carbohydrates test

• starch

  • add iodine solution (in potassium iodide) to a sample
  • if starch is present, you will see a colour change from yellow-brown to blue-black

  due to colour change :

  • when dissolved in potassium iodide, the iodine (I2) forms a triiodide ion I3- , which slips into the middle of amylase helix
• Reducing sugars

  • these include all monosaccharides + some disaccharides

  -> they can reduce, or give electrons to, other molecules

  • if you heat a reducing sugar with Benedict's solution (alkaline copper(II) sulphate), there's a colour change from blue to green to yellow to orange red
  • Benedict's solution contains Cu 2+ ions, which forms orange-red copper (II) oxide (Cu2O)

  -> this is a precipitate as it comes out of solution + forms a suspended solid
• Reducing sugars (continued)

  • if you use Benedict's Solution in excess, the intensity of the red colour is porportional to the concentration of sugar

  -> reaction mix will appear green if a little precipitate is formed, and orange red if a lot precipitate is formed
• Reducing sugars (continuedd)

  • it is also possible to use commerciallly manufacted test strips to test for reducing sugars by simply dipping into strip the test solution, and comoare colour with calibration card supplied
  • thesee are often used to test for glucose in urine of diabetic patients
• Non-reducing sugars

  • hydrolyse the bond first to, 'free up' these 'reducing sugars' and then test for reducing sugar as normal
  • first test a sample for reducing sugars to check there are none there in the first place
  • take a separate sample + boil it with HCl to hydrolyse the sucrose into glucose + fructose
  • cool solution + use sodium hydrogencarbonate solution to neutralise it
• test for reducing sugars (again)

  
  

  • a positive result (green-yellow-orange-red) indicates that non reducing sugar (e.g sucrose) was present in the original sample