Open main menu
Home
Random
Recent changes
Special pages
Community portal
Preferences
About Wikipedia
Disclaimers
Incubator escapee wiki
Search
User menu
Talk
Dark mode
Contributions
Create account
Log in
Editing
Chromate and dichromate
(section)
Warning:
You are not logged in. Your IP address will be publicly visible if you make any edits. If you
log in
or
create an account
, your edits will be attributed to your username, along with other benefits.
Anti-spam check. Do
not
fill this in!
=== Acid–base properties === In aqueous solution, chromate and dichromate anions exist in a [[chemical equilibrium]]. :{{Chem2|2 CrO4(2-) + 2 H+ ⇌ Cr2O7(2-) + H2O}} The [[predominance diagram]] shows that the position of the equilibrium depends on both [[pH]] and the analytical concentration of chromium.<ref group=notes>pCr is equal to the negative of the decimal logarithm of the [[molar concentration]] of chromium. Thus, when pCr = 2, the chromium concentration is 10<sup>−2</sup> mol/L.</ref> [[File:Predominance diagram Cr.png|thumb|left|[[Predominance diagram]] for chromate]] The chromate ion is the predominant species in alkaline solutions, but dichromate can become the predominant ion in acidic solutions. Further condensation reactions can occur in strongly acidic solution with the formation of '''trichromates''', {{chem2|Cr3O10(2−)}}, and '''tetrachromates''', {{chem2|Cr4O13(2−)}}.<ref>{{Cite journal |last1=Nazarchuk |first1=Evgeny V. |last2=Siidra |first2=Oleg I. |last3=Charkin |first3=Dmitry O. |last4=Kalmykov |first4=Stepan N. |last5=Kotova |first5=Elena L. |date=2021-02-01 |title=Effect of solution acidity on the crystallization of polychromates in uranyl-bearing systems: synthesis and crystal structures of Rb2[(UO2)(Cr2O7)(NO3)2] and two new polymorphs of Rb2Cr3O10 |url=https://www.degruyter.com/document/doi/10.1515/zkri-2020-0078/html |journal=Zeitschrift für Kristallographie - Crystalline Materials |language=en |volume=236 |issue=1–2 |pages=11–21 |doi=10.1515/zkri-2020-0078 |s2cid=231808339 |issn=2196-7105|url-access=subscription }}</ref> All poly[[oxyanions]] of chromium(VI) have structures made up of tetrahedral {{chem2|CrO4}} units sharing corners.<ref>{{Greenwood&Earnshaw2nd|page = 1009}}</ref> The hydrogen chromate ion, {{chem2|HCrO4−}}, is a [[weak acid]]: :{{chem2|HCrO4− ⇌ CrO4(2−) + H+}};{{spaces|5}} p''K''<sub>a</sub> ≈ 5.9 It is also in equilibrium with the dichromate ion: :{{chem2|2 HCrO4− ⇌ Cr2O7(2−) + H2O}} This equilibrium does not involve a change in hydrogen ion concentration, which would predict that the equilibrium is independent of pH. The red line on the predominance diagram is not quite horizontal due to the simultaneous equilibrium with the chromate ion. The hydrogen chromate ion may be protonated, with the formation of molecular [[chromic acid]], {{chem2|H2CrO4}}, but the [[acid dissociation constant|p''K''<sub>a</sub>]] for the equilibrium :{{Chem2|H2CrO4 ⇌ HCrO4- + H+}} is not well characterized. Reported values vary between about −0.8 and 1.6.<ref>[http://www.acadsoft.co.uk/scdbase/scdbase.htm IUPAC SC-Database]. A comprehensive database of published data on equilibrium constants of metal complexes and ligands.</ref> The dichromate ion is a somewhat weaker base than the chromate ion:<ref>{{cite journal |last=Brito |first=F. |author2=Ascanioa, J. |author3=Mateoa, S. |author4=Hernándeza, C. |author5=Araujoa, L. |author6=Gili, P. |author7=Martín-Zarzab, P. |author8=Domínguez, S. |author9=Mederos, A. |year=1997 |title=Equilibria of chromate(VI) species in acid medium and ab initio studies of these species |journal=Polyhedron |volume=16 |issue=21 |pages=3835–3846 |doi=10.1016/S0277-5387(97)00128-9}}</ref> :{{Chem2|HCr2O7- ⇌ Cr2O7(2-) + H+}},{{spaces|5}} p''K''<sub>a</sub> = 1.18 The p''K''<sub>a</sub> value for this reaction shows that it can be ignored at pH > 4.
Edit summary
(Briefly describe your changes)
By publishing changes, you agree to the
Terms of Use
, and you irrevocably agree to release your contribution under the
CC BY-SA 4.0 License
and the
GFDL
. You agree that a hyperlink or URL is sufficient attribution under the Creative Commons license.
Cancel
Editing help
(opens in new window)