Abstract
Rate of precipitation and quantitative separation of nickel sulfide in solutions buffered with ammonia and ammonium nitrate wherein a stoichiometric equation CH3CSNH2+2NH3+Ni2+→CH3C(=NH2)NH2+NiS+NH4+ persists have been studied over the temperature range from 50 to 80°C with the concentration of ammonia from 0.4 to 1.2M. An apparent rate equation -d[Ni]/dt=k[CH3CSNH2]=(kOH{OH-}+kb[NH3]) [CH3CSNH2] was obtained where kOH=27.4l/mol·min, kb=1.26×10-2l/mol·min at 65°C and ionic strength of 0.5, and the energy of activation regarding rate const. of k was found to be 16, 000cal/mol.
It has been concluded that the precipitation reaction of nickel sulfide in ammoniacal solution is a general base catalysis by hydroxyl ion and ammonia, the experimental evidence of which rests essentially on the form of the observed rate constant, k, which is the sum of the two terms, namely, k=kOH{OH-}+kb[NH3]. This supports the present view that the acid-base reaction between thioacetamide and the bases is a rate-determining step in the precipitation of sulfide.
The sequence of the reactions is
CH3-C-S-=NH2+NH3→∧[CH3-C_??_S_??_NH]-+NH4+ (I)
then, [CH3-C_??_S_??_NH]-+Ni(NH3)m2+→CH3-C-NH2+NiS+(m-1)NH3 (II)=+NH2
The crystal structure of nickel sulfide obtained by the thioacetamide method was found to belong to Hexagonal-NiAs type by means of X-ray powder method.
