6.4: Classifying Separation Techniques

We can separate an analyte and an interferent if there is a significant difference in at least one of their chemical or physical properties. Table 6.4.1 provides a partial list of separation techniques, organized by the chemical or physical property affecting the separation.

Table 6.4.1 . Classification of Separation Techniques

basis of separation	separation technique(s)
size	filtration; dialaysis; size-exclusion chromatography
mass or density	centrifugation
complex formation	masking
change in physical state	distillation; sublimation; recrystalization
change in chemical state	precipitation; electrodeposition; volatilization
partitioning between phases	extraction; chromatography

Separations Based on Complexation Reactions (Masking)

One widely used technique for preventing an interference is to bind the interferent in a strong, soluble complex that prevents it from interfering in the analyte’s determination. This process is known as masking. As shown in Table 6.4.3 , a wide variety of ions and molecules are useful masking agents, and, as a result, selectivity is usually not a problem.

Technically, masking is not a separation technique because we do not physically separate the analyte and the interferent. We do, however, chemically isolate the interferent from the analyte, resulting in a pseudo-separation.

Table 6.4.3 . Selected Inorganic and Organic Masking Agents for Metal Ions

masking agent	elements whose ions are masked
CN^–	Ag, Au, Cd, Co, Cu, Fe, Hg, Mn, Ni, Pd, Pt, Zn
SCN^–	Ag, Cd, Co, Cu, Fe, Ni, Pd, Pt, Zn
NH3	Ag, Co, Ni, Cu, Zn
F^–	Al, Co, Cr, Mg, Mn, Sn, Zn
\(\text{S}_2\text{O}_3^{2-}\)	Au, Ce, Co, Cu, Fe, Hg, Mn, Pb, Pd, Pt, Sb, Sn, Zn
tartrate	Al, Ba, Bi, Ca, Ce, Co, Cr, Cu, Fe, Hg, Mn, Pb, Pd, Pt, Sb, Sn, Zn
oxalate	Al, Fe, Mg, Mn
thioglycolic acid	Cu, Fe, Sn
Source: Meites, L. Handbook of Analytical Chemistry, McGraw-Hill: New York, 1963.

Example 6.4.1

Using Table 6.4.3 , suggest a masking agent for the analysis of aluminum in the presence of iron.

Solution

A suitable masking agent must form a complex with the interferent, but not with the analyte. Oxalate, for example, is not a suitable masking agent because it binds both Al and Fe. Thioglycolic acid, on the other hand, is a selective masking agent for Fe in the presence of Al. Other acceptable masking agents are cyanide (CN^–) thiocyanate (SCN^–), and thiosulfate (\(\text{S}_2\text{O}_3^{2-}\)).

Exercise 6.4.1

Using Table 7.6, suggest a masking agent for the analysis of Fe in the presence of Al.

Answer

The fluoride ion, F^–, is a suitable masking agent as it binds with Al³⁺ to form the stable \(\text{AlF}_6^{3-}\) complex, leaving iron in solution.

As shown in Example 6.4.2 , we can judge a masking agent’s effectiveness by considering the relevant equilibrium constants.

Example 6.4.2

Show that CN^– is an appropriate masking agent for Ni²⁺ in a method where nickel’s complexation with EDTA is an interference.

Solution

The relevant reactions and formation constants are

\[\mathrm{Ni}^{2+}(a q)+\mathrm{Y}^{4-}(a q)\rightleftharpoons \mathrm{NiY}^{2-}(a q) \quad K_{1}=4.2 \times 10^{18} \nonumber\]

\[\mathrm{Ni}^{2+}(a q)+4 \mathrm{CN}^{-}(a q)\rightleftharpoons \mathrm{Ni}(\mathrm{CN})_{4}^{2-}(a q) \quad \beta_{4}=1.7 \times 10^{30} \nonumber\]

where Y^4– is an abbreviation for EDTA. Cyanide is an appropriate masking agent because the formation constant for \(\text{Ni(CN)}_4^{2-}\) is greater than that for the Ni–EDTA complex. In fact, the equilibrium constant for the reaction in which EDTA displaces the masking agent

\[\mathrm{Ni}(\mathrm{CN})_{4}^{2-}(a q)+\mathrm{Y}^{4-}(a q) \rightleftharpoons \mathrm{NiY}^{2-}(a q)+4 \mathrm{CN}^{-}(a q) \nonumber\]

\[K=\frac{K_{1}}{\beta_{4}}=\frac{4.2 \times 10^{18}}{1.7 \times 10^{30}}=2.5 \times 10^{-12} \nonumber\]

is sufficiently small that \(\text{Ni(CN)}_4^{2-}\) is relatively inert in the presence of EDTA.

Exercise 6.4.2

Use the formation constants in Appendix 12 to show that 1,10-phenanthroline is a suitable masking agent for Fe²⁺ in the presence of Fe³⁺. Use a ladder diagram to define any limitations on using 1,10-phenanthroline as a masking agent. See Chapter 6 for a review of ladder diagrams.

