Selenate in Solution

As mentioned in the technical discussions, selenite was often determined by difference between total inorganic Se and selenate this measurement was. 

1 mL sample was mixed with 0.5% NaBH4, followed by HPLC separation (Hamilton PRP-XlOO), on-line microwave reduction of selenate to selenite in HCl (2 mol L ) and total determination as selenite by quartz furnace AAS. Calibration was by matrix-matching, using Na2Se03 and Na2Se04 calibrants. 

---

Selenates(IV) contain the [Se03] " and related sp>ecies. They are hydrolysed in solution and give complexes. 

A number of substances, such as the most commonly used sulfur dioxide, can reduce selenous acid solution to an elemental selenium precipitate. This precipitation separates the selenium from most elements and serves as a basis for gravimetry. In a solution containing both selenous and teUurous acids, the selenium may be quantitatively separated from the latter by performing the reduction in a solution which is 8 to 9.5 W with respect to hydrochloric acid. When selenic acid may also be present, the addition of hydroxylamine hydrochloride is recommended along with the sulfur dioxide. A simple method for the separation and deterrnination of selenium(IV) and molybdenum(VI) in mixtures, based on selective precipitation with potassium thiocarbonate, has been developed (69). 

Chemical Reactivity - Reactivity with Water Reacts vigorously with water forming selenic acid solution Reactivity with Common Materials Corrodes all metals in the presence of water Stability During Transport Stable Neutralizing Agents for Acids and Caustics Flush with water and rinse with dilute solution of sodium bicarbonate or soda ash Polymerization Not pertinent Inhibitor of Polymerization Not pertinent. 

Separation of Se and Te can also be achieved by neutralizing the alkaline selenite and tellurite leach with H2SO4 this precipitates the tellurium as a hydrous dioxide and leaves the more acidic selenous acid, H2Se03, in solution from which 99.5% pure Se can be precipitated by S02 ... 

In searching to formulate a mechanism of CuInSc2 phase formation by one-step electrodeposition from acid (pH 1-3) aqueous solutions containing millimolar concentrations of selenous acid and indium and copper sulfates, Kois et al. [178] considered a number of consecutive reactions involving the formation of Se, CuSe, and Cu2Se phases as a pre-requisite for the formation of CIS (Table 3.2). Thermodynamic and kinetic analyses on this basis were used to calculate a potential-pH diagram (Fig. 3.10) for the aqueous Cu+In-i-Se system and construct a distribution diagram of the final products in terms of deposition potential and composition ratio of Se(lV)/Cu(ll) in solution. 

Fig. 9.1. Sorption of selenate (SeO ) to a loamy soil, showing mass sorbed per gram of dry soil, as a function of concentration in solution. Symbols show results of batch experiments by Alemi et al. (1991 their Fig. 1) and lines are fits to the data using the reaction KA, reaction Freundlich, and Langmuir approaches.

We consider as a first example sorption of selenate (SeO ), as predicted by the reaction K(, reaction Freundlich, and Langmuir approaches (Sections 9.1-9.3). Alemi et al (1991) observed the partitioning of selenate in batch experiments between 10 g of a loamy soil and 20 ml of a pH 7.5 solution containing small amounts of Na2Se04 their results are shown in Figure 9.1. 

In contrast to TeFe, mixtures of SeFg and H20 appear not to interact over long time intervals at room temperature (57). HOSeFs and possibly HSe03F, however, are detected by, 9F-NMR spectroscopy in solutions of selenic acid in AHF (57). 

Add potassium permanganate and dichromate solutions respectively to two test tubes containing selenous acid solutions acidified with sulphuric acid. Heat the contents of the test tubes. What do you observe Write the equations of the reactions. What properties does selenous acid exhibit in this case ... 

Selenic Acid, HaSe04.—This acid was first prepared by Mitscher-lich in 1827, who suspended lead selenate in water and precipitated the lead by means of hydrogen sulphide. By the oxidation of aqueous solutions of selenious acid with suitable reagents, e.g. chlorine or bromine, selenic acid may also be obtained.6 When chlorine is used, any hydrogen chloride must be removed as soon as it is formed, for selenic acid is reduced by hot hydrogen chloride to selenious acid, with liberation of chlorine the process is best carried out6 by passing a... 

By the action of vanadium pentoxide on selenous acid solutions, or by reduction of a solution of vanadium pentoxide in selenic acid, red crystals of a free vanado-selenous acid have been obtained, the composition of which is SV206.4Se02.4H20.j aq. It contains four molecules of water of constitution, and, according to Prandtl, should be formulated H4Ve017.4H2Se03.(aj—2) aq. 

