A level-readyAll subjects · Biology · WBI11 (Unit 1)
Core practical 1: estimating reducing sugar and starch
Core practical 1 uses Benedict's reagent and iodine solution with a set of colour standards to estimate how much reducing sugar or starch a sample contains.
Making colour standards
1. Make a dilution series of glucose solutions of known concentration, for example 0, 0.2, 0.4, 0.6, 0.8 and 1.0%, from a stock solution and distilled water.
2. Add the same volume of Benedict's reagent to the same volume of each solution.
3. Heat all tubes in a boiling water bath for the same time.
4. The colours run from blue (no reducing sugar) through green, yellow and orange to brick red (high concentration). These are your colour standards.
For starch, add the same volume of iodine solution to a dilution series of starch: colour runs from orange-brown through to blue-black.
Testing the unknown
Treat the unknown sample in exactly the same way (same volumes, same heating time) and match its colour to the nearest standard. This is semi-quantitative: it gives a range, not an exact value.
Reducing sugars include glucose, fructose, maltose and lactose. Sucrose is a non-reducing sugar, so it gives a negative (blue) result unless it is first hydrolysed.
Variables and improvements
- Independent variable: concentration of the sugar (or starch) standard.
- Dependent variable: the colour produced.
- Control: volume of sample and reagent, temperature and time in the water bath.
Colour matching is subjective. To make it more quantitative, use a colorimeter to measure absorbance or transmission, or filter and weigh the red precipitate, and plot a calibration curve.
Worked example
You need 10 cm3 of 0.4% glucose from a 1.0% stock. What volumes of stock and water do you mix?
- Volume of stock = (0.4 / 1.0) x 10 = 4 cm3
- Water = 10 - 4 = 6 cm3
Answer: 4 cm3 of stock and 6 cm3 of distilled water.
Check yourself
- How many cm3 of a 1.0% glucose stock solution are needed to make 20 cm3 of 0.25% glucose?
Show answer
5 cm3. Volume = (0.25/1.0) x 20 = 5 cm3, made up to 20 cm3 with water.
- A sample heated with Benedict's reagent stays blue. What does this show?
- A high concentration of glucose
- Starch is present
- No reducing sugar is present
- A protein is present
Show answer
No reducing sugar is present. Blue is the colour of the unreacted reagent.
- Why does pure sucrose solution give a negative Benedict's test?
- Sucrose is a non-reducing sugar
- Sucrose is a polysaccharide
- Sucrose is insoluble
- Sucrose reacts only with iodine
Show answer
Sucrose is a non-reducing sugar. Sucrose must be hydrolysed into glucose and fructose before it gives a positive result.
- Why is this method described as semi-quantitative?
- It gives an exact concentration in g dm-3
- It only shows whether sugar is present or absent
- It needs no standards
- Colours are matched to standards, giving an estimated range rather than an exact value
Show answer
Colours are matched to standards, giving an estimated range rather than an exact value. Matching to the nearest standard gives an estimate within a range.
- Which change would make the estimate more quantitative?
- Using a larger test tube
- Measuring each tube with a colorimeter and using a calibration curve
- Heating the unknown for longer than the standards
- Judging the colour by eye in dim light
Show answer
Measuring each tube with a colorimeter and using a calibration curve. A colorimeter gives a numerical reading, removing subjective colour judgements.
- Which variable must be kept the same for every tube?
- Glucose concentration of the standards
- The colour produced
- The final result
- Time in the boiling water bath
Show answer
Time in the boiling water bath. Heating time affects how much precipitate forms, so it must be controlled.
- Iodine solution is added to a sample and it turns blue-black. What is present?
- Glucose
- Starch
- Lipid
- Protein
Show answer
Starch. Iodine turns blue-black with starch.
Sources
Last updated 7 November 2026. Spotted a mistake? Tell us.