Se-species in solution

Techniques developed for the determination of selenite and selenate involve a succession of several analytical steps (e.g. reduction, separation, detection) which are often far from being validated. In addition, the knowledge related to the stability of the species is still very scarce. A project has hence been launched within the BCR programme with the aim to evaluate the stability of Se-species in solution [42] this feasibility study has been continued by an interlaboratory study for the evaluation of method performance [43]. Both investigations were designed to improve the state-of-the-art of Se-speciation prior to the tentative certification of solution candidate reference materials as described in this section. As a follow-up, artificial freshwater solutions containing inorganic Se-species were prepared (RMs 602 and 603) [40,41]. 

---

Camara, C., Comelis, R., Quevauviller, P. Assessment of methods currently used for the determination of Cr and Se species in solutions. Trends Anal. Chem. 19, 189-194 (2000)... 

The project was started in 1992 by a feasibility study on the optimal storage conditions for solutions containing selenate and selenite [142], The verified stability of Se species in solutions enabled the organization of an interlaboratory study in 1993-94 [143], which was followed by a certification campaign in 1994-95 [144,145]. 

Average concentrations of Se species in solution (pg L" ) at field capacity (FC) and at submerged conditions... 

Fig. 3.11 Theoretical condition of corrosion, immunity, and passivation of CulnSe2, Cu Se, and Se phases in an aqueous solution. Concentrations of Cu, In, and Se species in the aqueous solution at 25 °C were assumed to be 10 M. (Reproduced from [179], Copyright 2009, with permission from Elsevier)...

It can, however, be questioned if the oxidation of cyclohexene in the pre.sence of several transition metals is a free-radical chain mechanism. Arzoumanian et al. have reported that the catalytic decomposition of cyclohexene hydroperoxide by a rhodium complex in benzene is not a free-radical chain reaction. They found that the decomposition of each cyclohexene hydroperoxide gives rise to only one free radical [9]. In this ca.se the decomposition products were found to be 18 % l-ol (2), 33 % 1-one (1) and 35 % polymers together with small amounts of water and oxygen. They were also able to identify two free-radical species in solution, namely the cyclohexenylperoxy radical (6) and the cyclohexenyloxy radical (7). 

Mechanisms of electrochemical and photoelectrochemical deposition of metal selenide clusters (Me = Pb, Cd, Zn, Bi, In) onto the surface as well as into the selenium films have been studied. These clusters are formed as a result of underpotential and overpotential deposition of the metals onto Se. Photoinduced underpotential deposition of Bi onto Se was used to cover selenium colloidal particles with BiiSes clusters. The PbSe and Bi2Sc3 clusters modify the Se surface and form electronic surface states in the Se bandgap, thus promoting electron exchange processes between the valence band and redox species in solution and the rise of the subbandgap photocurrent. 

Specific chapters then focus on different projects on speciation analysis. Chapter 4 deals with interlaboratory studies on methylmercury in fish and sediment Chapter 5 describes the collaborative projects to certify organotins in sediment RMs and mussel tissues Chapter 6 gives an overview of the certification project on trimethyllead in simulated rainwater and urban dust Chapter 7 describes the certification project on arsenic species in fish tissues Chapter 8 focuses on the intercomparison and tentative certification of Se(IV) and Se(VI) in simulated freshwater Chapter 9 deals with a feasibility study to stabilize Cr species in solution followed by the certification of Cr(III) and Cr(VI) in lyophilized solutions and welding dust Chapter 10 gives a review of methods used for A1 speciation Chapter 11 develops the overall collaborative project to standardize single and sequential extraction procedures for soil and sediment analysis, followed by interlaboratory studies and certification of soil and sediment reference materials. 

More recently, photo-induced generation of the hydrides has been reported utilizing UV-irradiation of the sample in an aqueous medium spiked with low molecular weight acids such as formic, acetic, ma-lonic, etc. The efficiency of the process is greater than 70% for Se and, depending on the species in solution, radical reactions can give rise to various products, including the simple hydrides or alkylated derivatives, as outlined below ... 

