Bomb Calorimeter

Calorimetry with Constant Volume
Experiment Notes
Combustion
Input Data
g
%
%
kcal/kg-°C
Result
Result goes here

What is a Bomb Calorimeter?

A Bomb Calorimeter (or an Oxygen Bomb Calorimeter) is used to measure the heat created by a sample of fuel or any substance burned under an oxygen atmosphere in a closed container, which is surrounded by water, under controlled conditions. The temperature change of the water is used to calculate the heat of combustion reaction. The measurement result is called the Combustion-, Calorific- or BTU-value (more common in the USA).

Bomb Calorimeter

Structure

A Bomb Calorimeter structure consists of the following:

  1. A high-pressure resistant thick-wall stainless steel and firmly sealed Bomb container in which combustion process takes place, b) Sample holder and the sample (reactant)
  2. Inner container (or Bucket) filled with a measured quantitiy of water in which the Bomb is immersed and to which the heat of reaction is transferred from the Bomb
  3. Outer container (insulating jacket or simply Jacket) with inlet and outlet water nozzles
  4. Resistance thermometer
  5. A motorized stirrer
  6. Ignition fuse wires with their protection tubes (sealed by welding)

The Bomb Calorimeter is sealed or isolated from its surroundings. The Jacket temperature is controlled and held constant throughout the determination procedure while the Bucket temperature is rising. The isolation by keeping the Jacket temperature constant does not allow creating a “perfect isolation”. There are small temperature exchanges between Bucket and Jacket containers. The influence from the environment (room) has to be kept small by using air-condition to keep the error as little as possible. A correction factor will be calculated after the experiment that takes these temperature exchanges into account.

Procedure

The combustion process takes place in the Bomb Calorimeter at constant volume. Through the combustion reaction, the pressure and temperature rises in the Bomb due to the conversion from chemical energy to thermal energy. To ensure complete combustion, the experiment is carried out in the presence of oxygen above atmospheric pressure. This requires that the combustion be confined to a constant volume. A Bomb Calorimeter works by the ignition of a sample in an excess of oxygen. This is achieved by pressurising a number of times up to 30 atmospheres. These pressurisations are needed to ensure that the Bomb has had all the air (and moisture) removed from the Bomb and be replaced by an excess of oxygen. The oxygen is required to ensure complete combustion of a sample.

After filling the Bomb with pressurized oxygen at 30 bar and closing, it is immersed into the warm water inside the Bucket. The water temperature in the Bucket is so adjusted that the temperature of the Calorimeter (Bomb with internals + Bucket with water) is stabilized at slightly below 77 °F. After the calorimeter setup (Jacket cover) is closed with all parts installed and ready for the test procedure, the heating of the Jacket starts with hot water entering into the Jacket from the inlet nozzle. The temperature of incoming water is so adjusted and controlled automatically ( few degrees over 77 °F) by a water heating / cooling systems integrated to the device that after a while, the temperature of water in the Bucket is stabilized at a temperature of 77 °F (T2). At that moment, the ignition of sample is done by the fuse wires which are in contact with the sample in the Bomb. After combustion, the final temperature (T4) of the water and the final temperature of the products in the Bomb are observed and measured, and then the change in temperature of the water / reactants can be determined with necessary corrective calculations.

Bomb Calorimeter

About 1g of solid or liquid matter is weighed into a crucible, and placed inside the stainless steel container of the Bomb Calorimeter filled with 435 psi (30 bar) of oxygen (Quality: technical oxygen 99.98%). Then the sample is ignited through an fuse wire inside the Bomb and burned (combusted). During the combustion the core temperature in the crucible can go up to 1800 °F (1000 °C), and the pressure rises for milliseconds to approximately 2900 psi (200 bar). All organic matter is burned under these conditions, and oxidized. Even inorganic matter will be oxidized to some extent.

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