CfE Higher Chemistry

Unit 3: Chemistry in Society

Chemical Analysis as part of quality control

06/12/2017

Learning Outcomes :

Composition and purity 06/12/2017

• I can explain the basic principle of how chromatography works, defining the mobile and stationary phases. for the

• I can read and interpret retention/time graphs from results of chromatography experiments.

• I can interpret chromatograms using Rf values

Lesson Starter: Redox Reactions

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Chromatography

• Chromatography is a powerful technique that allows chemists to separate the components of a mixture.

• Paper chromatography illustrates the basic principles of the separation technique.

Chromatography

• Every type of chromatography has a mobile phase and a stationary phase. In the case of paper chromatography, the wet paper is the stationary phase and the solvent is the mobile phase.

• The separation occurs because the molecules in the mixture have different affinities for the stationary and mobile phases.

• Their affinity for the stationary and mobile phases will depend on polarities and / or size of the molecules.

Paper chromatography Stationary phase: paper

Mobile phase : solvent

Substances that are more soluble in the solvent travel further

Substances that have more of an affinity for the paper (stationary phase) travel less far

Chromatography

Solvent

Ink is a mixture of dyes (liquids)

As the solvent travels up the paper the different liquids travel different distances

Indigo ink contains yellow and green dyes – the mixture of inks has been separated

Calculating Rf.

Experiment:

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Experiment:

• Place a few g of tomato puree into your test tube.

• Add 3 cm3 of the Propanone : PET ether solvent. Replace the lid tightly (or put the bung in the test tube) and shake vigorously.

• Let it settle for a minute or so.

• Take a capillary tube, or a very narrow Pasteur pipette and extract a little of the supernatant.

• Cut a strip of filter paper narrow enough to fit inside your test tube. Make a mark about 1cm from the bottom in pencil.

• Spot this on the line on the strip of filter paper. Dry this and then re apply several times, drying between each one.

• Put about 0.5 cm3 of hexane in the bottom of the test tube and insert the paper.

• Place the tube in a rack and leave it to run for a few minutes.

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What you do

Experiment:

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You will see that the yellow β-carotene, being less polar, travels faster up the paper. In fact it goes with the solvent front. Behind it, rather more spread out, is the orange-red of the lycopene.

Experiment:

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Thin layer chromatography • Stationary phase: glass or Al supports thin layer

of cellulose or silica

• Mobile phase : solvent

• TLC quicker and prevents spots spreading

• Fluorescent dye can be incorporated to enable spots to be located by UV

• Common use – cannabis analysis and amino acid separation.

Column chromatography • The column is packed with inert material

• Substances take a different time to travel through the column according to their relative affinity for the solvent and the column material

• Each substance has a specific retention time that can be used to identify it.

Column

• http://chemsite.lsrhs.net/FlashMedia/html/columnChrom.html

• Stationary phase – usually silica.

• Mobile phase – eluent and sample.

• Common use – separating food dyes.

Gas Liquid chromatography • Widely used in many industries

• Locally used by Diageo to test every mash produced by a distillery and every batch of alcohol produced

• Ensures the distillery is converting as much sugar to alcohol as possible

• Checks for impurities in the whisky

Gas-Liquid Chromatography

• Stationary: High bp liquid held on an inert powdered support material, packed into a coiled column in an oven!

• Mobile phase : inert gas eg Helium or nitrogen

• Heat vaporises sample, carried by gas through hot column

Gas Liquid chromatography

GLC cont • Rate of travel through column depends on relative affinity for

gas or inert material

• As substance emerged, analysed by detector, producing a graph

• Identification is by ‘retention time’

• Height of peak indicates amount of substance

Gas column

• Stationary phase- the column which contains a liquid stationary phase.

• Mobile phase – flow of inert gas. Usually Ne, Ar, N2

• Common use – drink driving – breath test analysis.

GC Chromatograms

Retention Time • The retention time of a

solute is taken as the elapsed time between the time of injection of a solute and the time of elution of the peak maximum of that solute.

Identification

• The Retention times of known compounds can be measured and recorded in a data base

• The Retention times of substances in the sample can then be compared to the data base and identified

Chromatography is used to identify drugs in urine samples taken from athletes.

The mixture of liquids in the urine is injected, samples of known drugs have already been analysed.

Steroid A takes 18mins 50 sec to pass through the chromatography machine.

Has the athlete tested below been using Steroid A?

Chemical analysis

• Analysis is very important in industry for quality control purposes

• Checking for contaminants

• Checking the level of product e.g alcohol in whisky, aspirin in tablets etc

Analysis Analysis is widely carried out for a range of applications.

A few examples are:

1. soil analysis

2. samples of blood, urine etc in health

3. Water purification analysis

4. swimming pools etc

5. environmental health

Types of Chromatography • Column Chromatography

• Paper Chromatography

• Ion Exchange

• Thin Layer

• Liquid Chromatography

• Gas Chromatography

• High Performance Liquid Chromatography

Case Studies

http://media.rsc.org/Chemistry%20at%20the%20races/RSC%20Horseracing%20Part%205.pdf

(neigh chance!!)

