Extraction-Chromogenic System for Silver(I) Based on Tetrazolium Violet and Semicarbazide Hydrochloride

Abstract

In the presence of semicarbazide hydrochloride (SCH) and traces of Ag^I, the tetrazolium salt 2,5-diphenyl-3-(1-naphmyl)-2H-tetrazolium chloride (Tetrazolium violet, TV) is reduced in alkaline medium to intensely colored chloroform-extractable 3,5-diphenyl-1-(1-naphthyl)formazan (Tetrazolium violet formazan, TVF). The following experimental parameters have been examined for finding optimal conditions for color development and extraction-spectrophotometric determination of Ag^I: concentration of reagents, extraction time, volume of the two phases, presence of dissolved oxygen, and light. Under the optimal conditions (c_TV = 4.0 × 10⁻⁵ mol/dm³, c_SCH = 2.4 × 10⁻³ mol/dm³, c_NaOH = 4 × 10⁻¹ mol/dm³, V_aq.phase = 25 cm³, V_chloroform = 4 cm³, extraction time t = 5 min) an isomer of TVF with λ_max = 511–512 nm is predominantly extracted. The calibration curve (at 512 nm) is linear in the concentration range of 0.013–0.028 µg cm⁻³ of Ag^I. The following ions do not interfere in determination of 0.65 µg of Ag^I; Li⁺, Na⁺, K⁺, \(\rm{NH}_4^+\), \(\rm{H}_2PO_4^-\), F−, Cl−, \(\rm{SO}_4^{2-}\), \(\rm{B}_4O_7^{2-}\), \(\rm{ReO}_4^-\), tartrate, and citrate (5000-fold excess); Mg^II and EDTA (1000-fold excess); Pb^II and Ba^II (500-fold excess); Al^III (300-fold excess); Zn^II and Ca^II (100-fold excess); V^V (50-fold excess); Hg^II, Hg^I, Mo^VI and Fe^III (25-fold excess); and Cu^II and W^VI (10-fold excess). Cr^III interferes in the process at a mass ratio of 1 : 1, however small amount of this cation (1.5-fold excess) can be masked by EDTA.