Dehydration curves

X-ray diffraction of 114 analcite at 25 °C revealed two weak reflections at d = 3.80 (at 23.4° 20) and 3.24 A (at 27.5 26) indexed as (320) and (411), respectively. These forbidden, weak reflections disappear from the pattern on heating the zeolite above 200° C. Their disappearance correlated closely with the pronounced second-order break in the dehydration curve at 200° C and suggests a randomization of the partially ordered distribution of 16Na+ ions over the 24Na+ sites (Vs, 0, x/4) in the unit cell. At the decomposition temperature, 916° C, the apparent cell edge is 13.69 A. 

Caex 114 Analcite. The dehydration curves obtained with Ca2+-containing samples, Caex82114 analcite, and natural wairakite (not shown) were displaced toward higher temperature in accordance with the higher heat of hydration of Ca2+ (Table I). Complete dehydration was obtained only above 500° C. Total H20 contents are similar to or slightly less than the parent analcite. 

The trihydrate, FeS04.8H20,9 and dihydrate, FeS04.2H20,10 have also been obtained—the former, by solution of the heptahydrate in concentrated hydrochloric acid the latter by separation from a concentrated solution of ferrous sulphate on addition of sulphuric acid in small quantities at a time. The existence of this latter hydrate is clearly indicated by a break in the time-dehydration curve at 100° C.11... 

Summary.— With regard to double salts we have learned that their formation from and their decomposition into the single salts is connected with a definite temperature, the transition temperature. At this transition temperature two vapour pressure curves cut, viz. a curve of dehydration of a mixture of the single salts and the solubility curve of the double salt or the dehydration curve of the double salt and the solubility curve of the mixed single salts. The solubility curves, also, of these two systems intersect at the transition point, but although the formation of the double salt commences at the transition point, complete stability in contact with water may not be attained till some temperature above (or below) that point. Only when ike temperature is beyond the transition interval will a double salt dissolve in water without decomposition (e.g. the alums). 

Prismatic, tetragonal crystals. The first mole of water of crystallization apparently Is more readily given off than the others. The dehydration curve shows that, at p = 10 mm. HaO, Sr(OH)a is stable from 100 to 450°C. M.p. 375 C. 

In some cases the fact that the hydroxometalates are chemical complexes is indicated by the color of the salts and of their solutions. The proof of structure is based on their thermal dehydration curve, their ability to form mixed halo-hydroxo salts, data on isomorphic relations, and some powder pattern studies. 

Nutting, P.G. 1943. Soil standard thermal dehydration curves of minerals. United States Geological Survey, Professional Paper 197-E, Washington, DC, pp. 197-217. 

Figure 6.13(b) shows the dehydration curves. The coverage 0 of a polymer chain by H-bonded water molecules is plotted against the temperature. The cooperative parameter... 

Thermograms of COD and COT differ from those of COM and from one another both qualitatively and quantitatively in the part corresponding to dehydration. COT loses water in two distinct steps, i.e., between 75°C and 100°C (about two water molecules) and between 100°C and 200°C (approximately one water molecule), whereas the dehydration of COD starts at 25-30°C and proceeds more gradually [44], A comparison of the qualitative differences of various dehydration curves gives information on differences in the modes of water incorporation resulting from different modes of crystallization. 

An example is given in Fig. 4, which shows partial thermogravimetric curves (dehydration only) obtained from calcium oxalate precipitates prepared at different initial reactant concentrations. Curve 1 represents dehydration curves typically obtained from samples of COM of different morphologies formed by heterogeneous nucleation (including compact crystals and dendrites) curve 2 is... 

At low and medium supersaturations, hydrophilic cations form different crystal hydrates by heterogeneous nucleation and subsequent crystal growth and phase transformation. Dehydration curves give information on the modes of water incorporation resulting from different modes of crystallization. A useful application of thermal analysis is the analytical approach by determining the mass loss due to dehydration, it was possible to quantitatively determine the proportion of different calcium oxalate hydrates in mixtures, which have been qualitatively analyzed by other techniques (X-ray powder diffraction, IR spectroscopy, etc.). The method yielded excellent results in studies of the kinetics of phase transformation and has been successfully used to demonstrate the potential of surfactant micelles to control the nature of the crystallizing phase. 

What has been said will be sufficient to make the dehydration curves or isotherms clear. Let us consider gel in the curves OOi Oo O2 0. We may imagine the gel in point 0 to contain numberless tiny capillaries filled with liquid. The surface of the liquid in each capillary is a half sphere with the concave surface toward the gas, and a very small radius of curvature because of the miniature dimensions of the capillaries. According to (1) such an arrangement causes an enormous pull on the liquid in the direction of the upright arrows in Pig. 24 and a corresponding pressure in the opposite direction on the walls. The latter would press the amicrons in the walls close together and cause a contraction of the gel, while the former would favor the escape of tipy air bubbles from the interior of the gel. These bubbles would therefore cause the turbidity in the transition point. 

A low red heat for a short time left the gel in such a state that the dehydration curve was very similar to that of the unheated gel except that the interval of constant pressure OOi is very much shorter. Longer heating had the same effect as may be seen from Fig. 28, taken from the article by van Bemmelen.t Strong ignition robs the gel of its property of taking up water to any large extent. 

Figure 19.4 Dehydration curves representing dehydration rates (DR) dtuing a period of 70 minutes for samples of three materials sample A (low EWC), sample B (medium EWC) and sample C (high EWC).

The dehydration curve can be divided in three steps that successively correspond to the evolution of 1) the zeolitic and adsorbed water on external surface (T < 7S°C), 2) coordination water linked at the edge magnesium atoms inside of the channels (in two times, domains 75-150°C and 150-370°C, two molecules each and weight losses 10.48% of the final mass), 3) structural (2.61 %), one molecule due to two hydroxyls from the octahedral layer of the talc ribbon) and decomposition of calcite. According to ref. 3, the structure folds when approximately half of the coordination water is removed, i.e. here under 2 Pa residual pressure between 100 and 130 C. 

Several dehydration curves for vermiculites have been published (Walker [1951], Lopez-Gonzalez and Cano-Ruiz [1959], Bradley and Serratosa [I960]), and a number of differential thermal curves (Barshad [1948], Walker and Cole [1957], Ernst et al. [1958]). A detailed study of the low-temperature peak system on the differential thermal curve of Mg-vermiculite was made by Walker and Cole [1957], who related the observed endothermic... 

Dehydration curves of some allophanes in New Zealand soils are shown in Figure 10. Three of these curves are obtained from clays consisting solely of allophane and illustrate the features already described. The fourth curve, obtained from a strongly weathered soil from basalt, contains kaolin and gibbsite. At the decomposition temperatures of the latter two minerals, a rapid loss in weight takes place. 

Figure 23. Dehydration curves for illites from A—Roxburgh, Scotland B—Fithian, Illinois C— Ballater, Scotland (Mackenzie [1957d]).

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