Disproportionation of Zn(H2PO4)2·2H2O Crystals (Topochemical

The topochemical transformations of Zn(H2PO4)2·2H2O crystals in the medium of acetone, ethanol, and diethyl ether have been studied by chemical analy...
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Ind. Eng. Chem. Res. 1997, 36, 4791-4796

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Disproportionation of Zn(H2PO4)2‚2H2O Crystals (Topochemical Particularities) Valery V. Samuskevich, Olga A. Lukyanchenko,* and Ludmila N. Samuskevich Institute of General and Inorganic Chemistry, Belarus Academy of Sciences, Surganov st. 9, 220072 Minsk, Belarus

The topochemical transformations of Zn(H2PO4)2‚2H2O crystals in the medium of acetone, ethanol, and diethyl ether have been studied by chemical analysis and X-ray powder diffraction methods in the temperature range of 0-40 °C. It was established that under these conditions crystalline zinc hydrophosphate monohydrate (ZnHPO4‚H2O) and going into a liquid phase phosphoric acid were formed. The influence of the chemical nature of the organic solvent, the residual quantities of water in it, the size of the initial crystals on localization particularities, and kinetics of the disproportionation process has been considered. The mechanism of the process which includes a solid-state proton transfer from one dihydrophosphate ion to another, the nucleation and growth of ZnHPO4‚H2O crystals, the removal of phosphoric acid and of part of crystallization water from the reaction zone has been proposed. The experimentally found values of activation energy E ) 125-145 kJ/mol are related to the overall process, the limiting stage of which is crystallization. Introduction The ability of dihydrophosphates of polyvalent metals by the action of water or aqueous solutions to transform in less-proton salts with the release of free phosphoric acid is their characteristic property. It is assumed that this process is associated with the hydrolysis of initial phosphates (Elmore and Farr, 1940; Corbridge, 1980). However, in actuality, as follows from the stoichiometric equations H 2O

M(H2PO4)2 98 MHPO4 + H3PO4 H 2O

3M(H2PO4)2 98 M3(PO4)2 + 4H3PO4

(1) (2)

water is the only medium in which reaction proceeds, but it does not take place there directly. As a rule, the transformations (1) and (2) occur at the expense of spontaneous recrystallization and include the next stages; the dissolution of initial salt and the disproportionation of dihydrophosphate anion in solution followed by the precipitation of a solid product. The rate of the overall process proceeding according to the same scheme and the depth of transformation depend on the solubility of the initial phosphate and the resulting product, liquid/solid ratio, pH value, temperature, etc. (Prodan and Samuskevich, 1994; Vasserman, 1980). There is evidence that disproportionation may occur without dissolution of dihydrophosphate too. In particular, it has been found that corresponding hydrophosphates are generated from Pb(H2PO4)2 and Zn(H2PO4)2‚ 2H2O on long standing in air (Jost et al., 1990; Samuskevich and Prodan, 1992) and from Ca(H2PO4)2 when acted upon by organic solvents (Grower et al., 1981; Bayramoglu and Keskinler, 1992). True, in the latest case the authors assume the partial dissolution of dihydrophosphate. However, our data show that the process may proceed in solid state. The analysis of scientific and patent literature shows that the processes of solid-state disproportionation are * Author to whom correspondence is addressed. E-mail: [email protected]. S0888-5885(97)00166-8 CCC: $14.00

capable of latent going with the induction period of different lengths and are responsible for the changes of composition and properties of phosphoric salts during their production and storage. It was also established that the transformations of types (1) and (2) are the basis for such important technological processes as a solidification of phosphate binders (Belous et al., 1981) and the forming of protective phosphate coatings on metal surfaces (Hain, 1973); however, they are not taken into account in the corresponding technological procedures. This is because the process of solid-state disproportionation is poorly studied and the data for the nature of factors determining the rate and the depth of conversion are lacking. In this connection we are engaged in systematic research on the kinetics and mechanism of transition metal dihydrophosphate topochemical transformations. The purpose of the present work is to investigate the process of solid-state disproportionation of Zn(H2PO4)2‚ 2H2O crystals in the medium of acetone, ethanol, and diethyl ether. We report data about the influences of different factors on the character of the localization process, kinetics and mechanism of chemical reaction, and attendant phase transformations. Experimental Section Two samples of zinc dihydrophosphate dihydrate with identical chemical compositions but different crystal sizes were used in the work. The small-sized crystals of dihydrate (D ) 0.05-0.1 mm) were precipitated from the viscous solution of ZnO in 75% H3PO4 (23 g of oxide/ 100 mL of acid) by stirring at room temperature for 30 min. The big crystals (D ) 2-3 mm) were prepared by letting the same solution set without stirring for 4 days. The precipitates were washed from the remains of the mother liquor on the glass filter with acetone and then with ether, keeping out of contact with air. These crystals are free from any impurities and have a long shelf-life when the relative humidity of air RH ) 3050%. X-ray diffraction patterns of small- and big-sized crystals coincide with one another and with literature data (Powder Diffraction Files. Card No. 27-987) for Zn(H2PO4)2‚2H2O. It is significant that the ordinary © 1997 American Chemical Society

