5.2: Separation and confirmation if individual ions in group III precipitates
Separating and confirming nickel(II) ion
Acid like \(\ce{HCl}\) dissolves precipitates of group III cations, i.e., \(\ce{Cr(OH)3(s, gray-green)}\), \(\ce{Fe2S3(s, yellow-green)}\), and \(\ce{FeS(s, black)}\), by the following series of reactions:
\[\ce{Cr(OH)3(s, gray-green) <=> Cr^{3+}(aq) + 3OH^{-}(aq)}\nonumber\]
\[\ce{FeS(s, black) <=> Fe^{2+}(aq) + 2S^{2-}(aq)}\nonumber\]
\[\ce{Fe2S3(s, yellow-green) <=> 2Fe^{3+}(aq) + 3S^{2-}(aq)}\nonumber\]
\[\ce{2Fe^{3+}(aq) + 2S^{2-}(aq) + 2H3O^{+} <=> 2Fe^{2+}(aq) + H2S(aq) + S(s) + 2H2O(l)}\nonumber\]
\[\ce{3OH^{-}(aq) + 3H3O^{+} <=> + 6H2O(l)}\nonumber\]
\[\ce{3S^{2-}(aq) + 6H3O^{+} <=> 3H2S(aq) + 6H2O(l)}\nonumber\]
\[\text{Overall reaction:}\ce{~Cr(OH)3(s, gray-green) + FeS(s, black) + Fe2S3(s, yellow-green) + 11H3O^{+} <=> Cr^{3+}(aq) + 3Fe^{2+}(aq) + 4H2S(aq) + S(s) + 14H2O(l)}\nonumber\]
Removal of basic \(\ce{OH^{-}}\) and \(\ce{S^{2-}}\) ions from products by acid-base neutralization drives these reactions in the forward direction. \(\ce{Fe^{3+}}\) is reduced to \(\ce{Fe^{2+}}\) by \(\ce{S^{2-}}\) under the acidic condition.
The solubility of \(\ce{NiS}\) is very low and it does not dissolve in non-oxidizing acid like \(\ce{HCl}\).
Therefore, the supernatant separated at this stage contains \(\ce{Cr^{3+}}\) and \(\ce{Fe^{2+}}\) and precipitate, if present is \(\ce{NiS}\), as shown in Figure \(\PageIndex{1}\).
Aqua regia , i.e., a mixture of \(\ce{HCl}\) and \(\ce{HNO3}\), can dissolve \(\ce{NiS}\) precipitate by removing \(\ce{Ni^{2+}}\) as soluble coordination anion \(\ce{[NiCl4]^{2-}}\) and, at the the same time, removing \(\ce{S^{2-}}\) by oxidizing it, using \(\ce{NO3^{-}}\) as oxidizing agent in the acidic medium.
\[\ce{NiS(s, black) <=> Ni^{2+}(aq) + 2S^{2-}(aq)}\nonumber\]
\[\ce{Ni^{2+}(aq) + 4Cl^{-}(aq) <=> [NiCl4]^{2-}(aq)}\nonumber\]
\[\ce{3S^{2-}(aq) + 2NO3^{-}(aq) + 8H3O^{+}(aq) <=> 3S(s)(v) + 2NO(g)(^) + 12H2O(l)}\nonumber\]
Nitrogen oxide (\(\ce{NO}\)) evaporates from the liquid mixture further driving the equilibrium to the forward direction. Most of the \(\ce{NO}\) in air is oxidized to nitrogen dioxide (\(\ce{NO2}\)) that forms brown color fumes over the liquid mixture as shown in Figure \(\PageIndex{2}\):
\[\ce{2NO(g) + O2(g) <=> 2NO2(g, red-brown)}\nonumber\]
The S precipitates are removed by centrifugation and decantation. The \(\ce{[NiCl4]^{2-}}\) coordination anion is converted to [Ni(NH 3 )] 2+ coordination cation by making the solution alkaline by ammonia addition:
\[\ce{[NiCl4]^{2-}(aq) + 6NH3(aq) <=> [Ni(NH3)6]^{2+}(aq, blue) + 4Cl^{-}(aq)}\nonumber\]
Dimethyl glyoxime \(\ce{(CH3)2C2(NOH)2}\) is a ligand that is capable of forming two coordinate covalent bonds with transition metal ions. The ligands like \(\ce{Cl^{-}}\), \(\ce{NH3}\), \(\ce{H2O}\), etc. that form one coordinate covalent bond with transition metals are called mono-dentate, and the chelates like dimethyl glyoxime form two coordinate covalent bonds are called bidentate. The ligands that can form two or more coordinate covalent bonds are called chelates or chelating agents. Coordination complexes with chelates are usually more stable, i.e., have higher formation constants than with mono-dentate ligands.
The addition of dimethyl glyoxime \(\ce{(CH3)2C2(NOH)2}\) to the liquid mixture containing \(\ce{[Ni(NH3)6]^{2+}}\) in an alkaline medium forms an insoluble coordination compound \(\ce{NiC8H14O4}\) that separates as a red color precipitate, as shown in Figure \(\PageIndex{3}\):
The structure of the dimethyl glyoxime chelating agent and its coordination complex with nickel is illustrated in Figure \(\PageIndex{4}\) below.
