Mass dehydration mechanisms

Crystalline CaHPO 2H20 loses both water molecules in a single step at moderately elevated temperature or upon storage to yield the anhydrous salt. The presence of free moisture accelerates this dehydration, which results in anhydrous dicalcium phosphate, often as a hard mass. Addition of a few percent of tetrasodium pyrophosphate or trimagnesium phosphate, Mg2(P0 2> stabilizes the dihydrate. The mechanism, however, is not well understood. Nonetheless, these materials are used widely to stabilize CaHPO 2H20, particulady for toothpaste appHcations. 

Such a classification of technetium cluster compounds, in our opinion, reflects the relationship between the thermal stability and structure of the clusters quite well. Moreover, on the basis of this classification it is easier to follow the mechanism of the main thermochemical transitions of technetium clusters, such as (1) dehydration (2) disproportionation and related processes occurring without changes or with only small changes in mass (3) one-stage processes of thermolysis. We shall now consider these main mechanisms of the thermochemical reactions of technetium clusters in greater detail. 

Cellulose pyrolysis kinetics, as measured by isothermal TGA mass loss, were statistically best fit using 1st- or 2nd-order for the untreated (control) samples and 2nd-order for the cellulose samples treated with three additives. Activation parameters obtained from the TGA data of the untreated samples suggest that the reaction mechanism proceeded through an ordered transition state. Sample crystallinity affected the rate constants, activation parameters, and char yields of the untreated cellulose samples. Various additives had different effects on the mass loss. For example, phosphoric acid and aluminum chloride probably increased the rate of dehydration, while boric acid may have inhibited levoglucosan... 

Pt(l 11), and 0.5 V on Pt(100), C02 is detected using IRAS.113 On polycrystalline Pt, C02 is detected using electrochemical mass spectroscopy beginning at ca. 0.4 V. These data support the mechanism below, in which methanol is completely dehydrated to form COad, which then reacts with H20. 

Mitrovic and Knezic (1979) also prepared ultrafiltration and reverse osmosis membranes by this technique. Their membranes were etched in 5% oxalic acid. The membranes had pores of the order of 100 nm, but only about 1.5 nm in the residual barrier layer (layer AB in Figure 2.15). The pores in the barrier layer were unstable in water and the permeability decreased during the experiments. Complete dehydration of alumina or phase transformation to a-alumina was necessary to stabilize the pore structure. The resulting membranes were found unsuitable for reverse osmosis but suitable for ultrafiltration after removing the barrier layer. Beside reverse osmosis and ultrafiltration measurements, some gas permeability data have also been reported on this type of membranes (Itaya et al. 1984). The water flux through a 50/im thick membrane is about 0.2mL/cm -h with a N2 flow about 6cmVcm -min-bar. The gas transport through the membrane was due to Knudsen diffusion mechanism, which is inversely proportional to the square root of molecular mass. 

During gas-liquid chromatographic-mass spectrometric analysis of the acid-catalyzed dehydration reaction of D-mannitol, 1,4 3,6-dianhydro-2-chloro-2-deoxy-D-mannitol was found among the reaction products.35 Contrary to the postulated Sn2 mechanism, according to which 2,5-di-endo oriented leaving-groups are substituted by different nucleophiles, resulting... 

Of particular interest in the present study is the apparent existence of an activated sorption mechanism for nitrogen in partially hydrated zeolite NaA, a further study of which is planned. The less definitely established irreversibly sorbed nitrogen on dehydrated zeolites NaA and NaX, but not on CaA and NaY, is in need of confirmation, and of further characterization. A practical utility of the results is the quantifying of nitrogen uptake from air, which persists even with appreciable sorbed water. This amount can be approximated from the isotherms as the capacity at 0.8 atm. inasmuch as the contribution due to oxygen is very small. Conversely, the isotherms can be used to assess the state of hydration of a zeolite, with the aid of a measurement of the amount of sorbed nitrogen, either by mass loss on evacuation at room temperature in a balance, or by collection of gas evolved on immersion of a sample in water. 

Hygro-themo-chemo-mechanical behaviour of concrete is of great practical importance in many fields of civil engineering. Modelling these phenomena, especially in fresh concrete structures or concrete elements exposed to fire, is a complex problem. Several non-linear phenomena, like heat and mass sources associated with hydration or dehydration processes, phase changes, hysteresis of sorption isotherms, material properties dependent on moisture content, tem-... 

Maleic acid, 338 Maionic ester, 380 Markovnikoff s rule, 96 Mass spectroscopy, 247/T Mechanism, alcohol dehydration, 92 alkane halogenation, 56 benzyne, 217 El and E2, UOff SnI and Sn2, 122 ... 

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