[Study on the complex formation reactions between rare earths (La3+, Pr3+) and DBS-Arsenazo with absorption spectrophotometry].

In this paper, ways of determining the composition and formation constants for rare earths (La3+, Pr3+)-DBS-Arsenazo (2-(2-arsonophenylazo)-7-(2,6-dibromo-4-sulphophenylazo)-1,8-dihydroxy-3, 6-naphthalene-disulfonic acid) complexes were discussed. La3+ or Pr3+ (colorless) reacts with DBS-Arsenazo which absorbs at 535 nm, blue-violet products absorbing at 630 nm are formed in 1 mol.L-1 HCl solution. To study the reactions, a series of spectra of the complexes were obtained by spectrophotometric titration of DBS-Arsenazo with La3+ or Pr3+. The number of the components contributing to the color systems was obtained by using factor analysis method. It shows that only two color components are in the system. Combined with structural consideration, it is possible that the reactions is as follows: La3+ or Pr3+ + nDBS-Arsenazo = La3+ or Pr(3+)-(DBS-Arsenazo)n Coloress absorbs at 535 nm absorbs strongly at 630 nm A linear regression method was proposed to determine the compositions, the formation constants and the concentration of the color reagent. The results show that La3+ or Pr3+ reacts with DBS-Arsenazo, and form 1:3 complexes in 1 mol.L-1 HCl solution. The conditional formation constants (log beta 3) determined are 15.36 and 15.34 respectively.