KEGS Biology Blog

Wednesday 12th September 2012 Section 2
September 13, 2012, 8:23 am
Filed under: Uncategorized

Reducing sugar Benedicts test and Starch test using Iodine in a Potassium Iodide solution


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In this lesson we were testing starch, glucose 1%, glucose 5% and sucrose using iodine or Benedict’s solution depending on the sugar you were testing
To test for starch:
1. Add two drops of starch solution to a well in the spotting tile using a dropping pipette.
2. Add two drops of iodine solution to the well using a clean dropping pipette. Observe and record the colour.
3. Repeat the test replacing the two drops of starch with two drops of distilled water, again using a clean pipette. This is the control test. Observe the colour and compare with the colour of the starch-iodine complex.

Results-When the iodine was added to the starch, it went from an orangey brown colour to a blacky blue colour. When the iodine was added to the distilled water though, there was no change in the colour and it remained an orangey brown colour.

Limitations-drops aren’t a very accurate measurement for liquids and therefore there may have been slight differences in peoples perceptions of the colours the solutions went.

To test for reducing sugars:
1. Use a syringe to measure 1 cm3 of glucose solution into a boiling tube.
2. Add 1 cm3 of Benedict’s solution using a clean syringe to the glucose and swirl to mix.
3. Place in a hot water bath and leave for about 5 minutes until there is a colour change. Record this colour change.
4. Repeat these steps using 1 cm3 of distilled water, sucrose and whatever percentage glucose solution you didn’t use the first time, using a clean syringe for each one.

Results-There was no change in the colour of the distilled water, it remained sky blue throughout. The glucose 1% turned green after 1 minute 30 seconds and then turned an orangey red after 4 minutes. This was slower than the glucose 5% which turned green after 1 minute 15 seconds and turned orangey red after 3 minutes 20 seconds. The sucrose remained sky blue from the Benedict’s solution up until 3 minutes when it turned green and remained that colour until the 5 minutes were up.

Limitations-There may have been air bubbles in the syringes when measuring out the volumes meaning there was less of one of the solutions than there was meant to be and this may have affected the results. Also a tiny bit of solution was getting left in the end of the syringe and this would have caused the same effect. The water bath would have cooled down from the original 80 degrees meaning the reaction would have been slower towards the end of the 5 minutes so expected results may not have been reached within the time limit. If test tubes were placed directly into the water bath after all components had been added then the Benedict’s solution may not have mixed sufficiently with the glucose, sucrose or distilled water causing the colour change to take longer.

Comment by charlieprudence

Useful website with information on the Benedict’s Test for Reducing Sugars-

Comment by sarahroche12

I found a video as a reminder of the experiment

Comment by graciemarlow

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