Analytical Separation Methods Notes

Analytical Separation Methods

Overview

• The primary goal of analytical separation is to isolate either the analyte or the interferent from the sample matrix.

• Sample clean-up using separation methods is crucial for minimizing errors caused by interferences present in the sample matrix.

• While effective, most separation methods are time-intensive and carry a risk of analyte loss or sample contamination.

• In many scenarios, separation is the only viable approach to eliminate interfering species.

• Most analytical methods incorporate steps to mitigate the effects of interferences.

• Interferences are species in the sample matrix that produce signals indistinguishable from the analyte's signal.

• The analyte can be separated from interferences if there is a significant difference in at least one of their chemical or physical properties.

Classification of Separation Techniques

• Separation techniques are classified based on differences in chemical and physical properties.

  • Size: Filtration, Dialysis.

  • Mass and Density: Centrifugation.

  • Complex Formation: Masking.

  • Change in Physical State: Distillation, Sublimation & Recrystallization.

  • Change in Chemical State: Precipitation, Volatilization & Ion- Exchange.

  • Partitioning Between Phases: Extraction and Chromatography.

Separations Based on Size

• The simplest physical property for separation is size.

A. Filtration

• Filtration involves using a porous medium that allows only the analyte or interferent to pass through.

• Particulate interferents can be separated from dissolved analytes using a filter with a pore size that retains the interferent.

• Filtration is a mechanical/physical process used to separate solids from fluids (liquids or gases).

• A medium is placed in the fluid flow, allowing the fluid to pass while retaining the solids (or part of the solids).

• This technique is important in the analysis of natural water and in gravimetric analysis to isolate precipitate particulates from the solution.

B. Dialysis

• Dialysis is a process where small solutes diffuse from a high concentration solution to a low concentration solution across a semi-permeable membrane until equilibrium is reached.

• The membrane selectively allows smaller solutes to pass while retaining larger species, making it an effective separation process based on size rejection.

• Dialysis membranes are typically made of cellulose, with pore sizes of 1–5 nm (nanometer = $1 \times 10^{-9}$ meter).

Dialysis Applications:

• Macromolecular Purification (e.g., proteins, hormones, and enzymes).

• Solute Fractionation.

• Contaminant Removal.

• pH Change.

• Desalting.

Diffusion, Dialysis, and Osmosis

• Diffusion: The movement of particles from an area of high concentration to an area of low concentration.

• Dialysis: The process of separating molecules in solution by the difference in their rates of diffusion through a semi-permeable membrane. It aids in separating waste and excess water from the blood to maintain body equilibrium by removing waste, salt, and excess water, maintaining chemical and nutrient levels (e.g., potassium, sodium, chloride, calcium, phosphorus, magnesium, and sulfate), and controlling blood pressure.

• Osmosis: The natural tendency for water molecules to pass through a semi-permeable membrane from the side low in dissolved impurities to the side high in dissolved impurities.

Reverse Osmosis

• Reverse osmosis forces water with a higher concentration of contaminants into a tank containing water with an extremely low concentration of contaminants.

• A reverse osmosis membrane removes impurities and particles larger than 0.001 microns.

• Factors affecting the performance of a Reverse Osmosis System:

  • Incoming water pressure

  • Water Temperature

  • Type and number of total dissolved solids (TDS) in the tap water

  • The quality of the filters and membranes used in the RO System

Separations Based on Mass or Density

• When there is a difference in the mass or density of the analyte and interferent, separation using centrifugation may be possible.

• Centrifugation involves using centripetal force for the separation of mixtures and is used in industry and laboratory settings.

• In chemistry and biology, a suspension of solid in liquid is poured into a centrifuge tube and spun around very fast in a centrifuge. The spinning motion forces the solid to the bottom of the tube, and then the liquid can be poured off from the solid.

• Centrifugation is commonly used in dairies to separate milk from cream because milk has less density than cream.

• The remaining solution is called the "supernatant" or “supernatant liquid “.

Separations Based on Complex Formation Reaction (Masking)

• Masking is a widely used separation technique for preventing interferences.

• Masking involves binding the interferent as a soluble complex, preventing it from causing errors in the determination of the analyte.

