formation of dimeric and higher order polymeric hydroxy species complicate studies in seawater and presently our knowledge of the actual speciation in seawater and the potential role of organic complexation is very limited. Furthermore Nb and Zr belong to the “high field strength elements” (HFSE) (Barth et al. 2001) and are being investigated as oceanographic watermass tracers (McKelvey and Orians 1998). Thus there is some urgency to develop new methods to determine Nb and Zr speciation at low concentrations in seawater to improve our understanding of the basic biogeochemistry of these metals. Wang et al. (1992a) published a sensitive electrochemical method for the detection of Nb with 5 μmol L-1 cupferron in 0.1 mol L-1 acetate buffer at pH 4. Their method was based on differential pulse cathodic stripping voltammetry (DPCSV). In the present work this method was extended to the pH range of 4.4-6.6 though the use of increased cupferron concentration was necessary for the achievement of the same sensitivity. With an increasing pH in both water and seawater the peak position of Nb was shown to move to a more negative potential, as observed previously by Ferrett et al. (1955). In order to apply this method to the determination of Nb in seawater, the cupferron concentration had to be further increased. This was due to the fact that there is a competition with Mg and Ca in seawater which are also able to form complexes with cupferron. In seawater it was found that Nb can be determined by DPCSV with cupferron in the pH range of 3.9-6.5. An increased sensitivity of S = 0.55 nA mol-1 L can be obtained by adding 1 mmol L-1 cupferron as a chelator and 5.4 mmol L-1 KBrO3 as an oxidizing agent at pH 6.4. The overall reaction is enhanced catalytically by the presence of an oxidizing agent and while cupferron itself can act as both complexing agent and oxidizing agent, KBrO3 is more effective as an oxidizing agent under these pH conditions. Generally the catalytic cycle is the reduction of the Nb(V)-cupferron complex to Nb(IV) with reoxidation back to Nb(V) in the presence of an oxidizing agent. At pH 7 another peak interfered at the same peak position as the Nb peak. The results of this work suggest that a 1:1 Nb:cupferron complex is formed at pH 6 whereas a 1:2 Nb:cupferron complex is formed at a higher pH (6.5-7.1) in MQ. For the polarographic determination of Nb an analogue of cupferron was also evaluated, N-benzoyl-N-phenylhydroxylamine (BPHA), as a chelator. BPHA is a stronger chelator for Nb than cupferron, but the catalytic cycle as with cupferron cannot be formed, because of the easy reduction of the Nb-BPHA complex. It was previously published that Nb forms ternary red-coloured complexes with PAR and either tartrate, oxalate or citrate at pH 5-6. These ternary complexes can be determined spectrochemically, because the light is absorbed at a wavelength of λmax = 550 nm. The composition of the formed Nb:PAR complex is 1:1 (Belcher et al. 1962a; Yamada et al. 1988; Yamada et al. 1990). The results of this thesis suggest that a 1:1 Nb-PAR complex is still formed at pH 7, but the formation takes far longer than at pH 5-6. The formation of the Nb:PAR complex without added chelator was significantly quicker indicating the absence of any competing complexation reactions. Zr and Nb are often being complexed by the same organic compounds. Thus the electrochemical determination of Zr with the related complexing agents cupferron and BPHA was also tested, but it was not possible to find the corresponding Zr peak. Zr obviously forms no catalytic cycle and has no redox chemistry [Zr(IV)/Zr(III)] under the conditions used here. Little is known about the marine chemistry of Nb and Zr. The oxidation state of Nb which dominates in seawater is probably +5 and the predominating hydrolyzed species could be primarily Nb(OH)6 - and secondarily Nb(OH)5 (Greenwood and Earnshaw 1990; Sohrin et al. 1998a; Byrne 2002). In acidic streams (pH = 4.5-6.2) Astrom et al. (2008) found that the dissolved Nb is present in an anionic form. The results of this thesis are consistent with an anionic Nb species in seawater for pH higher than 5.8.
The dominating oxidation state of Zr under seawater onditions is presumably +4 and the predominant species is Zr(OH)5 - with a small fraction of Zr(OH)4 (Byrne 2002). In a plot that is based on the pK-values published by Turner et al. (1981) it was found that with increasing pH the hydrolysis of Zr is increased and the dominating Zr species under seawater conditions agree with those published by Byrne (2002).

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