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Figure 1. Kinetic curves of the release of phosphoric acid during the process of isothermal holding of (a) small-sized and (b) bigsized Zn(H2PO4)2‚2H2O crystals at 20 °C. (1) In the flow of dehydrated ethanol (0.1 vol % H2O). (2) In the flow of rectified ethanol (4.0 vol % H2O). (3) In the flow of ether (0.5 vol % H2O). (4) In the flow of acetone (0.5 vol % H2O).

procedure of 3-4-fold treatment of freshly precipitated dihydrophosphate crystals by acetone, ethanol, or ether (when the total washing time is no more than 2-3 min) is accompanied with intensive collapse of the crystals, yielding the products of disproportionation, mainly ZnHPO4‚H2O. During the investigations of disproportionation kinetics in dynamics, acetone (0.5 vol % H2O), rectified ethanol (4.0 vol % H2O), ether (0.5 vol % H2O) (all the chemical reagent grade), and dehydrated ethanol (0.1 vol % H2O) were passed with fixed speed (1 mL/min) through the batch of 100 mg of dihydrate distributed by thin layer on the porous plate of the glass filter. The portions of eluate were collected at certain intervals, and then the released portions, according to the reaction of disproportionation phosphoric acid, was titrated with a 0.1 N aqueous solution of NaOH in the presence of bromocrezol green. The solid product which remained on the plate was analyzed in parallel. As the obtained results have shown, zinc contained in the solid phase did not go into solution under these conditions. During the investigations of disproportionation kinetics in the static regime, a series of the samples was allowed to stand under the layer of organic solvent for various periods of time. Thereafter, the solid phase was separated and the quantity of the acid released in solution was determined. The phase composition of the initial crystals and of the solid products forming at the different reaction steps was analyzed by an X-ray powder diffraction method using Cu KR radiation. NaCl was employed as the internal standard, when the quantitative estimate of phase composition was made. In the course of the reaction, changes of the crystals were viewed through an optical microscope. The most characteristic localization figures were fixed on the film at definite intervals. Results and Discussion The disproportionation rate of zinc dihydrophosphate in an organic liquids medium is determined among other factors by the chemical nature of the organic solvent, the residual quantities of water in it, the size of the initial crystals, and temperature. Effect of the Organic Solvent Nature and Its Water Content. Figure 1 shows the kinetic curves “the quantity of the releasing phosphoric acid vs time”, which have been obtained during the isothermal holding of small- and big-sized crystalline samples of dihydrophosphate in the flow of acetone, ethanol, and diethyl ether

Figure 2. Kinetic curves of the release of phosphoric acid during the process of isothermal holding of small-sized Zn(H2PO4)2‚2H2O crystals in the flow of acetone at 20 °C. The water content of the acetone is (1) 0.5, (2) 1, (3) 1.5, (4) 2.5, and (5) 4.5 vol %.

