The inclusion of 3d-impurities Mn(II), Co(II), Ni(II) and Cu(II) in a crystalline precipitate of ZnC 2 O 4 .2H 2 O is investigated. This study is a part of the systematic one deal with the mechanism of inclusion of 3d-ions in sparingly soluble oxalate systems. The experiments are carried out in bi-end multi-component systems at two different mediums -one with deficiency of oxalate ions, another with excess. Good agreement between included impurity and the concentration of its complex in the solution is established. The stability constant of monooxalato complex affects the impurity inclusion. This effect depends on the medium nature. In the deficiency of oxalate ions the factor determining the inclusion is thermodynamic one -stability of monooxalato complexes. In the excess of oxalate ions inserted amount depends on kinetic factor -the formation rate of these complexes. In the term of that the insertion of Mn(II) is definitely different in the two mediums while that of the Ni (II) does not depend on the medium. The copper shows deviation from overall dependence in the two mediums due to the Jahn-Teller distortion. Its double decreasing insertion in the excess of oxalate ions is related with stabilization of [Cu(C 2 O 4 )2] 2-. The conclusions presume that by varying the background medium and taking in view the ions present in the solution, the amount of inserted impurities can be predicted and controlled. IntroductionThe study of the sparingly soluble oxalate systems is related to their use as precursors in the synthesis of hightemperature superconductive ceramic materials, in the preparation of nanomaterials and a number of other new materials due to the fact that the precipitation processes provide the possibility of controlling the chemical and physical properties of the final products. Oxalate systems are traditionally used for the separation and concentration of elements, especially rare earth and transuranium elements. Detailed studies on the insertion of impurities upon the spontaneous crystallization of sparingly soluble oxalate systems are limited. Some of them are directed to the investigation of the effect of impurities on the properties of the materials [1][2][3][4]. Others deal with the effect of the process kinetics on the inserted amount of impurities [5][6][7] and with the determination of the distribution coefficients [8][9][10][11][12][13]. However, all these studies are carried out on bicomponent systems, comprising just a macro-and a micro-component. For this reason, a detailed investigation of the kinetics of the cocrystallization in oxalate systems directed to the elucidation of the mechanism of inclusion of impurities would clear up this problem. It would undoubtedly help the elucidation of the mechanism of the coprecipitation processes as well, since "the chemistry of oxalate coprecipitatiom processes and the nature of the products have not been well defined" [14]. The present study deals with the sparingly soluble compound ZnC 2 O 4 .2H 2 O, which is used as a starting material for the pr...

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A study of the sorption of 3d-transition elements onto ZnC 2 O 4 .2H 2 O has been carried out.To reveal the mechanism of the process, the experiments were performed in two media of different oxalate ion concentrations -1.10 -2 and 4.10 -4 mol/l, by the separate and by the joint presence of the microcomponents in the system. In the latter case, copper was also added, the sorption of which has been previously well studied. Different mechanisms of sorption in the different media were found, the determining role of the neutral oxalate complexes having been manifested. In the case of manganese and cobalt, in the medium with oxalate ions deficit, sorption by neutral complexes in the adsorption layer A is supplemented by ion-exchange sorption in the first surface layer S. The sorption of nickel under oxalate ion excess is proved to be an irreversible process, and the concept of its sorption by a mixed complex with the participation of the macrocomponent, Zn[Ni(C 2 O 4 ) 2 ], is advanced. The experimental data show the occurrence of segregation of Mn and Ni by their sorption onto the growth steps, and the absence of segregation of Co by its sorption. An interesting result is the extension of the validity of the Paneth-Fajans rule in the case of so-called complex oxalates, growing by neutral oxalate complexes, such as the oxalates of the macro-and microcompoments, studied in the present work.

The crystallization kinetics of zinc oxalate a t different supersaturations was studied by a turbidimetry method. It was found that the curves optically registered by this method can be quantitatively characterized a t initial supersaturations lower than 4.51. The graphically differentiated curves showed that the maximum crystallization rate could be fitted by equation Vmax z K ( C -Co)2, where C and C, are the initial and the equilibrium concentrations, respectively. The crystallization rates after the maximum follow the same equation depending on the absolute supersaturation a t a given moment. A linear dependence between the rate constants and the supersaturations a t the maximum crystallization rates is found, This dependence is determined by the inversely proportional correlation of the distances between the steps of growth with the supersaturation given by equation (8). The activation growth energy E* = 7.1 kcal/mole is determined from the straight line obtained. Die Kristallisationskinetik von Zinkoxalat bei verschiedenen Ubersattigungen wirdanhand einer turbidimetrisehen Methode untersucht. Es wird festgestellt, da8 die nach dieser Methode optisch registrierten Kurven bei Ausgangsiibersattigungen unter 4,51 quantitativ charakterisiert werden konnen. Die graphisch differenzierten Kurven zeigen, daB fur die maximale Kristallisationsgeschwindigkeit die Gleichung Vmax = K ( C -C,)2 giiltig ist, in der C und C, die Ausgangs-bzw. Gleichgewichtskonzentration bezeichnen. Nach dem Maximum folgen die Kristallisationsgeschwindigkeiten derselben Gleichung in Abhiingigkeit von der absoluten Ubersattigung in einem bestimmten Augenblick. Zwischen den Geschwindigkeitskonstanten und den Ubersattigungen bei den maximalen Kristallisationsgeschwindigkeiten besteht eine lineare Abhiingigkeit, die durch das umgekehrt proportionale Verhiiltnis der Abstiinde zwischen den Wachstumsstufen zu der von Gleichung (8) angegebenen Ubersiittigung bestimmt wird. Die Aktivierungsenergic des Kristallwachstums E * = 7 , l kcal/mol wird aus der erhaltenen Gerade bestimmt,