• http://media.rsc.org/Classic%20Chem%20Demos/CCD-24.pdf

Practice Question

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Practice Question

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• A typical chromatogram obtained from GLC (and HPLC) shows the quantity of component on the y-axis and the retention time on the x-axis.

• The retention time is the time it takes the component to reach the detector.

• The retention times can give us some information about the components if we know the nature of the stationary and mobile phases.

• For example, if the stationary phase is a polar liquid we would expect polar compounds to have a longer retention time than non-polar compounds since the polar components would be attracted to the stationary phase. Non-polar molecules would not be attracted to the stationary phase. They would be carried very quickly through the column by the carrier gas resulting in a very short retention time.

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Practice Question

• A suspicious house fire was found to have started in a chair. An almost empty bottle of paint thinner was found in a suspect’s car. In the house there were two cans of furniture polish which might have been used to clean the chair at some time.

• The chromatograms obtained from the remains of the chair, the paint thinner and the furniture polishes are shown.

• Which of the substances tested were present on the armchair?

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Practice Question

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2015 NH 4

2015 NH 4

Success Criteria:

Next Lesson:

I can explain the basic principle of how chromatography works, defining the mobile and stationary phases. for the

I can read and interpret retention/time graphs from results of chromatography experiments.

I can interpret chromatograms using Rf values

Composition and purity 06/12/2017

Learning Outcomes :

Volumetric titrations 06/12/2017

• I can state the principle of volumetric analysis using titration.

• I can give a definition of an indicator and name some examples.

• I know what the “end point” of a reaction is.

• Use the results of titrations and balanced redox equations to calculate the concentration of a reactant, given the concentration of the other.

• Explain what a standard solution is and how to make up a standard solution.

• Explain the relative accuracy of a range of volumetric measuring equipment.

Success Criteria:

Next Lesson:

I can state the principle of volumetric analysis using titration. I can give a definition of an indicator and name some examples. I know what the “end point” of a reaction is. Use the results of titrations and balanced redox equations to calculate the concentration of a reactant, given the concentration of the other. Explain what a standard solution is and how to make up a standard solution. Explain the relative accuracy of a range of volumetric measuring equipment.

Volumetric titrations 06/12/2017

Redox Titrations: Good practice

Take a watch glass and place it on the balance. Tare the balance (set it to zero). Carefully weigh out the required mass of substance.

Transfer this amount to a beaker. Add water from a wash bottle to dissolve it. Use some of the water to rinse all the substance off the watch glass. Do this at least twice.

Redox Titrations: Good practice

Stir with a glass rod until all the solid is dissolved, then transfer the solution to the volumetric flask. Use more water from the wash bottle to rinse out the beaker and the glass rod. Do this at least twice.

Add water to just below the line on the volumetric flask. Add the final drops with a teat pipette to ensure that the bottom of the meniscus is on the line.

Redox Titrations: Good practice

Put the lid on the flask and turn the flask over a couple of times to mix the solution

Redox Titrations: Good practice

Rise the BURETTE with the solution going to be put in it.

Use a FILTER FUNNEL to put solution into burette

Fill above the scale and the remove the filter funnel. The value will change on the scale when you do this. Open the tap and allow to run some solution off to get rid of any air bubbles

Read scale from BOTTOM of meniscus

Redox Titrations: Calculations Vit C

I2 (aq) + 2e- 2I - (aq)

C6H8O6 C6H6O6 + 2H+ (aq) + 2e-

reduction

oxidation

I2 (aq) + C6H8O6

colourless

C6H6O6 + 2H+ + 2I- (aq)

Blue/Black (in the presence of starch)

Iodine, the concentration is known (in the burette) acts as an oxidising agent. Vitamin C, the unknown concentration (in the conical flask) is a reducing agent.

Starch is added to show when the end-point is reached.

Redox Titrations: Calculations Vit C

I2 (aq) + C6H8O6

colourless

C6H6O6 + 2H+ + 2I- (aq)

A solution of Vit C was prepared and 25cm3 of this solution (250cm3 standard flask) was titrated against 0.031 mol/L of iodine solution using startch indicator. The average titre was 17.6cm3 Work out the mass of Vic C in tablet.

1 : 1

I2 (aq) C6H8O6

n= c x v

n= 0.031 x 0.0176

n= 0.0005456 n= 0.0005456

Redox Titrations: Calculations Vit C

25cm : 0.0005456moles

C6H8O6

m= n x GFM

250cm : 0.005456moles

MUST SCALE FOR FULL SOLUTION

m= 0.005456 x 176

m= 0.960g

2014 W 13

Redox Titrations: Calculations Vit C

1 : 1

I2 (aq) C6H8O6

n= c x v

n= 0.00125x 0.0254

n= 0.00003175 n= 0.00003175

20.0cm : 0.00003175moles

C6H8O6

1000cm : 0.00159moles

m= n x GFM

m= 0.00159 x 176

m= 0.279g