Chemistry 11–12 · Year 12

A calibration line from published atomic absorption data for copper in water

Module 8: Applying Chemical Ideas

Practical, model not builtMedium risk

School laboratory, not for home

In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

This site has no interactive model of its own. Where a step or a material names a Concept Studio model, simulation or tool, it has not been built; an external simulation a step names (for example PhET) is not part of this site.

The idea

Atomic absorption spectroscopy measures how much light free atoms of one metal remove from a beam at that metal's own wavelength, so a line fitted through standards of known concentration turns a sample's absorbance into a concentration. A school may have no atomic absorption spectrometer, as the NSW Department of Education Module 8 guide recognises, so the standards are prepared by hand and the absorbance readings are taken from published data, which the syllabus allows, since its Module 8 content accepts processing data as well as conducting an investigation.

Safety card

Medium riskLearners carry it out, with a teacher supervising

Setting: In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

Hazards

commercial atomic absorption standards are supplied in dilute nitric acid, which irritates skin and eyes
copper salts are harmful if swallowed and are toxic to aquatic life
volumetric glassware breaks and cuts

Controls

the teacher or technician dispenses the 1000 mg/L standard; learners handle only the diluted standards
eye protection and a wash bottle at every bench
copper solutions collected in a labelled waste container, never poured to the sink
no instrument and no flame is operated by learners in this activity

Note

NSW Department of Education Chemical Safety in Schools (CSIS) package, 2021 Technical Update: Section 1.7 (risk assessment) and Volume 2 Appendix D (generic assessment advice and DoE chemical categories); record a RiskAssess (riskassess.com.au) risk assessment before the lesson and check the school's hazardous chemical register (CSIS Section 1.9) for local restrictions.

What you need

Copper standard solution, 1000 mg/L, 5 mL (a commercial atomic absorption standard, or one the technician prepares and labels with its concentration)
Six 100.0 mL volumetric flasks with stoppers: one for the diluted stock and five for the working standards
Bulb pipettes 2.00 mL, 5.00 mL, 10.00 mL and 20.00 mL, and a 50 mL burette for the 2.50 mL and 15.00 mL transfers
Deionised water, about 1 L, and a wash bottle
Published atomic absorption absorbance readings for the five standards and for at least one sample, from a past HSC examination question or a published water-quality method, with the source recorded
The NHMRC Australian Drinking Water Guidelines fact sheets for copper and for lead, printed
Graph paper, or a spreadsheet set to fit a line through the origin

How to do it

Pipette 2.00 mL of the 1000 mg/L copper standard into a 100.0 mL volumetric flask and make up to the mark with deionised water. This diluted stock is 20.0 mg/L.
Transfer 2.50, 5.00, 10.00, 15.00 and 20.00 mL of the diluted stock into five 100.0 mL flasks and make each up to the mark, giving working standards of 0.50, 1.00, 2.00, 3.00 and 4.00 mg/L; the five transfers use 52.50 mL of the 100.0 mL of diluted stock. Record the tolerance printed on each flask, each pipette and the burette.
Take the published absorbance reading for each standard and tabulate concentration against absorbance.
Plot absorbance against concentration in mg/L, fit a straight line through the origin by least squares, and record the gradient and the correlation coefficient.
Read the sample's concentration from the line. Where the sample's absorbance sits above the highest standard, apply the dilution factor the published method states and recalculate.
Compare the result with the NHMRC Australian Drinking Water Guidelines values for copper, 2 mg/L on health grounds and 1 mg/L on aesthetic grounds, and state which of the two, if either, is exceeded.
Refit the line twice, once with the lowest standard removed and once with the highest removed, and report how far the sample result shifts each time. That shift, not the correlation coefficient alone, is the measure of how far the line may be trusted.
Repeat the whole calculation for the lead fact sheet's value of 0.005 mg/L and state why the guidelines name inductively coupled plasma mass spectrometry for lead rather than flame atomic absorption.

What you should see

The five standards fall on a straight line through the origin across 0.50 to 4.00 mg/L, because absorption by free atoms in the flame follows the Beer-Lambert relationship over a narrow range of absorbance. As a check on the arithmetic, standards of 0.50, 1.00, 2.00, 3.00 and 4.00 mg/L reading 0.030, 0.060, 0.120, 0.180 and 0.240 absorbance units give a sum of products of 1.815 and a sum of squares of 30.25, so the gradient is exactly 0.0600 absorbance units per mg/L and the correlation coefficient is 1.000; a sample reading 0.093 is then 1.55 mg/L. That figure sits above the 1 mg/L aesthetic guideline value for copper and below the 2 mg/L health guideline value, which is the two-part conclusion the learner must state rather than a single pass or fail. The same sample diluted ten-fold before measurement would place the original water at 15.5 mg/L. With real readings, dropping the top standard moves the gradient and the reported sample concentration by an amount the learner computes and reports, which is the point of the step; with the exact example readings above nothing moves, because all five points lie on the line.

What changes

What you change: concentration of copper in the prepared standards
What you measure: absorbance at the copper resonance wavelength
What you keep the same: wavelength, set to the copper resonance line
lamp current and slit width
flame type and the fuel to oxidant ratio
sample uptake rate
zeroing on a deionised water blank
the same instrument within one session

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

Atomic absorption spectroscopy reports every metal in a sample from one reading (each element needs its own lamp and wavelength, so the instrument is set for one element at a time).
The calibration line may be extended past the top standard (the line bends as absorbance rises, so a sample above the top standard is diluted and measured again).
A reading below the lowest standard can be quoted as a number (below the lowest standard the line has not been established, so the result is reported as less than that standard).
The instrument measures the compound the metal arrived in (the flame breaks the sample into free atoms, and it is the free copper atoms that absorb, whatever compound carried them).
A high correlation coefficient proves the result (five points can lie on a line and still give a wrong answer if a standard was made up wrongly, which is why removing a standard and refitting matters).
A concentration under a guideline value means the water is safe (one element was measured, and the guidelines set values for many others).

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Chemistry Stage 6 Syllabus (2017), NESA; the current syllabus, taught in 2026 (codes read from the syllabus document)CH12-15 CH11/12-2 CH11/12-3 CH11/12-4
Chemistry Stage 6 Syllabus (2017), NESA; the current syllabus, taught in 2026 (codes read from the syllabus document); a Working Scientifically outcome not among those the syllabus targets in Module 8, which it allows in any moduleCH11/12-5 CH11/12-6
Chemistry 11-12 Syllabus (2025), NESA; implemented from 2028, not yet taught; read at source in the Year 12 focus area Applying chemical ideas, which names atomic absorption spectroscopy as a method of elemental analysisCH-12-04
Chemistry 11-12 Syllabus (2025), NESA; implemented from 2028, not yet taughtCH-12WS-02 CH-12WS-03 CH-12WS-04 CH-12WS-05
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

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