Study of the behavior of chromium(III) in potassium nitrate solutions at 25°C

Abstract

458Potassium nitrate of high purity grade is used in several new fields of industry (optics, fiber optics, etc.) that impose high requirements to the content of impu rities that give color to glass (including chromium) [1].No solubility data for the KNO 3 -Cr(NO 3 ) 3 -H 2 O system at 25°С have been found in literature. Only the solubilities of potassium and chromium nitrates in water at 25°С have been determined; their values are 27.39 [2], 27.52 [3], 27.94 [4], and 44.80 wt % [5, 6], respectively. Cr(NO 3 ) 3 ⋅ 9H 2 O of pure grade and pure for analysis grade are commercially available (GOST 4471) [7].In aqueous solutions, chromium(III) in microcon centrations (≤1 × 10 -5 mol/L) exists in the form of a hydrated cation [Сr(H 2 O) 6 ] 3+ at pH 1.0-3.5; the for mation of a mononuclear hydroxo complex Cr(OH) 2+ was found at pH 3.5-4.0. The accumulation of Cr(OH) 2 + and complexes begins at pH > 5 and pH > 7, respectively [8].In this work the behavior of chromium(III) was studied in a wide range of concentrations in potassium nitrate solutions at 25°С. EXPERIMENTALTo gain data on the chromium(III) behavior in potassium nitrate solutions, the macro and micro concentration regions were studied.In the chromium(III)microconcentration range, the study was carried out by the solubility method. KNO 3 (high purity grade) (technical specifications 6 09 354) and chromium(III) nitrate (pure for analy sis grade; State Standard 4471) preliminarily recrystal lized from aqueous solutions and dried were used as initial materials. According to chemical analysis and IR spectra, the compounds corresponded to potas Cr(OH) 0 3 sium nitrate [9, 10] and chromium(III) nitrate non ahydrate [11].Potassium contents in samples of liquid phases and solid residues were determined by a gravimetric method [12], and chromium(III) contents, by a com plexonometry method [13]. The potassium and chro mium(III) contents when they were present together were found by analyzing two solution aliquots. To determine potassium, 0.6% aqueous solution of sodium tetraphenylborate was added to the first ali quot; the precipitate of potassium tetraphenylborate was transferred to a glass filter (POR 16), dried at 105°С to a constant weight, and weighed. To deter mine chromium(III), a precisely measured amount of 0.05 M Trilon B solution was added to the second ali quot and the mixture was boiled for 20 min. The solu tion was cooled, and the excess of 0.05 M Trilon B solution was titrated with 0.05 M zinc nitrate solution at pH 10.5-11.0 (ammonia buffered solution) in the presence of sulfarsazene (change in color from green to wine red). The metrological characteristics of the procedures (permissible discrepancies between two parallel definitions of 0.5 relative % and the relative value of a total analysis inaccuracy of 1.5%) were determined by variance analysis.The IR spectra of solid phases were taken on a Per kin Elmer 983 G spectrophotometer in the range of 200-4000 cm -1 . Samples were prepared pressing tab lets with potassium bromide.Chromium microamounts were ...