Calcium selenates

The selected solubility will be used for an estimate of the solubility product of calcium selenate dihydrate  

The mean activity coefficients of the sulphates of Cd, Cu, Mg, Mn, Ni, and Zn are quite similar [50HAR/OWE]. The variation at a concentration of 0.5 m is less than 10%. Ojkova and Staneva [890JK/STA] made isopiestic measurement on aqueous solutions of the selenates of Co, Cu, Mg, Ni, and Zn. The mean activity coefficients obtained from these measurements also show a moderate variation between the salts. The value of for MgSe04, = 0.12, will be used to extrapolate the conditional solubility product to / = 0. The selected result is  

Parker, Tice, and Thomason [97PAR/T1C] measured the Ca activity in solutions containing total Ca concentrations in the range 20 x 10 to 200 x 10 M and the selenate concentration 0.004, 0.01, or 0.03 M with a carefully calibrated ion-selective Ca electrode. Activity coefficient corrections were made by Davies equation. The equilibrium constant of the reaction  

This value is selected and the uncertainty has been estimated by the review. The quality of the work is supported by the parallel determination of the value of the constant for the sulphate complex, which agreed with literature data. 

The standard Gibbs energy of formation of calcium selenate dihydrate is calculated from the Gibbs energy of Reaction (V.178) and the selected values of AfG° for the ions and water to be  

---

The normal selenates of potassium, sodium, calcium and magnesium are very similar in properties to the corresponding sulphates, except that the potassium salt is much more soluble in water than potassium sulphate.3 Sodium selenate gives a decahydrate which effloresces, and the transition-point between this and the anhydrous salt is 31-8° C., above which point the solubility falls as the temperature rises. Calcium selenate gives a hemihydrate resembling plaster of Paris, and also a hydrate of composition CaSe04.l-5H2O.4... 

Write formulas for (a) magnesium arsenate, (b) calcium selenate, and (c) acetic acid. 

BeCI2 beryllium chloride 7787-47-5 25.00 1.9000 1 739 CaH406Se calcium selenate dihydrate 7790-74-1 25.00 2.7500 1... 

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. 

SEL/SHN3] Selivanova, N. M., Shneider, V. A., Physico-chemical properties of selenates V. Heat of formation of calcium selenate from the elements, Izv. Vyssh. Uchebn. Zaved., Khim. Khim. Tekhnol, 2, (1959), 475-479, in Russian. Cited on pages 402,453. 

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. 

---