Cryst. Res. Technol. 36 2001 12 1357â1364 © WILEY-VCH Verlag Berlin GmbH, 13086 Berlin, 2001 0232 -1300/01/1212-1357 $ 17.50+.50/0 In solution, the growth rate and the crystal habit are influenced by a number of factors such as supersaturation, temperature, pH of the solution, cooling rate, agitation, viscosity, initial state of the seed crystal and the presence of impurities. The crystallization of orthorhombic b-MgSO4·7H2O, from low temperature aqueous solution by slow cooling process was studied. The metastable zone width, the induction periods (t) for different supersaturations and the effect of pH on the growth rate of the crystals were investigated. The increase of pH yielded bigger crystals. The structural, optical, thermal and mechanical properties of b-MgSO4·7H2O have been studied using FT-IR, X-ray diffraction, TGA- DTG and micro hardness analyses. Keywords: epsomite, orthorhombic, solubility, metastable zone width, induction period (Received May 22, 2001; Accepted August 2, 2001) 1. Introduction In many industrial crystallization processes, the size and shape of the crystal is an important factor, since the undesirable habits such as plate like or neddle -like [CANO et al.] causes the problems of separating, washing and drying. The properties such as packing density [TER HORST et al.], agglomeration and re-dissolution mainly depends on the shape of the crystals. Many studies have been reported on the factors, which influences the morphology of the crystal both by experimental and predictive methods [MULLIN; DAVEY et al.]. The epsomite (MgSO4·7H2O), as a source of Mg 2+ ions has wide application in medical (acute management of cardiac arrhythmia) and agricultural industry (fertilizer). It forms a raw material for manufacturing various chemicals containing magnesium. This material, when doped with selected activators yields efficient luminescent properties [RAYAN et al.; YAMASHITA and HAMADA]. Recently, new ESR dosimetric materials, have been developed with due attention to the tissue-equivalence requirement and salts of these metals with organic acids may find further applications in ESR dosimetric imaging plates [IKEYA et al.]. Since epsomite crystals are small and often exhibit needle like habits, the study of nucleation and control on morphology are essential for industrial application. A supersaturated solution will often nucleate more readily in the presence of a crystal of the solute than in its absence. This catalyzing effect of the b-MgSO4·7H2O has been discussed by [MASON and STRICKLAND- CONSTABLE; STRICKLAND-CONSTABLE] and the breeding of crystal nuclei of b-MgSO4 ·7H2O in aqueous solution was reported by LAL et al. MgSO4·7H2O exists as a and b-forms, above and below 800°C respectively [RAYAN et al.]. b-MgSO4·7H2O crystallizes with orthorhombic structure with space group P212121 and unit cell dimensions a = 11.87 Ã
, b = 12.00Ã
and c = 6.86 Ã
[JCPDS DATA]. There is no S. RAMALINGOM, J. PODDER*, S. NARAYANA KALKURA Crystal Growth Centre, Anna University, Chennai, India *Department of Physics, Bangladesh University of Engineering & Technology, Dhaka, Bangladesh Crystallization and Characterization of Orthorhombic b-MgSO4·7H2O 1358 S. RAMALINGOM et al.: Orthorhombic b-MgSO4·7H2O previous report on the study of nucleation, metastable zone width (MSZW), effect of pH on the growth rate and the crystal habit of b-MgSO4·7H2O. This paper reports the MSZW, induction period (t) and the crystallization of b-MgSO4 ·7H2O at low temperature. Keeping all other variables constant, the effect of pH on the growth rate and crystal habit was investigated by in situ observations. 2. Experimental Metastable zone width measurements The solubility of the recrystallised MgSO4·7H2O in triple distilled water has been measured for different temperatures, viz. 30-50°C in steps of 5°C. The temperature, at which the first visible speck found, was noted by visual observation, in the non-agitated system by super cooling the solution using a cryostat. The MSZW, the difference between the saturation temperature and the temperature at which the first visible speck found was measured for different saturations. The induction period, which is the time interval between the system attaining supersaturation and the formation of first nucleus in the supersaturated state, was measured for three different supersaturation ratios (S = 1.1, 1.2 and 1.3) [KASHCHIEV et al.]