• Masking isn't strictly a separation method because the analyte and interferent are never physically separated; hence, it's considered a pseudo-separation technique.

• A wide variety of ions and molecules have been used as masking agents.

Example:

• The use of cyanide ion $CN^-$ as a masking agent for $Fe^{3+}$ (interference) in the volumetric determination of $Ca^{2+}$ by titration with EDTA ($Y^{4-}$-).
  $Fe^{3+} + CN^- \rightleftharpoons Fe(CN)^{2+}$
  $Ca^{2+} + CN^- \rightleftharpoons Ca(CN)^{2+}$
  $Ca^{2+} + EDTA \rightleftharpoons Ca(EDTA)$

• Masking: A pseudo-separation method where species acting as interferences in chemical analysis are prevented from interfering by adding a masking agent.

• Masking Agent: A complexing agent that reacts selectively with a component in a solution to prevent that component from interfering in a determination.

Selected Inorganic and Organic Masking Agents for Metal Ions

• $CN^-$: Ag, Au, Cd, Co, Cu, Fe, Hg, Mn, Ni, Pd, Pt, Zn

• $SCN^-$: Ag, Cd, Co, Cu, Fe, Ni, Pd, Pt, Zn

• $NH_3$: Ag, Co, Ni, Cu, Zn

• $F^-$: Al, Co, Cr, Mg, Mn, Sn, Zn

• $S<em>2O</em>3^{2-}$: 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

Separations Based on a Change of State

• When the analyte and interferent are in the same phase, separation can be achieved by inducing a change in one of their physical or chemical states.

A) Changes in Physical State

• Changes in physical state used for separations include phase transitions depending on the phases of analyte and interferent.

  • Liquid to Gas Phase Transition

  • Solid to Liquid Phase Transition

  • Solid to Gas Phase Transition

Separations Based on a Change of Physical State

1) Liquid to Gas Phase Transition:

• When the analyte and interferent are liquids, separation based on distillation may be possible if their boiling points are significantly different.

• Distillation: A method of separating chemical substances based on differences in their volatilities in a boiling liquid mixture. The component with the low boiling point will be distilled first and collected (separated), and then the higher boiling point component (analyte or interferent) will be collected.

• Fractional distillation: The separation of a mixture into its component parts, or fractions, such as in separating chemical compounds by their boiling points by heating them to a temperature at which several fractions of the compound will evaporate. It is a special type of distillation. Generally, the component parts boil at less than $25^\circ C$ from each other under a pressure of one atmosphere (1 atm). If the difference in boiling points is greater than $25^\circ C$, a simple distillation is used.

2) Solid to Liquid Phase Transition:

• The solid sample can be separated from interferences (purified) by dissolving the sample in a suitable selected solvent to form a solution for the sample containing analyte and interferent.

• The solution is then applied to a low temperature to produce the purified, re-crystallized solid analyte. This process is called Recrystallization.

3) Solid to Gas Phase Transition:

• When the sample is a volatile solid, the separation of the analyte and interference by sublimation may be possible.
  Sublimation: is a process by which the sample is heated and the solid analyte vaporizes without passing through the liquid state and the vapor is then condensed to recover the purified solid analyte.

B) Changes in Chemical State

• The main method used for the change in chemical state is precipitation with suitable modification that can be achieved by the following processes:

  • Controlling of Acidity (pH)

  • Controlling of Oxidation State

  • Formation of Complex Ion

1) Controlling of Acidity (pH):

• Iron (III), zinc, manganese, and cobalt are precipitated from alkaline solution as hydrous oxides. Only iron (III) is precipitated if the solution is adjusted to pH 3.

2) Controlling of Oxidation State:

• Aluminium (III) and chromium (III) ions are precipitated from alkaline solution as hydrous oxides, but if the chromium (III) is oxidized to chromate ion (VI) ion, only aluminium will precipitate.

3) Formation of Complex Ion:

• Both iodide and chloride ions form precipitates when silver nitrate (AgNO3) is added. In the presence of ammonia hydroxide (NH4OH), only silver iodide (AgI) precipitates because the excess amount of (NH4OH) will form a soluble ammonium-chloride complex.