A calorimeter ( lat. Calor , heat '; / (lat.) "Metari", measure) is a measuring device (or an apparatus) for determining the amount of heat that is released or absorbed during physical , chemical or biological processes. With the help of a calorimeter, the specific heat capacity of a substance can also be determined. A distinction is made between calorimeters according to operating modes such as adiabatic or isothermal or according to the measuring principle such as power compensation or heat conduction principle. The measurement process itself is called calorimetry .
Calorimeter for measuring the resulting heat of reaction in a chemical reaction are as Reaction designated.
A distinction is made between:
- Heat flow calorimeter,
- Heat balance calorimeter and
- adiabatic calorimeters
Heat flow and heat balance calorimeters are mostly operated isothermally , i. H. the internal temperature is kept constant with a regulator. Then the dissipated heat output is identical to the output generated by the sample.
In the heat flow calorimeter, the heat is transported by conduction and the temperature difference required for this is measured. The thermal resistance of the wall must be known or the calorimeter must be calibrated. Small heat flows are advantageously conducted as completely as possible through a thermopile in the otherwise well-insulated wall.
In the heat balance calorimeter, the temperature difference between the flow and return of the temperature control jacket and the mass flow of the temperature control medium are measured and the heat output transported through the wall is calculated from this.
With the adiabatic (more precisely: anisothermal ) calorimeter, the sample is located in an insulated vessel or in a vacuum. The thermal energy generated by the reaction is calculated from the measured temperature increase. The heat capacity of the sample must be known. In dynamic differential calorimetry , the same temperature profile is imposed on a similar but non-reacting sample by supplying energy. The electrically supplied power is easy to measure.
The calorimeter is thermally insulated from the environment. The heat exchange takes place with a liquid (liquid calorimeter) or with a metal (metal block calorimeter). This type of device is the most common in calorimetry. When working properly, it can achieve accuracies of up to 0.01%. This procedure is used when the heat exchange takes a maximum of 20 minutes.
It consists of a double-walled copper container, the space between which is filled with water and is intended to ensure a temperature-constant environment in the inner calorimeter. The calorimeter vessel made of thin sheet metal is placed on a thermally insulated base. Ordinary water is used as the calorimeter liquid, but other liquids can also be used. A stirrer, the speed of which must remain constant, ensures a better heat exchange. The change in temperature is measured with a thermometer. see also : Bomb calorimeter for determining the calorific value.
With these devices, the temperature difference between the calorimeter liquid and the vessel jacket is constantly compensated by heating or cooling. Both processes must take place at the same speed. This is easier to achieve the slower the heat transfer to the calorimeter (20 to 60 minutes).
With these devices, the amount of heat is taken from certain substances that undergo a phase change. The temperatures therefore remain constant during the experiment. These devices are also known as phase change calorimeters. They are used for slow reactions that take several hours.
This calorimeter is one of the most accurate for measurements of heat quantities at 0 ° C. With the amount of heat to be measured, ice is melted. Since the heat of fusion of the water is known, the amount of heat can be determined from the amount of melt water.
A classic ice calorimeter consists of a funnel-shaped inner container that is surrounded by an outer container. In preparation, the inner container is filled with distilled water and the outer container with a cold mixture , so that a layer of ice forms on the inner wall of the inner container. The cold mixture and the remaining, non-frozen water are then drained off and the entire calorimeter is brought to the temperature of the water's melting point. The experiment is placed in the inner container and the calorimeter is closed with a lid. The melt water that forms runs out of an outlet of the inner container for measurement.
This calorimeter, often also called steam calorimeter, is mainly used to determine the specific heat capacity of a substance between 100 ° C and 20 ° C. Water vapor is used as the condensing gas. The body K to be examined is suspended from a sensitive scale by means of a fine wire and is located inside the calorimeter. If one suddenly introduces saturated water vapor, which has been freed of dripping liquid, into this space, a certain amount of vapor will condense on the initially cold body until the body has assumed the temperature of the vapor. A quantity of heat is transferred to the body (with : quantity of heat;: heat of condensation; mass of condensed steam).
A thin-walled platinum dish attached to the bottom of the body protects against water dripping. The buoyancy that occurs due to the steam flow must be taken into account. The method can provide very precise values.
Heat exchange calorimeter
In the case of reactions that extend over several hours to a few months, a quick and complete heat exchange with the environment is ensured. The speed is measured as a function of time.
Reaction calorimeters are calorimeters optimized for chemical applications. They are used in chemical process development to measure the heat generated during a reaction and the time course of the power ( heat flow ). The thermal data obtained in this way are required for the safety assessment of the process and the design of the reactor cooling systems. Particularly sensitive caloric measurements can be carried out using chip calorimeters, the heat flow preferably being determined by a series of thermocouple pairs. With this measuring principle, micro-flow calorimeters also enable rapid serial measurements.
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