been reported to proceed rather faster than with the thoriurri tartrate. The alteration of the tartrate rotation in response to this direct attack on one of the centers of optical activity needs no comment. The quantitative relation with alkalinity suggests a quantitative correlation with completeness of the transformation to the ionized form or forms. Darmois and Heng3 presented evidence for a 1: 1 complex of tartrate and thorium in relatively acid solutions, but the lack of pH control greatly diminishes the significance of this evidence. Their data also show there must be another complex, probably with two tartrates per thorium, but again little more could be said. Bobtelsky (7) E.g , S. Kirschner, Abstracts of Papers, 129th Meeting, American Chemical Society, Dallas, Texas, April 8-13 (1956), p. 21-Q; M. E. Tsimbler, Sbornzk Statez Obshchei Kham., Akad. Nauk S.S.S.R., 1, 330 (1953) ( C . A, 49, 868d (1955)).
and Grauss have also published a paper on thoriumtartrate complexes, which is based on “heterometric” (nephelometric) titrations. We can say little about this work other than that the authors did not seem to be aware that thorium tends to hydrolyze in all but reasonably acid solutions, and proceeded to mix solutions of very different pH values to obtain their results. Further, we do not see horn the authors could have obtained the quantitative results reported, in view of the qualitative behavior we observed on adding thorium solutions to tartaric acid solutions and vice versa. At the least, their results must be highly dependent on the exact details of solution concentration, etc. , and the interpretation correspondingly uncertain. (8) M. Bobtelsky and B. Gratis, Bull. Res. Council Israel, 3, 83 (1953).
THE SORPTION OF WATER VAPOR O N DEHYDRATED GYPSUM BY R. I. RAZOUK, A. SH. SALEM AND R. SH. MIKHAIL Chemistry Department, Faculty of Science, Ain Shams University, Abbassia, Cairo, Egypt, Ll.4 R Received November 18, 1969
Sorption isotherms of water vapor on completely and partially dehydrated native and precipitated gypsum are similar. Quick uptake is noted a t very low vapor pressures followed by an almost horizontal plateau in the isotherm until saturation pressure, when the uptake becomes a function of time. Sorption-desorption along the plateau is reversible, but hysteresis becomes pronounced when desorption is carried out from sorption values a t saturation. The isotherm is then parallel to the sorption plateau but displaced to higher values depending on the time of exposure to saturation pressure. When dehydration of gypsum is conducted below 400°, the initial quick uptake of water corresponds to the formation of the hemihydrate. The dihydrate is formed after several days’ exposure of the hemihydrate to saturated water vapor. But exposure for several days to water vapor a t a pressure 3.5% short of saturation results in an uptake only slightly greater than corresponds to the formation of the hemihydrate, although raising the pressure to the saturation value induces further uptake to an amount exceeding that required to form the dihydrate. Dehydration above 500’ renders the anhydrite incapable of forming the dihydrate] although the hemihydrate may still be formed. Sorption isotherms on partially dehydrated gypsum show a linear relation between the amount of formed hemihydrate and the percentage of decomposition when dehydration is a t 150”. It is concluded that the transformation of the anhydrite into the hemihydrate in presence of water vapor is a quick process, whereas the transformation of the hemihydrate into the dihydrate is a slower process which takes place in presence of the saturated vapor of water, and which is more readily affected by the temperature of dehydration. Experiments on the rate of sorption of water vapor as well as infrared absorption spectra and X-ray analysis of various states of the system CaS04H?O confirm the above vie--s.
Introduction Extensive work has been done on the uptake of water vapor by dehydrated gypsum. Particular attention may be drawn to the work of Gaudefroy,’ Kishimoto,* Linck and J ~ n g Budnikov,4 ,~ Hammond and TVithrosv,5 Turtsev16Gregg and Willing,’ and Jury and Light.* But in spite of the immense literature on the subject, no comprehensive study of the isothermal uptake of water vapor by the anhydrite formed from gypsum by dehydration in vacZto has been undertaken. Moreover, the effect of partial dehydration on the isothermal uptake of mater vapor has not yet been investigated. The present work includes a study of the equilibrium uptake of water vapor on partially (1) C. C:audefroy, Compl. rend., 168, 2006 (1914). (2) K. Kishimoto, J . Japan Ceram. Assoc., 36’7, 201 (1922). . Chem., 13’7,407 (1924). (3) G. I i n c k and H. Jung, Z. a n o ~ g allgem. (4) P. P. Budnixov, Kolloid-Z., 46, 95 (1928). (5) W. A . Hammond and J. R. Withrow, I n d . Eng. Chem., 26, 633 (1933). (6) A. A. Turtsev, Bull. Akad. Sci.. URSS, Geol., 4, 180 (1939). (7) S. J. Gregg and E. G. J. Willing, J . Chem. Soc., 2916 (1951). ( 8 ) S. H. J u r y and W.Light, J I . , Ind. E n g . Chem., 44, 591 (1952).
