Chapter 3- 3.4- Testing for carbohydrates Flashcards
What is the test for reducing sugars?
Benedict’s test - Benedict’s reagent is used, an alkaline solution of copper(II) sulphate.
In chemistry reduction is a reaction involving the gain of electrons. All monosaccharide’s and some disaccharides (for example maltose and lactose) are reducing sugars.
This means that they donate electrons, or reduce another molecule or chemical.
How is the benedict’s test carried out ?
Place the sample to be tested in a boiling tube. If it is not in liquid form, grind it up or blend in water.
Add an equal volume of Benedict’s reagent.
Heat the mixture gently in a boiling water bath for five minutes.
Reducing sugars will react with the copper ions in Benedict’s reagent. This results in the addition of electrons to the blue Cu(2+) ions, reducing them to brick red Cu(+) ions.
When a reducing sugar is mixed with Benedict’s reagent and warmed, a brick-red precipitate is formed indicating a positive result.
The more reducing sugar present, the more precipitate formed and the less blue Cu(2+) ions are left in solution, so the actual colour seen ill be a mixture of brick-red (precipitate) and blue (unchanged copper ions) and will depend on the concentration of the reducing sugar present. This makes it a qualitative test.
Explain how to use Benedict’s test for non-reducing sugars
Non-reducing sugars do not react with Benedict’s solution and the solution will remain blue after warming, indicating a negative result.
If sucrose is first boiled with dilute hydrochloric acid it will then give a positive result when warmed with Benedict’s solution.
This is because the sucrose has been hydrolysed by the acid to glucose and sucrose has been hydrolysed by the acid to glucose and fructose, both reducing sugars.
How do you test for starch?
Iodine.
The iodine test is used to detect the presence of starch.
To carry out the test, a few drops of iodine dissolved in potassium iodide solution are mixed with a sample.
If the solution changes colour from yellow/brown to purple/black starch is present In the sample.
If the iodine solution remains yellow/brown it is negative result and starch is not present.
What are reagent strips used for?
Manufactured reagent strips can be used to test for the presence of reducing sugars, more commonly glucose.
The advantage is that, with the use of a coloured-coded chart, the concentration of the sugar can be determined.
What is the quantitative method to determine concentration?
Colorimetry.
In a benedict’s test, the colour produced is dependent on the concentration of reducing sugar present in the sample.
A colorimeter is a piece of equipment used to quantitatively measure the absorbance, or transmission, of light by a coloured solution.
The more concentrated a solution is the more light it will absorb and the less light it will transmit.
This can be used to calculate the concentration of reducing sugar present.
What is the procedure for colorimetry?
1) A filter was placed in the colorimeter.
2) The colorimeter was calibrated using distilled water.
3) Benedict’s test was performed on a range of known concentrations of glucose.
4) The resulting solutions were filtered to remove the precipitate.
5) The % transmission of each of the solutions of glucose was measured using the colorimeter.
6) Using this information a calibration curve was plotted. Step 3-6 were repeated using the solutions with the unknown concentration of glucose.
What are biosensors?
Biological components to determine the presence and concentration of molecules such as glucose.
The analyte is the compound under investigation.
- Molecular recognition, a protein (enzyme or antibody) or single strand of DNA (ssDNA) is immobilised to a surface, for example a glucose test strip.
This will interact with, or bind to, the specific molecule under investigation.
- Transduction, this interact in will cause a change in a transducer. A transducer detects changes, for example, in pH, and produces a response such as the release of an immobilised dye on a test strip or an electric current in a glucose-testing machine.
- Display, this then produces a visible, qualitative or quantitative signal such as a particular colour on a test strip or reading on a test machine.