The structure of species in solution depends on numerous parameters (pH, temperature, nature of the cation of the ba.se, Al/Si ratio, etc.) and remains uncertain. The interpretation of AI and Si NMR spectra is complicated and uncertain because of the high lability of Al-O-Si bonds, 2-3 orders of magnitude higher than that of Si-O-Si bonds [96,97]. In the presence of alkaline cations (Na, Cs ), the complexes [(H0)3A10Si(0H)20Si03H3 J - - " and [(HOlaAlOSiOjHj-j are detected in. solution. With tetraalkylammonium cations, cages similar to those observed in silicate solution were found with different substitution ratios of silicon by aluminum. 

The solution retains a larger amount of dissolved material, the greater the volume of solution at constant alkali concentration. This result suggests that zeolites exhibit a kind of solubility in their mother liquor. However, the ionic species in solution are not necessarily directly related to structural units of the zeolite framework. In silicate solutions, various silicate anions can co-exist in mobile equilibrium inter se. Aluminate solutions at high pH are simpler in that A1(0H)4 is the dominating species. In alumino-silicate solutions oxy-anions containing both A1 and Si are possible [75]. 

Cathodic electrodeposition of microcrystalline cadmium-zinc selenide (Cdi i Zn i Se CZS) films has been reported from selenite and selenosulfate baths [125, 126]. When applied for CZS, the typical electrocrystallization process from acidic solutions involves the underpotential reduction of at least one of the metal ion species (the less noble zinc). However, the direct formation of the alloy in this manner is problematic, basically due to a large difference between the redox potentials of and Cd " couples [127]. In solutions containing both zinc and cadmium ions, Cd will deposit preferentially because of its more positive potential, thus leading to free CdSe phase. This is true even if the cations are complexed since the stability constants of cadmium and zinc with various complexants are similar. Notwithstanding, films electrodeposited from typical solutions have been used to study the molar fraction dependence of the CZS band gap energy in the light of photoelectrochemical measurements, along with considerations within the virtual crystal approximation [128]. 

The first examples of mononuclear disulfur and diselenium complexes of platinum have been described.330 Reduction of the sterically hindered complex trans- PtC 2( P M e2A r)2] (Ar = 2,4, 6-tris[bis(trimethylsilyl)methyl]phenyl, 2,6-bis[bis(trimethylsilyl)-methyl]-4-[tris(trimethylsilyl) methyl]-phenyl) with lithium naphthalide in THF solution affords the platinum(0) species [Pt(PMe2Ar)2]. Oxidative addition of elemental sulfur or selenium yields the dichalcogenatoplatinum(II) complexes of the type [PtE2(PMe2Ar)2] (E = S, Se) containing a unique PtE2 ring system. The complexes are stable to air in the solid state, but slowly decompose in solution after several days at room temperature. 

The simplest possible coordination compounds of Cd and Hg with chalcogen donors are the anions MCh22. Multinuclear NMR studies of these species (Ch = Se, Te) in NH3 or en solutions with [K(2.2.2-cryptand)]+ counterions have been reported.314 Structural studies31, also combined with new multinuclear NMR measurements, have been performed.316... 

Accordingly, isotopic equilibration for Cr and Se species is expected to be much slower than for the aqueous Fe(III)-Fe(II) couple, which reaches equilibrium within minutes in laboratory experiments (Beard and Johnson 2004). Additionally, Cr(III) and Se(0) are highly insoluble and their residence times in solution are small, which further decreases the likelihood of isotopic equilibration. In the synthesis below, isotopic fractionations are assumed to be kinetically controlled unless otherwise stated. However, definitive assessments of this assumption have not been done, and future studies may find that equilibrium fractionation is attained for some reactions or rmder certain conditions. 

This is an example of conproportionation. The net results were CdSe deposits that required thermal treatments to obtain optimal photoactivity due to the presence of elemental Se [129]. The initial solution to this problem was to search for a Se precursor in a lower oxidation state, such as selenosulfite, SeSOs [132]. The nominal oxidation state of Se in this species is zero, as it is formed by the reaction of elemental Se with sulfite ... 

---