. Fig. 1: Solubility and nucleation curve of MgSO4·7H2O. Crystal growth procedure 450 ml of saturated solution of b-MgSO4·7H2O was prepared using the recrystallized salt at 40°C by constant stirring for 12 hours using the predetermined solubility data. The solution was heated to a temperature 1.5 - 2.5°C higher than the saturated temperature to attain homogeneity and to avoid any spurious nucleation during filtration. The solution was filtered with borosil filter sheet and the filtrate was taken into three identical crystallizers (150ml each). Maintaining all the other parameters constant, the pH of the mother solution was maintained at three different values (2.0, 3.5 and 5.0) by adding few drops of concentrated sulphuric acid. The seed crystals of MgSO4·7H2O were grown by isothermal slow Cryst. Res. Technol. 36 (2001) 12 1359 evaporation method. Good quality seed crystals of 2mm size were suspended in the middle of the charged crystallizers. The crystallizers were sealed and placed inside a constant temperature bath maintained at 40 ± 0.01°C. Super cooling was done by reducing the temperature of the bath by 0.1°C per day to initiate crystallization process. Over a period of 15~20 days, well faceted, optically transparent large single crystals with dimensions of about 2 x 2 x 3.5 cm3 were obtained. Characterization The crystallized samples were characterized by Fourier Transform Infrared spectroscopy using Perkin-Elmer spectrum RX1 FT-IR system. Samples were prepared by KBr pellet technique. Thermogravimetric analysis were performed with a Mettler M3 balance TG furnace and microprocessor TA-3000 system. Powder X-ray diffraction (XRD) has been recorded using a Seifert diffractometer (model 2002) with CuKa radiation (l= 1.5418 Ã
). Microhardness studies were carried out using Leitz Wetzlar Metallus II Vicker's micro- indentation tester. 3. Results and discussion Solubility The solubility of the recrystallized salt of MgSO4·7H2O in triple distilled water at various temperatures 30°C, 35°C, 40°C, 45°C and 50°C were determined. It was found that the solubility increased almost linearly with the increase of temperature. It implies that the coefficient of solubility is positive [Fig.1]. Fig. 2: Temperature Vs induction period for different supersaturations. Metastable zone width and induction period The metastable zone width at various temperatures under supersaturated conditions were determined. At low temperature, the metastable zone width was found to be large and narrow 1360 S. RAMALINGOM et al.: Orthorhombic b-MgSO4·7H2O at higher temperatures. It was revealed that, wider the metastable zone width, higher the stability of the mother phase for the crystallization mechanism. To investigate the effect of temperature on the induction period, experiments were carried out in the non-agitated system for different supersaturations (S = 1.1, 1.2 and 1.3). The supersaturation was kept constant in each case and the results are reproduced in Figure 2. It was found that, at a given supersaturation the induction period decreased with an increase in temperature. The effect of pH on the growth rate and the crystal habit It was observed that the growth rate mainly depended on the pH of the mother phase. The average growth rate was measured as 1.2, 1.3 and 1.6 mm/day for pH values 2.0, 3.5 and 5.0 respectively [Figure 3]. As a measure to indicate crystal habit, the ratio l/w was used, where l is the length along the c-axis and w is the width perpendicular to c-axis [TAKUBO and KUME]. The measured values of l/w ratio for the grown crystals were 1.346, 1.727 and 2.037 respectively for the said pH values. Higher pH values increased the growth rate and size of the crystals. Fig. 3: Photographs of MgSO4·7H2O single crystals for different pH. FT-IR studies The vibrational spectral characteristics of the crystals were studied using FT-IR spectroscopy [Figure 4]. The SO4 2- ion possesses Td symmetry [HERZBERG] with four active fundamentals in the infrared, n1 at 981 cm -1, n2 at 451 cm -1, n3 at 1104 cm -1 and n4 at 613 cm -1. Twisting and rocking of water molecule were found at 690 and 834 cm1. The absorption bands at 745 and 1074 cm-1 confirmed the presence of n3 SO4 and n4 SO4 respectively. The band at 1632 cm -1 and the broad band at 3100 cm-1 might be due to the presence of n2 H2O II and n1 H2O III respectively. X-ray diffraction studies The XRD pattern of MgSO4â¢7H2O single crystal was as shown in Figure 5. The observed XRD patterns of the grown crystal were found to be in good agreement with the JCPD data. The lattice parameters of the grown crystals are a = 11.689 Ã
