Equilibria and kinetics of chromium(VI) speciation in aqueous solution – A comprehensive study from pH 2 to 11

Mária Szabó, József Kalmár, Tamás Ditrói, Gábor Bellér, G. Lente, Nina Simic, I. Fábián

Research output: Contribution to journalArticle

9 Citations (Scopus)

Abstract

A coherent and comprehensive speciation study was conducted in the aqueous Cr(VI) system in order to clarify several discrepancies in the literature. Using pH potentiometric UV–vis titrations and stopped-flow kinetic measurements, it was shown that the dominant Cr(VI) species are Cr2O7 2−, HCrO4 and CrO4 2− in an aqueous solution of Cr(VI) under ambient conditions ([Cr(VI)] = 7.7 mM–155 mM, pH = 2–11, I = 1.00 M NaClO4, T = 25.0 °C). This paper provides a comprehensive interpretation of the kinetic features of the dimerization equilibria of Cr(VI) in the acidic – alkaline pH range from pH 2 to 11. Under acidic conditions, the conversion of HCrO4 and Cr2O7 2− is catalyzed by H+. This observation is consistent with the formation of a protonated intermediate, which is presumably HCr2O7 . The forward and backward rate constants for the 2 HCrO4 ⇌ Cr2O7 2− + H2O reaction were determined at various pH values in concentration jump experiments. In the absence of catalysts, the slowest dimerization reaction was observed at pH 7 with a lifetime of a few minutes. At basic pH, OH reacts with Cr2O7 2− to give CrO4 2− on stopped-flow timescale. The basic hydrolysis of Cr2O7 2− is irreversible, and the reaction is first order with respect to both Cr2O7 2− and OH. The results presented here are consistent with some of the previously published experimental observations, and resolve the inconsistencies in the literature of Cr(VI) speciation.

Original languageEnglish
Pages (from-to)295-301
Number of pages7
JournalInorganica Chimica Acta
Volume472
DOIs
Publication statusPublished - Mar 1 2018

Fingerprint

Dimerization
chromium
Chromium
aqueous solutions
Kinetics
kinetics
Titration
Hydrolysis
Rate constants
Catalysts
dimerization
Experiments
chromium hexavalent ion
titration
hydrolysis
catalysts
life (durability)

Keywords

  • Chromium(VI)
  • Speciation equilibrium
  • Speciation kinetics

ASJC Scopus subject areas

  • Inorganic Chemistry
  • Physical and Theoretical Chemistry
  • Materials Chemistry

Cite this

Equilibria and kinetics of chromium(VI) speciation in aqueous solution – A comprehensive study from pH 2 to 11. / Szabó, Mária; Kalmár, József; Ditrói, Tamás; Bellér, Gábor; Lente, G.; Simic, Nina; Fábián, I.

In: Inorganica Chimica Acta, Vol. 472, 01.03.2018, p. 295-301.

Research output: Contribution to journalArticle

Szabó, Mária ; Kalmár, József ; Ditrói, Tamás ; Bellér, Gábor ; Lente, G. ; Simic, Nina ; Fábián, I. / Equilibria and kinetics of chromium(VI) speciation in aqueous solution – A comprehensive study from pH 2 to 11. In: Inorganica Chimica Acta. 2018 ; Vol. 472. pp. 295-301.
@article{3361e12dace1420e8ca9078b4131bb06,
title = "Equilibria and kinetics of chromium(VI) speciation in aqueous solution – A comprehensive study from pH 2 to 11",
abstract = "A coherent and comprehensive speciation study was conducted in the aqueous Cr(VI) system in order to clarify several discrepancies in the literature. Using pH potentiometric UV–vis titrations and stopped-flow kinetic measurements, it was shown that the dominant Cr(VI) species are Cr2O7 2−, HCrO4 − and CrO4 2− in an aqueous solution of Cr(VI) under ambient conditions ([Cr(VI)] = 7.7 mM–155 mM, pH = 2–11, I = 1.00 M NaClO4, T = 25.0 °C). This paper provides a comprehensive interpretation of the kinetic features of the dimerization equilibria of Cr(VI) in the acidic – alkaline pH range from pH 2 to 11. Under acidic conditions, the conversion of HCrO4 − and Cr2O7 2− is catalyzed by H+. This observation is consistent with the formation of a protonated intermediate, which is presumably HCr2O7 −. The forward and backward rate constants for the 2 HCrO4 − ⇌ Cr2O7 2− + H2O reaction were determined at various pH values in concentration jump experiments. In the absence of catalysts, the slowest dimerization reaction was observed at pH 7 with a lifetime of a few minutes. At basic pH, OH− reacts with Cr2O7 2− to give CrO4 2− on stopped-flow timescale. The basic hydrolysis of Cr2O7 2− is irreversible, and the reaction is first order with respect to both Cr2O7 2− and OH−. The results presented here are consistent with some of the previously published experimental observations, and resolve the inconsistencies in the literature of Cr(VI) speciation.",
keywords = "Chromium(VI), Speciation equilibrium, Speciation kinetics",
author = "M{\'a}ria Szab{\'o} and J{\'o}zsef Kalm{\'a}r and Tam{\'a}s Ditr{\'o}i and G{\'a}bor Bell{\'e}r and G. Lente and Nina Simic and I. F{\'a}bi{\'a}n",
year = "2018",
month = "3",
day = "1",
doi = "10.1016/j.ica.2017.05.038",
language = "English",
volume = "472",
pages = "295--301",
journal = "Inorganica Chimica Acta",
issn = "0020-1693",
publisher = "Elsevier BV",

