ERT207 ANALYTICAL CHEMISTRY CHAPTER 3 : Preparing Samples for Analysis Pn Syazni Zainul Kamal

CO2 : Ability to classify and use separation techniques and gravimetric methods for mass determination

What is Analytical Chemistry? Concerned with the chemical characterization of matter and the answer of : 1) What is it (Qualitative) –identification of elements, ions or compound 2) How much is it (Quantitative) “everything is made of chemicals. Analytical chemists determine what and how much”

Separation process almost every element or compound naturally found in an impure state these impure raw materials must be separated into their purified components before they can be put to productive use a separation process is used to transform a mixture of substances into two or more distinct products.

Important in synthesis, chemical industries, biomedical The separated products could differ in chemical or physical properties, such as size, or crystal modification or other separation into different components The mixture at hand could exist as a combination of any two or more states: solid-solid, solid-liquid, solid-gas, liquid-liquid, liquid-gas, gas-gas, solid-liquid-gas mixture, etc.

Petroleum refinery plant 1st step – petroleum separation into fraction on the basis of boiling point

Separation isolate the analyte from interfering constituents Interferent – chemical sp. that causes a systematic error in an analysis Separation isolate the analyte from interfering constituents Separation can be either complete or partial Complete separation A B C Mixture ABC Partial separation Mixture AB C Mixture ABC

Classifying separation techniques Separation based on …. Size – filtration, dialysis, ion-exchange chromatography Mass/density – (if there is diff. in the mass/density) centrifuge Complexation reaction – masking

Change of state – (if an analyte and interferent in the same phase) – distillation, sublimation, recrystallization, precipitation Partitioning between phase – (analyte and interferent are in two immiscible phases) – extraction, chromatography

Filtration – (size)

Centrifuge (mass/density) Centrifugation is used when we want to separate small amounts of suspension. The suspension of solid in liquid is poured into a centrifuge tube, then spin around very fast in a centrifuge. The spinning motion forces the solid to the bottom of the tube. Then the liquid can be poured off from the solid. Centrifugation is commonly used in dairies to separate milk from cream to make skimmed milk. It is possible because milk has less density than cream.

Masking – (complexation reaction) Used a masking agent which reacts with chemical species that may interfere in the analysis. Masking agent react rapidly to form strong complexes with the ion to be masked

Sublimation – (change of state) Some solids can change to vapor state without melting when heated. When the vapor is cooled, the solid forms again. Eg iodine from a mixture of sand. When heated, only iodine changes to vapor. The vapor changes back to solid on the side of the funnel. Substances which sublime include anhydrous aluminum chloride, iodine and benzoic acid, anhydrous iron (III) chloride and anhydrous aluminum chloride.

Distillation – (change of state) When a solution of solid in liquid is heated, the liquid will evaporates. The hot vapor that formed can be condensed back to liquid again on a cold surface. A way is to recover water from a salt solution. The solution is heated and the stream is to be condensed back to water. The solute and solvent can both be collected. Evaporation + Condensation = DISTILLATION

Separation based on partitioning between phase

2 techniques can be use to separate analyte and interferent: Extraction (is which a solute is transferred from one phase to a new phase) Chromatography (will be discussed in chapter 6)

Type of extraction Liquid-liquid extraction Solid-phase extraction Continuous extraction

Liquid-liquid extraction Very useful for isolate analytes from complex sample matrices prior to chromatographic analysis and spectrophotometric determination. Method to separate compounds based on their relative solubility in two different immiscible liquids. (water & solvent) Liquid-liquid extraction involve distribution of a solute between two immiscible liquid phases ‘Solvent extraction’ – perfume, biodiesel, vege oil

Distribution coefficient If a solute is in aqueous phase and is extracted into an organic phase: A solute S will distribute itself between two phases (after shaking/mixing). Ratio of [S] in the two phases will be constant. KD – distribution coefficient KD = [S]1 solvent 1 eg. organic solvent [S]2 solvent 2 eg. water

Distribution process in liquid-liquid extraction Using separatory funnel The mixture is put into a separatory funnel, The mixture is shaken for about a minute, and the phases are allowed to separate. A solute is extracted from an aqueous solution into an immiscible organic solvent. A solute is an organic compound, will distribute from water into organic solvents. The principle is “like dissolves like”. at bottom layer, the solvent is denser and will be drawn off after the separation is completed.

