Calorimeters, What They Are and How They Work

Measuring Heat Energy Bound in the Bonds of Chemical Compounds

Jun 22, 2009

Candle Wax Releases Calories as It Burns, Building and Fire Research Laboratory (U.S.).

Knowledge of the bonded energy within chemicals is important to scientists interested in ballistics, fuels and foods. Calorimeters make such measurements a reality.

Energy always has intrigued scientists. Energy is a measure of the capacity to do work. Work is defined as a force applied over a measured distance.

Whether it is the potential electrical energy in a battery, or potential heat within trinitrotoluene (TNT) or a hamburger, scientists have devised unique ways to get real estimates of the potential energy within compounds. One such instrument is the calorimeter, a device used to determine heat energy or calories.

Lavoisier Discovered Oxygen, Investigated and Measured Burning, Combustion

Lavoisier, a French scientist, made some amazing discoveries in the 1700s. He determined the existence of oxygen for combustion. Later, using a simple and very well-engineered combustion bomb or chamber, he proceeded to measure heat output during the production of carbon dioxide (see photo below of Lavoisier-type calorimeter). This marked the beginnings of measuring the energy contained within different compounds.

Calorimetry Measures Food and Fuel Energy, Burning and Decomposition

Calorimetric measurements are relatively simple to do. A combustible (burnable) substance is ignited and totally burned in the presence of oxygen in a decomposition vessel (aka "bomb"). The emitted heat energy is trapped, usually in water, and measured by a thermometer or thermocouple.

A typical calorimetric procedure is: (Reference)

1 gram of a solid or liquid compound is weighed in a crucible.
the crucible is added to a thick-walled, stainless-steel vessel with pressurized oxygen at 30 bar (barometric pressures=435 psi). The vessel is doubly-insulated with outer, water-filled walls connected to a heat-measuring device (thermometer, or thermocouple)
the sample is ignited via an ignition wire inside the vessel.
crucible temperatures rise to 1,000 °C (1800 °F), and pressures of 200 bar (2900 psi) as the gases expand in the chamber. All the organic matter is burned and oxidized. Inorganic matter (minerals) are oxidized; often, vitrification occurs. The heat created during the burning is measured.

Modern, Specialized Calorimeters and Calorimetry

Among the newest are:

rotating bomb calorimeter, measures energies and enthalpies of combustion for organic and organo-metallic compounds with a precision of 0.01%. Other possible determinations include: carbon content measurement, Gibb's energy of formation and chemical equilibrium. The calorimeter can be used to: predict runaway reactions, determine chemical process design, optimize energy and coolant requirements and equipment size, analyze and certify modern engine fuels (e.g. isooctane) and rocket fuels such as JP-10 cruise missile fuel.
differential scanning calorimeter, measures thermal responses of an unknown specimen compared to a standard when both are heated at a uniform, constant rate in a furnace with 2 identical crucibles. Centered under each crucible is a Pt vs. Pt-10 Rh differential thermocouple. Temperature differences between the two specimens are influenced by heats of reaction, phase transitions, mass, or specific heat. Control baselines are plotted first. This reveals the temperature difference of the two empty crucibles as the temperature is changed at a constant rate over the experimental temperature range. Thermal responses are measured for a standard material (e.g. sapphire) and an unknown under controlled conditions. The specific heat is determined from the ratio of the departure of the standard and unknown from the baseline. The scanning calorimeter is the most rapid method for specific heat determination. The device can function with oxygen, air or inert gas environments to 1,500 °C, requires small volumes (0.5 to 20 cubic mm), has a small error rate of +/- 3.5% and is automated and computer controlled.(Calorimeters Reference)

Read more about calories and energy in biological systems.

Sources

Harris, N. C. and E. M. Hemmerling. 1963. Introductory Applied Physics. Second Ed., McGraw-Hill, N.Y.

Hewitt, P. G. 1992. Conceptual Physics. Second Ed., Addison-Wesley, Menlo Park, Ca.

The copyright of the article Calorimeters, What They Are and How They Work in Scientific Research Methods is owned by Donald Reinhardt. Permission to republish Calorimeters, What They Are and How They Work in print or online must be granted by the author in writing.