5.1: Separation of group III cations
- Page ID
- 369542
\( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \)
\( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)
\( \newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\)
( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\)
\( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\)
\( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\)
\( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\)
\( \newcommand{\Span}{\mathrm{span}}\)
\( \newcommand{\id}{\mathrm{id}}\)
\( \newcommand{\Span}{\mathrm{span}}\)
\( \newcommand{\kernel}{\mathrm{null}\,}\)
\( \newcommand{\range}{\mathrm{range}\,}\)
\( \newcommand{\RealPart}{\mathrm{Re}}\)
\( \newcommand{\ImaginaryPart}{\mathrm{Im}}\)
\( \newcommand{\Argument}{\mathrm{Arg}}\)
\( \newcommand{\norm}[1]{\| #1 \|}\)
\( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\)
\( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\AA}{\unicode[.8,0]{x212B}}\)
\( \newcommand{\vectorA}[1]{\vec{#1}} % arrow\)
\( \newcommand{\vectorAt}[1]{\vec{\text{#1}}} % arrow\)
\( \newcommand{\vectorB}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \)
\( \newcommand{\vectorC}[1]{\textbf{#1}} \)
\( \newcommand{\vectorD}[1]{\overrightarrow{#1}} \)
\( \newcommand{\vectorDt}[1]{\overrightarrow{\text{#1}}} \)
\( \newcommand{\vectE}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash{\mathbf {#1}}}} \)
\( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \)
\( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)
\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Group II cations form sulfides that have very low solubility. After group II cations are removed under a low concentration of \(\ce{S^{2-}}\) in an acidic medium, the solution is made alkaline. Remember that like sulfides, hydroxides are also insoluble according to insoluble ions rule#1 of solubility guidelines described in chapter 1 states “Hydroxide (\(\ce{OH^{-}}\)) and sulfides (\(\ce{S^{2-}}\)) are insoluble except when the cation is a heavy alkaline earth metal ion: \(\ce{Ca^{2+}}\), \(\ce{Ba^{2+}}\), and \(\ce{Sr^{2+}}\), alkali metal ions, and ammonium ion.”
Table 1 lists solubility product constants of hydroxides of group III & IV cations at 25 oC, maximum hydroxide (\(\ce{OH^{-}}\)) concentration, and the maximum pH that can exist in a saturated solution containing 0.1M cation solutions that may be present in the test solution at this stage. It can be observed that the ions listed in table 1 will not precipitate as hydroxides during the precipitation of group II cations under the acidic pH range of 0.5 to 1.
\(\ce{Fe{3+}}\) forms the most insoluble hydroxide, but it is reduced to \(\ce{Fe^{2+}}\) by \(\ce{H2S}\) during precipitation of group II cations:
\[\ce{2Fe^{3+}(aq) + S^{2-}(aq) <=> 2Fe^{2+}(aq) + S(s)}\nonumber\]
\(\ce{Fe^{3+}}\) may be present only if precipitation of group III starts from a fresh sample that has not been subjected to group II separation.
It can be observed from Table 1 that if the pH of the sample solution is increased to a range of 7 to 10, \(\ce{Fe^{3+}}\), \(\ce{Cr^{3+}}\), \(\ce^{Ni{2+}}\), and \(\ce{Fe^{2+}}\) will precipitate as \(\ce{Fe(OH)3(s, rusty)}\), \(\ce{Cr(OH)3(s, gray-green)}\), \(\ce{Ni(OH)2(s, green)}\), and \(\ce{Fe(OH)2(s, green)}\), leaving behind in the solution rest of the ions that may still be present at this stage. Group III comprise of , \(\ce{Fe^{3+}}\), \(\ce{Cr^{3+}}\), \(\ce{Ni^{2+}}\), and \(\ce{Fe^{2+}}\) ions.
