Preparation and Properties of Sulfatotris(Thiourea)Zinc(II) The phase rule studies on the system zinc sulfate-thiourea-water a t 30'C showed that only one complex of compwitiou [Zn(SC(NH2)2)3OSOa] is formed' between zinc sulfate and thiourea. The low and congruent solubility of the complex, 4.990, in comparison with those of ZnSOc7H20 (38.1%) and thiourea (17.8%) and its occupancy of the largest ares in the saturation region of the phase diagram is helpful to its easy isolation in pure state. We have been giving an exercise to the graduate students of chemistry involving the preparation and characterization of sulfatotris(thiourea)zinc(II), not only from the preparative aspects, but also from the point of view of structure and reactivity of c ~ r d i n a t i o ncompounds. The title compound is prepared as follows: Dissolve 8.6 g ZnS04.7HzO in 30 ml water and add to a thiourea solution containing 6.9 g. Mix the two solutions thoroughly by stirring with a glass rod. Allow the crystals separating out to settle out for 10 min. Collect the crystals on a filter crucible, wash successively with ice-cold water, acetone, and ether. Yield 6 g. The compound is very stable and relatively large crystals can be grown without any diff~eulty.It is possible to quantitatively analyze for thiourea, zinc, and sulfate by the hypoiodite,z oxidation to urea and sulfate and oxinate3 and barium sulfate methods respectively, as the complex dissociates in dilute aqueous solution. The infrared spectrum of the complex together with those of thiourea and ZnS01.7HzO is helpful in delineating the four coordination around Zn. It could he easily inferred that thiourea is bonded to Zn through the S atom and not through the N atom. The Tdsymmetry of the ionic SO4 islowered to CB"in the complex, due toits coordination to Zn, asevidenced by these splitting into two bands of the va and the appearance of the intense ur hand of SO stretching frequencies4 in the region 12M)-900 cm-I. The thermogravimetric curves obtained with 200 mg of the sample in air using mullite and platinum crucibles as containers reveal that the compound starts decomposing at 2W°C and the final product, ZnO, is obtained at 650°C in the mullita crucible and at 980°C in the platinum crucible through ZnS as one of the intermediates. The formation of ZnO at lower temperatures is due to the reaction, 3ZuSOn ZnS 4Zn0 4S02, occurring around 550% This reaction is hindered in platinum crucible due to the catalytic activity of platinum in the formation of SO3 in air which combines with ZnO to give ZnSOb. This alsoexplains the gain in weight noticed in the temperature range 520-720°C in the TG curve of the compound in platinum crucible. The preparation and anlapis of the compound comprise one laboratory experiment of 6-hr duration while the spectral and thermal behavior studies constitute another exercise of equal duration. All the students are able tocomplete the exercise with the excellent results within the specified time.
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'Udupa, M. R., Ph.D. Thesis, Indian Institute of Technology, Madras, India, 1967. 2Kolthoff, I. M., Belcher, R., Stenger, V. A. and Matsuyama, G., "Volumetric Analpis," Vol. 111, Interscience, New York, 1957, p. 291. %gel, A. I., "A Textbook of Quantitative Inorganic Analysis," h n g m a n Ine., New York, 1978. 'Nakamoto, K., "Infrared Spectra of Inorganic and Coordination Compounds," Wiley-Interscience, New York, 1969. Indian Institute of Technology Madras-600 036, India
M. R. Udupa
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