Selenous acid determination

Discussion. This gravimetric determination depends upon the separation and weighing as elementary selenium or tellurium (or as tellurium dioxide). Alkali selenites and selenious acid are reduced in hydrochloric acid solution with sulphur dioxide, hydroxylammonium chloride, hydrazinium sulphate or hydrazine hydrate. Alkali selenates and selenic acid are not reduced by sulphur dioxide alone, but are readily reduced by a saturated solution of sulphur dioxide in concentrated hydrochloric acid. In working with selenium it must be remembered that appreciable amounts of the element may be lost on warming strong hydrochloric acid solutions of its compounds if dilute acid solutions (concentration <6M) are heated at temperatures below 100 °C the loss is negligible. 

Comprehensive accounts of the various gravimetric, polarographic, spectrophotometric, and neutron activation analytical methods have been published (1,2,5,17,19,65—67). Sampling and analysis of biological materials and oiganic compounds is treated in References 60 and 68. Many analytical methods depend on the conversion of selenium in the sample to selenous acid, H SeO and reduction to elemental selenium when a gravimetric determination is desired. 

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 tellurous acids, the selenium may be quantitatively separated from the latter by performing the reduction in a solution which is 8 to 9.5 N with respect to hydrochloric acid. When selenic acid may also be present, the addition of hydroxy] amine hydrochloride is recommended along with the sulfur dioxide. A simple method for the separation and determination of selenium(IV) and molybdenum(VI) in mixtures, based on selective precipitation with potassium thiocarbonate, has been developed (69). 

F. Li, W. Goessler, K. J. Irgolic, Determination of trimethylselenonium iodide, selenomethionine, selenious acid and selenic acid using high performance liquid chromatography with on-line detection by inductively coupled plasma mass spectrometry or flame atomic absorption spectrometry, J. Chromatogr. A, 830 (1999), 337-344. 

A sensitive gas-chromatographic determination of selenium is possible as 5-chlor-or 5-nitro-piazselenol. These compounds are extractable into benzene or toluene for direct injection into a gas-chromatograph with BCD or TCD After treatment of steel samples with diluted mixtures of equal volumes of hydrochloric and nitric acids in the presence of perchloric acid, the selenium is completely converted into the quadrivalent state. Large amounts of Fe(III) are masked with phosphoric acid. In pure sulfuric acid ultramicro amounts of selenium are converted into seleneous acid with a bromine-bromide redox buffer solution For high-purity... 

Gelbach and King [42GEL/K1N] prepared their specimen of Ag2Se04(s) by slow precipitation from 10% silver nitrate and magnesium selenate solutions. The solubility in six solutions of selenic acid in the concentration range from 0.00 to 0.12 M was determined. Equilibrium was approached from under- as well as supersaturation. The solubility in water was found to be 2.42 x 10 M. No solubility product was calculated from the data. This has been done here with log K° = 1.75 for the protonation constant... 

Ag2Se04 was prepared by mixing stoichiometric quantities of silver nitrate and selenic acid. The salt contained 60.04% Ag which is close to the theoretical value of 60.14%. The preparation was shown to be crystalline by X-ray diffraction. The solubility was measured after 15 to 20 days in a thermostat. No increase in solubility was observed between day 15 and day 20. The silver concentration was determined by a tur-bidimetric method. 

The heat of precipitation silver selenate was measured by mixing 0.04241 moles of silver nitrate dissolved in 325 g of water with 0.02122 moles of selenic acid in an isothermal calorimeter equipped with a sensitive thermometer. The acid (7.07 M) was contained initially in a bulb that was broken in order to start the reaction. The Ag2Se04 formed was shown to be crystalline by X-ray diffraction. The water equivalent was determined after each run. The experimental results and the evaluation of the standard enthalpy of formation of Ag2Se04(cr) are shown in Table A-17. 

The solubility of SrSe04(cr) in water has been determined over the temperature interval 273 to 373 K. Crystalline strontium selenate was prepared by mixing stoichiometric amounts of strontium nitrate and selenic acid. Under the light microscope the crystals appear as very thin needles. The strontium concentration in saturated solution was obtained by polarographic (half-wave potential at -2.1 V) and gravimetric measurements. Both methods gave concordant results, Table A-18. 

Nakashima and Toei showed that inorganic selenium in the form of selenous acid reacts with 4-chlorololphenylenediamine hydrochloride to produce 5-chlorobenzene-2,13 selenadiazole which is then extracted into toluene preparatory to determination on a column consisting of SE-30 on Chromosorb W operated at 200 C. Nitrogen is used as a carrier gas and detection was achieved by an electron capture detector. 

Shimoishi and Toei determined ultramicroamounts of selenium in pure sulphuric acid by oxidizing it to selenous acid with bromine-bromide redox buffer solution followed by quantitative conversion to 5-nitropiaselenol, by reaction with 4.nitro-o-phenyldiamine, and extraction into toluene and estimation of the 5-nitropiaselenol under the same conditions as described above. 

Lingane and Niedrach have claimed that the h-VI states of tellurium (or selenium) are not reduced at the dropping electrode under any of the conditions of then-investigation however, Norton et al. [42] showed that under a variety of conditions, samples of telluric acid prepared by several different procedures do exhibit well-defined (though irreversible) waves, suitable for the analytical determination of the element. The reduction of Te(H-VI) at the dropping electrode was found coulometri-cally to proceed to the -II state (whereas selenate, Se(-i-VI), was not reduced at the dropping electrode in any of the media reported). 

With the exception of the calcium, strontium, barium and mercurous salts, the normal selenates are readily soluble in water. Barium chloride and mercurous nitrate are therefore convenient precipitation agents.6 Barium selenate is, however, more soluble than barium sulphate, and also differs from the latter salt in being slowly reduced to selenite by hydrochloric acid 7 for these reasons precipitation with barium chloride is not applicable to the quantitative determination of selenie acid. A concentrated solution of selenie acid which has been saturated with barium selenate deposits crystals of barium selenie acid, H2[Ba(Se04)2].8... 

A standard official method is based on fluorimetry [65-67]. In this method, the plant sample is digested with 60% m/m perchloric acid 70% nitric acid, 3 1 v/v, and the residue is dissolved in 2 M hydrochloric acid. Any selenate present in this solution is converted to selenite by boiling. The concentration of selenate is then determined fluorimetrically as a dekalin extract of the complex formed with 2,3-diaminonaphthalene. 

Cane sheath and neem leaf samples were used and the final acid digest was heated with concentrated hydrochloric acid to reduce any selenate to selenite. The solutions were finally made up to 100 ml and 5 and 10 ml aliquots were analysed. The samples did not contain any selenium, and recoveries were determined by the addition of 50 and 100 xg of selenium to the plant samples before decomposition. The results were all satisfactory. When these experiments were repeated with 1.0 and 2.5 xg of selenium as spikes, the recoveries were 80 -120% at the 1 xg level and 96-120% at the 2.5 pg level (per five grams of sample). 

Shimoishi and Toei [766] have described a gas chromatographic determination of selenium in non saline waters based on l,2-diamino-3,5-dibromobenzene with an extraction procedure that is specific for selenium (IV). Total selenium is determined by treatment of non saline water with titanium trichloride and with a bromine-bromide redox buffer to convert selenide, elemental selenium and selenate to selenious acid. After reaction, the 4,6-dibromopiazselenol formed from as little as lng of selenium can be extracted quantitatively into 1 ml of toluene from 500ml of natural water up to 2ng L 1 of selenium(IV) and total selenium can be determined. The percentage of selenium(IV) in the total selenium in river water varies from 35 to 70%. 

---