Lesson 9 of 9 / Practical and data skills
Food tests, indicators and risk
Which reagent answers which question, and how do you make it quantitative?
I can carry out and interpret food tests and indicators, make the Benedict's test semi-quantitative, and assess risk with a rating.
About 8 min
Make a guess. You are not marked.
You compare glucose solutions with Benedict's. Why must the reagent be in excess?
Show the answer
So the sugar, not reagent, limits colour.
If Benedict's runs out, stronger solutions look the same as weaker ones and comparison fails.
The key idea
Each test answers one question with a colour change written "from ... to ...". Benedict's becomes semi-quantitative when you compare against standards of known concentration, keep volumes and heating fixed, and keep the reagent in excess.
A reducing sugar, such as glucose, can reduce copper(II) ions in Benedict's solution. A non-reducing sugar, such as sucrose, cannot until it is broken down into reducing sugars. A redox indicator changes colour when it is reduced or oxidised.
Work with the evidence
Write every result from ... to ...
Starch: Iodine solution changes from yellow-brown to blue-black.
Colours are approximate drawings. Benedict's colours run blue, green, yellow, orange, brick-red as reducing sugar increases; compare against standards for a concentration.
Explanation
Write every result as a colour change "from ... to ...". Starch: iodine solution changes from yellow-brown to blue-black. Protein: biuret reagent changes from blue to purple. Lipid: shake the sample with ethanol, then pour the liquid into water; a cloudy white emulsion forms. Reducing sugar: add an equal volume of Benedict's solution and heat in a boiling water bath for a fixed time; the colour changes from blue through green, yellow and orange to a brick-red precipitate as the concentration increases.
To test for a non-reducing sugar, first show that the sample gives a negative Benedict's test (stays blue). Then boil a fresh sample with dilute hydrochloric acid to hydrolyse the sucrose into glucose and fructose, cool it, and neutralise it with sodium hydrogencarbonate until fizzing stops, because Benedict's does not work in acid. A positive Benedict's test now shows that a non-reducing sugar was present.
Benedict's can be made semi-quantitative. Make a set of standard glucose solutions of known concentration, for example by serial dilution. Test each standard and each unknown with the same volume of Benedict's solution, in excess, heated for the same time at the same temperature. Match the unknown's colour to the standards to estimate its concentration, or time how long each takes to first change colour. A colorimeter reading of the remaining blue solution after filtering removes the judgement of colour by eye.
Three indicators track gases and redox. Hydrogencarbonate indicator is red at the carbon dioxide level of normal air; it turns yellow when carbon dioxide increases and purple when it decreases. Methylene blue changes from blue to colourless when it is reduced, so it can compare the respiration rate of yeast at different temperatures by timing decolourisation. DCPIP also changes from blue to colourless when reduced: it detects vitamin C, and it shows the light-dependent reaction in isolated chloroplasts, which reduce it in light.
A risk assessment needs four parts for each hazard: the hazard, the risk (what could happen), a precaution and a rating. A boiling water bath can scald: medium risk; use a test-tube holder and keep the tube mouth pointing away. Dilute hydrochloric acid is an irritant to eyes and skin: low to medium risk; wear eye protection and wipe up spills. Ethanol is highly flammable: medium risk; keep it away from flames and use a water bath, not a Bunsen burner, to warm anything near it.
Step by step
- 1
Pick the test
Match the reagent to the substance or process asked about.
- 2
Control the conditions
Same volumes, temperature and time; reagent in excess; fresh sample for each test.
- 3
Record from ... to ...
State the starting and final colour, and compare with standards if quantitative.
- 4
Assess the risk
Hazard, what could happen, the precaution and a rating.
Worked example
Work through the evidence
A fruit extract stays blue after heating with Benedict's solution. After boiling with dilute hydrochloric acid and neutralising, it gives an orange precipitate. What do you conclude?
One way to explain it
No reducing sugar was present at first, but a non-reducing sugar was. Acid hydrolysis broke it into reducing sugars, which then gave a positive Benedict's test.
Why this answer works
- A negative first test is needed before the acid step means anything.
- Neutralising is essential because Benedict's does not work in acid.
- The final colour shows that reducing sugars are now present.
Watch out for this
A student says: "A brick-red result means there is exactly twice as much sugar as an orange result." What is wrong with this?
Show the answer
Benedict's colours give a rough order of concentration only. To estimate a concentration, compare against standards of known concentration tested under the same conditions.
Check your understanding
Why must Benedict's solution be in excess when comparing glucose concentrations?
Show the answer
So the amount of precipitate is set by the sugar, not by reagent running out.
If the copper(II) ions ran out, two high concentrations could give the same colour and could not be told apart.
Quick recall
Which reagent answers which question, and how do you make it quantitative?