GCSE · Chemistry · Edexcel · Spec 1CH0
Flame photometer data
Put a sample in a flame and a flame photometer gives you two answers: which metal ions are there, and how much.
A sample reads 42. What is its concentration?
Slide the point along the line until its height is 42. Then read the concentration it sits above.
Inside flame emission spectroscopy
Step through it. Each stage hands something to the next.
Is the metal in the sample?
The unknown sample's spectrum has lines at positions 1, 2, 4, 5, 7 and 8. Each lettered metal gives the lines listed. Is each metal present or not present?
Still to sort
Present (0)
Every one of the metal's lines appears in the unknown sample.
Where the line is: The line between the two columns: all of its lines, not just some of them.
Not present (0)
At least one of the metal's lines is missing from the unknown sample.
Where the line is: A metal with some lines matching still goes here, because some are missing.
Made-up line positions for practice. They are not real wavelengths.
WHAT YOU'VE LEARNED
A quick recap of today's lesson.
One flame, two answers: which metal ions are in the sample, and how much of each.
What you need to know
- Flame emission spectroscopy (FES) is an instrumental method for analysing metal ions, and it uses a flame photometer.
- The sample goes into a flame, which makes the metal ions emit light.
- A spectroscope splits that light into a spectrum of lines, and the lines correspond to different elements.
- Each positive metal ion emits a unique set of lines, its flame emission spectrum, so spectra act like fingerprints.
Have a goYour friend Sam says: 'Two different metal ions could easily give exactly the same set of lines.' What does the fingerprint idea say back to Sam?
Each positive metal ion emits its own unique set of lines.
A fingerprint is only useful for identifying if no two are the same, and each metal ion's set of lines is unique.
- To identify metal ions, compare the unknown sample's spectrum with the spectra of known metal ions.
- A metal is present only if all its lines appear in the unknown. If some are missing, it is not present.
Have a goMetal W gives three lines. Two of them show up in an unknown sample's spectrum, and the third doesn't. Could W still be present?
No. W is not present.
One of its lines is missing, and a metal is present only if all of its lines appear.
- FES avoids the masking problem of flame tests: several metal ions in one solution can each be detected and measured.
- Readings for known concentrations are plotted to make a calibration curve, which is used to find a concentration in a sample.
Have a goYou have flame photometer readings for three solutions whose concentrations you already know. Can you build a calibration curve from them?
Yes. That is exactly what a calibration curve is made from.
Known concentrations give the points; the line through them is the ruler you later use on the unknown.
- To find a concentration, take the sample's reading, go across to the line of best fit, read off the concentration, and add units.
The big picture
Flame emission spectroscopy (FES) is an instrumental method for analysing metal ions, and it uses a flame photometer. The sample goes into a flame, the metal ions emit light, and a spectroscope splits that light into a spectrum of lines. Each metal ion's set of lines is its fingerprint: a metal is present only if all of its lines appear in the unknown sample. Readings for known concentrations make a calibration curve, and you use it to turn a sample's reading into a concentration, written with its units.
Key points
Worked example
Problem
Known solutions of a metal ion give these flame photometer readings: 0 g/dm³ gives 0, 5 g/dm³ gives 8, 10 g/dm³ gives 16, 15 g/dm³ gives 24 and 20 g/dm³ gives 32. A sample of unknown concentration gives a reading of 20. Find its concentration.
⚠ Watch out
Deciding a metal is present because one of its lines matches. All of its lines must appear. A second slip is stopping at the number: a concentration needs its units.
Memory hook
Fingerprint for WHICH, ruler for HOW MUCH. A fingerprint only matches if every ridge lines up, and a calibration curve is the ruler that turns a reading into a concentration.
Check yourself
An unknown shows lines 2, 4, 5 and 9. Metal R gives lines 2 and 9; metal S gives lines 4 and 7. Which is present, and why?
Flashcards
(12)What is flame emission spectroscopy (FES)?
What happens to the sample in FES?
What does the spectroscope do?
Why does a flame emission spectrum act like a fingerprint?
How do you identify metal ions in an unknown sample?
When is a metal present in the unknown sample?
A metal's lines match the unknown in some places but not all. Present or not?
Which problem of flame tests does FES avoid?
What is a calibration curve?
How is a calibration curve made?
How do you find a concentration from a calibration curve?
How must a concentration answer be written?
Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.
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