and completely dehydrated gypsum, together with measurements of the rate of uptake at saturation vapor pressure of water. The infrared absorption spectra and X-ray diffraction patterns of different states of the calcium sulfate-mater system have also been determined in order to throw more light on the mechanism of the uptake. Experimental The uptake of water vapor was determined with the aid of a spring balance of the McBain-Bakr type,g the sensitivity of which was 0.045 mm./mg. The infrared absorption spectra were determined with the aid of a Perkin-Elmer Infracord Spectrophotometer Model 137, using the Nujol mull technique. The X-ray diffraction patterns were made in the Centre National de la Recherche Scientifique (Paris), using DebyeSchemer technique with monochromatic Cu KLYradiation and curved crystal monochromator. Crystalline gypsum (selenite) was kindly presented by Basic Dolomite Inc., Cleveland, Ohio. I t was transparent and very pure with lamellae-like structure. Precipitated gypsum was a pure Schering-Kahlbaum preparation. Both the native and precipitated forms contained the stoichio(9) J. W. McBain and A . &I. Bakr, J . A m . Chem. Sac., 48, 690 (1926).
Oct., 1960
SORPTION OF WATERVAPORON DEHYDRATED GYPSUM
1351
metric amoiint of water according to the formula CaS04. 2H20.
Results and Discussion I. Uptake of Water Vapor on Partially and Completely Dehydrated Gypsum. (i) Sorption-Desorption Isotherms on Completely Dehydrated Gypsum.--Naturally occurring gypsum (selenite), as well a:3 precipitated gypsum, was dehydrated in vacuo at different temperatures, and the sorption isotherms of water vapor on the products of dehydration were determined. Desorption isotherms 8:; well as resorption isotherms on the products obtainad by outgassing at room temperature also were determined. In all cases the sorpt ioa-desoi ption--resorption isotherms are similar. The results of a, typical complete set of isotherms on a specimen of calcium sulfate formed from precipitated gypsum by dehydration in vacuo at 150" are drawi graphically in Fig. 1. Water is taken up readily at the lowest pressures until the uptake reaches a value which corresponds very closely t o the stoichiometric amount required to form the hemihydrate. I n this region the isotherm is almost vertical. taking this amount of water, the isotherm turns abruptly and runs along an almost horizontal straight line, so that the amount of water taken increases only very slightly a+,the vapor pressure is raised until saturation is attained, when the uptake increases appreciably with time of exposure (curve I). When desorption iq carried out from a point on the plateau of the isotherm (say a, curve I), the deqorption curve coincides with the almost horizontal wrptioii limh of the isotherm, so that sorption and desorption are reversible in this region, but the amount ret,iined on evacuation at room temperature corresponds to the formation of the hemiilresis becomes more pronounced i is conducted from a point on the vertical limb of the isotherm which is obtained by exposing the material to saturation vapor pressure for variable intervals of time. The extent of hysteresi> depend. on the saturation uptake, being greater the higher is the uptake. Curves 2 , 3, 4,5 and (3 are the desorption isotherms obtained after exposing the sample to the saturated vapor of water for one day, two, four, seven and ten days, respectively. Curves 11, I11 and IV, on the other hand, are resorption isotherms obtained when resorptioii is conducted after completing the desorptioii isotherms 3, 4 and 5 , reepectively. The system behaves in a similar manner on desorption iollowing resorption. It is to be noted that the laqt de::orption isotherm 6 gives a retained d u e s o m c n h t higher than the value required for the formarion of the dihydrate. To test the effect of the time of contact of the sorbent with the saturated water vapor on the hystere-is a specimen of gypsum was dehydrated at 150" and the isotherm vias determined as usual up to saturation vapor pressure. A quick uptake was then effected tiy slightly cooling the container of the sorhent, wheii point b was obtained after one day's exposure, the amount taken up being very close to the amount taken up after five days' exposure iC I sntiirated vapor under ordinary condi-
tb
-& 150 v
rz:
100
1
50
7 0
0.2
0.4 0.6 0.8 1.0 P/Po. Fig. 1.-Sorption-desorption-resorption isotherms of v-ater vapor on calcium sulfate prepared 1)) the dehvdratiori of precipitated gypsum a t 150". Isotherms at 30": 0, sorption and resorption; 0 , desorption. Curve 1,sorption on original dehydrated gypsum; curvc I, dcxsorption from point a ; curves 2, 3, 4, 5 and 6, desorption isotherms obtained after exposing the system to the saturated vapor of water for one day, 2, 4, 7 and 10 days,respectively. curves 11, I11 and IV, resorption isotherms ohtailled ahen the process is conducted after completing the desorption 140therms 3, 4 and 5, retpcctively.
