🔎 Which metal ions?
Compare the lines in the sample spectrum with a reference set. If the sample has all the lines of the reference for a metal, that metal ion is in the sample.
Identifying Ions » Instrumental Methods and Flame Emission Spectroscopy
What you'll learn this session
Study time: 30 minutes
AQA spec: 4.8.3.6, 4.8.3.7
- What instrumental methods are and why chemists use them
- The advantages of instrumental methods over chemical tests
- How flame emission spectroscopy works
- How to read a line spectrum to find metal ions and their concentrations
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Unlock This CourseInstrumental methods
So far you have identified substances using chemical tests, such as adding sodium hydroxide solution and looking for a precipitate. These tests use test tubes and your own eyes. Chemists also use machines to do the job. Elements and compounds can be detected and identified using instrumental methods.
A machine does not rely on a person judging a colour. It measures the sample and gives a result, often as a chart or a table of numbers.
Key terms:
- Instrumental method: a way of detecting and identifying elements and compounds using a machine.
- Accurate: the result is very close to the true value.
- Sensitive: able to detect very small amounts of a substance.
- Rapid: quick, so many samples can be tested in a short time.
Why use instrumental methods?
Machines like this are accurate, sensitive and rapid - they can spot tiny amounts that a test tube and your eyes would miss
Instrumental methods are accurate, sensitive and rapid. These are their advantages compared with the chemical tests you have met.
🎯 Accurate
A machine gives a measured result. It does not depend on someone deciding what colour they can see.
🔬 Sensitive
They can detect tiny amounts of a substance, even when only a very small sample is available.
⚡ Rapid
A result can come in a short time, so lots of samples can be tested.
A chemical test can be hard to judge. In a flame test, a bright yellow flame can hide a lilac one. A machine can pick out each metal separately.
Common mistakes
Writing only "it is better" or "it is easier" will not score marks. Name the advantage: accurate, sensitive or rapid, and say what it means.
Flame emission spectroscopy
Taking river water for flame emission spectroscopy - the flame's light reveals which metal ions are in it and how much
Flame emission spectroscopy is an example of an instrumental method. It is used to analyse metal ions in solutions. It works like this:
- The sample solution is put into a flame.
- The flame gives out light.
- The light is passed through a spectroscope.
- The output is a line spectrum.
The line spectrum can be analysed to identify the metal ions in the solution and to measure their concentrations.
It is the same idea as a flame test, where metal ions give a colour to a flame. The difference is that a spectroscope splits the light up. Each metal ion gives its own pattern of lines, a bit like a fingerprint. This is why the method can identify several metals in the same sample.
Key terms:
- Flame emission spectroscopy: an instrumental method that analyses metal ions in solution using the light given out by a flame.
- Spectroscope: an instrument that the light passes through to produce a spectrum.
- Line spectrum: the output, made up of separate lines.
- Concentration: how much of a substance is dissolved in a given volume of solution.
Reading a line spectrum
📏 How much?
The concentration of the ions is measured from the spectrum. The sample is compared with the results for solutions of known concentration.
In exams you are given a chart or a table for the sample and a reference set in the same form. You do not need to remember any real spectra. You just match them up.
Worked example
The table below uses made-up data. A tick means a line is present.
| Line | Reference: ion P | Reference: ion Q | Reference: ion R | Sample |
|---|---|---|---|---|
| 1 | ✓ | ✓ | ||
| 2 | ✓ | ✓ | ✓ | |
| 3 | ✓ | |||
| 4 | ✓ | ✓ | ✓ | |
| 5 | ✓ | ✓ |
Step 1: ion P has lines 1 and 2. Both are in the sample, so P is present.
Step 2: ion Q has lines 3 and 4. Line 3 is missing from the sample, so Q is not present.
Step 3: ion R has lines 2, 4 and 5. All are in the sample, so R is present.
Answer: the sample contains ions P and R.
Common mistakes
Ticking a metal as present because only one of its lines matches. Every line in the reference must appear in the sample. Another mistake is forgetting that one line can belong to more than one metal, as line 2 does above.
Exam-style question
A scientist tests a sample of river water for metal ions. Give two advantages of using flame emission spectroscopy instead of chemical tests. The line spectrum of the sample has lines at the same positions as the reference spectrum for ion X, and no others. What does this show? (3 marks)
Model answer
Advantages (any two): it is accurate, it is sensitive so it can detect very small amounts, it is rapid. The sample has the same lines as the reference for ion X, so the sample contains ion X and none of the other ions in the reference set.
Exam tip
When asked to interpret a spectrum, check each metal's reference lines against the sample, one metal at a time, and cross out any metal with a line missing.