at 20 °C. The disproportionation rate was a maximum with the use of ethanol. Thus, the process was completed after 40 min when rectified ethanol was passed through the layer of small-sized crystals. In this case, the quantity of liberated phosphoric acid (0.34 mmol of H3PO4/mg of Zn(H2PO4)2‚2H2O corresponds to the total transformation of dihydrophosphate to hydrophosphate. The formation of ZnHPO4‚H2O was confirmed by the results of X-ray studies too. The big crystals which have the lesser specific surface and as a result the lesser contact space with a solvent are more stable; but after 10-12 h, they also completely transform into zinc hydrophosphate monohydrate. It can be assumed that more pronounced, as compared with other solvents, initiating action of ethanol is explained by the high content of water in it (4 vol %); however, with absolute ethanol (water content as little as 0.1 vol %), disproportionation goes on even faster (curve 1). At this point, the ability of dehydrated organic solvent to take away a part of crystallization water from dehydrophosphate and thereby to stimulate the transformation reveals itself. A complicated relationship between the rate of acid release and the water content of the organic solvent was observed when the process was carried out in the flow of acetone too. With acetone (0.5 vol % H2O) dihydrophosphate disproportionation proceeded at an appreciable rate, and 0.12 mmol of H3PO4/mg of Zn(H2PO4)2‚ 2H2O liberated during 4 h (Figure 2, curve 1). At the same time, when supplementary 0.5 vol % H2O was added to the initial acetone, it ceased to exert any action on dihydrophosphate crystals. Even after 4 h of contact no evidence of disproportionation was found. This effect originally revealed in the course of an isothermal experiment too when the initial acetone employed as an eluent was periodically changed for the acetone containing 0.5 vol % H2O in addition. In this case the kinetic curve (Figure 3) consists of the fragments of rate increasing and of the horizontal fragments corresponding to the ceasing of reaction. The addition of 1, 2, and 4 vol % H2O to acetone (0.5 vol % H2O) produces the reverse action (Figure 2, curves 3-5). In this case the disproportionation process noticeably speeds up and completes in 250, 110, and 25 min, respectively. In the examples cited above the presence of water in the organic solvent affects the rate of the process being investigated, but it has no influence on the chemical and phase compositions or on the morphology of the resulting products. These latter, in spite of proceeding structural transformation of Zn(H2PO4)2‚2H2O into

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Figure 3. Release of phosphoric acid during the process of isothermal holding of small-sized Zn(H2PO4)2‚2H2O crystals at 20 °C when the acetone containing 0.5 vol % H2O (O) was periodically changed for the acetone containing 1 vol % H2O (b).

Figure 4. Kinetic curves of the release of phosphoric acid during the process of isothermal holding of big-sized Zn(H2PO4)2‚2H2O crystals in the flow of ether at 20 °C. The water content of the ether is (1) 0.5, (2) 0.6, (3) 0.7, (4) 0.8, (5) 0.9, and (6) 1.1 vol %.

ZnHPO4‚H2O, present pseudomorphose structures retaining the form and the facetting of the initial crystals. The situation is different if diethyl ether is used as an eluent. The data shown in Figure 4 illustrate the influence of minor water additions to ether (water content of 0.5 vol %). The disproportionation rate of crystals (D ) 2-3 mm) rises steeply when 0.1 and 0.2 vol % H2O are added. However, a further increase of the water content of ether causes the progressive deceleration of phosphoric acid release. As this takes place, the form of the kinetic curves changes, which suggests the development of diffusion impediments. When ether with an overall water content of 1.1 vol % was employed, under 0.03 mmol of H3PO4/mg of dihydrophosphate was given off in a matter of 60 min, whereas in the subsequent portions of eluate, the acid was not detected at all (Figure 4, curve 6). The change of the character of structural transformations is one of the reasons for the development of diffusion impediments. As X-ray diffraction studies demonstrated, the phase composition of the resulting products changed as the water content of the ether was increased. So, when the ether held more than 0.7 vol % H2O, instead of well-crystallized ZnHPO4‚H2O, the mixtures of several phases (namely, ZnHPO4‚3H2O, Zn3(PO4)2‚4H2O, and ZnHPO4‚1.5H2O) appeared. It is necessary to note that in the case of ethanol and acetone all these phases were found when the water content was over 6 and 10 vol %, respectively; at one’s lesser values the sole product ZnHPO4‚H2O was identified. The simultaneous formation of several phases makes their crystallization difficult, as indicated by the broadening of X-ray diffraction reflections. The products become amorphous and constitute the loose bulky mass as the water content of ether increases to 1.1 vol %. This

