engleski

Contribution of Optical Microscopy to the Study of Magnesium Phosphate Transformation

Magnesium ammonium phosphate hexahydrate, MgNH_4PO_4&middot ; 6H_2O, commonly known as struvite, naturally appears as a mineral (guano). This compound has often been found as a constituent of scale deposits in pipes and on heat exchangers. Struvite is also a frequent constituent of renal and vesicle calculi related to the urinary tract infections. In distinction from struvite, magnesium hydrogen phosphate trihydrate, MgHPO_4&middot ; 3H_2O, known as newberyite, has rarely been found in human and animal calculi. In order to contribute to a better understanding of MgNH_4PO_4&middot ; 6H_2O and MgHPO_4&middot ; 3H_2O formation, the systematic physical-chemical investigations have been undertaken [1, 2, 3]. In this work, the transformation of struvite into newberyite was studied at the initial reactant concentrations ci(PO_43^-) = 1.0⋅ 10^-1 mol dm^-3 and ci(Mg2^+) = 2.3⋅ 10^-1 mol dm^-3 and all experiments were carried out at a temperature of 25°C. Precipitation systems were prepared by mixing equal volumes (60.0 cm^3) of magnesium chloride and ammonium dihydrogenphosphate solutions preadjusted to pHi = 6.50 or pHi = 7.05 by adding required amounts of NaOH. The progress of the transformation process was followed by measuring the pH of the solution. The morphology of the precipitate was observed by optical microscopy and the composition of the precipitate was analyzed by infrared spectroscopy. Figure 1 shows changes in morphology of the solid phase during the transformation process at different initial pHi. Thus, in the system at pHi = 6.50, the first precipitate appeared in the form of rod-like crystals (Fig. 1a1), while the first precipitate formed at pHi = 7.05 consisted of a mixture of twins, rod-like crystals and dendrites (Fig. 1b1). All these morphologies of crystals were found to be the forms of struvite, as determined by the Fourier transform infrared (FT-IR) spectroscopy. During the process the pH values in both systems decreased and tiny crystals made their appearance (Figs. 1a2, 1a3, 1b2 and 1b3). As FT-IR spectra showed, these new-formed crystals were composed of newberyite. At the end of the process of transformation, i.e. 160 min after mixing the reactants in the experiments initiated at pHi = 6.50 and 360 min after in the experiments initiated at pHi = 7.05, only FT-IR bands for newberyite were observed and tiny crystals were the only shape that remained in the systems (Figs. 1a4 and 1b4). References: (1) V. Babić-Ivančić, J. Kontrec, D. Kralj, Lj. Brečević, Croat. Chem. Acta, 75 (2002) 89. (2) V. Babić-Ivančić, J. Kontrec, Lj. Brečević, Urol. Res., 32 (2004) 350. (3) J. Kontrec, V. Babić-Ivančić, Lj. Brečević, Coll. Antropol., in press

magnesium ammonium phosphate hexahydrate; struvite; newberyite; transformation; scale deposits; renal calculi

nije evidentirano