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# Making a calibration curve for starch concentration

##### Demonstration

To follow the progress of a starch digestion or starch synthesis reaction, you could track the changing concentration of starch with a colorimeter.

To convert colorimeter absorbance readings to concentration, you need to make up some standard solutions of starch, add a fixed volume of iodine solution, and take colorimeter readings. Plotting the colorimeter readings against concentration gives you a curve. Then, during the course of your experiment, you can convert colorimeter readings (from the starch solution in your reaction vessel) into concentrations by comparison with the curve and interpolation of the values.

#### Lesson organisation

This could be a simple demonstration to show students how to make up solutions at different concentrations, how to use the colorimeter and/ or how to plot and use a calibration curve. If all your students make up solutions and add iodine following instructions as closely as possible, you can discuss how reliable any one calibration curve is.

#### For the demonstration

Colorimeter – set to read absorbance in the ‘orange’ range of the spectrum (610 nm)

Pipette filler, 1

Test tubes, 10

Soluble starch (0.5–1.0 g) – to make up to 100 cm3 of 1% starch

Distilled water

Iodine solution

Cuvettes

#### Health & Safety and Technical notes

Wear eye protection when dispensing iodine solution.

1 Starch solution  Add a weighed amount of starch (0.5 g or 1.0 g) to a little heated water, mix to a paste, then dilute to 50 or 100 cm3.

2 Iodine solution  Iodine is only sparingly soluble in water (0.3 g per litre); it is usual to dissolve it in potassium iodide solution (KI) to make a 0.01 M solution (by tenfold dilution of a 0.1 M solution) to use as a starch test reagent. Refer to CLEAPSS Recipe card 33.

For 100 cm3 of 0.1 M solution, measure out 3.0 g of potassium iodide (KI) into an appropriate beaker. Moisten the potassium iodide with a few drops of water. Measure out 2.54 g of iodine (see Hazcard 54: iodine is harmful) and add to the moistened potassium iodide. Add a small volume of water and stir. When no more iodine appears to dissolve, add more water and stir. Repeat until all the iodine has dissolved. Pour the solution into a measuring cylinder and dilute to the final volume. If there are any bits of iodine remaining, return the solution to the beaker, and leave it on a magnetic stirrer for several minutes. Add the solution to a labelled bottle and mix well.

#### Procedure

SAFETY: Wear eye protection when dispensing iodine solution.

a Make up 50 cm3 or 100 cm3 of 1% soluble starch in distilled water. Use a graduated pipette to measure 0.5 cm3, 1 cm3, 2 cm3, 3 cm3, 5 cm3, 7 cm3 and 10 cm3 of the solution into a series of test tubes.

b Make each tube up to 10 cm3 with distilled water from another clean graduated pipette.

c Place 10 cm3 of water in a final test tube.

d Add one drop of iodine solution to each tube and mix thoroughly.

e With each solution in turn, transfer enough of the solution to fill a clean colorimeter cuvette. Take a reading of absorbance at ‘orange’ wavelengths (610 nm).

f Plot a graph of absorbance against concentration. The spreadsheet you can download below gives a sample set of results.

g Use the graph to calculate concentration from absorbance readings gained during an investigation.

Health & Safety checked, May 2009