Liquid-liquid extraction

The distribution ratio Many substances, partially ionized in aqueous layer as weak acid Eg extraction of benzoic acid from an aqueous solution. Part of HBz in aqueous layer exist as Bz-. HBz is a weak acid in water with ionization constant Ka Distribution ratio: is the ratio of the concentrations of all the species of the solute in each phase. D = [HBz]e [HBz]a + [Bz-]a Organic solvent Aqueous solvent

Ka = [H+]a[Bz-]a [HBz]a Equation that relates D, KD and Ka is D = KD 1 + Ka/[H+]a the extraction efficiency will be independent of the original concentrations of the solute.

Percent extracted Fraction of the solute extracted will depend on the volume ratio of the two solvents Fraction of solute extracted is equal to milimoles of solute in the organic layer divided by the total number of milimoles of solute. The milimoles are the molarity times the mililiters. Therefore, the percent extracted is given by, %E = [S]oVo [S]oVo + [S]aVa X 100%

Vo and Va are the volumes of the organic and aqueous phases, respectively. Relation between %E and D is given by, %E = 100D D + (Va/Vo) If Vo = Va, then %E = 100D D + 1 If D is less than 0.001, the solute can be considered quantitatively retained. The percent extracted changes only from 99.5% to 99.9% when D is increased from 200 to 1000.

Example 1 20 ml aqueous solution of 0.10 M butyric acid is shaken with 10 ml ether. After the layers are separated, it is determined by titration that 0.5 mmol butyric acid remains in the aqueous layer. What is the distribution ratio, and what is the percent extracted?

Solution Starting in aqueous = 20 ml x 0.10 M BA = 2.0 mmol BA Left in aqueous sol. = 0.5 mmol BA Extracted in ether = 1.5 mmol BA [BA]e = 1.5 mmol/10ml = 0.15M [BA]a = 0.5 mmol/20ml = 0.025M..therefore D = 0.15 = 6.0 0.025 %E = 100D = 100 x 6.0 = 75% D + (Va/Vo) 6.0 + (20/10)

Solid-phase extraction Liquid-liquid extraction – extracting solvents used limits to those that are water immiscible, Large volume of solvents used – waste disposal problem SPE – become widely used for sample cleanup prior chromatographic analysis compounds that are dissolved or suspended in a liquid mixture are separated from other compounds in the mixture according to their physical and chemical properties.

SPE uses the affinity of solutes dissolved or suspended in a liquid (mobile phase) for a solid through which the sample is passed (stationary phase) to separate a mixture into desired and undesired components. The result either - desired analytes of interest or undesired impurities in the sample are retained on the stationary phase.

The stationary phase comes in the form of a packed syringe-shaped cartridge Solid phase extraction cartridges and disks are available with a variety of stationary phases, each of which can separate analytes according to different chemical properties. Most stationary phases are based on silica that has been bonded to a specific functional group.

Some of these functional groups include hydrocarbon chains of variable length (for reversed phase SPE), quaternary ammonium or amino groups (for anion exchange), and sulfonic acid or carboxyl groups (for cation exchange)

Working principle Conditioning Powdered phase (sorbent extraction bed) placed in a small cartridge Powdered phase eg silica bonded with hydrophobic phase, must be conditioned in order to interact with aqueous sample Accomplish by passing methanol through the sorbent bed. This penetrate into bonded layer and permit water mol. and analyte to diffuse into bonded phase Water is passed to remove excess solvent prior to adding sample

Loading sample/retention The sample (dissolve in liquid) is passed through sorbent extraction bed to separate a mixture into desired and undesired components. Analytes adsorbed to sorbent extraction bed Rinsing/washing Rinse the column to remove undesired components Elution Remove desired analytes

Continuous extraction For component of interest that has unfavorable partition coefficient (desired compound insoluble in solvent). Extraction is accomplished by continuously passing the extracting phase through the sample until a quantitative extraction is achieved Involving solid samples are carried out with a Soxhlet extractor

Working principle solid material containing some of the desired compound is placed inside a thimble made from thick filter paper, which is loaded into the main chamber of the Soxhlet extractor. The Soxhlet extractor is placed onto a flask containing the extraction solvent. The Soxhlet is then equipped with a condenser.

The solvent is heated to reflux. The solvent vapour travels up a distillation arm, and floods into the chamber housing the thimble of solid. The condenser ensures that any solvent vapour cools, and drips back down into the chamber housing the solid material.

The chamber containing the solid material slowly fills with warm solvent. Some of the desired compound will then dissolve in the warm solvent. When the Soxhlet chamber is almost full, the chamber is automatically emptied by a siphon side arm, with the solvent running back down to the distillation flask. This cycle may be allowed to repeat many times, over hours or days.

After extraction the solvent is removed, typically by means of a rotary evaporator, yielding the extracted compound. The non-soluble portion of the extracted solid remains in the thimble, and is usually discarded.