Infra-red and Raman spectral study of the aqueous nickel(II)–nitrite system. Evidence for photochemical alteration of the chemical equilibrium

Abstract

Infra-red and Raman spectra of aqueous nickel(II)–nitrite systems have been obtained for a wide range of compositions. Stability constants for the nickel(II)–nitrite system have been measured by infra-red and Raman techniques. The infra-red and Raman data give completely different stability constants. Raman studies indicate the existence of Ni(NO₂)₂(H₂O)₄ exclusively with β₂= 0.15 ± 0.02 in excellent agreement with previous work. Infra-red data indicate the more usual step-wise formation of the complexes Ni(NO₂)(H₂O)⁺₅, Ni(NO₂)₂(H₂O)₄ and Ni(NO₂)₃(H₂O)^–₃ with β₁= 2.2 ± 0.5, β₂= 0.45 ± 0.2 and β₃= 0.65 ± 0.2 but the existence of higher order complexes could not be excluded. To rationalize the difference between the two sets of data it is postulated that visible light in the 400–600 nm region alters the chemical equilibrium through rapid photochemical aquation. The infra-red data more closely describe the nickel(II)–nitrite system in the absence of light.