Calorimeters are a tool used in calorimetric testing (calorimetry), which measures heat, enthalpy or specific heat capacity for both chemical reactions and physical changes in a given system.
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For both the academic researcher and the industry professional, there are various types of calorimeters out there in the marketplace.
Regardless of the intended application, how and why each type could (and should) be used is vitally important in the choice process. This article aims to shed some light on how different types of calorimeters operate, to help both researchers and industry purchasers make an informed choice.
Calorimeters - Applications and the Working Principle
Adiabatic calorimeters have a few uses, but the most prevalent are when you want to measure the thermal runaway, thermal instability or the energy from reaction and decomposition processes.
Essentially, the aim of an adiabatic calorimeter is to measure the enthalpy change in a system, whether it is during a crystallization, mixing, dilution or some other form energy changing process. These calorimeters give off no heat to their surroundings, instead work under zero heat exchange conditions i.e. adiabatic conditions.
Because the measurement of the enthalpy in a system requires the consideration of both the kinetic and thermodynamic loss of heat measurements, adiabatic calorimeters are generally more robust than many other types and can be used for systems that are prone to explosions, i.e. Li-ion batteries.
There are two common types of Adiabatic calorimeter- Accelerating Rate Calorimetry (ARC) and Adiabatic Pressure Dewar Calorimetry (ADC). ARCs are commonly used for measuring the decomposition of unstable materials.
ARCs use high temperatures and pressures (typically around 500°C and 200 barg, respectively). This method is not a scalable process but mathematical data can be correlated to predict larger systems using the thermal inertia.
ADCs operate at much lower pressures (40 barg), but these also operate under a much lower thermal inertia and are scalable up to at least 1 liter. These calorimeters are generally tailored for most applications, although a common area is for runaway reactions, and have the ability to rapidly change the internal conditions- with respect to the temperature, pressure and mechanical agitation.
There are traditionally four types of reaction calorimeter- heat-flow, heat-balance, constant flux and power compensation calorimeters. These calorimeters can be used to measure both the exothermic and endothermic changes within a reaction system to build a better picture of what is occurring during the reaction. The measurements can either be a chemical reaction or a physical process.
Reaction calorimetry is a non-invasive, non-destructive, real-time calorimeter technique that yields important kinetic data, as well as provides safety data for scale-up and hazardous processes.
For anyone looking at taking their reactions to the larger scale, the use of a calorimeter in this way improves understanding of the heat generated on the small scale. It also gives an indication of the heat release on the industrial scale and provides the user with the necessary information as to whether the process is viable from a health and safety perspective.
Bomb calorimetry is typically used to measure the enthalpy of combustion in a reaction. As such, these types of calorimeters are robust, generally made of steel and use an oxygenated atmosphere (for combustion purposes). Due to the rigidity of the calorimeter itself, most reactions in bomb calorimeters occur at a constant volume. These calorimeters can withstand the explosive effects of both the induced pressure and the exothermic release during the reaction process.
An internal pressure of up to 1500 psig can be generated inside the calorimeter. So, for safety reasons, any bomb calorimeter that is purchased should be able to withstand up to 3000 psig.
Learn More About Calorimeters
These calorimeters work by determining the enthalpy of combustion through a substitution method, where the heat observed from the sample is measured against the heat release of a known standard. The heat released is absorbed within the calorimeter and the temperature change is measured.
The high concentration of oxygen in the calorimeter during the combustion process even allows for non-volatile compounds to become combustible. However, because their normal nature is not to combust, these methods generally lead to incomplete combustion and will not always give an accurate result.
Whilst not as widely-used, calvet-type calorimeters can be used to measure the enthalpy change during sublimation reactions and the behavior of a material. This type of calorimeter uses sensors to determine the latent heat of transitions or the heat capacity of a system.
With this type of calorimeter, there is no calibration or standards required and the system can manage temperatures up to 1600 °C. They can also be used under isothermal conditions to perform drop calorimetry experiments, which outputs the calorific capacity and enthalpies of formation. It is more of a niche type of calorimeter, few manufacturers.
Constant-pressure, as the name suggests, measures the change in enthalpy of a solution-based reaction whilst maintaining a constant pressure. These are the simplest types of calorimeters, and a common example is the one used by high-school students to measure the heat of a reaction using a polystyrene cup, a lid with two holes, a thermometer and a stirring rod.
The simplicity of this calorimetric method is achievable because the calorimeter holds a known amount of solution, and as the pressure is constant, the heat transferred from the solution is equal to the change in enthalpy.
Differential Scanning Calorimeters (DSCs)
Differential scanning calorimeters, or DSCs, are a very common class of calorimeter found in both academic and industrial laboratories. DSCs are used across a wide range of scientific fields and industries, and employ a multi-use approach that can be used to measure a variety of properties, including the melting temperature, heat of fusion, latent heat of melting, reaction energy, reaction temperature, glass transition temperature, crystalline phase transition temperature and energy, precipitation energy, precipitation temperature, denaturation temperature, oxidation induction times and the specific heat capacity.
One of the biggest advantages of DSCs is the easiness and quickness in which they can measure the transition of materials, which is an especially large asset for polymeric materials. DSCs measure the change in the endothermic and exothermic processes by measuring the heat released from a sample.
There are two main approaches to measuring a sample- heat flow and heat flux. In heat flow measurements, the differential scanning calorimeters directly measure the heat that enters and leaves the sample. The calorimeter uses a feedback loop to maintain a constant temperature within the material and measures the required power to do this against known standards. This approach is useful if you want precise control of the temperature and gives accurate enthalpy and heat capacity measurements.
Heat flux DSCs work by measuring the temperature changes, or heat flux, between a sample and a reference material, and calculates the heat flow from the calibration data. This approach is less sensitive to small transitions and generally produces less accurate results than its heat flow counterparts.
Isothermal Titration Calorimeter
Isothermal titration calorimetry is used to measure the reactions, and interactions, between biomolecules. This method of calorimetry allows for the binding affinity, stoichiometry, entropy and enthalpy of a solution-based binding reaction to be calculated. It is a useful method for studying biological interactions, including drug delivery systems.
The method is titrimetric-based and produces a quantitative analysis when the titrated solution reacts with a substrate. When the binding of a molecule occurs, heat, which is either absorbed or released, is measured by the calorimeter. This method is performed under constant pressure and temperature and can even measure the binding efficiency of a substrate to an enzyme. As the environmental variables are kept constant, the heat of the reaction is proportional to enthalpy change associated with the binding process and proportional to the advance of the binding process, which allows for very accurate results.
This method is also non-destructive, has no requirement for biological labels, has the ability to measure unusual reaction systems and is fast. However, it can only determine thermodynamic quantities (and not kinetic) and requires a large amount of sample to run. However, for the biologically-inspired fields, it is a valuable tool.
So, whether you are looking for a calorimeter for an academic laboratory, industrial/ larger-scale laboratory, or even at home, there is a wide range of calorimeters on the market today to suit any need.
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References and Further Reading
Mettler Toledo: http://www.mt.com/us/en/home/products/L1_AutochemProducts/Reaction-Calorimeters-RC1-HFCal.tabs.custom5.html
Hope College: http://www.chem.hope.edu/~polik/Chem345-2000/bombcalorimetry.htm
“Measurement of enthalpies of sublimation by drop method in a Calvet type calorimeter: design and test of a new system”- Santos L. M. N. B. F., et al, Thermochimica Acta, 2004
Malvern Panalytical: http://www.malvern.com/en/products/technology/isothermal-titration-calorimetry/
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