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Chemical tests for Monosaccharides

There are several chemical tests that can be performed to detect the presence of monosaccharides, specifically reducing sugars. A common test is the Benedict’s test. Here’s how it works:

1. Prepare the sample: Take a small amount of the solution or food sample that may contain monosaccharides and dissolve it in water.

2. Perform the test: Add a few drops of Benedict’s reagent (a solution of copper sulfate and sodium citrate) to the test tube containing the sample.

3. Heat the mixture: Place the test tube in a water bath and heat it to a gentle boil for a few minutes.

4. Observe the color change: If the sample contains reducing sugars (such as glucose or fructose), a color change will occur. Initially, the solution may be blue. If monosaccharides are present, the solution will turn green, yellow, orange, or brick-red, depending on the amount of reducing sugar present. The intensity of the color change correlates with the concentration of reducing sugars.

 The Benedict’s test is a qualitative test, meaning it provides a visual indication of the presence or absence of reducing sugars but does not give precise quantitative results.For quantification of monosaccharides, more precise methods like spectrophotometry or chromatography are typically employed.

Benedict’s test is specific to reducing sugars, including most monosaccharides and some disaccharides. However, it may not detect non-reducing sugars such as sucrose (table sugar) and certain polysaccharides like starch. For the detection of non-reducing sugars, additional steps involving acid hydrolysis and re-testing with Benedict’s reagent may be necessary.

 Chemical tests to identify presence of starch

Substances containing starch turn a blue-black colour in the presence of iodine solution. An observable colour change is therefore the basis of a chemical test for the compound.

In the following investigation we will test a few different foods for the presence of starch.

Test for the presence of starch 

Aim

To test for the presence of starch.

Apparatus

• piece of potato or bread
• lettuce leaf
• petri dish
• iodine solution
• dropper
• other food items of your choosing

Method

1. Place a piece of potato or bread, the lettuce leaf, and your other food samples in separate petri dishes.
2. Using the dropper add a few drops of iodine solution to the food item in each petri dish.

Observations

Record your observations.

Questions

Can this method be used to determine how much starch is present? Explain your answer.

Chemical test to identify presence of reducing sugars

Certain monosaccharides, such as glucose, are known as reducing sugars. These are defined as sugars that can easily undergo oxidation reactions (i.e. lose an electron or gain an oxygen atom) and act as a reducing agent. In order to test for carbohydrates we typically test for the presence of reducing sugars using either the Benedict’s or Fehling’s test. Both solutions (Benedict’s and Fehling’s) contain copper sulphate which reacts with reducing sugars to produce a colour change.

Testing for the presence of reducing sugars 

Aim

To test for presence of sugars using Benedict’s or Fehling’s test.

Apparatus

• 4 heat resistant test tubes
• 1 beaker
• Bunsen burner or water bath with hot water (+50^∘C)
• test tube rack (if using a water bath)
• glucose solution
• albumen solution or egg white
• starch solution
• water
• Benedict’s solution OR Fehling’s solution
• marking pen to mark the test tubes
• thermometer
• 10ml syringe or measuring cylinder

Safety precautions

1. Follow the safety procedures when lighting your Bunsen burner. Do not light it in a shelf or enclosed space. Remove all notebooks, papers and excess chemicals from the area. Tie back any long hair, dangling jewelry and loose clothing and never leave an open flame unattended while it is burning.
2. When heating your test tubes in the heated water in the beakers ensure that the mouth of the test tubes point away from you and fellow learners.
3. When handling the test tubes, especially when they are hot, use a test tube holder and wear goggles.

Method

Prepare a water bath by filling a beaker to the halfway mark with water. Place the beaker on a tripod stand over a Bunsen flame.This will serve as your water bath.

Whilst waiting for the water to reach the desired temperature, carry out the following instructions:

1. Label the test tubes 1–4.
2. Using the syringe or measuring cylinder, add the following to the test tubes:
   • test tube 1: 55 ml of 1% starch solution
   • test tube 2: 55 ml of 10% glucose solution
   • test tube 3: 55 ml  of 11% albumen solution
   • test tube 4: 55 ml water.
3. Add 55 ml Benedict’s solution to each tube.
4. Place the test tubes in the beaker of hot water on the tripod.
5. Use a thermometer to monitor the water temperature and adjust the flame to maintain the water temperature at approximately 50^∘
6. If using the water bath, place the test tubes into the test tube rack and place into the water bath with temperature set to 50^∘
7. After about 55 minutes, when a colour change has occurred in some of the test tubes, extinguish the flame, or remove the test tubes from the water bath.
8. Place the four test tubes in a test tube rack and compare the colours.

Results

Draw a table to record the results of this experiment. It is important to observe and record any changes that have taken place.