|
Ion |
Salt |
Ksp at 25 oC |
Minimum [OH-] and pH needed to precipitate |
|---|---|---|---|
| \(\ce{Fe^{3+}}\) |
\(\ce{Fe(OH)3}\) |
\(\mathrm{K}_{\mathrm{sp}}=\left[\mathrm{Fe}^{3+}\right]\left[\mathrm{OH}^{-}\right]^{3}=2.8 \times 10^{-39}\) |
\(\left[\mathrm{OH}^{-}\right]=3.0 \times 10^{-13}~M=\mathrm{pH} ~1.5\) |
| \(\ce{Cr^{3+}}\) |
\(\ce{Cr(OH)3}\) |
\(\mathrm{K}_{\mathrm{sp}}=\left[\mathrm{Cr}^{3+}\right]\left[\mathrm{OH}^{-}\right]^{3}=1.0 \times 10^{-30}\) |
\(\left[\mathrm{OH}^{-}\right]=2.2 \times 10^{-10}~M=\mathrm{pH} ~4.3 \) |
| \(\ce{Ni^{2+}}\) |
\(\ce{Ni(OH)2}\) |
\( \mathrm{K}_{\mathrm{sp}}=\left[\mathrm{Ni}^{2+}\right]\left[\mathrm{OH}^{-}\right]^{2}=5.5 \times 10^{-16}\) |
\(\left[\mathrm{OH}^{-}\right]==7.4 \times 10^{-9}~M=\mathrm{pH} ~4.3 \) |
| \(\ce{Fe^{2+}}\) |
\(\ce{Fe(OH)2}\) |
\( \mathrm{K}_{\mathrm{sp}}=\left[\mathrm{Fe}^{2+}\right]\left[\mathrm{OH}^{-}\right]^{2}=4.9 \times 10^{-17}\) |
\( \left[\mathrm{OH}^{-}\right]=2.2 \times 10^{-9}~M=\mathrm{pH} ~5.6\) |
|
\(\ce{Ca^{2+}}\) |
\(\ce{Ca(OH)2}\) |
\( \mathrm{K}_{\mathrm{sp}}=\left[\mathrm{Ca}^{2+}\right]\left[\mathrm{OH}^{-}\right]^{2}=5.0 \times 10^{-6}\) |
\(\left[\mathrm{OH}^{-}\right]=7.1 \times 10^{-4}~M=\mathrm{pH} \mathrm{} ~10.9 \) |
|
\(\ce{Ba^{2+}}\) |
\(\ce{Ba(OH)2}\) |
\( \mathrm{K}_{\mathrm{sp}}=\left[\mathrm{Ba}^{2+}\right]\left[\mathrm{OH}^{-}\right]^{2}=2.6 \times 10^{-4}\) |
\(\left[\mathrm{OH}^{-}\right]=5.1 \times 10^{-3}~M=\mathrm{pH} \mathrm{} 11.7 \) |
- * Following cations that may be present in the initial solution are not listed in this table due to the reason: i) \(\ce{Pb^{2+}}\), \(\ce{Hg2^{2+}}\), and \(\ce{Ag^{+}}\) are already removed as chloride precipitates of group I cations, ii) \(\ce{Sn^{4+}}\), \(\ce{Cd^{2+}}\), \(\ce{Cu^{2+}}\), and \(\ce{Bi^{3+}}\) has been removed as group II sulfides under pH 0.5 to 1, iii) \(\ce{Na^{+}}\) and \(\ce{K^{+}}\) form soluble compounds with all anions according to rule#1 of solubility described in chapter 1. Source: Engineering ToolBox, (2017). Solubility product constants. [online] Available at: https://www.engineeringtoolbox.com/s...sp-d_1952.html [Accessed Feb. 5th, 2022]
Buffers, that resist change in pH are employed in such a situation where pH needs to be maintained in a narrow range. Buffers are a mixture of a weak acid and its conjugate base or a mixture of a weak base and its conjugate acid. Ammonia (\(\ce{NH3}\)), i.e., a week base and ammonium ion (\(\ce{NH4^{+}}\)) is its conjugate acid.
The \(\ce{NH3}\)/\(\ce{NH4^{+}}\) is a suitable buffer that can maintain pH of around 9. The buffer is prepared by adding 2 drops of 6M \(\ce{HCl}\) into 15 drops of the sample and then adding 6M \(\ce{NH3}\) drop by drop to neutralize the acid.
\[\ce{HCl(aq) + H2O(l) -> H3O^{+}(aq) + Cl^{-}(aq)}\nonumber\]
\[\ce{NH3(aq) + H3O^{+}(aq) -> NH4^{+}(aq) + H2O(l)}\nonumber\]
\[\text{Overall reaction:} \ce{~HCl(aq) + NH3(aq) -> NH4^{+}(aq) + Cl^{-}(aq)}\nonumber\]
Then 5 drops more of 6M \(\ce{NH3}\) are added after the \(\ce{HCl}\) has been neutralized to make a mixture of \(\ce{NH3}\) and \(\ce{NH4^{+}}\) that maintains pH ~9 and OH- at around 1 x 10-5 M.
