L. L. Simmons, L. F. Lowden, and T. C. Ehlert
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A Mass Spectrometric Study of K2C03and K 2 0 L. L. Simmons, L. F. Lowden, and 1. C. Ehlert" Chemistry Department, Marquette University, Milwaukee, Wisconsin 53233 (Received September 1, 1976) Publication costs assisted by the International Minerals and Chemical Corporation
Mass spectrometric analyses of the vapors over K2CO3 between 1037 and 1184 K show that the sublimation pressure is described by In Pat, = [(-3.67 f 0.09 X 104)/Z'l+ 15.65 f 0.9. Decomposition to K20(c)and COz(g) is more important than sublimation and obeys the equation In P,,,(CO,) = [(-3.045 f 0.1 X 104)/q + 14.69 f 0.9. Sublimation of K20(c) obeys the equation In P,,,(K,O) = [(-3.817 f 0.08 X 104)/Z'l+ 17.28 f 0.7. The equilibrium constant for K2O(g) * 2K(g) + '/202(g) obeys the equation In K = [(-3.86 f 0.11 X 104)/T]+ 17.9 f 1.0. Derived AHl1o~~ values are AHf(K2C03,g)= -234, AHsubl(K2C03)= 73.0 f 2, AHf(K20,g)= -76.8 f 2, AH,tOm(K2O,g) = 137 f 2, ~ s u b l ( K 2 0=)75.8 f 1.4, AHf(Kz0,c)= -153 f 4 kcal mol-'. It appears KzC03 is not detectably soluble in K20.
Introduction Although potassium compounds generally constitute less than 1% of the ore, coke, limestone, and sinter materials used in blast furnaces, it has been that potassium in several forms reaches a much higher steady state concentration in certain regions of an operating blast furnace. Predictions of the extremely complex processes present and the mechanisms by which potassium is transported in these furnaces are hampered by lack of reliable thermodynamic data for many of these simple compounds. The high temperature behavior of K2C03,for example, which can exist in blast furnaces under certain conditions and is an important raw material in glass making, is still inadequately under~tood.~,~ It is generally recognized that K&O3 decomposes at elevated temperatures, and although certainly C02is one of the products, previous investigators5-' were unable to tell if K20(g)or K(g) and 02(g) or all three species are also formed. It appeared, then, that knowledge of the behavior of K20was essential to interpreting the available data on K2C03. Since K 2 0 is difficult to prepare and, because of its hygroscopicity, even more difficult to load into an effusion cell without exposure to H 2 0 or C02, we attempted to prepare it in situ in our apparatus by thermally decomposing K2C03. Method The apparatus used was a near duplicate of that described previously' and consisted of a quadrupole mass spectrometer used to identify and monitor the constituents of a molecular beam from a Knudsen effusion cell. The only important differences between the present apparatus and that described previously were the use of Pt-Pt, 13% Rh thermocouples for cell temperature measurements and a modification to the analyzer power supply to reduce the mass dependence of the ion transmission efficiency. Certified, ACS grade, anhydrous K2C03 was used without further purification. Since our sensitivity calibration is based on sample weight loss, impurities are tolerable if they do not contribute to the weight loss or alter the activity of the K2CO3 Neither problem appeared in these experiments. The effusion cell, made of 0.005-cm thick platinum foil, was essentially an oblate sphere with a 1cm long diameter and 0.8 cm short diameter. A circular cm2 in area and 0.96 Clausing effusion orifice, 3.7 X f a c t ~ rwas , ~ located on the short axis. The sample area was over a hundred times greater than the orifice area to enhance achievement of equilibrium. The cell was secured The Journal of Physical Chemistry, Vol. 81, No. 8 , 1977
in the center of a massive, cylindrical tantalum jacket which opened to a diameter of 1cm to permit unimpeded escape of the effusing vapors. The jacket and cell were placed in the beam source furnace so the beam was sampled normal to the orifice plane, a geometry which minimizes discrimination in sampling." The power applied to two separate filaments was adjusted until the cell top and base temperatures differed by less than 3 K. All four thermocouples were calibrated at the melting point of 99.999% A1 in the Pt effusion cell. A thermocouple welded to the bottom of the Pt cell indicated 943 K vs. the accepted value 934 K (IPTS-68) with
