Editing Superoxide (section)

==Salts==

Superoxide forms salts with [[alkali metal]]s and [[alkaline earth metal]]s. The salts [[sodium superoxide]] ({{chem2|NaO2}}), [[potassium superoxide]] ({{chem2|KO2}}), [[rubidium superoxide]] ({{chem2|RbO2}}) and [[caesium superoxide]] ({{chem2|CsO2}}) are prepared by the reaction of {{chem2|O2}} with the respective alkali metal.<ref>{{cite book|last1=Holleman|first1=A.F.|title=Inorganic chemistry|date=2001|publisher=Academic Press, W. de Gruyter|location=San Diego, CA & Berlin|isbn=0-12-352651-5|edition=1st English|editor-first=Nils|editor-last=Wiberg}}</ref><ref>{{cite journal|title= The Preparation of Calcium Superoxide from Calcium Peroxide Diperoxyhydrate|first1=E.|last1=Vernon Ballou|first2=Peter|last2=C. Wood|first3=LeRoy|last3=A. Spitze|first4=Theodore|last4=Wydeven|date=1 July 1977|journal=Ind. Eng. Chem. Prod. Res. Dev.|volume=16|issue=2 |pages=180–186 |doi=10.1021/i360062a015}}</ref>

The alkali salts of {{chem2|O2-}} are orange-yellow in color and quite stable, if they are kept dry. Upon dissolution of these salts in water, however, the dissolved {{chem2|O2-}} undergoes [[disproportionation]] (dismutation) extremely rapidly (in a [[pH]]-dependent manner):<ref>{{Cotton&Wilkinson5th|page=461}}</ref>

:{{chem2|4 O2- + 2 H2O → 3 O2 + 4 OH-}}

This reaction (with moisture and carbon dioxide in exhaled air) is the basis of the use of [[potassium superoxide]] as an oxygen source in [[chemical oxygen generator]]s, such as those used on the [[Space Shuttle]] and on [[submarine]]s. Superoxides are also used in [[firefighter]]s' [[oxygen tank]]s to provide a readily available source of oxygen. In this process, {{chem2|O2-}} acts as a [[acid-base reaction theories|Brønsted base]], initially forming the [[hydroperoxyl]] radical ({{chem2|HO2}}).

The superoxide anion, {{chem2|O2-}}, and its protonated form, [[hydroperoxyl]], are in [[chemical equilibrium|equilibrium]] in an [[aqueous solution]]:<ref name="pka">{{cite journal |url=https://openlibrary.org/b/OL14350787M/Reactivity_of_HO2_O2_Radicals_in_Aqueous_Solution |title=Reactivity of HO<sub>2</sub>/O<sub>2</sub><sup>−</sup> Radicals in Aqueous Solution |journal=J. Phys. Chem. Ref. Data |date=1985 |volume=14 |issue=4 |pages=1041–1091 |doi=10.1063/1.555739 |bibcode=1985JPCRD..14.1041B |url-access=<!-- free -->|last1=Bielski |first1=Benon H. J. |last2=Cabelli |first2=Diane E. |last3=Arudi |first3=Ravindra L. |last4=Ross |first4=Alberta B.}}</ref>
:{{chem2|O2- + H2O ⇌ HO2 + OH-}}

Given that the hydroperoxyl radical has a [[pKa|p''K''<sub>a</sub>]] of around 4.8,<ref>{{cite web|url=http://www.sens.org/files/pdf/manu10.pdf|title={{chem|HO|2|•}}: the forgotten radical Abstract|archive-url=https://web.archive.org/web/20170808150022/http://www.sens.org/files/pdf/manu10.pdf|archive-date=2017-08-08|url-status=dead}}</ref> superoxide predominantly exists in the anionic form at neutral pH.

Potassium superoxide is soluble in [[dimethyl sulfoxide]] (facilitated by [[crown ether]]s) and is stable as long as protons are not available. Superoxide can also be generated in [[aprotic]] solvents by [[cyclic voltammetry]].

Superoxide salts also decompose in the solid state, but this process requires heating:

:{{chem2|2 NaO2 → Na2O2 + O2}}