Editing Neptunium (section)

===Solution chemistry===
[[File:Np ox st.jpg|thumb|upright=1.6|right|Neptunium ions in solution]]
When it is in an aqueous solution, neptunium can exist in any of its five possible oxidation states (+3 to +7) and each of these show a characteristic color.<ref name="Dutkiewicz2017" /> The stability of each oxidation state is strongly dependent on various factors, such as the presence of [[oxidizing agent|oxidizing]] or [[reducing agent]]s, [[pH]] of the solution, presence of [[coordination complex]]-forming [[ligand]]s, and even the concentration of neptunium in the solution.<ref name="Yoshida753">Yoshida et al., pp. 752–4.</ref>

{| class="wikitable" style="float:right; clear:right; margin-left:1em; margin-top:0;"
|-
! Oxidation <br />state !! Representative compound
|-
| +2 || [K(2.2.2-crypt)][NpCp'<sub>3</sub>]
|-
| +3 || [[Neptunium(III) chloride]], NpCl<sub>3</sub>
|-
| +4 || [[Neptunium(IV) oxide]], NpO<sub>2</sub>
|-
| +5 || [[Neptunium(V) fluoride]], NpF<sub>5</sub>
|-
| +6 || [[Neptunium(VI) fluoride]], NpF<sub>6</sub>
|-
| +7 || [[Neptunium(VII) oxide-hydroxide]], NpO<sub>2</sub>(OH)<sub>3</sub> 
|}
In [[acid (chemistry)|acidic]] solutions, the neptunium(III) to neptunium(VII) ions exist as Np<sup>3+</sup>, Np<sup>4+</sup>, {{chem|NpO|2|+}}, {{chem|NpO|2|2+}}, and {{chem|NpO|3|+}}. In [[base (chemistry)|basic]] solutions, they exist as the oxides and hydroxides Np(OH)<sub>3</sub>, NpO<sub>2</sub>, NpO<sub>2</sub>OH, NpO<sub>2</sub>(OH)<sub>2</sub>, and {{chem|NpO|5|3-}}. Not as much work has been done to characterize neptunium in basic solutions.<ref name="Yoshida753" /> Np<sup>3+</sup> and Np<sup>4+</sup> can easily be reduced and oxidized to each other, as can {{chem|NpO|2|+}} and {{chem|NpO|2|2+}}.<ref name="Yoshida759">Yoshida et al., p. 759.</ref>

;Neptunium(III)
Np(III) or Np<sup>3+</sup> exists as hydrated complexes in acidic solutions, {{chem|Np|(H|2|O|)|''n''|3+}}.<ref name="lanl" /> It is a dark blue-purple and is analogous to its lighter [[congener (chemistry)|congener]], the pink [[rare-earth metal|rare-earth]] ion [[promethium|Pm<sup>3+</sup>]].<ref name="lanl" /><ref name="Greenwood">{{Greenwood&Earnshaw|page=1265}}</ref> In the presence of [[oxygen]], it is quickly oxidized to Np(IV) unless strong reducing agents are also present. Nevertheless, it is the second-least easily [[hydrolysis|hydrolyzed]] neptunium ion in water, forming the NpOH<sup>2+</sup> ion.<ref name="Yoshida768" /> Np<sup>3+</sup> is the predominant neptunium ion in solutions of pH&nbsp;4–5.<ref name="Yoshida768">Yoshida et al., p. 766–70.</ref>

;Neptunium(IV)
[[File:Np(IV) in 8 M HCl..jpg|thumb|Np(IV) in 8 M HCl.]]Np(IV) or Np<sup>4+</sup> is pale yellow-green in acidic solutions,<ref name="lanl" /> where it exists as hydrated complexes ({{chem|Np|(H|2|O|)|''n''|4+}}). It is quite unstable to hydrolysis in acidic aqueous solutions at pH&nbsp;1 and above, forming NpOH<sup>3+</sup>.<ref name="Yoshida768" /> In basic solutions, Np<sup>4+</sup> tends to hydrolyze to form the neutral neptunium(IV) hydroxide (Np(OH)<sub>4</sub>) and neptunium(IV) oxide (NpO<sub>2</sub>).<ref name="Yoshida768" />

;Neptunium(V)
Np(V) or {{chem|NpO|2|+}} is green-blue in aqueous solution,<ref name="lanl" /> in which it behaves as a strong [[Lewis acid]].<ref name="Yoshida753" /> It is a stable ion<ref name="Yoshida753" /> and is the most common form of neptunium in aqueous solutions. Unlike its neighboring homologues {{chem|UO|2|+}} and {{chem|PuO|2|+}}, {{chem|NpO|2|+}} does not spontaneously [[disproportionation|disproportionate]] except at very low pH and high concentration:<ref name="Yoshida759" />

:2 {{chem|NpO|2|+}} + 4 H<sup>+</sup> ⇌ Np<sup>4+</sup> + {{chem|NpO|2|2+}} + 2 H<sub>2</sub>O

It hydrolyzes in basic solutions to form NpO<sub>2</sub>OH and {{chem|NpO|2|(OH)|2|-}}.<ref name="Yoshida768" />

;Neptunium(VI)
Np(VI) or {{chem|NpO|2|2+}}, the neptunyl ion, shows a light pink or reddish color in an acidic solution and yellow-green otherwise.<ref name="lanl" /> It is a strong Lewis acid<ref name="Yoshida753" /> and is the main neptunium ion encountered in solutions of pH&nbsp;3–4.<ref name="Yoshida768" /> Though stable in acidic solutions, it is quite easily reduced to the Np(V) ion,<ref name="Yoshida753" /> and it is not as stable as the homologous hexavalent ions of its neighbours uranium and plutonium (the [[uranyl]] and [[plutonyl]] ions). It hydrolyzes in basic solutions to form the oxo and hydroxo ions NpO<sub>2</sub>OH<sup>+</sup>, {{chem|(NpO|2|)|2|(OH)|2|2+}}, and {{chem|(NpO|2|)|3|(OH)|5|+}}.<ref name="Yoshida768" />

;Neptunium(VII)
Np(VII) is dark green in a strongly [[Base (chemistry)|basic]] solution. Though its [[chemical formula]] in basic solution is frequently cited as {{chem|NpO|5|3-}}, this is a simplification and the real structure is probably closer to a hydroxo species like {{chem|[NpO|4|(OH)|2|]|3-}}.<ref name="lanl" /><ref name="Greenwood" /> Np(VII) was first prepared in basic solution in 1967.<ref name="Yoshida753" /> In strongly [[acid (chemistry)|acidic]] solution, Np(VII) is found as {{chem|NpO|3|+}}; water quickly reduces this to Np(VI).<ref name="Yoshida753" /> Its hydrolysis products are uncharacterized.<ref name="Yoshida768" />