2244 C. H. LIU, JOHN HASSON, AND G . PEDRO SMITH
Inorgunic Chemistry
CONTRIBUTIOS FROM THE OAK AND
RIDGENATIOSAL LABORATORY, OAK RIDGE, TENNESSEE, TLABORATORY, ~ UPTON, ~ NETT ~YORK ~
BROOKHAVES h
Potentiometric and Spectrophotometric Studies of Nickel(I1)-Nitrate Complexes i n Molten Dimethyl Sulfone and in the Molten LiN03-NaN03-KN03Eutectic'" BY C. H. LIU, JOHX HASSOS,")
AND
G. P E D R O SMITH1c
Recezeed Jlaich 20, 1968 Kickel(I1) was shown potentiometrically t o form successive 1: 2 and 1: 3 complexes with nitrate in dimethyl sulfone a t 1%5", and values of the formation constants were determined. An amalgamated nickel coil served as the indicator electrode, 2nd the silver(1)-silver system was used as the reference electrode. These potentiometric results were confirmed by spectrophotometric measurements, and the ligand field spectra of the nickel(I1) species involved were determined. T h e spectrum of nickel(I1) in the molten LiSO3-h-aS03-KN03 eutectic a t 125' was measured and found t o correspond closely t o t h a t of the 1 : 3 complex.
Introduction Molten nitrates have been widely used as solvents in electrochemical and spectrophotometric investigat i o n ~ . ~In, ~these media nickel(I1) is coordinated to nitrate oxygens with a more or less octahedral geometry4 but the structure and composition of the complex have not been identified. As an initial step toward determining the complex in nitrate salt melts, we studied the formation of successive nickel(I1)nitrate complexes in molten dimethyl sulfone a t 125" by potentiometric and spectrophotometric methods and measured the nickel(I1) spectrum in the molten LiN03-NaN03-KN03 eutectic a t the same temperature. The results, reported here, show the formation of 1 : 2 and 1 : 3 complexes in molten dimethyl sulfone and the near-identity of the spectrum of the complex in the nitrate salt eutectic with that of the 1: 3 complex. Experimental Section Chemicals. Dimethyl Sulfone.-Commercial reagent XTas recrystallized once from water and three times from methanol and then vacuum dried. Lithium Perchlorate .-Perchloric acid was allowed t o react with lithium carbonate t o yield the hydrated salt which was then dried in the molten state at 240'. A detailed preparative procedure is given e1sewhere.j Silver Perchlorate .-Perchloric acid was slowly added t o silver carbonate. After filtration, the solution was evaporated t o dryness, and t h e white product was dried a t 170". Nickel Perchlorate Hexahydrate .-Perchloric acid was slowly added t o excess nickel carbonate. After filtration, t h e solution was evaporated until it appeared saturated. T h e green needles, which separated upon cooling, were filtered out and stored while moist in a desiccator over anhydrous calcium sulfate, which does not remove the water of hydration. Nickel Nitrate Hexahydrate .-Commercial reagent was twice recrystallized from water. Lithium Nitrate .-Commercial reagent was twice recrystallized from water and dried at 1.50'. (1) (a) This research was sponsored by the United States Atomic Energy Commission. Parts of this paper are based on t h e Ph.D. Thesis of John Hasson, Polytechnic Institute of Brooklyn, 1967, (b) Nuclear Chemistry Division, Brookhaven National Laboratory. (c) Metals and Ceramics Division, Oak Ridge National Laboratory. (2) C. H. Liu, K . E . Johnson, and H. A. Laitinen in "h.Iolten Salt Chemist r y , " R.1. Blander, E d . , Interscience Publishers, Xew York, N . Y . , 1964, p p 681-i33. (3) G . P. Smith in "Molten Salt Chemistry," M.Blander, Ed., Interscience Publishers, Xew Yor-k, S . Y . , 1964, pp 427-503. (4) I ) . i\I. Gruen and 12. L. McBeth. J . P h y s . Chevz., 63, 3'33 (1959). (5) C. H. Liu, L. S e w m a n , and J. Hasson, t o be submitted for publication.
Nickel Perchlorate and Nitrate Solutions in Dimethyl Sulfone. --A weighed amount of the hydrated nickel salt was added t o a desired amount of dimethyl sulfone. T h e mixture !vas heated t o 130-150", and dry argon or nitrogen was passed vigorously through the solution overnight. After t h e flushing was c o n pleted, the solution was brought to volume with enough dimethyl sulfone and lithium perchlorate t o give the desired concentration of nickel(I1). -4nalysis indicated a water content less than O.Ol';.l in these preparations. In several cases, the nickel contents were analyzed t o confirm the calculated concentrations. Potentiometric Measurements.-In these measurements, a silver perchlorate-silver reference electrode and an amalgamated nickel wire indicator electrode were used. T h e suitability of these electrodes in dimethyl sulfone has already been demonstrated .e Individual electrode compartments were separated from the main bath by sintered-glass frits, which also served as salt bridges. Lithium perchlorate a t a concentration of 2 M was used as the inert electrolyte. The volume of the solution in each compartment was calculated from weight and density data. Silver(I ) was added t o t h e reference electrode compartnient either as silver perchlorate or b y coulometric generation. In some cases, t h e nickel perchlorate solution in dimethyl sulfone, previously described, was used in the sample compartment; nickel(I1) could also be generated coulometrically. T h e potential of an amalgamated nickel indicator electrode in the nickel(l1) solution was measured against the silver reference electrode. Lithium nitrate was then added in successive weighed portions, and the potential measurement was repeated after each addition. X detailed description of the experimental setup and procedure is given elsewhere.5 Spectrophotometric Measurements.-The spectra were obtained on a Cary Model 14H spectrophotometer with a furnace which has been described elsewhere.6 Square silica cells with a 1-cm path length were used. T h e absorption curves of the solvent media and t h e solutions of interest m-ere measured against air as reference, and the absorbances of the solutions were obtained by difference. A weighed amount of dimethyl sulfone solution of nickel perchlorate or nickel nitrate in 3 JI lithium perchlorate was placed in a cell, and the absorption curve was measured. Successive weighed portions of lithium nitrate were added t o the solution, and the spectral measurement was repeated after each addition. T h e volumes of t h e solutions were calculated from their total weights and densities. T h e latter were determined b y the float method.' T h e molar concentrations of nickel(I1) ranged from 0.01 to 0.07 M , and the highest nitrate concentration examined was 1.5 Jf. In separate experiments, the absorption spectra of solutions of similar compositions, except for the absence of nickel(II), were measured. Kickel(I1) solutions in t h e LiS03-KSOs-NaN03 eutectic were ( 6 ) C. 12. Boston and G . P. Smith, Rev. Sci. I ; r s h , 3 6 , 206 (1Y63) ( 7 ) C. I
