Oct., 196%
FREEZING POIXTSOLUBILITY CURVEOF AQUEOUSSODIUM HYDROXIDE
2051
THE FREEZIXG POIST-SOLUBILITY CURVE OF AQUEOUS SODIUM HYDROXIDE I N THE REGIOX NEAR THE ANHYDROUS-RIONOHYDRATE EUTECTIC1 BY G. E. BRODALE AND W. F. GIAUQUE Low Temperature Laboratory, Deparlments of Chemistry and Chemical Engineering, Universily of Calijornza, Berkeley, Cal. Recereed April 87,196'8
The composition and melting point of the SaOH-NaOH.Hz0 eutectic have been found to be 73.1 wt. % NaOH and 62.61", respectively. This melting point is in good agreement with the more accurate value of 62.63' obtained by Murch and Giauque. No evidence of a hydrate intermediate between NaOH and NaOHsHZ0 was found. The measurements cover the range 69.8 to 75.4 wt. % and 62.61 to 100.6O.
I n coniiection with a recent investigation of the thermodynamic properties of XaOH and NaOH H 2 0 in this Laboratory, it became important to have a more accurate det,erminat'ionof the eutectic composit'ion of these substances, Experimental
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Preparat:ion of Sodium Hydroxide.-Sodium hydroxide was prepared in a nitrogen-filled glove box in a manner similar to that described by Murch and Giauque,z except that cold nitrogen from the liquid nitrogen supply was passed over the met,allic sodium, and water, which had been boiled to remove COa, was dropped slowly onto the metal. 'The smaller heat of reaction with liquid, as compared with gaseous water, and the cooling effect of the nitrogen moderated the reaction (which stjillrequires a great deal of caut'ion). The maximum concentration of SaOH used, 77%, was obtained by evaporation in the nitrogen-filled glove box by means of a heat lamp. Lower concentrations were then obtained by dilution. It had been shown by the previous work2 that this method of preparation produces a very small concentration of carbonate (-0.01%). In the present work 5-10-g. samples were taken with a pipet constructed with a removable tip containing a 'rcoarBe" fritted glasis filter. A "fine" fritted glasa filter was tried but proved to be too slow. The sample weights were corrected for buoyancy. The temperatures were measured with a strain free platinum resistance thermometer (L. and N. no. 718169) calibrated a t the National Bureau of Standards. It was checked at the ice point and a correction of -,O.O5O was applied. The platinum thermometer was encased in a Pyrex glass tube. Since glass can be attacked by sodium hydroxide, the tube wa8 checked for loss in weight and the effect of such impurity was shown to be negligible. The experiment was carried out mTith samples of about 350 g. in a platinum bottle. The platinum bottle contained a Teflon coated spin bar. It was supported by a '/8 in. aluminum plate and about 0.5 in. of spun glass insulation b'etween it and the magnetic stirrer drive. It was enclose'd in a 10 in. length of 6 in. aluminum pipe, The pipe was wound with an electric heater and wrapped with 0.5 in. of spun glass. The top was similarly enclosed. The pipe temperature was controlled to within 0.5". The whole assembly was in 1atm. of nitrogen.
Results The Freezing Point-Solubility Curve.-Most of the early work on the phase relationships in aqueous sodium hydroxide was done b:y Pickering.3 He prepared various concentrations, solidified the material, and t'hen, on melting, n.oted the temperature a t wh.ich the last evidence of solid could be detected. He comments on the fact that the monohydrate "crystallizes very easily in large, pointed, (1) This work was supported in part by the National Science Foundation. (2) I,. E. Afurch and W. F. Giauque, J . Phys. Chem., 66, 2052 (1962). (3) S. U. Pickering.
J. Chem. Soe., 63,890
(1893).
and semi-transparent crystals." Apparently, the monohydrate was easily observed and although there is a considerable scatter in his points in the monohydrate melting region, a curve through his observations is in reasonably good agreement with the present results. However, we were very suspicious concerning the accuracy with which one could observe the disappearance of the last crystals of anhydrous NaOH from the very concentrated aqueous solution. The tendency would be to assign too low a temperature to a given concentration. We have found that Pickering's observed temperatures along the NaOH curve are about 20" too low. Pickering did not measure either the composition or temperature of the eutectic between NaOH and IfaOH.HzO and a curve through his observations on the anhydrous phase would give a eutectic mixture too rich in NaOH at too low a temperature. The results which cover the range from 62.61 to 100.6" and concentrations from 69.8 to 75.4% by weight are given in Table I. TABLE I FREEZING POINTSOLUBILITY CURVEFOR AQUEOUSSODIUM HYDROXIDE Wt. % NaOH
T, ' C .
Phase
68.95" Monohydrate 65.10" NaOH.H20 69.8 65.1 NaOH.HZ0 71.3 64.3 NaOHaH20 73.1 62.61 NaOH.Hz0-NaOH 73 2 66.4 NaOH 73.5 72.1 NaOH 74.5 85.2 NaOH 75.4 100.6 NaOH a Taken from Murch and Giauque, ref. 2.
By comparison with the results of Murch and Giauque,2 me suspect that the measurements a t 65.1 and 64.3" are about 0.1" too high. The stability of the eutectic temperature made it possible to observe it with greater accuracy. The results indicate that no hydrate intermediate between KaOH and KaOH.€120, such as has been suggested by de Forcrand4 and Mauret,6 appeared during these measurements. This observation is in agreement with the experiments and conclusion of Murch and Giauque. (4) R. de Forcrand, Compt. rend., 1.93, 223 (1901). ( 5 ) M. Mauret, ibid., 240, 2151 (1956).