A k & 150
i
\
A
50
m
I 0
0.2
0.4
HI
0.6
0
-
-
:L&rzr2 0.8
1.0
P l Po.
Fig. %.--Effect of the temperature of dehydration of precipitated gypsum on the water isotherms: curve I, samples dehydrated at 100, 113, 1500 and 200"; curve 11, samples dehydrated a t 90 and 300 ; curve 111, sample dehydrated a t 400'; curve IT', sample dehydrated at 600".
tions. The desorption isotherm starting from this point b is shown in curve 7 of Fig. 1, and it indicates that mater \yhich has been taken rapidly is readily given off on lowering the vapor pressure. In this case, the amount of retained water is just above the value required for the formation of the hemihydrate and very close to the amount obtained after one day's exposure (curve 1, Fig. I). Similar results were obtained with native gypsum that had been dehydrated at the same temperature. The results obtained in the present investigation resemble those obtained by Gregg and Willing7 and Jury and Lights in so far as their isotherms are also square-shaped with the plateau corresponding to the formation of the hemihydrate, but in neither case were the measurements extended to saturation vapor pressure. Furthermore, their desorption isotherms which were carried out from points short of saturation lie always above the sorption isotherms. This may be accounted for by the different conditions of experimentation, especially
R. I. RAZOUK, A. SH.SALEM AND 1%.SH. MIKHAIL
1352
i--
30 47% ~~~
0
0.2
0.4
0.6
0.8
1 .o
PlPO. Fig. 3.--Sorption of water vapor on partially dehydrated gypsum prepared at 150"; isotherms a t 35'.
-i-
400 I
v-
__1
I
l l
150 100
50
40 60 80 IO0 Time in hours. Fig. 4.--Effect of the temperature of dehydration of precipitated gypsum on the rate of hydration and rehydration of calcium sulfate when exposed to the saturated water vapor a t 35": curves I, 11, 111 and IV, rate of hydration for samples dtchydrated a t 150, 400, 500 and 800°,respectively. Curves 1, 2, 3 and 4, rate of rehydration for samples originally dehydrated a t 150, 400, 500 and SOO", respectively, arid then 1iydratc.d and outgassed a t room temperature.