phenomenon is obviously due to the formation of a swelling net-shaped structure in which mass transfer is essentially hard. From the kinetic curve 6 (Figure 4) it follows that the process comes to a halt when the transformation degree is relatively small (about 10%). However, these data are in rather poor agreement with the direct crystal observations in the course of the reaction. Even within 60 min after the start of the process, the presence of unreacted parts of the initial crystals cannot be detect either by visual inspection or with an optical microscope. It is apparent that the reaction is practically complete by that time. Really, if at this stage the ether (0.5 vol % H2O) in the eluent is substituted for acetone (0.5 vol % H2O), more than 0.30 mmol of H3PO4/mg of Zn(H2PO4)2‚2H2O is found in the first portions of eluate, confirming the total transformation of dihydrophosphate. The ability of acetone to extract the phosphoric acid tightly held in the bulk of the swelled net-shaped structure is attributable to different reasons. This question (as well as the above-mentioned effects related to the influence of trace impurities of water on the properties of a composite syste: organic liquid-phosphoric acid-salt) invites more detailed investigations. In light of the concepts developed in the present work about the topochemical nature of the process being studied, the fact that the role of organic medium in some cases does not reduce to the dissolving ability with respect to phosphoric acid is significant by itself. Presented data show that the solvents employed affect the phase composition, the degree of crystallinity, and the morphology of the resulting products. They have influence on the character of the localization process too. Localization Particularities. Figure 5 gives the characteristic figures which formed on the faces of Zn(H2PO4)2‚2H2O crystals (D ) 2-3 mm) as a result of isothermal holding in the flow of acetone (0.5 vol % H2O), ether (0.5 vol % H2O), and ethanol (4 and 0.1 vol % H2O) when the transformation degree was about 5%. The reaction begins at the whole of the surface practically simultaneously only with the use of absolute ethanol. In the other cases clearly defined spot localization of the process occurs; in doing so the quantity of figures, their shape, and the localization sites are dictated by the nature of the organic solvent. However, it should be pointed out that the process of localization in different organic mediums has a characteristic common feature. In all instances a powerful orienting effect of the crystal lattice of the initial dihydrophosphate can be followed. The resulting figures grow in definite crystallographic directions, and as a consequence of this gain the contours to a greater or lesser extent repeat the contours of the face from which they arise. The obtained results testify that the disproportionation of Zn(H2PO4)2‚2H2O in the medium of liquids harnessed in the work is a representative topochemical reaction. One is accompanied by the localized topotactic transformation of the initial dihydrophosphate crystals to zinc hydrophosphate monohydrate and by the release of forming acid and of part of crystallization water into a liquid phase: organics

Zn(H2PO4)2‚2H2O(cryst) 98 ZnHPO4‚H2O(cryst) + H3PO4(liq) + H2O(liq) (3) Kinetics and Mechanism. The typical form of the kinetic curve R-τ (the liberation of 0.34 mmol of

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Figure 5. Characteristic figures formed on the faces of Zn(H2PO4)2‚2H2O crystals (D ) 2-3 mm) as a result of isothermal holding when the transformation degree was about 5%. (a) In the flow of acetone (0.5 vol % H2O). (b) In the flow of ether (0.5 vol % H2O). (c) In the flow of rectified ethanol (4.0 vol % H2O). (d) In the flow of dehydrated ethanol (0.1 vol % H2O).

Figure 6. Disproportionation of big-sized Zn(H2PO4)2‚2H2O crystals in the flow of acetone (0.5 vol % H2O) at 37 °C. (a) Kinetic curve R-τ. (b) Photomicrographs of the crystal face within 10 (1), 15 (2), 19 (3), and 24 min (4) after the onset of the reaction.

H3PO4/mg of Zn(H2PO4)2‚2H2O is taken as R ) 100%), and corresponding localization effects observed when the disproportionation of big-sized crystals proceeds in the flow of acetone (0.5 vol % H2O) at 37 °C are shown in Figure 6. The course of the reaction with the induction period and succeeding self-acceleration agrees well with the topochemical nature of the process. X-ray diffraction studies of the samples at different reaction steps indicate that the crystallization of the solid product begins practically contemporaneously with the onset of acid release. The disproportionation of big-sized crystals proceeds with the convenience for measurement rates at a higher temperature as compared with the disproportionation of a fine sample. The determination of a quantitative relationship between these rates presents difficulties due to the comparatively low reproducibility of the results obtained with single crystals. Averaged data for 5-6 runs point to the fact that the disproportionation of small-sized crystals goes on in 10-fold quick time

compared to the case of single crystals. By these means satisfactory correlation between the process rate and the specific surface of the initial crystals is observed. The fine sample with good reproducibility of the results was used to estimate the gross kinetic characteristics of the disproportionation process. The reaction in the flow of acetone occurs with the convenience for measurement rates at t ) 15-40 °C. The kinetic curves R-τ (Figure 7a) are described satisfactorily by the familiar topochemical equation R ) 1 - exp(-kτn) (Delmon, 1969). In this situation a log[-log(1 - R)]log τ plot yields a straight line. The quantity k is determined from the length intercepted by this line on the ordinate axis. The kinetic parameter n, as it is called, is calculated from the slope of the line. This parameter is variously interpreted by different authors. We hold to this idea that the quantity n is useful for the estimation of the process limiting stage. According to Prodan (1976), n < 1 points to the diffusion impediments, n ) 1 suggests that the reaction proceeds in the kinetic regime, and n > 1 implies that the process has

