ANALYTICAL AND LABORATORY TECHNIQUES

Question 1

separation

Answer 1

process used to transform a mixture of substances into two or more distinct products

Question 2

purification

Answer 2

the physical separation of a chemical substance of interest from contaminating substances.

Question 3

Separation Techniques

Answer 3

1. Filtration
2. Distillation
3. Chromatography
4. Crystallisation (Precipitation)

Question 4

Filtration

Answer 4

• Filtration is a mechanical operation that separates the components of a heterogeneous mixture on the basis of differences in particle size.
• It is used most often to separate a liquid from a solid.
• One component is in solution while the other is not. The soluble component will pass through the filter while the other component will be retained.

Question 5

In the Lab

Answer 5

• Fluids flow through the filter due to a difference in pressure – fluid flows from the high pressure side to the low pressure side of the filter, leaving solid material behind.
• The simplest method to achieve this is by gravity e.g. the coffee filter to keep the coffee separate from the grounds.

Question 6

At Home

Answer 6

• The solid remaining in the filter paper is called the residue and can be dried by spreading it out on the filter paper and allowing the liquid to evaporate.
• The liquid which has passed through the filter paper is called the filtrate.

Question 7

Vacuum Filtration

Answer 7

• In the technique of vacuum filtration, the pressure of the flask is lowered to speed up the process.

Question 8

types of Filter

Answer 8

1. Surface filter

2. depth filter

Question 9

surface filter

Answer 9

a solid sieve which traps the solid particles, with or without the aid of filter paper.

Question 10

depth filter

Answer 10

a bed of granular materials which retains the solid particles.

Question 11

Distillation

Answer 11

• This is a method used for purifying a volatile liquid product.
• The crude liquid reaction product is heated to a boil, the vapours rise and are condensed into a receiver.
• Nonvolatile impurities remain in the still pot.

Question 12

Fractional Distillation

Answer 12

• Homogeneous liquid mixtures can be separated by fractional distillation.
• Distillation requires the different liquids to have different boiling points.
• In essence, each component of the mixture is boiled and collected.
• The lowest boiling more volatile component distills first, followed by the higher-boiling material.
• A good separation can usually be achieved if the components differ in boiling point by more than 5oC.
• Separating components with small volatility differences requires many vapourization-condensation steps until only the volatile component reaches the condenser.

Question 13

Chromatography

Answer 13

• Chromatography comes from the Greek root chroma, meaning “colour and was chosen because the original chromatographic separations involved colouredsubstances.
• Chromatography is used to separate mixtures that have different abilities to adhere/adsorb to solid surfaces.
• The mixture to be separated is dissolved in a mobile phase and the components are separated as this phase moves over a solid surface called the stationary phase

Question 14

Chromatography 2

Answer 14

• Partition equilibria are the basis of this technique.
• Chromatography is a continuous extraction process in which solute species are exchanged between two phases.
• The mobile phase moves with respect to the stationary phase.
• The partition ratio K of a solute A between the stationary and mobile phases is [A]stationary/[A]mobile=K

Question 15

Chromatography 3

Answer 15

• As the mobile phase containing solute passes over the stationary phase, the solute molecules move between the two phases. True equilibrium is never fully established because the motion of the fluid phase continually brings fresh solvent into contact with the stationary phase.
• The greater the affinity a component has for the stationary phase, i.e. the greater K is, the slower its progress through the separation system.
• The greater affinity a component has for the mobile phase the faster it moves.

Question 16

Chromatography 4

Answer 16

• Because different compounds adsorb to the stationary phase to differing extents, they migrate through the solid phase at different rates and are separated as they emerge.

Question 17

Thin Layer Chromatography

Answer 17

• The mobile phase or eluent is either a pure solvent or a mixture of different solvents, and is chosen so that the different compounds can be separated effectively.
• The eluent is optimized in small scale pretests, often using thin layer chromatography (TLC) with the same stationary phase.
• After the sample has been applied on the plate, the solvent or solvent mixture is drawn up the plate via capillary action.
• Because different analytes ascend the TLC plate at different rates, separation is achieved.
• Thin layer chromatography is performed on a sheet of glass, plastic, or aluminum foil, which is coated with a thin layer of adsorbent material, usually silica gel, aluminium oxide or cellulose.

