International A Level Biology / Chemistry for biologists
Core practical 1: estimating sugar and starch concentrations revision notes
Edexcel International A Level Biology (YBI11), chapter 1: Chemistry for biologists, specification point 1.3. Free lesson.
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Compare an unknown sugar or starch solution with standards of known concentrations to estimate its concentration. The method is semi-quantitative, so you report a range rather than an exact value.
How to make concentration standards
A standard is a solution with a known concentration. Start with glucose stock of a known concentration, then halve the concentration at each step by mixing equal volumes of solution and distilled water. Mix each tube well before transferring liquid, so the next standard has the concentration you expect. A 0% standard is distilled water only. It shows what no sugar looks like.
How to estimate reducing sugar with Benedict's
Benedict's reagent tests for reducing sugars, such as glucose, but not sucrose. Treat every standard and the unknown in the same way. If the sample is food, grind it with distilled water and filter it first, so you test the liquid. As glucose concentration rises, the colour changes from blue through green, yellow and orange to brick red. A coloured precipitate, which is a solid formed in the liquid, also appears. Compare the unknown against the standards in order of concentration, against a white background. The two standards it lies between give the estimate.
How the starch test is different
Use iodine solution to test for starch, not Benedict's. Iodine works at room temperature, so it does not need heating. Make starch standards of known concentration and add the same volume of iodine to each one. Iodine changes from orange-brown to blue-black when starch is present. More starch gives a darker blue-black colour. Match the unknown's shade against the starch standards, just as you compare sugar with glucose standards.
What to control and what the result means
For Benedict's, the independent variable is the glucose concentration of the standards, set by serial dilution. The dependent variable is the colour after heating, matched by eye against the standards on a white background. Keep the volume of each solution the same. A larger volume contains more sugar and could change the colour. Keep the volume of Benedict's the same and in excess. If it runs out, stronger solutions could end up looking alike. Keep the water bath temperature and heating time the same. Temperature affects how far the reaction goes, and the colour continues developing…
How to make the estimate more accurate
If an unknown lies between the 0.5% and 1% glucose standards, report that range. The standards do not show that its concentration is exactly 0.75%. Make extra standards within the gap, such as 0.6%, 0.7%, 0.8% and 0.9%, to narrow the range. A colorimeter measures absorbance and makes the method quantitative. Repeating the test can show repeatability, but does not by itself make the estimate more accurate.