, b = 12. 101 Ã
, c = 6.747 Ã
. Cryst. Res. Technol. 36 (2001) 12 1361 Fig. 4: FT-IR spectrum for MgSO4â¢7H2O crystal Fig. 5: X-ray Diffraction pattern of MgSO4·7H2O. Thermal analysis Figure 6 shows the different decomposition stages of MgSO4·7H2O crystal. In the TGA trace, a broad endothermic peak was found in the range of 116 - 224 °C. This peak belongs to the dehydration step. As the crystal contains 7H2O, the weight loss is expected due to the 1362 S. RAMALINGOM et al.: Orthorhombic b-MgSO4·7H2O elimination of seven water molecules. In the second stage, further decompositions are due to the elimination of SO4 2- ions in the form of SO2 and oxygen leaving behind MgO. MgSO4·7H2O ® MgSO4 + 7H2O MgSO4 ® MgO + SO2 + O2 Fig. 6: TGA-DTG curve of MgSO4·7H2O. Microhardness studies Plane crystal surfaces were selected for the microhardness studies. Indentations were carried out using Vickerâs pyramidal indentor for various loads (P) ranging from 10 to 100 g [Figure 7]. The duration of the indentation time was kept constant at 10 seconds. For each load, several indentations were made and the average value of the diagonal length (d) of the indentation mark was used. Using the relation HmV = 1.8544 P/d2 [MOTT] Vickerâs hardness number was calculated for each indentation. The microhardness of the sample was found to decrease with the increasing load. Cryst. Res. Technol. 36 (2001) 12 1363 Fig. 7: Load Vs Vickerâs microhardness number of MgSO4·7H2O. 4. Conclusions The solubility and the metastable zone width for the saturated solution of MgSO4·7H2O at various temperatures were measured. The induction period for various supersaturations was determined on the growth of MgSO4·7H2O crystals. At pH = 2.0, the growth rate was lowest and the size of the crystal was the least. When the pH was changed to 5.0, the growth rate increased yielding bigger crystals. The crystals grown were highly transparent and well faceted. Faster growth rates were achieved for higher pH values. The microhardness study revealed that the grown crystals were mechanically soft. Acknowledgement One of the authors, S.Ramalingom gratefully acknowledges the University Grants Commission, India, for the award of Teacher Fellowship. This work was supported by a grant from AICTE, New Delhi, India. References CANO, H., GABAS, N., CANSELIER, J.P.: J. Crystal Growth. 224 (2001) 335 Crystal data. Determinative tables Vol. II (US Department of Commerce, National Bureau of Standards and the Joint Committee on Powder Standards) (1972) DAVEY, R.J., MULLIN, J.W., WHITING.M, J.L.: J. Crystal Growth 58 (1982) 304 HERZBERG, G.: âInfrared and Raman spectra of Polyatomic Moleculesâ. D. Van Nostrand, New York (1945) 489 IKEYA, M., HASSAN, G. M., SASAOKA, H., KINOSHITA, Y., TAKAKI, S., YAMANAKA, C.: Appl.Radiat.Isot. 52 (2000) 1209 KASHCHIEV, D., VERDOES, D, VAN ROSMALEN, G.M.: J. Crystal Growth 110 (1991) 373 LAL, D.P., MASON, R.E. A., STRICKLAND-CONSTABLE, R. F.: J. Crystal Growth 5 (1969) 1 MASON, R. E. A., STRICKLAND-CONSTABLE, R. F.: J. Faraday Soc. 62 (1966) 455 1364 S. RAMALINGOM et al.: Orthorhombic b-MgSO4·7H2O MOTT, B. W.: âMicroindentation Hardness Testingâ, Butterworths, London (1956) 9 MULLIN, J.W.: âCrystallizationâ, 3 rd Edition, Butterworth- Heinemann, Oxford 1993 RAYAN, F. M., LEHMANN, W., FELDMAN, D. W., MURPHY, J.: J. Electrochem. Soc. (1974) 1475 STRICKLANDâCONSTABLE, R. F.: âKinetics and Mechanism of Crystallizationâ, Academic Press, London (1968) TAKUBO, H., and KUME, S.: J. Crystal Growth 67 (1984) 217 TER H ORST, J.H., GEERTMAN, R.M., VAN DER HEIJDEN, A.E., VAN ROSMALEN, G.M.: 14th International symposium on Industrial Crytallization, Cambridge 1999 YAMASHITA, N., HAMADA, T.: Jpn. J. Appl. Phys. 38 (1999) 1326 Contact information: S. RAMALINGOM, J. PODDER, S. NARAYANA KALKURA* Crystal Growth Centre Anna University Chennai-25 India *corresponding author e-mail:
[email protected],
[email protected]