Calorimeter requirements

Among the most important features of a calorimeter for plant studies are  

Multiple. sample capabilities. Plant studies routinely require examination of many. samples, particularly when the objectives of the study include identification of phenotypes with desired growth, stability, or yield characteristics. Screening large populations for individuals with desired metabolic characteristics is impossible in single-sample calorimeters. Current commercial multiwell calorimeters allow up to three samples to be run simultaneously. A calorimeter with as many as 100 sample wells could be effectively employed for plant studies. 

Ampules that allow ready, repeated access to samples. During the course of plant measurements, the calorimeter must be opened and ampules must be 

Ampules sufficiently large to contain adequate tissue and headspace gases for supporting respiration throughout the time required for measurements. 

Rapid equilibration time. Minimizing sample equilibration time is important to obtaining the high throughput required for most plant calorimetry studies. 

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Together, the water and the calorimeter require 231 cal/°C, so the total energy required by them for 4.00°C is... 

Values of COT) can be derived from a constant volume calorimeter by measuring AU for small values of Tj - TO and evaluating AU/(T2 - T ) as a fiinction of temperature. The energy change AU can be derived from a knowledge of tlie amount of electrical energy required to change the temperature of the sample + container... 

Hence, it is necessary to correct the temperature change observed to the value it would have been if there was no leak. This is achieved by measuring the temperature of the calorimeter for a time period both before and after the process and applying Newton s law of cooling. This correction can be reduced by using the teclmique of adiabatic calorimetry, where the temperature of the jacket is kept at the same temperature as the calorimeter as a temperature change occurs. This teclmique requires more elaborate temperature control and it is prunarily used in accurate heat capacity measurements at low temperatures. 

With most non-isothemial calorimeters, it is necessary to relate the temperature rise to the quantity of energy released in the process by determining the calorimeter constant, which is the amount of energy required to increase the temperature of the calorimeter by one degree. This value can be detemiined by electrical calibration using a resistance heater or by measurements on well-defined reference materials [1], For example, in bomb calorimetry, the calorimeter constant is often detemiined from the temperature rise that occurs when a known mass of a highly pure standard sample of, for example, benzoic acid is burnt in oxygen. 

The selection of the operating principle and the design of the calorimeter depends upon the nature of the process to be studied and on the experimental procedures required. Flowever, the type of calorimeter necessary to study a particular process is not unique and can depend upon subjective factors such as teclmical restrictions, resources, traditions of the laboratory and the inclinations of the researcher. 

Accurate enthalpies of solid-solid transitions and solid-liquid transitions (fiision) are usually detennined in an adiabatic heat capacity calorimeter. Measurements of lower precision can be made with a differential scaiming calorimeter (see later). Enthalpies of vaporization are usually detennined by the measurement of the amount of energy required to vaporize a known mass of sample. The various measurement methods have been critically reviewed by Majer and Svoboda [9]. The actual teclmique used depends on the vapour pressure of the material. Methods based on... 

Suppose we wish to evaporate one mole of water, as expressed in equation (7). One mole contains the Avogadro number of molecules (6.02 X 1023) and has a weight of 18.0 grams. Using a calorimeter, as you did in Experiment 5, you could measure the quantity of heat required to evaporate one mole of water. It is 10 kilocalories per mole. This value is called the molar heat of vaporization of water. This is the energy required to separate 6.02 X 1023 molecules of water from one another, as pictured in Figure 5-1. 

To change the temperature of a particular calorimeter and the water it contains by one degree requires 1550 calories. The complete combustion of 1.40 grams of ethylene gas, CJti,(g), in the calorimeter causes a temperature rise of 10.7 degrees. Find the heat of combustion per mole of ethylene. 

Different methods are required to get Af// for substances that will not burn in oxygen. Sometimes the reaction can be made to proceed directly in a calorimeter, and a measurement of q gives Af//. For example, at T = 298.15 K... 

The implication of this equation is that, because chemical reactions typically take place at constant pressure in vessels open to the atmosphere, the heat that they release or require can be equated to the change in enthalpy of the system. It follows that if we study a reaction in a calorimeter that is open to the atmosphere (such as that depicted in Fig. 6.11), then the measurement of its temperature rise gives us the enthalpy change that accompanies the reaction. For instance, if a reaction releases 1.25 kj of heat in this kind of calorimeter, then we can write AH = q — —1.25 kj. 

We can determine calorimeter front its temperature change using Equation, which is similar to Equation calorimeter cal Here, is the total heat capacity of the calorimeter. That is, Ccal is the amount of heat required to raise the temperature of the entire calorimeter (water bath, container, and thermometer) by 1 °C. 

Constant-pressure calorimetry requires only a thermally insulated container and a thermometer. A simple, inexpensive constant-pressure calorimeter can be made using two nested Styrofoam cups. Figure 6-16 shows an example. The inner cup holds the water bath, a magnetic stir bar, and the reactants. The thermometer is inserted through the cover. The outer cup provides extra thermal insulation. 

Muscle activity is accompanied by cellular pumping of sodium ions. The energy requirements of the sodium pump have been studied on an individual cardiac muscle mounted inside a tiny differential calorimeter and stimulated by electrical impulses. The heat evolved was different in the presence and absence of a known inhibitor of the sodium pump. 

This sequence was then repeated with a different titer composition as many times as necessary to cover the required range of amphiphile/water compositions. An MS-DOS desk-top computer with 640K of random-access memory (RAM) was used to control the calorimeter and to collect thermistor voltages at the fixed time intervals. Spread-sheet macros running on this same computer were used to make various plots, including thermistor voltage y . time and corrected heat ys time. 

The lack of calorimetric data is particularly evident in the case of the adsorption of gases on oxides or on oxide-supported metals, i.e., on solids similar to most industrial catalysts. Moreover, adsorption calorimeters are generally used at temperatures that are much lower than those usually found in industry, and it would be difficult indeed to adapt most usual adsorption calorimeters for the measurement of heats of adsorption of gases on industrial catalysts at elevated temperatures. The present success of gas chromatographic techniques for determining heats of reversible adsorption may be explained by the gap between the possibilities of the usual adsorption calorimeters and the requirements of industrial catalysis research. 

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