Specification Point 9.8C:
Describe that instrumental methods of analysis are available and that these may improve sensitivity, accuracy and speed of tests
- Advancements in technology and computing have allowed for the development of instruments designed to analyse chemical substances.
- Methods of analysis include X-ray, Infra-Red and Mass Spectroscopy, Gas Chromatography and Flame Photometry.
- These analytical techniques require modern day instruments which are a vital part of busy chemistry laboratories.
- The advantage of using these instruments over more traditional methods include:
- They provide greater accuracy.
- They are faster and easier to use.
- Modern instruments are very sensitive and can work with very small sample sizes.
Specification Point 9.9C:
Evaluate data from a flame photometer:
a) to determine the concentration of ions in dilute solution using a calibration curve
b) to identify metal ions by comparing the data with reference data (no knowledge of the instrument or how it works is required)
- When substances are heated they often emit energy in the form of light.
- Flame photometry works by exposing the sample to a very hot flame and then measuring the intensity and wavelength of the light emitted.
- An apparatus inside the machine then converts the light into electrical signals.
- The output is an emission spectrum in which different ions produce lines in different parts of the spectrum.
Diagram of an emission spectrum for mercury obtained from flame photometry
- The emission spectrum consists of brightly coloured thin lines on a dark background and each element ion produces a unique spectrum.
- Flame photometry also works for mixtures of ions.
- This is a major advantage over flame testing which can only analyze one ion at a time.
- The concentration of an unknown sample can be determined by creating a calibration curve.
- Solutions of different but known and concentrations of the metal ion are prepared and analysed using the flame photometer.
- The readings obtained are used to prepare a calibration curve which plots the instrument reading against the known concentrations.
- This is then used to determine the concentration of the unknown solution using construction lines from the instrument reading of the unknown sample.
- An example is shown below for a solution of calcium ions that produced a reading of 3.8 units and whose concentration is thus calculated at being 1.85 Ca mol dm-3.
Calibration curve of calcium solutions for determining the concentration of the sample solution
- Ions in unknown samples can be identified by comparing the sample spectrum to reference spectra.
- This is particularly useful if the sample contains a number of different ions.
- The following flame spectrum for example was obtained for solution containing an unknown metal:
- When compared to the reference spectra below we can see that the solution must contain sodium ions:
Edexcel GCSE Chemistry Notes
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