Bibliographic Information

"High-Temperature Heat Contents of V₂O₃, V₂O₄, and V₂O₅," O.A. Cook, Journal of the American Chemical Society, Vol. 69, pp. 331-333 (1947), published by American Chemical Society.

General Materials Processing Notes

"The original source material was commercial ammonium vanadate (96.7% pure). This material was purified by the following procedure: Concentrated hydrochloric acid was saturated with ammonium vanadate at about 100 °C and the resulting solution was diluted 3:1 with distilled water and filtered. The filtrate was heated to boiling and oxidized with ammonium persulfate in the presence of a small amount of nitric acid. Reprecipitation of ammonium vanadate was obtained on neutralizing with ammonium hydroxide in the presence of an excess of ammonium chloride. The precipitate was filtered, washed repeatedly with 1:40 ammonium hydroxide and once with distilled water, and finally dried twenty-four hours at 75 °C - 80 °C. Analysis gave 43.55% vanadium, which is the theoretical value. Vanadium pentoxide was prepared from the purified ammonium vanadate by heating in a platinum vessel in a stream of pure oxygen at 440 °C - 460 °C for seven days. Analysis for vanadium gave 55.96%, as compared with the theoretical 56.02%. To prepare vanadium trioxide, pure vanadium pentoxide was heated in a silica flask at 800 °C in a stream of pure hydrogen until no further water vapor was evolved. Analysis of the product gave 67.89% vanadium, as compared with the theoretical 67.98%. Vanadium tetroxide was prepared from pure vanadium trioxide by controlled oxidation with air, in a platinum vessel, starting at 300 °C. At this temperature combustion became self-supporting and additional heat was not required. The blue-black product was removed, placed in a silica flask, evacuated at room temperature, and given a prolonged heat treatment at temperatures below 600 °C to assure uniformity of composition. Analysis gave 61.45% vanadium, as compared with the theoretical 61.42%. Analyses of ammonium vanadate and vanadium pentoxide were made by dissolving samples in 10% sulfuric acid, reducing with sulfur dioxide, expelling the excess sulfur dioxide with carbon dioxide, and titrating against potassium permanganate. Analyses of vanadium trioxide and tetroxide were made similarly except that fusion with sodium bisulfate was required before dissolving in 10% sulfuric acid."

Measurement Methods

The authors cite K.K. Kelley et al., Bureau of Mines Technical Paper 686, 24 (1946) and summarize the procedure as follows. "Frequent calibrations of the furnace thermocouple were made at the gold and palladium points... Sample weights were corrected to vacuum by means of the following densities: 4.87 g/cm³ for V₂O₃; 4.40 g/cm³ for V₂O₄ and 3.36 g/cm³ for V₂O₅. During the measurements the materials were contained in platinum-rhodium alloy capsules. These capsules were filled through small necks that subsequently were pinched shut and sealed by platinum welding. The heat contents of the empty capsules had been determined separately and only a small correction was required for platinum used in sealing. ... the equations were derived to represent the molal heat content results. ... The corresponding heat capacity equations (were) obtained by differentiation..." Values in original units [cal/mol·K] were converted to SI.

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