KS4 Science GCSE/IGCSE Industrial Chemistry Revision Notes
6. Instrumental Methods of Chemical Analysis
Fast, sensitive and accurate instrumental methods of chemical analysis are required to meet the demands of the modern chemical industry. These methods include mass spectroscopy, atomic emission spectroscopy, gas-liquid chromatography, nuclear magnetic resonance spectroscopy, infra-red spectroscopy, ultra-violet spectroscopy. In most cases only a small sample is required and modern analytical techniques have the advantages of greater sensitivity, more accurate data analysed by computer, automation of analysis, multi-samples efficiently analysed, a greater range and versatility of analytical techniques with greater reliability and consistency of analysis data.
Index of sections: 1. Limestone, lime - uses, thermal decomposition of carbonates, hydroxides and nitrates * 2. Enzymes and Biotechnology * 3. Contact Process, the importance of sulphuric acid * 4. How can metals be made more useful? (alloys of Al, Fe, steel etc.) * 5. The importance of titanium * 6. Instrumental Methods of Chemical Analysis * 7. Chemical & Pharmaceutical Industry Economics & Sustainability * 8. Products of the Chemical & Pharmaceutical Industries & impact on us * 9. The Principles & Practice of Chemical Production - Synthesising Molecules and other web pages of industrial chemistry notes: Ammonia synthesis/uses/fertilisers * Oil Products * Extraction of Metals * Halogens - sodium chloride Electrolysis * Transition Metals * Extra Electrochemistry
Instead of testing for chemicals using standard laboratory equipment such as test tubes etc. Special instruments have been developed to carry out such testing. These are quick, accurate and can be used on very small samples.
Ultra-violet spectroscopy can be used to the determine purity or concentration of solution of a substance that absorbs uv light.
Gas-liquid chromatography (gc/glc) can be used to analyse liquid mixtures which can be vapourised (e.g. petrol, blood for alcohol content).
A sample of the substance under investigation is injected and vapourised into a tube containing a carrier gas (called the mobile phase, it moves). The gas carries the vaporised substance through a long 'separating' tube or column wound around inside a thermostated oven.
The substances in the mixture are partially and temporarily absorbed by an absorbent material held in the column.
The gases emerge from the oven into a detector system which electronically records the different signal as each substance comes through. A printout or computer display of the results from the gas chromatograph, called the gas chromatogram, shows a series of peaks in the graph line imposed on a steady baseline when only the carrier gas is passing through the detector.
The time it takes for a substance to come through is called the retention time and is unique for each substance for a particular set of conditions (flow rate, length of separating column, nature of separating column material, temperature etc.). Generally speaking, the greater the molecular mass of the mixture molecule, the longer the retention time. This is because the component molecule - immobile phase intermolecular force of attraction increases with the size of the component molecule, so it is absorbed/retained temporarily a bit more strongly (see right of diagram).
The height of the peak, or more strictly speaking, the area under the peak, is proportional to the amount of that particular substance in the mixture.
Therefore it is possible to identify components in a mixture and calculate their relative proportions in the mixture.
The chromatogram shown above (right of diagram) illustrates the separation of some alkane hydrocarbons in petrol (in reality it is far more complicated with dozens of hydrocarbon molecule peaks on the chromatogram). The different peak heights give the relative proportions i.e. hexane >pentane > heptane.
The retention time order follows the trend of increasing molecular mass gives increasing retention time i.e. in time heptane C7H16 > C6H14 > C5H12
The gas chromatographic instrument can be calibrated with known amounts of known substances.
Don't confuse with 'non-instrumental' paper/thin layer chromatography.
You can also have more sophisticated analysis by attaching a mass spectrometer to the gas chromatograph and analyse each separated molecule as they exit the separating column. You can get the molecular mass of each component from the molecular ion peak (see mass spectroscopy further up the page)
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