How to Determine Sugars by Calorimetric Method

All carbohydrates produce roughly the same amount of heat per gram when burned. However, closer analysis uncovers slightly different enthalpies of combustion among various sugars. Measuring a sample's enthalpy of combustion lets you determine which sugar it contains.



A bomb calorimeter measures temperature rise following complete oxidation of a fuel. Simple calculation then converts this to the enthalpy of combustion of that fuel.

Things You'll Need

• 6 volt battery
• Copper calorimeter
• Magnesium wire
• Scale
• Steel bomb
• Steel crucible
• Stirrer
• Thermometer

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Instructions

• 1

  Weigh the sugar sample.

• 2

  Weight the calorimeter, the bomb and the rest of your apparatus.

• 3

  Place the sample in the crucible.

• 4

  Place the crucible over the ring attached to the bomb's electrodes.

• 5

  Stretch the magnesium wire between the electrodes so that it touches the sample.

• 6

  Screw the lid of the bomb shut.

• 7

  Open the bomb's oxygen valve.

• 8

  Pour a known mass of water into the calorimeter.

• 9

  Lower the bomb into the calorimeter.

• 10

  Measure the water's temperature with the thermometer.

• 11

  Connect the electrodes to the battery. The sugar will ignite.

• 12

  Stir the water continuously as the sample burns. Keep the thermometer in the water.

• 13

  Note the maximum temperature the water reaches.

• 14

  Subtract the water's initial temperature from its final temperature. For example, if the temperature rises from to 25.5 to 35.6 degrees Celcius: 35.6 - 25.5 = 10.1

• 15

  Divide the specific heat capacity of the bomb calorimeter by the specific heat of water. For example, if the calorimeter's specific heat capacity is 0.42 joules per gram per degree, you would have: 0.42 / 4.186 = 0.1

• 16

  Multiply the answer to Step 15 by the weight of the calorimeter, thermometer and the rest of your apparatus. For example, a weight of 9,000 g would yield: 0.1 * 9,000 = 900

• 17

  Add the answer to Step 16 to the water's mass. If you used 3,000 g of water, the calculation would be: 3,000 + 900 = 3,900

• 18

  Multiply the answer to Step 17 by the answer to Step 14: 3,900 * 10.1 = 39,390

• 19

  Divide your answer by the mass of the fuel you used. If you used 10 g, you would have: 39,390 / 10 = 3,939 calories / gram

• 20

  Compare your measured enthalpy of combustion to standard values for different sugars. 3.94 kcal per gram nearly equals 3.95 kcal per gram, the enthalpy of combustion of sucrose. This suggests that the sample was sucrose.