Studies on the hydrolysis of metal ions

Author

Paul Leslie Brown, University of Wollongong

Year

1984

Degree Name

Doctor of Philosophy

Department

Department of Chemistry

Recommended Citation

Brown, Paul Leslie, Studies on the hydrolysis of metal ions, Doctor of Philosophy thesis, Department of Chemistry, University of Wollongong, 1984. https://ro.uow.edu.au/theses/1179

Abstract

The potentiometric titration technique has been used to study the hydrolytic properties of six metal ions. Indium(III), thorium(IV), scandium(III), beryllium(II), and aluminium(III) were studied at 25°C and magnesium(II) was investigated at higher temperatures from 60° to 200°C.

Data analysis of the experiments performed at 25°C using a modified version of the MINIQUAD 75 program indicated the likely presence of the following species and formation constants: Indium(III): In(OH)²⁺ -4 .53(0.008), In(OH)₂ ⁺ -9.34(0.02), and In₂(OH)₂ ⁴⁺ -5 .48(0.007); Thorium(IV): Th(OH)³⁺ -2.98(0.007), Th₄(OH)₁₂ ⁴⁺ -30.55(0.03), and Th₆(OH)₁₅ -34.41(0.03); Scandium(III): Sc(OH)²⁺ -4.840(0.008), Sc₂(OH)₂ ⁴⁺ -6.096(0.004), and Sc₃(OH)₅ ⁴⁺ -17.567(0.006); Beryllium(II): Be(0H)₂ -11.320(0.008), Be₂(OH)³⁺ -2.955(0.007), and Be₃(0H)³⁺ -8.804(0.002); Aluminium(III): Al(OH)²⁺ -5.27(0.005), Al(0H)₂ ⁺ -10.88(0.02), and Al₃(OH)₄ ⁵⁺ -13.31(0.01). Data analysis of the high temperature experiments using the ORGLS program indicated the presence of Mg(OH)₂(solid) as the sole product. The following results were obtained at varying temperatures and ionic strengths: 60°, 0.10, -15.29(0.02); 100°, 0.10, -13.45(0.02); 150°, 0.10, -11.62(0.007); 200°, 0.10, -10.21(0.01); 60°, 1.00, -15.52(0.01); 100°, 1.00, -13.64(0.02); 150°, 1.00, -11.88(0.006), 200°, 1.00, -10.44(0.01). The standard deviations of the formation constants have been given in parentheses for all species.

These results together with previous literature results for other metal ions have been analysed with the aim of rationalising the hydrolytic behaviour of the ions in terms of a common, theoretical basis. A correlation between the formation constants of the hydrolytic species formed and the charge, ionic radius, and electronic structure of the hydrolysing metal ion was found and the following equation for predicting the formation constant of the species M_p(0H)q (ignoring charges) has been developed.

-log β_pq = [(p-1)int₁ - q(int₂)] + [(p-l)slp₁ - q(slp₂)] x [g₁(z/r² + g₂)]

In the equation, z is the charge, r the ionic radius, int₁ and slp₁ , and int₂ and slp₂, are the least squares intercept and slope values of linear equations for the M(0H) and polymeric M_p(0H)_q species, respectively, and g₁ and g₂ are functions dependent only upon the charge and electronic structure of the metal ion.

The results also suggested that a systematic testing mechanism for hydrolysis schemes could be developed. A subroutine was written for interfacing with the MINIQUAD program, which used a cluster search technique for this purpose.

Experimental design is discussed in detail. It is shown that the acquisition of a very large amount of very precise data in the appropriate concentration ranges (if experimentally accessible) is mandatory if meaningful results are to be obtained.