Oct., 1956
THERMAL DISSOCIATION PRIGSSURII OF CALCIUM CARBOXATE
on phase studies and on high-pressure compressibilities, range from 1.3 to about 2.0 kcal. mole-1, with 1.4619 and 1 . 3 2 0 being given in recent cornpilations. Feick" has calculated a value of 45.6 cal. mole-' deg.-l for the specific heat of crystal modification (19) K. K. Kelley, Bull. U.S. Bur. Mined, 888, 82 (1936). (20) Nat. Bur. Standards. (U.S.), Cir., 600,564 (1952).
i3Si
I (442.8-398°K.) of ammonium nitrate, using his
measured value of the enthalpy of the liquid, and other necessary data from the literature. The present value of 1.53 kcal. mole-' for the heat of fusion, when used in these calculations, gives a specific heat of 40.6 mole-l deg.-l for c,I, The value for the absolute entropy of ammonium nitrate calculated by Feick is not, however, affected appreciably.
THERMAL DISSOCIATION PRESSURE OF CALCIUM CARBONATE BY K. J. HILL.AND E. R. S. WINTER John & E . Sturge Limited, Birmingham, Erigland Received February $8, 1068
The thermal dissociation pressure of calcium carbonate has been measured over the temperature range 449-904'. results fit the equation log,, ?I(-.) = -8792.3/T 10.4022, and are compared with those of other authors.
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The thermal dissociation of calcium carbonate has long been the subject of investigation. Most workers have confined their study of the variation of the thermal dissociation pressure with temperature to the range of temperature in which the pressure can be measured by a simple mercury manometer. An investigation of the behavior a t higher temperatures and pressures, up to and beyond the eutectic point, was reported three decades ago by Smyth and Adams,' but no dissociation pressures have been reported at temperatures below about 600" when the pressure is of the order of 1 mm. It has been the object in the present work to extend the range to as low temperatures as possible. Experimental The calcium carbonate contained in a platinum boat inside a silica tube was heated in a tubular furnace 1.5 inches in diameter and 14 inches long, which had a central constanttemperature zone of some three inches, in which was placed the sample. During the major part of the work the furnace tube was wound with three separate windings insulated from each other with asbestos paper; first a platinum resistance thermometer was wound non-inductively onto the tube using 0.1 mm. platinum wire, the other two windings were of 25 S.W.G. nichrome wire. The resistances of these windings at room temperature were about 18, 60 and 60 ohms, respective1 . In operation an a x . voltage of up to 250 volts was appEed to the inner of the two nichrome windings from a Variac variable transformer. The voltage was adjusted to supply the majority of the heat required to maintain the furnace a t the desired temperature. The resistance thermometer was then used in an a.c. Wheatstone bridge circuit to control the supply to the second nichrome winding by means of an electronic controller of the on-off type. This method of control was used a t temperatures up to just over 800' and it was possible to maintain the temperature constant to within f0.5' for periods of up to three weeks or more. At higher temperatures the tube was run with a single winding, the energy supplied being controlled by a Sunvic Simmerstat energy controller, which maintained the selected temperature constant to within f1'over periods of up to 12 hours. The temperature of the furnace was measured by means of a platinum/platinum-13% rhodium thermocouple using a commercial pyrometer, which could be read to 1'. The thermocouple used was standardized at the sulfur point a t intervals throughout the work. A small platinum resistance thermometer was used to check the constancy of the temperature of the furnace over long periods. This was placed (1) F. H. Smyth and L. H. Adams, J . Am. Cham. Sac., 4S, 1167 (1928).
The
with its sensitive tip just outside the silica tube alongside the platinum boat and beside the thermocouple; i t was used in a Wheatstone bridge network supplied by a single accumulator. The bridge was balanced at the approximate temperature of the furnace and the out of balance current was then supplied to a recording galvanometer of high resistance and sensitivity. I n this way a continuous record of the variation of the temperature was obtained and drifts and temperature changes of the order of 0.25' or less could be detected. The silica tube containing the platinum boat and sample was connected by a graded seal to the remainder of the vacuum system which was of Pyrex glass and contained no stopcocks or ground joints, being isolated from the pumps, etc., by a mercury cut-off. Thermal dissociation pressures over 1 mm. were measured by a mercury U-manometer with arms of about 1 cm. diameter, one of which was evacuated continuously; a cathetometer was used for pressures below -10 cm. Pressures below 3 mm. and above 0.04 mm. were measured by a McLeod gage, and a more sensitive McLeod gage was employed for pressures below 0.04 mm. Satisfactory cross-calibration was achieved in the pressure regions where the gage and the manometer overlapped. A thermostated zirani gage protected from mercury vapor by a trap at -78 , and which had been calibrated against a McLeod gage using carbon dioxide, was also used to check pressures where the ranges of the two McLeod gages met. The calcium carbonate used throughout these experiments was Grade 80 Pure Precipitated Calcium Carbonate (manufactured by John & E. Sturge Ltd.). It consisted of irregular calcitic rhombs about 40 p in diameter and had the analysis Chloride (Cl)
