Analytical Separation

The purpose of separation in analytical chemistry is to _______.

separate different analytes from each other to reduce interference

To deal with Interferences:

❑Using a masking agent to immobilize or chemically bind the

interference species.

❑Converting either the analyte or the interferences into a

separate phase that can be separated mechanically.

Extraction

• Transfer of solute from one phase to another in which is more soluble.

• Phases can be liquid, solid or gas.

Liquid/liquid extraction

Two immiscible layers: organic (S2) and aqueous (S1) in separatory funnel

• Immiscible liquids remain in separate phases

Solute “partitions” between two solvents:

❑Solute prefers the solvent that is more like itself

❑Polar and ionic compounds partition into water (s1)

❑Nonpolar compounds partition into organic solvents (s2)

Partition Coefficient (K) works for _______.

molecules that only have one form

• K = [S2]/[S1]

Distribution Coefficient (D) works for ______.

species that have multiple forms

• ex: acids and bases

Ideally, K is _______.

independent of the total quantity of S

Partition Coefficient (K)

Ratio of solute (S) concentrations in each phase (phase 1 and phase 2) at equilibrium

• K = (1-q) V1 / qV2

The larger the partition coefficient, the _______.

smaller amount of solute that remains in phase 1 (q)

q

fraction of S in phase 1 after extraction

• q = V1/V1+KV2

• pH dependent

V1

volumes of solvent in phase 1

V2

volumes of solvent in phase 2

m

number of moles of S (total)

How to move S to solvent 2 as much as possible?

K should be large and q should be small

The fraction of solute remaining in phase 1 depends on (q):

• V1, V2: →larger V2, q smaller ; larger V1, q larger

• K → larger K, q smaller

For extraction efficiency, perform _______.

more extractions with small volumes

• gives smaller q

To improve extraction efficiency, we want ______.

a small V1, large V2, and small q

Neutral species are more soluble in ______.

organic phase, while charged species are more soluble in water

Charge dictates solubility and ______.

pH dictates charge

The analyte present in more than one chemical form, its distribution between two phases is described by:

Distribution Coefficient, D.

❑Takes into account all chemical forms of a compound.

❑Different from partition coefficient (K) for those analytes that can be present in more than one chemical form.

If a solute is an acid or base its charge _______.

changes as the pH changes

Distribution coefficient D is used in place of the ______.

K in the equation for q when dealing with species that has more

than one chemical form

D equation in terms of HA

q in terms of D (use for weak acid and base)

D in terms of a base

αB is the fraction of weak base in the neutral form, B, in the

aqueous phase.

To extract a base into water, use a _______.

pH low enough to convert B into BH+.

To extract an acid HA into water, use a ______.

pH high enough to convert HA into A-.