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Figure 7. Kinetic curves of the disproportionation of small-sized Zn(H2PO4)2‚2H2O crystals in the flow of acetone (0.5 vol % H2O): (1) 40, (2) 30, (3) 22, (4) 18, (5) 15 °C. (b) Kinetic curves of the disproportionation of small-sized Zn(H2PO4)2‚2H2O crystals in the flow of ethanol (4 vol % H2O): (1) 19, (2) 13, (3) 10, (4) 7, (5) 0 °C. Table 1. Kinetic Characteristics of the Disproportionation Process of Zn(H2PO4)2‚2H2O Small-Sized Crystals in the Flow of Acetone (0.5 vol % H2O) temp, °C

n

K, min-1

15 18 22 30 40

2.0 2.1 2.0 1.9 2.1

14 × 10-4 32 × 10-4 68 × 10-4 29 × 10-3 16 × 10-2

spot localization. The calculated values of the kinetic parameter listed in Table 1 are on the order of 2.0; which is to say that the process proceeds in the kinetic regime with the spot localization predominant. The closely related values of n obtained at all temperatures indicate that the mechanism of process (in particular the nature of the limiting stage and the type of localization) is essentially temperature independent). It varies only slightly in the course of isothermal reaction too. This is evidenced by the fact that the values of activation energy Ev computed from the temperature dependence of the reaction rate VR)const at different transformation degrees are in close agreement. All these values fluctuate within 140-150 kJ/mol and check nicely with the magnitude of 145 ( 5 kJ/mol, calculated from the temperature dependence of the rate constant K (K was determined with the allowance made for kinetic parameter n using the Sakovich equation (Sakovich, 1955) K ) nk1/n): K ) 1023.3 exp(145200/RT) min-1. In the flow of ethanol disproportionation of smallsized crystals in zinc dihydrophosphate goes on with rates compared at lower temperatures (0-20 °C). At the same time the form of the curves R-τ (Figure 7b) together with the kinetic characteristics of the process is not radically different from that which is obtained when the reaction proceeds in the flow of acetone. The less pronounced induction period and the wider interval of kinetic parameter values (n varies between 2.0 and 2.8) are noteworthy only. Here, the rate constant temperature dependence is described by the equation K ) 1020.6 exp(125400/RT) min-1. The Arrhenius activation energy E ) 125 ( 10 kJ/ mol is somewhat below that for the process in the flow of acetone. However, it is hardly probable that both of these values belong to the chemical act of disproportionation (i.e., proton transfer from one H2PO4- ion to another in the dihydrophosphate lattice) because the typical activation energy value for such a process is about 40 kJ/mol.

Figure 8. Kinetic curves of the release of phosphoric acid during the isothemal holding of big-sized Zn(H2PO4)2‚2H2O crystals at 20 °C: (1) static conditions; (2) in dynamics.

