Calcium selenites

The solid phases studied in the solubility experiments are assumed by the review to have the composition CaSe03-H20 and the solubility equilibrium is written  

Chukhlantsev [56CHU] prepared calcium selenite by mixing 0.1 M solutions of calcium chloride and sodium selenite solution in stoichiometric amounts. Chemical analysis confirmed the 1 1 ratio between Ca(ll) and Se(IV). No X-ray diffraction measurements were performed. The solubility of the specimen in dilute solution of nitric or hydrochloric acid was measured at 293 K. The data have been recalculated as described in Appendix A, [56CHU], to yield log, ((V.177), 293.15 K) = - (6.15 0.22). The 

Sharmasakar, Reddy, and Vance [96SHA/RED] obtained log ((V.177), 298.2 K) = - 7.76 from measurements of the solubility of crystalline CaSe03-H20 in aqueous media. For reasons presented in Appendix A, this result has been given zero weight. 

The review considers the measurements by Savenko to be the most satisfactory and selects  

This value appears to be somewhat too large. The review therefore concludes that additional measurements would be desirable, preferably with better control of the solid phase. Note that the enthalpy value was calculated with non-TDB auxiliary data and therefore included in Appendix E. 

---

Calcium selenite, 22 73t Calcium silicate(s), 12 63 hydration, 5 475-477 Calcium silicate hydrate, 5 477t hydration, 5 475... 

D. 17 (a) telluric acid (b) sodium arsenate (c) calcium selenite (d) barium antimonate (e) arsenic acid ... 

The rather large solubility product of calcium selenite suggests that the reaction in the calorimeter was not complete. It is estimated that a few per cent of the components remained unreacted. No correction has been applied for this fact. The calculation of the standard enthalpy of formation of CaSe03 H20 with new auxiliary data is shown in Table A-31. 

The primary solubility data and the calculation of the solubility products, defined in the usual way, are presented. This presentation contains some unexpected results. The total solubility of the metal ion and selenite are approximately equal in water and in the inert salt solutions for the magnesium and manganese selenites. This is the expected result for a simple dissolution reaction. For calcium selenite, the metal ion concentration was about 100 times greater than the total selenite concentration whereas for zinc selenite the opposite was found. There is no comment in the paper on these results, which contradict the equilibrium reactions used in the paper to define the reported solubility products for calcium and zinc selenite. The review also noted that the calculation of the magnesium and selenite activities from the total concentrations introduces activity coefficients between 0.1 to 0.01 at moderate ionic strengths. Thus the values of these coefficients appear unreasonably small. On the whole, the activity coefficient corrections introduced appear to vary in an erratic way between the various systems studied. 

Ebert, M., Havlicek, D., Calcium selenites, Collect. Czech. Chem. Commun., 46, (1981), 1740-1747. Cited on pages 399,471. 

Savenko, V. S., Solubility product of magnesium and calcium selenites, Zh. Neorg. Khim., 40, (1995), 1254-1256, in Russian, English translation in [95SAV2]. Cited on pages 395, 400. 

Mahan DC, Magee PL. 1991. Efficacy of dietary sodium selenite and calcium selenite provided in the diet at approved, marginally toxic, and toxic levels to growing swine. J Anim Sci 69(12) 4722-4725. 

The calcium selenite thus formed (equation 10) is, however, soluble in water to about 500 mg/1. Oxidation of calcium selenite to calcium selenate does not proceed to any significant extent, and even if it did, calcium selenate is even more soluble in water. Selenite salts of most common metals are also relatively soluble. Therefore, the treatment of the weak acid bleed by lime precipitation cannot achieve high removal levels of selenium. 

Synonyms anhydrous calcium sulfate-anhydrite muriacite karstenite anhydrous gypsum anhydrous sulfate of lime hemihydrate-plaster of Paris annalin dried gypsum dried calcium sulfate dihydrate-gypsum alabaster satin spar mineral white terra alha satinite light spar selenite precipitated calcium sulfate native calcium sulfate... 

Although G. E. Stahl and Caspar Neumann both believed that alum contained lime, J. H. Pott was unable to prepare it from lime and vitriolic acid, but always obtained merely selenite (calcium sulfate) (74). When Stahl leached with water a broken clay tube he had used for distilling spirit of vitriol (sulfuric acid), he obtained crystals of alum (74). Pott, too, prepared alum from clay and sulfuric acid (74). 

In 1768 A. S. Marggraf made the first chemical investigation of fluorite, distinguished it from heavy spar and selenitic spar (sulfates of barium and calcium), and showed that it is not a sulfate (77, 78). When he distilled pulverized fluorspar with sulfuric acid from a glass retort, the glass was badly attacked and even perforated. He noticed that an earth [silica] appeared in the receiver, and therefore concluded that the sulfuric acid had liberated a volatile earth from the fluorspar (77). 

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... 

Selenium and tellurium The elements are present as selenite and tellurite in dilute nitric acid solution. The mixture is spotted upon paper and dried thoroughly in the air. The solvent is dry n-butyl alcohol containing 4 per cent (v/v) of dry methanol. The atmosphere in the separation vessel is saturated with respect to the solvent vapour and the relative humidity is also maintained at 50 per cent by means of a saturated solution of calcium nitrate. The solvent is allowed to diffuse 8-10 cm down the strip (c. 2 hours). After evaporation of the solvent, the strip is sprayed with 0 5m tin(II) chloride in dilute hydrochloric acid. The tellurium is indicated by a black band (RF 0 1) and the selenium as an orange band (RF 0 5). It is possible to detect 1-5 pg of Se in the presence of 1 mg of Te by this method (see also Fig. VI.5g). 

Examine specimens of selenite, gypsum, and plaster of Paris. Describe them. Recall or repeat the experimental study of the solubility of calcium sulphate. Mix a little plaster of Paris with enough water on a block of wood to form a thin paste. Let it stand undisturbed for ten or fifteen minutes, and then examine. Describe the change. What is meant by the set of plaster of Paris ... 

Solution. Dissolve lOO gm. of the commercial salt in 1000 cc. of water. Let the solution stand a day, and then filter if not clear. Calcium Sulphate, CaS04. — Solid. For water of crystallization use crystallized gypsum (selenite). For other work use the powdered variety. 

Calcium sulfate dihydrate alabaster Cal-Tab-, Compactroh, Destab-, E516 gypsum light spar mineral white native calcium sulfate precipitated calcium sulfate satinite satin spar selenite terra alba USG Terra Alba. 

VOC/BLA] Vochten, R., Blaton, N., Peeters, O., Deliens, M., Piretite, Ca(U02)3(Se03)2(0H)4.4H20, a new calcium uranyl selenite from Shinkolobwe, Shaba, Zaire, Can. Mineral, 34, (1996), 1317-1322. Cited on page 636. 

Calcium sulphate CaS04.2H2 Bihydrate of Itme calcic sulphate gypsum plaster of Paris selenite sulphate of lime. 

Dihydrate, native calcium sulfate precipitated calcium sulfate gypsum alabaster selenite terra alba satinite mineral white satin spar light spar. Lumps or powder, d 2.32. It loses only part of its water at 100-150. Sol in water very slowly sol in glycerol. Practically insol in most organic sol -vents. 

Ca5Si6017 3H20 5-CALCIUM 6-SILICATE 3-HYDRATE 468 CdSe03 CADMIUM SELENITE 505... 

---