PAG 9.3 qualitative testing for glucose Flashcards
method for testing for reducing sugar
1) use a dropping pipette and add 2cm3 of Benedict’s reagent to each of the samples
2) record observations
3) place boiling tubes into a water bath
4) record observations after 2 mins
5) after another 2 mins remove boiling tubes from the water bath and record final observations
method for testing for non-reducing sugars
1) use a dropping pipette and add 2cm3 of HCl into the samples
2) place all boiling tubes into a water bath for 2 mins
3) remove tubes from water bath and let them cool
4) neutralise acid by adding sodium hydrogen carbonate to each sample until no more bubbling occurs
5) check the pH with indicator paper if the pH is lower than 7 repeat step 4 until it isn’t
6) use a dropping pipette to add 2cm3 of Benedict’s reagent
7) record observations then put boiling tubes into water bath
8) after 2 mins record observations
9) after another 2 mins remove from water bath and record final observations
what improvements could you suggest to the method for this qualitative test?
use a colorimeter so the amount of light absorbed by the colour can be measured to see which solution is darker and therefore has more sugar
what changes can you make to obtain semi-quantitive results?
a scale can be produced which have the different colours that Benedicts solutions turns when in the presence of sugars on it, so each sample can be placed on this scale
outline a full method for a fully quantitative test for reducing sugars
1) create a dilution series of glucose using distilled water
2) place 2cm3 of each unknown samples into separate boiling tubes
3) add 2 cm3 of Benedict’s solution
4) place boiling tubes in 90 degree water bath for 4 mins
5) take tubes out and leave to cool
6) set colorimeter to red absorbance
7) use it to measure absorbance of each tube
suggest how you could distinguish a sample of glucose from a sample of maltose using Benedict’s reagent
they react differently with Benedict’s solution
glucose has a rapid red colour change
maltose turns green/yellow before turning red
