Ni-Ti layered double hydroxides, as nickel hydroxide, can be employed as catalysts for oxidation reactions, active materials for accumulators and supercapacitors, electrochromic materials etc. A detailed study on their properties is required for optimization of synthesis method.
In order to determine the solubility product for Ni-Ti layered double hydroxide the solutions of TiOCl2, Ni(NO3)2 and TiOCl2+Ni(NO3)2 (molar ratio of Ni2+/Ti4+=5) have been titrated with solution of NaOH. As a results, there titration curves in coordinates pH-ml of added titrant (Fig. 1).
Figure 1 – Dependency of pH of titrated solution on volume of added titrant.
The calculations were carried out taking into account the activity ion activity coefficient and changes in solution’s volume. Ion concentration and ionic strength of solution, were calculated assuming stoichiometric reaction between reactants according to assumed reactions :
TiO2++2OH-→ TiO(OH)2 (1)
Ni2++2OH- →Ni(OH)2 (2)
TiO(OH)2+ 5Ni(OH)2+2NO3-+nH2O→Ni5Ti(OH)12(NO3)2∙mH2O (3)
As a result of calculations, pKsp values were obtained for following compounds: TiO(OH)2= 28,97, Ni(OH)2= 9,40, Ni5Ti(OH)12(NO3)2=90,51. Because of major deviation of pKsp value for Ni(OH)2 from literature value 14,70 , an assumption has been made that for precipitation of Ni2+ a lower amount of base is required than for reaction (2).
In order to determined the stoichiometry for precipitation reaction of Ni2+ cation the combined method conductometeric and potentiometric titration method was used.
Based on result of conducted experiment the curves in coordinates pH–OH–/Ni2+ ratio and R–OH-/Ni2+ ratio (Fig.2) have been build. The resistance curve has three regions, that correspond to following OH-/Ni2+ ratio: region I – 0,3-0,88, region II – 0,88-1,87 and region III – 1,87-2,2. The decrease of solutions’ resistance in region is due to temperature increase caused by heat of precipitation reaction. The resistance increase in region I is related to the precipitate formation in dilution of solution. Sharp decrease of resistance in region III, signifies presence of free OH- ions and complete precipitation of Ni2+ cation from solution at point 1,87. This fact allows to assume that in regions I and II the formation of basic salt with composition Ni(OH)1,87(NO3)0,13 occurs, which during ageing transform into nickel hydroxide.
Figure 2 – Dependency of pH and R on OH-/Ni2+ ratio during titration.
Based on titration results, the calculation of solubility product for basic nickel nitrate have been conducted. As a results of calculations, the value of pKsp Ni(OH)1,87(NO3)0,13= 13,66 has been obtained. For Ni5Ti(OH)12(NO3)2 an additional calculation of pKsp value has been carried out. Taking into account the discovered stoichiometry oбыл проведен повторный расчет значения pПР с учетом обнаруженной стехиометрии реакции осаждения никеля, и было получено значение 101,98.
- Boclair J. W., Braterman P. S. Layered Double Hydroxide Stability. 1. Relative Stabilities of Layered Double Hydroxides and Their Simple Counterparts, Chem. Mater., 1999, 11, 298 – 302
- Lure Yu. Yu. SPRAVOCHNIK PO ANALITICHESKOY HIMII — M.: Himiya, 1971. — 456