2.1.2d Testing for Glucose (PAG 9.3) Flashcards
What does the term qualitative test mean
Only gives us a positive or negative result to tell us whether a particular substance is present. It does not tell us how much of the substance is present (or at what concentration).
What is the test for sugars (glucose and sucrose)
Benedict’s test
Aim
To test qualitatively for the presence of glucose (a reducing sugar) and sucrose (a non-reducing sugar)
Chemicals involved in the tests + hazard
- Benedicts reagent (low hazard but avoid contact w eyes)
- 2.0 mol dm^-3 hydrochloric acid (causes skin and eye irritation)
- Sodium hydrogen carbonate (low hazard)
- Samples (A-D)
Equipment needed for the tests
- Benedicts reagent
- 2 dropping pipettes
- 8 boiling tubes
- 8 x 5cm^3 syringes
- Samples in beakers, labelled A, B, C
- Water bath set to 100dc
- 2.0 mol dm^-3 hydrochloric acid
- Sodium hydrogen carbonate
- Small spatula
- pH paper
Health & safety of the tests
- Eye protection must be worn due to presence of boiling water & acid
- Benedicts reagent is low hazard but contact w eyes should be avoided
- Boiling water bath presents a scalding hazard
- 2.0 mol dm^-3 hydrochloric acid is an irritant
What is method 1 of testing sugars
Testing for reducing sugars
Method for testing reducing sugar (1)
- Label 4 boiling tubes A-D & place 4cm^3 of the corresponding samples into each using a fresh syringe each time
- Using dropping pipette, add 2cm^3 Benedicts reagent to each of 4 samples
- Record observations of each sample immediately after adding Benedict’s reagent
- Place all 4 boiling tubes into the water bath
- After 2mins record observations of each sample
- After a further 2mins (4mins total incubation time) remove tubes from water bath & record final observations of each sample & therefore which letter corresponds to the glucose solution
Method for testing for non-reducing sugars (2)
- Put 4cm^3 of each of the samples, A-D, into its own labelled boiling tube using a fresh syringe each time
- Use dropping pipette to add 2cm^3 of hydrochloric acid to each of 4 samples
- Place all 4 in water bath for 2mins
- Remove from water bath to cool
- Neutralise the acid by gradually adding sodium hydrogen carbonate to each sample until no more effervescence is observed
- Check pH w indicator paper. If pH is still lower than pH 7, repeat previous step.
- Use dropping pipette to add 2cm^3 Benedicts reagent to each of 4 samples
- Record observations of each sample immediately after adding
- Place all tubes into water bath
- After 2mins, record observations of each
- After further 2mins (4mins total incubation) remove tubes from water bath & record final observations & your conclusions abt the presence or absence of non-reducing sugar in the samples (& therefore which letter corresponds to the sucrose solution)
What improvements can be made to the method for this qualitative test
- Could ensure temp control. Maintaining a consistent heating temp of water bath would provide optimal reaction conditions (& ensure all test tubes are heated evenly)
- We could control samples by including positive & negative control samples. This would validate the accuracy of the results
What changes would you make to obtain semi-quantitative results
Using a colorimetric scale that correlates specific colours to glucose concentration levels would allow for more precise estimations. It would allow us to determine the glucose concentration level to be high, medium, low
Outline a method for a fully quantitative test for reducing sugars
- Prepare 4 test tubes of glucose solutions w known concentrations. Record initial concentration of reducing sugars in samples
- Add hydrochloric acid using a pipette to each sample & heat for 2mins in water bath
- Neutralise the acid by adding sodium hydrogen carbonate to the samples until bubbling ceases. Check w pH paper until solutions are neutral
- In separate test tubes, add a fixed vol of each glucose solution & an equal volume of of Benedict’s reagent. Mix & heat in water bath for 4mins
- Measure colour intensity of each solution using colorimetric scale
- Plot a graph of absorbance against glucose concentration to create a calibration curve. Measure the absorbance of the test samples & use the calibration curve to determine their concentrations of reducing sugars
- Calculate concentration of reducing sugars in each sample based on absorbance readings from calibration curve
Suggest how to distinguish a sample of glucose from a sample of maltose using Benedicts reagent
Prepare solutions of glucose & maltose, ensuring both are in equal concentrations. Add fixed vol of Benedict’s reagent to samples. Shake to mix and heat in water bath.
As GLUCOSE, as a monosaccharide reducing sugar, would react with Benedicts reagent, producing a colour change from green to red.
MALTOSE, as a disaccharide reducing sugar, would react in similar way.
Add hydrochloric acid to maltose sample & heat in bath. This would hydrolyse it into 2 glucose molecules. Neutralise the acid w sodium hydrogen carbonate. After this repeat Benedicts test on maltose sample
find simpler version
What is Benedicts solution
An alkaline copper (II) sulphate (CuSO4) which is a blue colour. Importantly, this solution changes colour in presence of reducing sugars
Colours of Benedicts solution
No reducing sugar –> BLUE
Very low –> GREEN
Low –> YELLOW
Medium –> BROWN
High –> BRICK-RED, ORANGE-RED