The formation of red color precipitate upon the addition of dimethyl glyoxime at this stage confirms the presence of nickel ion in the test sample.
Separating and confirming iron ions
The supernatant containing \(\ce{Fe^{2+}}\) and \(\ce{Cr^{3+}}\) ions is separated from \(\ce{NiS}\) precipitate after the addition of \(\ce{HCl}\) to the precipitates of group III cations. The supernatant is made alkaline to pH 9 to 10 by adding ammonia solution. A pH paper is used to determine the pH. Hydrogen peroxide (\(\ce{H2O2}\)) is added as an oxidizing agent to the alkaline solution. \(\ce{Fe^{2+}}\) is oxidized to \(\ce{Fe^{3+}}\) and precipitates out as rusty-brown solid \(\ce{Fe(OH)3}\), and \(\ce{Cr^{3+}}\) is oxidized to soluble chromate ion (\(\ce{CrO4^{2-}}\)) under this condition:
\[\ce{2Fe^{2+}(aq) + H2O2(aq) <=> 2Fe^{3+}(aq) + 2OH^{-}(aq)}\nonumber\]
\[\ce{Fe^{3+}(aq) + 3OH^{-}(aq) <=> Fe(OH)3(s, rusty-brown)(v)}\nonumber\]
\[\ce{2Cr^{3+}(aq) + 3H2O2(aq) + 10OH^{-}(aq) <=> 2CrO4^{2-}(aq) + 8H2O(l)}\nonumber\]
The mixture is centrifuged to separate supernatant containing \(\ce{CrO4^{2-}}\) ions and precipitate containing rusty brown precipitate \(\ce{Fe(OH)3}\), as shown in Figure \(\PageIndex{5}\).
The \(\ce{Fe(OH)3}\) precipitate is dissolved in \(\ce{HCl}\) solution:
\[\ce{Fe(OH)3(s, rusty-brown) <=> Fe^{3+}(aq) + 3OH^{-}(aq)}\nonumber\]
\[\ce{3OH^{-}(aq) + 3H3O^{+}(aq) <=> 6H2O(l)}\nonumber\]
Thiocyanate (\(\ce{SCN^{-}}\)) is a ligand that forms deep-red coordination complex ion \(\ce{[FeSCN]^{2+}}\) by reacting with \(\ce{Fe^{3+}}\), as shown in Figure \(\PageIndex{6}\).
\[\ce{Fe^{3+}(aq) + SCN^{-}(aq) <=> [FeSCN]^{2+}(aq, deep-red)}\nonumber\]
Turning the supernatant color to deep-red upon addition of thiocyanate confirms iron ions are present in the test sample.
Confirming chromium(III) ion
The supernatant obtained after removal of \(\ce{Fe(OH)3}\) precipitate contains \(\ce{CrO4^{2-}}\) ions in an alkaline medium. The solution is made acidic by the addition of nitric acid where \(\ce{CrO4^{2-}}\) converts to dichromate ion (\(\ce{Cr2O7^{2-}}\)):
\[\ce{2CrO4^{2-}(aq) + 2H3O^{+}(aq) <=> Cr2O7^{2-}(aq) + 3H2O(l)} \nonumber\]
\(\ce{H2O2}\) is a reducing agent in acidic medium. \(\ce{H2O2}\) is added to the acidic mixture to reduce \(\ce{Cr2O7^{2-}}\) to \(\ce{Cr^{3+}}\) through the following reactions:
\[\ce{2Cr2O7^{2-}(aq) + 8H2O2(aq) + 4H3O^{+}(aq) <=> 4CrO5(aq, dark-blue) + 14H2O(l)} \nonumber\]
\[\ce{4CrO5(aq) + 12H3O^{+}(aq) <=> 4Cr^{3+}(aq, light-blue) + 7O2(g)(^) + 18H2O(l)} \nonumber\]
Oxygen evolves from the mixture and can be observed as gas bubbles in the solution. \(\ce{CrO5}\) intermediate is a dark blue color in which one oxygen is in -2 oxidation state and the other four oxygen are in -1 oxidation state. \(\ce{CrO5}\) is unstable in solution and decomposes to \(\ce{Cr^{3+}}\) which is a light blue color. Residual \(\ce{H2O2}\) is destroyed by heating the mixture in a boiling water bath, which can be observed through oxygen gas bubbling out. Keep in mind that the destruction of \(\ce{H2O2}\) is significantly slower in an acidic medium than in an alkaline medium. It may take a longer time to destroy \(\ce{H2O2}\) in the acidic medium. Then the solution is changed from acidic to alkaline by adding 6M NaOH to the mixture. \(\ce{Cr^{3+}}\) precipitates out as gray-green \(\ce{Cr(OH)3}\) solid:
\[\ce{Cr^{3+}(aq) + 3OH^{-}(aq) <=> Cr(OH)3(s, gray-green)(v)} \nonumber\]
The formation of gray-green precipitate at this stage confirms \(\ce{Cr^{3+}}\) is present in the test sample, as shown in Figure \(\PageIndex{7}\).