The [2+4] cycloaddition of selenocarbonyl (selenoxo) or tellurocarbonyl (telluroxo) compounds to diene systems remains one of the most widely used reactions for the preparation of various six-membered rings containing selenium or tellurium (see CHEC-II(1996), Section 5.11.7.2.1). The extremely sterically hindered selenal 154 has been isolated in monomeric form in solution and reacts with 2,3-dimethylbutadiene to afford the corresponding dihydro-selenin cycloadduct 155 (Equation 62) C1996AGE660, 1997T12167, 1998PS633>. 

Phosphate, silicate, borate, arsenate, selenite, chromate, and fluoride are anions for which ligand exchange is important. Nitrate, chloride, bromide, and perchlorate are not held, while sulfate and selenate may be weakly held. As a consequence, leaching of nitrate and sulfate from soil in drainage water can be significant, but very little phosphate is lost in solution. Of the trace metals, Co, Cu, Ni, and Pb are strongly held on oxide surfaces by chemisorption, but the process is much less important for Cd and Zn. 

Iron Selenites.—Although metallic iron does not appear to be soluble in selenous acid, yet selenites of iron are readily obtained in a variety of ways. When sodium selenite is added to ferrous sulphate solution, a white precipitate of ferrous selenite, FeSe03, is obtained.4 This becomes darker on exposure to air in consequence of oxidation. If the white precipitate is dissolved in hydrochloric acid, a portion of the selenium separates out, whilst ferric chloride and selenous acid remain in solution. Thus —... 

Calculate the concentrations of H3O+, OH-, HSe04-, and Se04 - in 0.12 M H2Se04, selenic acid, solution. Some kidney stones are crystalline deposits of calcium oxalate, a salt of oxalic acid, (COOH)2. Calculate the concentrations of H3O+, OH-, COOCOOH-, and (C0Q-)2 in 0.12 M (COOH)2. Compare the concentrations with those obtained in Exercise 62. How can you explain the difference between the concentrations of... 

Figure 13. Effect of period of incubation on the concentration of selenite (a) and selenate (b) in solution after adding respectively selenite or selenate to a soil. Intermediate points have been omitted for clarity [80].

While the sorption curves are almost linear on a log log scale, the model fits a gentle curve as this is consistent with a bigger body of information (Fig. 9.). At any given level of sorption, the concentration of selenite in solution decreases with time and with increasing temperature. It is this decrease that is modelled as due to diffusive penetration. Selenate differs in that the sorption curves are steeper (as also shown in Fig. 9.) and, importantly, that the effects of time, though detectable, are much smaller. These two species therefore provide a test for the argument that apparent non-reversibility of sorption occurs because of the continuing reaction. 

Evaporation of the leach water from 1000 g of phosphogypsum and X-ray diffraction of the resulting crystals did not identify any compounds of trace elements. Trace elements may exist as sulfates, such as mercuric sulfate, or as calcium salts, such as calcium selenate, in equilibrium with a saturated solution of calcium and sulfate ions from gypsum. Further isolation of trace elements by extraction, precipitation, ion exchange, or other means would so alter this equilibrium that the original amounts and types of compounds present would not be determined. Literature studies of phase equilibria in saturated gypsum solutions were used to identify possible trace element compounds. 

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]. 

The effect of storage at 40°C was studied in 100 mL vessels (instead of 500 mL as used in the other experiments). Surprisingly, the stability was found to be much better for both species in solutions stored at pH 2 and pH 6 in polyethylene containers (with and without addition of chloride). Tests performed with samples stored in the dark and exposed to sunlight demonstrated that light had no significant effect on the stability of selenite and selenate for the period tested. 

Selivanova, Zubova, and Finkel stein [59SEL/ZUB] also made a calorimetric study of the reaction between a silver nitrate and a selenic acid solution with formation of Ag2Se04(cr). The standard enthalpy of formation of the compound has been evaluated from the data in Appendix A. The value obtained there ... 

Klein [40KLE] measured the solubility of cobalt selenate in water as a function of temperature and presented the results in a table and a graph. Several hydrates are formed and the transition temperatures observed were heptahydrate hexahydrate 284.6 K, hexahydrate tetrahydrate 306.7 K, and tefrahydrate —> monohydrate 346.7 K. CoSe04 6H20 readily forms metastable saturated solutions up to about 340 K. 

---