Lesson 7.5 - Testing for Carbohydrates Flashcards
Reducing sugars examples?
Maltose
Lactose
Glucose
Fructose
Ribose
Meaning of reducing sugars?
They can donate electrons, or reduce another chemical / molecule.
Reducing meaning in chemistry?
Reaction involving the gain of electrons
What is Benedicts reagent?
alkaline solution of Copper (II) sulfate.
What is used in test for reducing sugars?
Benedicts reagent
Test for reducing sugars:
- Place sample to be tested in a boiling tube. If it is not a liquid, grind it up with water in a mortar and pestle or blend it.
- Add equal volume of benedicts reagent.
- Heat the mixture gently in a boiling (100 degrees celcius) water bath for 5 minutes.
Colour change of Benedicts reagent?
Blue –> Brick-red
Why is there a colour change in the test for reducing sugars?
- The reducing sugars react with the copper (II) ions.
- Results in the addition of electrons to the blue Cu+2 ions. This reduces them to brick-red Cu+ ions.
What happens when reducing sugar is mixed with benedicts solution and warms?
Brick-red precipitate forms
How is the test for reducing sugars qualitative?
More reducing sugars present = more Cu+
Colour of solution is a mix of brick-red Cu+ ions and blue Cu+2 ions.
Colour changes of reducing sugars:
High: Brick-red
Medium: Orange / Yellow / Amber
Low: Green
None: Blue
Iodine test:
Test for starch
How does the iodine test work for starch?
Few drops of iodine dissolved in potassium iodine solution and are mixed with the solution.
If solution colour changes from yellow/brown to black/blue then starch is present.
Reagent strips?
Manufactured tests to test for reducing sugars. Most commonly glucose.
Advantage of reagent strips?
Colour-coded chart to determine the concentration of reducing sugars.
Non-reducing sugars examples?
Sucrose
Describe how the investigation could be changed to give quantitative results
- Use a colorimeter
- filter and dry precipitate, then weigh it.
Standardises the method
Colour change is subjective.
How to test for non-reducing sugars?
To test for these types of sugars, you must first hydrolyse them into their monosaccharide components before we can do the rest of the test.
Test for non-reduing sugars
- Carry out the test for reducing sugars, and if the result is negative (turns blue), continue with the next steps.
- Add 2 cm3 of the food sample to 2 cm3 of dilute hydrochloric acid.
- Heat the mixture in a gently boiling water bath for 5 minutes (the acid will hydrolyse disaccharides into monosaccharides).
- Neutralise the mixture by adding sodium hydrogencarbonate solution.
- Retest this mixture using the test for reducing sugars.
If non-reducing sugars were present at the start, the mixture will now change from a blue solution to a brick red precipitate.
Steps to test for iodine
- Place 2 cm3 of your food sample into a test tube.
- Add a couple of drops of iodine solution and shake.
- If starch is present, the solution will turn from orange to blue-black.
