Pré-concentração e determinação de Hg(II) com ditizona empregando sistema ternário homogêneo de solventes

Abstract

Using a homogeneous ternary solvent system water-ethanol-chloroform, denominated of single-phase solution, procedures were developed for the spectrophotometric determination of Hg21 with ditizone. In the evaluated procedures aqueous solution of Hg21 at a pH 1,5 (or sample) substitutes the portion water and portion chloroform is substituted by ditizone-chloroform solption. The first procedure, that we denominated direct determination in single-phase solution, uses the single-phase solution water-ethanol-chloroform 1:6:3 v/v, respectively, and ditizone-chloroform solution 0,0025 % m/v. In this single-phase solution, where happens the formation of the Hg(HDz)2 complex, ions Hg2+ is determined comparing the analytic sign in analytic curve of calibration, obtained to 486 nm. With this procedure it is determined Hg2+ in the interval of concentration of 0.25 pg cm'3 to 1.00 pg cm’3. The second appraised procedure uses the same solution of ditizone-chloroform of the previous procedure, even so with a single-phase solution water-ethanol-chloroform 50:51:7 v/v, respectively. After the complexation reaction happened in this new single-phase solution, the Hg(HDz)2 complex is extracted for the organic phase by means of the addition of 50.0 cm3 of water at 40 °C. To follow the mercury extracted for the organic phase it is determined, to 486 nm, comparing the analytic sign in na analytic curve of calibration. In this procedure a pre-concentration of 7 times is observed for the solution Hg2 h and the calibration graph is lineal up to 0.15 pg cm'3 , according to the equation AAbs = 3.7566 Cng21- + 0.0206 (r2 = 0.995). The detection limit for this linear concentration range was estimated as 0.01 pg cm'3. In both procedures, significant interference of Fe31 is observed due to oxidation reaction with ditizone and also the interference of the ions Cu2 ' and Ag+ because these ions also reacts with ditizone in the same experimental conditions of the reaction of the ion Hg . It was observed that a viable alternative to minimize the eftects of this interference is the application of the Standard Method of Addition. This alternative was evaluated using Solutions samples simulated in Iaboratory with aqueous Solutions Hg2 t with the three interference species (Fe3 ',Cu2 ’ and Ag+) in the maximum concentrations allowed in waters, according to Conselho Nacional do Meio Ambiente. It was observed that the effects of the interferences become insignificant for concentration of ion Hg2+ starting from 0.30 pg cm'3. The accuracy of the methodology with pre-concentration associated to the Standard Method of Addition was evaluated with samples simulated in laboratory contends the inteferences and also for dopagem of samples of mineral water obtained in the local trade. Experimental values of the concentration of Hg2+ were compatible with the expected values, so much for the samples simulated in laboratory as for the dopagem of samples.