The group III cations precipitate at this stage as hydroxides, as shown in Figure \(\PageIndex{1}\), except \(\ce{Ni^{2+}}\):
\[\ce{Fe^{3+}(aq) + 3OH^{-}(aq) -> Fe(OH)3(s, reddish-brown ~or ~rusty)(v),}\nonumber\]
\[\ce{Cr^{3+}(aq) + 3OH^{-}(aq) -> Cr(OH)3(s, gray-green)(v),}\nonumber\]
\[\ce{Fe^{2+}(aq) + 2OH^{-}(aq) -> Fe(OH)2(s, green)(v).}\nonumber\]
The concentration of \(\ce{Fe^{2+}}\), i.e., the most soluble hydroxide of group III cations, is reduced by more than 99.99%, i.e., from 0.1M to 4.9 x 10-7 M when pH is increased to 9 and \(\ce{OH^{-}}\) concentration is increased to 1 x 10-5 M:
\[\mathrm{Fe}^{2+}=\frac{\mathrm{K}_{\mathrm{sp}}}{\left[\mathrm{OH}^{-}\right]^{2}}=\frac{4.9 \times 10^{-17}}{\left(1 \times 10^{-5}\right)^{2}}=4.9 \times 10^{-7} \mathrm{~M}\nonumber\]
Nickle ion is not precipitated at this stage as it forms soluble coordination cation \(\ce{[Ni(NH3)6]^{2+}}\) with ammonia:
\[\ce{Ni^{2+}(aq, green) + 6NH3(aq) <=> Ni(NH3)6(aq, blue)}\nonumber\]
Therefore, \(\ce{S^{2-}}\) is introduced by adding thioacetamide and heating the mixture in a boiling water bath. Decomposition of thioacetamide produces ~0.01M \(\ce{H2S}\):
\[\ce{CH3CSNH2(aq) + 2H2O(l) <=> CH3COO^{-}(aq) + NH4^{+}(aq) + H2S(aq)}\nonumber\]
Nearly all of the \(\ce{H2S}\) dissociates to form ~0.01M \(\ce{S^{2-}}\) at pH ~9:
\[\ce{H2S(aq) + 2H2O(l) <=>2H3O^{+}(aq) + S^{2-}(aq)}\quad K_a = \frac{\left[\mathrm{H}_{3} \mathrm{O}^{+}\right]^{2}\left[\mathrm{~S}^{2-}\right]}{\left[\mathrm{H}_{2} \mathrm{~S}\right]}=1.3 \times 10^{-20}\nonumber\]
The ammonia complex of nickel, i.e., \(\ce{[Ni(NH3)6]^{2+}}\) precipitates out as \(\ce{NiS}\), and, at the same time, \(\ce{Fe(OH)3}\) and \(\ce{Fe(OH)2}\) also convert to \(\ce{Fe2S2}\) and \(\ce{FeS}\):
\[\ce{Ni(NH3)6^{2+}(aq, blue) + S^{2-}(aq) <=> NiS(s, black) + 6NH3(aq)}\nonumber\]
\[\ce{2Fe(OH)3(s, reddish-brown) + 3S^{2-}(aq) <=> Fe2S3(s, yellow-green) + 6OH^{-}(aq)}\nonumber\]
\[\ce{Fe(OH)2(s, geen) + S^{2-}(aq) <=> FeS(s, black) + 2OH^{-}(aq)}\nonumber\]
Chromium remains as \(\ce{Cr(OH)3}\) precipitate because chromium sulfide is unstable in water.
Group III precipitates, i.e., \(\ce{Cr(OH)3(s, gray-green)}\), \(\ce{NiS(s, black)}\), \(\ce{Fe2Se3(s, yellow-green)}\), and \(\ce{FeS(s, black)}\) in the mixture are separated as precipitates, and the rest of the ions, i.e, \(\ce{Ca^{2+}}\), \(\ce{Ba^{2+}}\), \(\ce{Na^{+}}\) and \(\ce{K^{+}}\), etc. remain dissolved in the supernatant, as shown in Figure \(\PageIndex{2}\). The color of the precipitate does not give a clear indication of what ions are present at this stage as several species of different colors may be mixed at this stage.