Question 18

Crystallization

Answer 18

• Crystallization is often used to purify a compound by separating it from trace impurities.• This technique is based on the differences in solubility of the components of a mixture.
• Many substances are more soluble in hot solvent than in cold, and this property is applied in the separation.
• The substance to be purified must be soluble in the solvent when hot, but insoluble when the solvent is cold. • The success of this technique is critically dependent on the correct choice of solvent.
• Any impurities present must be either insoluble in the hot solvent so that they can be removed by filtration, or soluble in the cold solvent so that the remain in solution.
• The recrystallized product can be separated from the solution in a purer form by filtration.

Question 19

Crystallization in the Health Sciences - GOUT

Answer 19

• Gout is a disease marked by elevated levels of uric acid in the bloodstream.
• Crystals of monosodium urate or uric acid are deposited on the articularcartilage of joints, tendons, and surrounding tissues.
• It is marked by transient painful attacks of acute arthritis initiated by crystallization of urates within and about the joints.

Question 20

Types of Quantitative Analysis

Answer 20

• Volumetric analysis
• Gravimetric analysis
• Spectroscopic analysis

Question 21

Gravimetric Analysis

Answer 21

• In a gravimetric analysis a soluble analyte is converted into a well defined insoluble product which can be isolated and weighed.Key Characteristics– produce precipitate– collect & dry– weigh

Question 22

Absorption Spectroscopy

Answer 22

• Absorption spectroscopy refers to a (wide) range of techniques employing the interaction of electromagnetic radiation with matter.
• The intensity of a beam of light measured before and after interaction with a sample is compared.
• The wavelength of the radiation being used is in the UV-visible range.

Question 23

UV-Visible Spectroscopy

Answer 23

• UV-visible spectroscopy refers to techniques where one measures how much light of a particular wavelength (color) is absorbed by a sample. Colour can be correlated with the presence and or structure of a particular chemical, and since absorbance is an easy and cheap measurement to make, absorbance spectroscopy is widely used for both qualitative, quantitative and structural work in a wide range of fields.
• For example: DNA absorbs light in the UV range, which is why sunlight is dangerous, so the amount of DNA in a sample can be determined by measuring the absorbance of UV light.

Question 24

Beer-Lambert Law

Answer 24

• The Beer–Lambert law relates the absorption of light to the properties of the material through which the light is traveling.
• The diagram shows Beer–Lambert absorption of a beam of light as it travels through a cuvette of width ℓ.
• The law tends to break down at very high concentrations, especially if the material is highly scattering.

Question 25

Calibration Curve

Answer 25

In analytical chemistry, a calibration curve is a general method for determining the concentration of a substance in an unknown sample by comparing the unknown to a set of standard samples of known concentration. The calibration curve is a plot of how the instrumental response, in this case absorbance, changes with the concentration of the analyte (the substance to be measured). The operator prepares a series of standards across a range of concentrations near the expected concentration of analyte in the unknown. Each of these standards is analysed using the chosen technique and a series of measurements is produced. For most analyses a plot of instrument response vs. analyte concentration will show a linear relationship. The operator can measure the response of the unknown and, using the calibration curve, find the concentration of analyte.

Question 26

Soluble ionic compounds

Answer 26

1. NO3-
2. CH3COO-
3. Cl-; Br-, I-
   [exceptions: Ag+; Hg2^2+]
4. SO4^2-
   [Sr^2+; Ba^2+; Hg2^2+; Pb^2+]

Question 27

Insoluble ionic compounds

Answer 27

1. S^2-
    [NH4+; alkali metal cations, Ca2+; Sr2+; Ba2+] 
2. CO3^2-  
    [NH4+; alkali metal cations]
3. PO4^3- 
     [NH4+; alkali metal cations] 
4. OH- 
     [NH4+; alkali metal cations, Ca2+; Sr2+; Ba2+]

Question 28

Chromatography desperation techniques:

Gas-liquid

Answer 28

Mobile phase: gas

Stationary phase: liquid absorbers on a porous solid in a tube

Question 29

Chromatography desperation techniques:

Gas-solid

Answer 29

Mobile phase: gas

Stationary phase: porous solid in a tube

Question 30

Chromatography desperation techniques:

Column

Answer 30

Mobile phase: liquid

Stationary phase: liquid adsorbed on a porous solid in a tubular column

Question 31

Chromatography desperation techniques:

Paper

Answer 31

Mobile phase: liquid

Stationary phase: liquid held in the pores of a thick paper

Question 32

Chromatography desperation techniques:

Thin layer

Answer 32

Mobile phase: liquid

Stationary phase: liquid or solid; solid = held in glass plate, liquid = may be adsorbed on it

Question 33

Chromatography desperation techniques:

Ion exchange

Answer 33

Mobile phase: liquid

Stationary phase: solid (finely divided ion-exchange resin) in a tubular column