6.2 Analytical Separations: Solvent Extraction, SPE, Ion Exchange, Precipitation, Masking, and Electrodeposition

6.2 Analytical Separations: Solvent Extraction, SPE, Ion Exchange, Precipitation, Masking, and Electrodeposition

Quick Answer: Analytical area D (10 items) lists solvent extraction, ion exchange, complexation, precipitation, electrodeposition, and solid-phase extraction. These separations isolate or preconcentrate an analyte before measurement. Be ready to calculate extraction efficiency, predict how pH controls extraction and precipitation, choose masking agents, and set electrolysis potentials. Chromatography, the most powerful separation family, follows in Section 6.3.

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Liquid-Liquid (Solvent) Extraction

The partition coefficient describes one species distributed between two immiscible phases. The distribution ratio counts all forms of the analyte, so it is what actually governs a separation:

$K_D = \frac{[A]_{org}}{[A]_{aq}}, \qquad D = \frac{C_{org}}{C_{aq}}\ (\text{all forms})$

Multiple Extractions

After $n$ extractions of $V_{aq}$ with fresh $V_{org}$ each time, the fraction left in the aqueous phase is

$q_n = \left(\frac{V_{aq}}{V_{aq} + D\,V_{org}}\right)^n$

The same total solvent extracts more when divided into several portions.

pH Control for Acids and Bases

For a weak acid that partitions only as neutral HA:

$D = \frac{K_D[H^+]}{[H^+] + K_a}$

At pH far below $pK_a$, $D \approx K_D$ and the acid moves into the organic phase. At pH far above $pK_a$ it stays in water as $A^-$. Benzoic acid ($pK_a$ 4.2) is extracted into ether from pH 2 but left behind at pH 8. Amines behave the opposite way: they extract from basic solution and are stripped back into acid. This is the basis of acid-base extraction schemes.

Metal Extraction

Metal ions become extractable when converted into neutral species:

• Chelates: dithizone, 8-hydroxyquinoline (oxine), APDC, and acetylacetone form neutral $ML_n$ complexes. Because $D$ depends on $[H^+]^{-n}$, pH selects between metals.
• Ion pairs: a bulky counter-ion forms an extractable neutral pair, as when $FeCl_4^-$ is extracted from 6 M HCl into ether.

The separation factor $\alpha = D_A / D_B$ must be large for a clean single-stage separation.

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Solid-Phase Extraction and Microextraction

Solid-phase extraction (SPE) passes the sample through a cartridge of sorbent in four steps:

1. Condition the sorbent (for example, methanol then water for C18).
2. Load the sample; the analyte is retained.
3. Wash off weakly held interferences.
4. Elute the analyte in a small volume of strong solvent, which also concentrates it.

Solid-phase microextraction (SPME) uses a coated fibre that is exposed to the sample or its headspace and then desorbed directly in a GC inlet, with no solvent. QuEChERS ("quick, easy, cheap, effective, rugged, safe") extracts pesticide residues from food with acetonitrile and salts, followed by dispersive-SPE cleanup.

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Ion Exchange

Ion-exchange resins are cross-linked polymers carrying fixed charged groups:

Selectivity rises with charge and with smaller hydrated radius: $Al^{3+} > Ca^{2+} > Na^+$, and for alkali metals $Cs^+ > Rb^+ > K^+ > Na^+ > Li^+$. Concentrated eluent can reverse the order, which is how resins are regenerated.

Applications:

• Deionized water: pass water through an $H^+$-form cation resin, then an $OH^-$-form anion resin.
• Total salt determination: passing a salt solution through an $H^+$ resin releases an equivalent amount of $H^+$, which is then titrated with NaOH.
• Water softening: $Na^+$-form resin exchanges $Ca^{2+}$ and $Mg^{2+}$.
• Ion chromatography with suppressed conductivity detection (Section 6.5).

Resin capacity is expressed in milliequivalents per gram (dry) or per millilitre (wet).

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Precipitation Separations

• pH-controlled hydroxide precipitation: $Fe(OH)_3$ precipitates from mildly acidic solution (around pH 3), well before $Mg(OH)_2$ (around pH 10). Buffers such as $NH_3/NH_4^+$ hold the pH between them.
• Sulfide separations depend on $[S^{2-}]$, which is controlled by pH through $H_2S$ ionization. Acidic $H_2S$ precipitates the very insoluble sulfides ($CuS$, $CdS$, $PbS$, $HgS$), while basic sulfide also precipitates $ZnS$, $NiS$, $CoS$, and $MnS$. This is the basis of the classical qualitative-analysis groups.
• Organic precipitants: dimethylglyoxime precipitates $Ni^{2+}$ selectively as a red chelate; 8-hydroxyquinoline precipitates many metals at controlled pH.
• Coprecipitation and collectors: a deliberate carrier precipitate such as $Fe(OH)_3$ sweeps trace metals out of large volumes for preconcentration. Section 5.3 covers unwanted coprecipitation.

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Complexation and Masking

A masking agent forms a stable complex with an interfering ion so that it no longer reacts, avoiding a physical separation.

Demasking releases an ion selectively: formaldehyde frees $Zn^{2+}$ from $Zn(CN)_4^{2-}$, so magnesium and zinc can be titrated in turn. Cyanide must never be acidified, because that releases toxic HCN. Section 5.5 covers EDTA titrations in detail.

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Electrodeposition (Electrogravimetry)

Metals are reduced onto a weighed platinum cathode and determined by mass gain, or plated out to remove them as interferences.

• In controlled-potential electrolysis, the cathode potential is held with a potentiostat so that only the target metal deposits. From the Nernst equation, lowering a metal's concentration a million-fold (from 1 M to 1 µM) requires a potential shift of $6 \times \frac{0.05916}{n}$ V: 0.18 V for $n = 2$. Two metals can therefore be separated quantitatively only if their deposition potentials differ by more than this.
• Example: Copper deposits from acidic solution at potentials where nickel and zinc stay in solution.
• Mercury-cathode electrolysis strips many heavy metals from solution before determining Al, Ti, or alkali metals. Its use is now limited because of mercury toxicity.
• Depolarizers such as nitrate prevent hydrogen evolution and keep deposits adherent.

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Distillation and Volatilization

Volatile analytes can be separated by distillation: $NH_3$ in Kjeldahl analysis, $CO_2$ from carbonates, $SO_2$ from foods, and phenols and cyanide in water analysis.