No abstract

A selective method for the spectrophotometric determination of V in the presence of other refractory elements is proposed. It is based on the re-examination of its redox reaction with 2,2'-iminodibenzoic acid in H2SO4 medium (11.1-11.4 mol 1-1) in the presence of H3P04. Up to 1 mg of TiIV, Z P , NbV, Mow and Wvl do not interfere. V should be pre-oxidized in H2SO4 medium (S6.5 rnol 1-1) at room temperature with a 0.1% m/v solution of potassium permanganate, the excess of which is removed by adding successively 1 ml of a 10% m/v solution of urea and a few drops of a 0.1% solution of sodium nitrite. The molar absorptivity of this system was found to be 1.2 X 104 1 mol-1 cm-1 at 610 nm. The Lambert-Beer law is obeyed over the concentration range 0.4-2.0 pg ml-1 of V with the linear regression equation A = 0.0188C-0.0229 (C = pg of V per 25 ml) and correlation coefficient r = 0.9990. The standard deviation ( n = 10) and relative standard deviation at the level of 20 pg of V were found to be 0.88 pg and 4.4%, respectively. The versatility of the proposed method was demonstrated by incorporation of the preconcentration of V with N-benzoyl-N-phenylhydroxylamine as precipitant and MoV1 as carrier.

An improved mole ratio method for the dual-wavelength spectrophotometric determination of the equilibrium concentration of a chromogenic ligand L in an aqueous binary system is proposed. Techniques involving the k factor and isoabsorbance points can be used to mask instrumentally the absorbance of the product formed. By varying x, the concentration of the metal ion M, and fixing y, that of L, added in excess to ensure virtual completion of the reaction under study, a set of AL values, the absorbance of free L, can be measured under conditions free from any side reaction. As AL = E ~/ ( Yxnlm), where E is the molar absorptivity, I is the optical pathlength and nlm is the mole ratio of M and L in the reaction product M , L , , a linear graph can be plotted with AL as the ordinate and x as the abscissa. By extrapolation it intersects the abscissa at xo(x = xo when AL = 0). Once xo has been found, the mole ratio nlm can be readily obtained from the above equation as yis known and AL = 0. This is an absolute method. Its special features are as follows. (1) A linear plot gives more reliable results. (2) Dispensation with the use of a reference solution ensures a precise correction for any concomitant interfering absorbance possibly present in the same solution. (3) It is feasible to determine AL, one of the two interdependent variables of absorption under the specified conditions, hence it is applicable also to systems in which the absorption spectra of L and M , L , cross or overlap each other.

A comparative study by elemental analysis, differential scanning thermal analysis, X-ray photoelectron spectrometry, 1H NMR spectrometry and infrared spectrometry revealed that the product of reaction of N-benzoyl-N-phenylhydroxylamine (BPHA) with Sn" is identical with that with Sn" and that BPHA acts merely as a chelating ligand, neither oxidizing Sn" nor reducing Sn". A reaction mechanism is proposed. The use of BPHA in a large excess facilitates the dissociation of anionic chlorocomplexes of Sn to the active species SnCI2*+ on the one hand, and shifts the redox equilibrium of Snn4nw on the other, favouring the Sn" state.