Chemistry 11–12 · Year 12

Colourimetry of copper(II) solutions: a calibration curve, the Beer-Lambert law and an unknown from a brass or ore digest

Module 8: Applying Chemical Ideas; no 2025 code: the Chemistry 11-12 Syllabus (2025) drops colourimetry and ultraviolet-visible spectrophotometry, and keeps atomic absorption spectroscopy only in Year 12 under CH-12-04, which this entry reaches as the generalisation of the calibration line rather than as the practical itself.

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

Absorbance is proportional to the concentration of a coloured species, so a calibration line from standards converts the absorbance of an unknown into its concentration; the same principle underlies UV-visible spectrophotometry and atomic absorption spectroscopy.

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

copper(II) sulfate harmful
nitric acid digestion (teacher, fume cupboard) releases nitrogen dioxide
cuvettes fragile

Controls

teacher performs the acid digestion in the fume cupboard
eye protection
copper waste collected

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

0.125 mol/L copper(II) sulfate stock, 100 mL (the five standards take 52.00 mL), and five 25.0 mL volumetric flasks for standards of 0.010, 0.025, 0.050, 0.075 and 0.100 mol/L
Colourimeter with a red filter (about 635 nm) or a spectrophotometer, or a smartphone colourimetry app in a fixed light box, and matched cuvettes
Unknown: a copper(II) solution from a brass filing (0.2 g) dissolved by the teacher in 5 mL of 6 mol/L nitric acid in the fume cupboard and diluted to 100.0 mL, or a copper ore digest as in the RSC method
Deionised water for the blank, pipettes, tissues

How to do it

Prepare the five standards by pipetting 2.00, 5.00, 10.00, 15.00 and 20.00 mL of stock into the flasks and making up to the mark.
Zero the instrument on the water blank; measure the absorbance of each standard twice and average.
Plot absorbance against concentration, fit the straight line through the origin, and record its gradient.
Measure the unknown's absorbance; if it lies above the highest standard, dilute it by a known factor and remeasure.
Read the concentration from the line, correct for dilution, and calculate the mass of copper in the original brass sample and its percentage.
In the simulation enter the standards' readings and the unknown's to reproduce the fit; explore why absorbance above about 1.5 departs from the straight line.

What you should see

Absorbance rises in proportion to concentration over the 0.010 to 0.100 mol/L range, the exact gradient depending on the instrument and filter. A brass sample of 0.200 g at 60 to 65 percent copper gives a 100.0 mL solution of 0.0189 to 0.0205 mol/L, between the two lowest standards, so the result is reported to two significant figures and compared with that 60 to 65 percent. An unknown that reads off the top of the line needs dilution before it can be read.

What changes

What you change: concentration of copper(II) in the standards
What you measure: absorbance
What you keep the same: wavelength or filter
cuvette path length
blank and zeroing
temperature
same instrument

Common misconceptions

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

Absorbance is the same as colour intensity seen by eye (the eye judges brightness logarithmically and is fooled by depth of colour; the instrument measures transmitted light).
The calibration line can be extended to any concentration (above about A = 1.5 the line bends; dilute instead).
The instrument measures how much copper metal is present (it measures the hydrated copper(II) ion at one wavelength; other coloured ions interfere).

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 CH11/12-4
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
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

All Concept Studio activities