Anhydrous crystals

Physical Properties. Mahc acid crystallines from aqueous solutions as white, translucent, anhydrous crystal. The S(—) isomer melts at 100-103°C (1) and the R(+) isomer at 98-99°C (2). On heating, D,L-mahc acid decomposes at ca 180°C by forming fumaric acid and maleic anhydride. Under normal conditions, malic acid is stable under conditions of high humidity, it is hygroscopic. 

For sodium palmitate, 5-phase is the thermodynamically preferred, or equiUbrium state, at room temperature and up to - 60° C P-phase contains a higher level of hydration and forms at higher temperatures and CO-phase is an anhydrous crystal that forms at temperatures comparable to P-phase. Most soap in the soHd state exists in one or a combination of these three phases. The phase diagram refers to equiUbrium states. In practice, the drying routes and other mechanical manipulation utilized in the formation of soHd soap can result in the formation of nonequilibrium phase stmcture. This point is important when dealing with the manufacturing of soap bars and their performance. 

Crystals of the dihydrate belong to the monoclinic system and have lattice parameters a = 659 pm, b = 1020 pm, and c = 651 pm. The anhydrous crystal belongs to the cubic system, a = 596 pm. Other physical properties of the anhydrous salt are Hsted iu Table 1. The anhydrous salt is hygroscopic but not dehquescent. 

Ammonium thiosulfate, (NH 2S2 3 commonly referred to as ammo hypo, has displaced sodium thiosulfate in photography. It is normally sold in the United States only as the aqueous solution. In addition, a crystal slurry and anhydrous crystal are available in Europe. 

Citric acid, anhydrous, crystallizes from hot aqueous solutions as colorless translucent crystals or white crystalline powder. Its crystal form is monoclinic holohedra. Citric acid is dehquescent in moist air. Some physical properties are given in Table 1 (1 3). The solubiUty of citric acid in water and some organic solvents is given in Table 2. The pH and specific gravity of aqueous solutions of citric acid are shown in Table 3. 

Alloxan [2,4,5,6(l//,3//]pyrimidine, tetrone] [50-71-5] M 142.0, m 170 (dec), pK 6.64. Crystn from water gives the tetrahydrate. Anhydrous crystals are obtained by crystn from acetone, glacial acetic acid or by sublimation in vacuo. 

A piperidene-based intermediate was found to crystallize as either an anhydrate or a hydrate, but the impurity profile of the crystallized solids differed substantially [26], Considerations of molecular packing led to the deduction that there was more void volume in the anhydrate crystal structure than in that of the hydrate form, thereby facilitating more clathration in the anhydrate than in the hydrate phase. This phenomenon was led to a decision to crystallize the hydrate form, since lower levels of the undesired impurity could be occluded and greater compound purity could be achieved in the crystallization step. 

The hydration equilibria and phase transformations associated with a cytotoxic drug, BBR3576, have been studied [72]. The initially hydrated form could be made to undergo a phase transition where it lost approximately half of its water content, but the hemidesolvated product could be easily rehydrated to regenerate the starting material. If however, the original sample was completely dehydrated, the substance first formed a metastable anhydrate phase that underwent an irreversible exothermic transition to a new anhydrate crystal form. The hydration of this latter anhydrate form yielded a new hydrate phase whose structure was different from that of the initial material. 

Solids that form specific crystal hydrates sorb small amounts of water to their external surface below a characteristic relative humidity, when initially dried to an anhydrous state. Below this characteristic relative humidity, these materials behave similarly to nonhydrates. Once the characteristic relative humidity is attained, addition of more water to the system will not result in a further increase in relative humidity. Rather, this water will be sorbed so that the anhydrate crystal will be converted to the hydrate. The strength of the water-solid interaction depends on the level of hydrogen bonding possible within the lattice [21,38]. In some hydrates (e.g., caffeine and theophylline) where hydrogen bonding is relatively weak, water molecules can aid in hydrate stabilization primarily due to their space-filling role [21,38]. 

Fig. 4.21. Thermogravimetric analysis of a compound capable of being isolated as an anhydrate crystal form (solid trace), and as the monohydrate (dashed trace) and dihydrate (dotted trace) solvatomorphs.

Fig. 7.8. Molecular packing in the theophylline anhydrate crystal phase. Adapted from Ref. [18].

Predicting What might you observe if the anhydrous crystals were left uncovered overnight ... 

Although the acidic antibiotics do not show the high selectivity characteristic of the neutral ones, there is still some preference, a strong one for monovalent as compared with divalent cations, and within the monovalent cations a variation from one acid to another. Monensin prefers sodium to potassium. The larger nigeridn, (X7), prefers potassium to sodium its silver, potassium, and sodium salts are isomorphous and anhydrous. Crystal structure determinations on the silver salt were carried out independently by workers in the U.S.A. (78) and in Japan... 

The authors note that in the two hydrated crystals, a system of stacked bases and strings of chloride ions are present. On the other hand, in the anhydrous crystal, an extensive hydrogen bonding network is not present. The guanines, instead, exist as ionic Gua2+Cl2 units and form parallel layers with interleaved chloride ions. EPR resonances from radicals other than thiyl radicals were observed near g = 2.0, but the radicals causing them were not the object of the investigation and were not characterized. 

Recrystd from H2O forming a dihydrate, or anhydrous crystals from AcOH. The... 

The effect of hydrogen bonds on the physical properties of crystals is shown in a striking way by oxalic acid. This substance exists in two anhydrous crystal forms. 1 One of these, the a form, contains layers of molecules held together by hydrogen bonds, the structure of a layer... 

NH2.K803 and (iii) the aq. soln. of the iso-salt is not coloured bluish-violet when treated with lead dioxide, or silver oxide, whereas the aq. soln. of the ordinary salt gives a coloration. F. Raschig also showed that (iv) the iso-salt is not hydrolyzed so readily as the ordinary salt and (v) whilst the potassium iso-salt separates from hot acidified soln. in anhydrous crystals, those of the ordinary salt are dihydrated. 

Anhydrous crystals from ether and petroleum ether, m.p. 124-125° (1). Monohydrate crystals m.p. 96 (1,2). 

Appearance Colorless anhydrous crystals sold in 60% solution. 

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