1.1.11Biological Molecules

Finding the Concentration of Glucose — AQA A Level Biology 7402 Revision Notes & Topical Questions

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Lesson summary

1.1.11 Finding the Concentration of Glucose — AQA A Level Biology 7402 notes: Plan and interpret a colorimetric or test-strip method that uses a calibration curve to find the concentration of glucose in an unknown solution.To find an unknown concentration, you measure a property …

serial dilution
red filter
Exam-style question 1
Exam-style question 2
Exam-style question 3
Colorimeter

Section 1 of 9The idea of a calibration curve

To find an unknown concentration, you measure a property that changes predictably with concentration for solutions of known concentration, then read off the unknown from the resulting graph.

Calibration curve

This is a calibration curve.

For glucose, the property can be the colour produced by Benedict's reagent, the colour produced by a glucose oxidase test strip,

or the absorbance of a coloured product measured in a colorimeter.

Section 2 of 9Making the standard solutions

Start with a stock solution of known concentration, for example 1.0 mol dm^-3 or 10 g per 100 cm³. A serial dilution produces a range:

Making the standard solutions

Use a pipette or measuring cylinder to take a measured volume of stock and make it up with distilled water to a total volume.

A simple series: 10, 8, 6, 4, 2 and 0 cm³ of stock made up to 10 cm³ with water gives 100, 80, 60, 40, 20 and 0 per cent of the stock concentration.

For a very wide range, a 1 in 10 dilution series is used, but the points then fall unevenly and a log scale is needed.

Concentration of diluted solution = stock concentration × (volume of stock ÷ final volume).

Section 3 of 9Colorimetric procedure

Add equal volumes of each standard (and the unknown) to separate tubes.

Add the same volume of Benedict's reagent (excess) to each and heat in a water bath at the same temperature for the same time.

Remove the red precipitate by filtering or centrifuging, leaving the unreacted blue copper(II) solution.

The more glucose in the sample, the more copper has been used up and the paler the blue filtrate.

Red filter

Zero the colorimeter with distilled water or the 100% glucose filtrate, and use a red filter,

which is absorbed strongly by blue solutions; measure absorbance or percentage transmission of each filtrate.

Plot the values for the standards against concentration and draw a smooth line or curve of best fit.

Read the unknown's concentration from its absorbance.

Section 4 of 9Using glucose test strips and biosensors

Test strips contain the enzyme glucose oxidase and a dye.

The enzyme oxidises glucose, producing hydrogen peroxide, which with a second enzyme (peroxidase) changes the dye colour, giving a colour scale.

A biosensor makes the same reaction produce a small electric current that is proportional to glucose concentration and is displayed digitally.

These methods are specific for glucose, quick, and require little sample, so they are used in diabetes monitoring.

Benedict's detects any reducing sugar, so it is less specific.

Section 5 of 9Reliability and sources of error

Variables to controlvolume of reagent and sample, temperature and time of heating, the filter, and cuvette cleanliness.

Repeat each standard at least three times and use the mean.

The calibration range must include the unknown, since reading outside it relies on extrapolation.

If results are not on the curve line, check for pipetting errors and anomalies.

Uncertainty can be estimated from the resolution of the equipment, for example ±0.5 cm³ on a measuring cylinder is a much bigger percentage error for 2 cm³ than for 10 cm³,

so a pipette or syringe is a better choice for small volumes.

Section 6 of 9Worked data approach

A common exam task gives a table of colorimeter readings for standards and asks you to find an unknown.

Follow a routine.

First, check whether the trend is increasing or decreasing with concentration, and whether the line is straight or curved.

Plot the points with concentration on the x-axis and the reading on the y-axis, draw a smooth line through them,

and then draw a line from the unknown's reading across to the curve and down to the x-axis.

Show these construction lines on the graph, because marks are often given for them.

For example, imagine absorbance values of 0.80, 0.60, 0.40, 0.20 and 0.00 for glucose at 0, 20, 40, 60 and 80 mmol dm^-3, since the filtrate gets paler as more copper is removed.

Worked data approach

An unknown reading of 0.30 would fall between 40 and 60, so its concentration is estimated at 50 mmol dm^-3.

If the unknown solution was diluted before testing, multiply by the dilution factor to give the concentration of the original sample.

The method can be improved by using more standard concentrations close to the expected value of the unknown, by repeating each reading, and by using a specific enzyme-based test strip or biosensor when the sample contains other reducing sugars that would interfere with Benedict's reagent.

Section 7 of 9Exam-style questions

Describe how to prepare 10 cm³ of 0.4 mol dm^-3 glucose from a 1.0 mol dm^-3 stock. [2]2 marks
A colorimeter reading of an unknown glucose sample gave 55% transmission. The calibration curve showed 50% at 0.6 and 60% at 0.4 mol dm^-3 (straight line between). Estimate the concentration. [2]2 marks
Explain why a red filter is used and why the blue precipitate is removed. [3]3 marks

Section 8 of 9Key terms

Term	Meaning
Calibration curve	A graph of a measured property against known concentrations used to find unknowns.
Serial dilution	A stepwise dilution producing a range of concentrations.
Colorimeter	An instrument that measures how much light a solution absorbs or transmits.
Absorbance	A measure of the light absorbed by a solution.
Stock solution	A concentrated solution of known concentration used for dilutions.
Glucose oxidase	Enzyme that specifically oxidises glucose, used in test strips and biosensors.
Biosensor	A device that uses a biological molecule to produce an electrical signal.

Section 9 of 9Exam technique

Using unequal volumes of reagent or different heating times, which make the standards and unknown not comparable.

Reading a concentration from outside the range of the standards without saying it is an extrapolation.

Forgetting a zero (water) blank when calibrating the colorimeter.

Saying more glucose gives a bluer filtrate. More glucose uses more copper, so the filtrate is paler.

Last updated: 18 October 2026

Section 1 of 9The idea of a calibration curve

Section 2 of 9Making the standard solutions

Section 3 of 9Colorimetric procedure

Section 4 of 9Using glucose test strips and biosensors

Section 5 of 9Reliability and sources of error

Section 6 of 9Worked data approach

Section 7 of 9Exam-style questions

Describe how to prepare 10 cm3 of 0.4 mol dm-3 glucose from a 1.0 mol dm-3 stock. [2]2 marks

A colorimeter reading of an unknown glucose sample gave 55% transmission. The calibration curve showed 50% at 0.6 and 60% at 0.4 mol dm-3 (straight line between). Estimate the concentration. [2]2 marks

Explain why a red filter is used and why the blue precipitate is removed. [3]3 marks

Section 8 of 9Key terms

Section 9 of 9Exam technique

Marked practiceQuizzes for this lesson

Describe how to prepare 10 cm³ of 0.4 mol dm^-3 glucose from a 1.0 mol dm^-3 stock. [2]2 marks

A colorimeter reading of an unknown glucose sample gave 55% transmission. The calibration curve showed 50% at 0.6 and 60% at 0.4 mol dm^-3 (straight line between). Estimate the concentration. [2]2 marks