The experimentally found E-values are related to the overall process. The latter involves, apart from a chemical stage, the nucleation and growth of a new phase as well as the removal of phosphoric acid and of part of crystallization water from the reaction zone. There are reasons to assume that the limiting stage is the crystallization of the product. This supposition is founded on experimentally revealed effects associated with the influence of phosphoric acid on the disproportionation process. When working with Zn(H2PO4)2‚ 2H2O big crystals, it was detected that the reaction initially localized on that part of the surface from whence the removal of releasing phosphoric acid was difficult, for example, on the lower crystal face. The addition of limited amounts of phosphoric acid to acetone or ethanol (1.5 mL of 86% H3PO4/100 mL of organic solvent) leads to instantaneous formation of the crystalline product (ZnHPO4‚H2O) on the whole surface of the initial crystals. By these means, phosphoric acid, in certain situations, may promote crystallization of the resulting product and thereby accelerate the overall process and not decelerate it following the rules of chemical equilibrium. This effect is similar to topochemically well-known mineralizing action of water vapors when they suddenly accelerate the dehydration of crystallohydrates (Young, 1966). Phosphoric acid forming by the disproportionation process also exerts crystallizing action when it is not removed from the reaction system. Indeed the reaction rate increased steeply after a prolonged induction period, and the process was over much faster than that in the dynamic regime, when Zn(H2PO4)2‚2H2O crystals were held under the layer of acetone at constant temperature in static conditions (Figure 8). In this case the reaction acceleration is due to the widening of the reaction zone not only at the expense of the growth of originally formed nuclei but at the expense of their permanent rise as phosphoric acid accumulates as well. The kinetic analysis of such a process and the interpretation of the obtained kinetic characteristics are hampered because the reaction mechanism at different stages varies constantly. Moreover, the kinetic characteristics, specifically the activation energy, are controlled by the procedure factors, such as the liquid/solid ratio. It seems likely that attempts by way of different experimental shifts to receive the kinetic characteristics relevant to the chemical stage of the disproportionation process are meaningless. One needs to recognize that, during conditions under study, the limiting stage is crystallization instead of chemical reaction and the values of the activation energy cited above are related only to this stage.

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Conclusions The investigations of the kinetics and of the localization particularities of the Zn(H2PO4)2‚2H2O crystals disproportionation process have revealed a solid-state nature of this process. It proceeds at the expense of proton redistribution between dihydrophosphate ions in the lattice of the initial crystals and must be accompanied by the topochemical transformation into crystalline ZnHPO4‚H2O with the release of phosphoric acid. The organic medium initiates disproportionation contributing to the removal of one of the products (acid) from the reaction zone. However, the limiting stage of the process is crystallization, which is characterized by the activation energy E ) 125-145 kJ/mol (depending upon the nature of the organic solvents). The results obtained are necessary for elaboration of technology to produce the dihydrophosphates of transition metals which are free from disproportionation products and possess predetermined properties. Literature Cited Bayramoglu, M.; Keskinler, B. Kinetics of the Dissociation of Ca(H2PO4)2‚H2O in Ethyl Alcohol. Ind. Eng. Chem. Res. 1992, 31, 1602. Belous, N. H.; Samuskevich, V. V.; Ermolenko, I. N. The Chemical Transformations of Cu-phosphate binder in the process of solidification. Proc. Belarus Acad. Sci., Chem. [Russ.] 1981, 2, 5. Corbridge, D. E. C. Phosphorus. An Outline of its Chemistry, Biochemistry and Technology; Elsevier: Amsterdam, The Netherlands, 1980. Delmon, B. Introduction a la cinetique heterogene; Technip: Paris, 1969.

Elmore, K. L.; Farr, T. D. Equilibrium in the system calcium oxide-phosphorus pentoxide-water. Ind. Eng. Chem. 1940, 32, 58. Grower, L. B.; Carl, B. D.; Amiova, H. R. A new process for production of purified phosphoric acid and/or fertilizer grade dicalcium phosphate from various grades of phosphoric materials. Ind. Eng. Chem. Process Des. Dev. 1981, 20, 47. Hain, I. I. Theory and practice of phosphate coatings; Khimia: Leningrad, 1973. Jost, K.; Schneider, M.; Wozzala, H. On the mechanisms of reactions of Me(II)-phosphates in the solid state. Phosphorus, Sulfur Silicon 1990, 51/52, 105. Powder Diffraction Files. Card No. 27-987. Prodan, E. A. The regularities of topochemical reactions; Nauka i technika: Minsk, Belarus, 1976. Prodan, E. A.; Samuskevich, V. V. Stability of phosphoric salts; Nauka i technika: Minsk, Belarus, 1976. Sakovich, G. V. Remarks about some kinetic equations for solidstate reactions used in the present time. Tomsk Univ. Trans., Chem. [Russ.] 1955, 26, 103. Samuskevich, V. V.; Prodan, E. A. Effect of moisture on the stability and transformation rate of zinc dihydrophosphate dihydrate crystals. Proc. Belarus Acad. Sci., Chem. [Russ.] 1992, 5/6, 29. Vasserman, I. M. Chemical precipitation from solutions; Khimia: Leningrad, 1980. Young, D. Decomposition of solids; Pergamon Press: Oxford, U.K., 1966.

Received for review February 21, 1997 Revised manuscript received July 2, 1997 Accepted July 2, 1997X IE970166H

X Abstract published in Advance ACS Abstracts, September 1, 1997.