}

TY - JOUR

T1 - Equilibria and kinetics of chromium(VI) speciation in aqueous solution – A comprehensive study from pH 2 to 11

AU - Szabó, Mária

AU - Kalmár, József

AU - Ditrói, Tamás

AU - Bellér, Gábor

AU - Lente, G.

AU - Simic, Nina

AU - Fábián, I.

PY - 2018/3/1

Y1 - 2018/3/1

N2 - A coherent and comprehensive speciation study was conducted in the aqueous Cr(VI) system in order to clarify several discrepancies in the literature. Using pH potentiometric UV–vis titrations and stopped-flow kinetic measurements, it was shown that the dominant Cr(VI) species are Cr2O7 2−, HCrO4 − and CrO4 2− in an aqueous solution of Cr(VI) under ambient conditions ([Cr(VI)] = 7.7 mM–155 mM, pH = 2–11, I = 1.00 M NaClO4, T = 25.0 °C). This paper provides a comprehensive interpretation of the kinetic features of the dimerization equilibria of Cr(VI) in the acidic – alkaline pH range from pH 2 to 11. Under acidic conditions, the conversion of HCrO4 − and Cr2O7 2− is catalyzed by H+. This observation is consistent with the formation of a protonated intermediate, which is presumably HCr2O7 −. The forward and backward rate constants for the 2 HCrO4 − ⇌ Cr2O7 2− + H2O reaction were determined at various pH values in concentration jump experiments. In the absence of catalysts, the slowest dimerization reaction was observed at pH 7 with a lifetime of a few minutes. At basic pH, OH− reacts with Cr2O7 2− to give CrO4 2− on stopped-flow timescale. The basic hydrolysis of Cr2O7 2− is irreversible, and the reaction is first order with respect to both Cr2O7 2− and OH−. The results presented here are consistent with some of the previously published experimental observations, and resolve the inconsistencies in the literature of Cr(VI) speciation.

AB - A coherent and comprehensive speciation study was conducted in the aqueous Cr(VI) system in order to clarify several discrepancies in the literature. Using pH potentiometric UV–vis titrations and stopped-flow kinetic measurements, it was shown that the dominant Cr(VI) species are Cr2O7 2−, HCrO4 − and CrO4 2− in an aqueous solution of Cr(VI) under ambient conditions ([Cr(VI)] = 7.7 mM–155 mM, pH = 2–11, I = 1.00 M NaClO4, T = 25.0 °C). This paper provides a comprehensive interpretation of the kinetic features of the dimerization equilibria of Cr(VI) in the acidic – alkaline pH range from pH 2 to 11. Under acidic conditions, the conversion of HCrO4 − and Cr2O7 2− is catalyzed by H+. This observation is consistent with the formation of a protonated intermediate, which is presumably HCr2O7 −. The forward and backward rate constants for the 2 HCrO4 − ⇌ Cr2O7 2− + H2O reaction were determined at various pH values in concentration jump experiments. In the absence of catalysts, the slowest dimerization reaction was observed at pH 7 with a lifetime of a few minutes. At basic pH, OH− reacts with Cr2O7 2− to give CrO4 2− on stopped-flow timescale. The basic hydrolysis of Cr2O7 2− is irreversible, and the reaction is first order with respect to both Cr2O7 2− and OH−. The results presented here are consistent with some of the previously published experimental observations, and resolve the inconsistencies in the literature of Cr(VI) speciation.

KW - Chromium(VI)

KW - Speciation equilibrium

KW - Speciation kinetics

UR - http://www.scopus.com/inward/record.url?scp=85019760471&partnerID=8YFLogxK

UR - http://www.scopus.com/inward/citedby.url?scp=85019760471&partnerID=8YFLogxK

U2 - 10.1016/j.ica.2017.05.038

DO - 10.1016/j.ica.2017.05.038

M3 - Article

AN - SCOPUS:85019760471

VL - 472

SP - 295

EP - 301

JO - Inorganica Chimica Acta

JF - Inorganica Chimica Acta

SN - 0020-1693

ER -