Answer

The relevant reactions and equilibrium constants are

\[\begin{array}{ll}{\mathrm{Fe}^{2+}(a q)+3 \mathrm{phen}(a q)} & {\rightleftharpoons\mathrm{Fe}(\mathrm{phen})_{3}^{2+}(a q) \quad \beta_{3}=5 \times 10^{20}} \\ {\mathrm{Fe}^{3+}(a q)+3 \mathrm{phen}(a q)} & {\rightleftharpoons \mathrm{Fe}(\mathrm{phen})_{3}^{3+}(a q) \quad \beta_{3}=6 \times 10^{13}}\end{array} \nonumber\]

where phen is an abbreviation for 1,10-phenanthroline. Because \(\beta_3\) is larger for the complex with Fe²⁺ than it is for the complex with Fe³⁺,1,10-phenanthroline will bind Fe²⁺ before it binds Fe³⁺. A ladder diagram for this system (as shown below) suggests that an equilibrium p(phen) between 5.6 and 5.9 will fully complex Fe²⁺ without any significant formation of the \(\text{Fe(phen)}_3^{3+}\) complex. Adding a stoichiometrically equivalent amount of 1,10-phenanthroline to a solution of Fe²⁺ is sufficient to mask Fe²⁺ in the presence of Fe³⁺. A large excess of 1,10-phenanthroline, however, decreases p(phen) and allows for the formation of both metal–ligand complexes.

Separations Based on a Partitioning Between Phases

The most important group of separation techniques uses a selective partitioning of the analyte or interferent between two immiscible phases. If we bring a phase that contains the solute, S, into contact with a second phase, the solute will partition itself between the two phases, as shown by the following equilibrium reaction.

\[S_{\text { phase } 1} \rightleftharpoons S_{\text { phase } 2} \label{7.1}\]

The equilibrium constant for reaction \ref{7.1}

\[K_{\mathrm{D}}=\frac{\left[S_{\mathrm{phase} \ 2}\right]}{\left[S_{\mathrm{phase} \ 1}\right]} \nonumber\]

is called the distribution constant or the partition coefficient. If K_D is sufficiently large, then the solute moves from phase 1 to phase 2. The solute will remain in phase 1 if the partition coefficient is sufficiently small. When we bring a phase that contains two solutes into contact with a second phase, a separation of the solutes is possible if K_D is favorable for only one of the solutes. The physical states of the phases are identified when we describe the separation process, with the phase that contains the sample listed first. For example, if the sample is in a liquid phase and the second phase is a solid, then the separation involves liquid–solid partitioning.

Extraction Between Two Phases

We call the process of moving a species from one phase to another phase an extraction. Simple extractions are particularly useful for separations where only one component has a favorable partition coefficient. Several important separation techniques are based on a simple extraction, including liquid–liquid, liquid–solid, solid–liquid, and gas–solid extractions.

Solid Phase Extractions

In a solid phase extraction of a liquid sample, we pass the sample through a cartridge that contains a solid adsorbent, several examples of which are shown in Figure 6.4.10 . The choice of adsorbent is determined by the species we wish to separate. Table 6.4.5 provides several representative examples of solid adsorbents and their applications.

Figure 6.4.10 . Selection of solid phase extraction cartridges for liquid samples. The solid adsorbent is the white or black material in each cartridge. From left-to-right, the absorbent materials are octadecylsilane, carbon, octadecylsilane, polyamide resin, and diol; see Table 6.4.5 for additional details. The size of the cartridges dictates the volume of sample used; from left-to-right, these cartridges use samples of 1 mL, 3 mL, 6 mL, 3 mL, and 1 mL.

Table 6.4.5 . Representative Adsorbents for the Solid Phase Extraction of Liquid Samples

absorbent	structure	properties and uses
silica		retains low to moderate polarity species from organic matrices fat soluble vitamins, steroids
aminopropyl		retains polar compounds carbohydrates, organic acids
cyanopropyl		retains wide variety of species from aqueous and organic matrices pesticides, hydrophobic peptides
diol		retains wide variety of species from aqueous and organic matrices proteins, peptides, fungicides
octadecyl (C–18)	—C18H37	retains hydrophobic species from aqueous matrices caffeine, sedatives, polyaromatic hydrocarbons, carbohydrates, pesticides
octyl (C–8)	—C8H17	similar to C-18

As an example, let’s examine a procedure for isolating the sedatives secobarbital and phenobarbital from serum samples using a C-18 solid adsorbent [Alltech Associates Extract-Clean SPE Sample Preparation Guide, Bulletin 83]. Before adding the sample, the solid phase cartridge is rinsed with 6 mL each of methanol and water. Next, a 500-μL sample of serum is pulled through the cartridge, with the sedatives and matrix interferents retained following a liquid–solid extraction (Figure 6.4.11 a). Washing the cartridge with distilled water removes any interferents (Figure 6.4.11 b). Finally, we elute the sedatives using 500 μL of acetone (Figure 6.4.11 c). In comparison to a liquid–liquid extraction, a solid phase extraction has the advantage of being easier, faster, and requires less solvent.

Figure 6.4.11 . Steps in a typical solid phase extraction. After preconditioning the solid phase cartridge with solvent, (a) the sample is added to the cartridge, (b) the sample is washed to remove interferents, and (c) the analytes are eluted.