0
20
with respect to the method of dehydration, being in vacuo in the present investigation, and in presence of air in the experiments of earlier authors. (ii) Effect of Temperature of Dehydration.The effect of the temperature of dehydration of native and precipitated gypsum on the water vapor isotherms has been studied also, and the results of experiments on the latter are shown in Fig. 2 . When gypsum is dehydrated a t temperatures lying between 100 and 200", it gives rise to the same isotherms (curve I), while samples dehydrated a t 90 and at 300" give a slightly lower isotherm (curve 11). But in all cases, the water uptake at the lowest pressures corresponds to the formation of the hemihvdrate. However. when
Yol. 64
ucts obtained by dehydration at 400 and 600", respect ivcly). (iii) Effect of Partial Dehydration.--The effect of partial dehydration of gypsum on the sorption isotherm of water vapor was studied also in case of natural and precipitated forms. The specimen was dehydrated until a certain fraction was decomposed and the isotherm determiiied on the product. The results of a typical set of experiments is shown in Fig. 3a, which gives the sorption isotherms of water vapor on partially dehydrated precipitated gypsum having lost varying amounts of its water of crystallization. Figure 3b represents the relation between the water uptake at p / p ~= 0 and a t p!p, = 1 as a function of percentage decomposition. Assuming that the amount of water taken up almost instantaneously at p i p o = 0, and which is retained on outgassing at room temperature, is an indication of the hemihydrate formation, it can be safely inferred from the direct proportionality between the water uptake and the percentage of decomposition that dehydration in 11acuo leads directly to the formation of the anhydrite, and does not proceed in two steps, namely, prior formatio? of the hemihydrate and then the anhydrite. This is in agreement with the idea which mas originally presented by van? Hoff, Hinrichsen and TT7egert,'" and which has been supported by the work of several other authors," that the dihydrate passes directly to the soluble anhydrite at the h v e r temperatures of dehydration. Experiments carried out in this Laboratory on the rate of dehydration of native and precipitated gypsum also confirm this view.I2 11. Rate of Hydration of Dehydrated Gypsum from the Saturated Vapor Phase.-Throughout the course of the present investigation. it has been noticed that the time factor plays an important role in the process of water uptake at saturation vapor pressure, and hence a study of this effect undoubtedly would throw some light on the mechanism of the process. Experiment L: were, therefore, conducted in order to measure the rate of water uptake when calcium sulfate. formed from precipitated gvpsum by dehydration in L'acm at 150, 400, 500 and 800", was r q o s e d to the saturated mpor of water at 33". The uptake was followed even until it exceeded the amouiit required for the stoichiometric formation cif the dihydrate, or else reached a constant limiting value. The product then was oiitgaesed cautiously at room temperature for several hours, the rptained amoiint determined, and finally the course of rehydr nt'ion was followed again after exposing tiic system to saturation vapor pressure. The ciirT-eP of the rate of uptake are parabolic in shape but tcnd to limiting values, and a rise of the dehydration temperature leads to a lowering of the rate of nptake. The results of a typical series of experiments are shown in Fig. 4. Thus, when gypsum is dehydrated, for instance, at 150", and then exnosed to saturated vapor, it takes up water, in the first
Oct., 1960
SORPTION OF WATER VAPORON DEHYDRATED GYPSUM
few minutes, more than is required to form the hemihydrate (curve I). The rate of uptake then decreases, but remains considerable, until the amount corresponding to the formation of the dihydrate is surpassed, then the curve tends to a limiting vdue. The amount of retained water obtained after outgassing a t room temperature for 5 hours IS slightly above that required to form the dihydrate. The rehydration curve (curve I) is also parabolic and meets the hydration curve at the limiting water uDtake. The sample dchydrated a t 400" reveals the same behavior (Fig. 4, curves I1 and 2), but the rate of hydration and the amount of water taken up at saturation are slightly less. However, the retained value is less than corresponds to the dihydrate, 2nd the hydration and rehydration curves meet also at a value higher than what IS required to form the dihydrate. When dehydrztion is conducted at 500°, however, the hehavior is different (Fig. 4, curve 111). Thus after a region of quick uptake, lower in itself than the corresponding values for the products of dehydration at 150 and 400°, but still exceeding the amount required to form the hemihydrate, the curve tends to reach slowly a limiting value whkh is Far 1eqs than what is required to form the dlhydra te. The amount of retained water 15 again close to that which corresponds to the formation of the heniihydrate. The rehydration curve (Fig;. 3 , ( w r w 3) IS also parabolic but it tends to hwome parallel to the time axis. similar belm\-ior is observed with gypsum dehydrated at 800" (Fig. 4, curve IT'), but the hydration r u r w is still lower, and the uptake which rnrreaponds to 1 he formation of the hemihydrate IS attailled only after 12 hours. About half molerille of \rater pel molecule of sulfate also is retained 111 this c < w . The rehydration curve (Fig. 4, cur\e 4) I F . as iiyual, above that of hydration and almost pni:iIlel to it, as with the prodiict prepared 'it 500". 111. Infrared Absorption Spectra.-Infrared absorption spcctr:i of gypsum, anhydrous calcium sulfate ant1 sorne hydration product5 were determined and the results obtained are fouiid to be similar in man!' respects to those o€ Jiiller and Wilkms l 3 The abwrption spectra of typical systems are qhor;ri in Fig. 5. The important bands present in the spectrim of the dihydrate are those absorbing at 9.85 and 8 85 1.1 (characteristic for the sulfate radicul,14together with those absorbing at G 13,
