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What is calorimetry?
Calorimetry is an "old" science with a distinguished past. It was used originally for the measurement of the specific heats of various substances and for the study and categorisation of physical phenomena such as melting, vapourisation, dilution, equivalence between work and heat, and, of course, chemical reactions. Measurements of the heats of adsorption were at first slow to develop owing to the relatively small amounts of heat evolved when adsorption on solid surfaces takes place. The early microcalorimeters were massive, had limited sensitivity, took days to reach thermal equilibrium and had a very long time constant, i.e. the time required to record a heat gain or loss was very long.
The advent of thermocouples and semiconductors has radically altered the construction of microcalorimeters which now have relatively small adsorption cells with a high thermal sensitivity of detection and low time–constant. However most of the microcalorimeters which have been applied to the study of adsorption operate under static (or closed system) conditions, which permit neither the measurement of adsorption / desorption cycles, the reversibility of surface interactions, the examination of adsorbent surfaces in the presence of different gaseous or liquid environments, nor the sequential application of several different probes, each perhaps with various different physico–chemical affinities for the surface being studied. These conditions are crucially important in the study of industrially significant surface interactions with substances such as catalysts, metals, surfactants, oxides, pigments, fillers, and biologically active compounds etc., where the processes in which their properties are to be are exploited must be modelled using a flow–through (or open system) approach.
To overcome the limitations of batch (static) calorimetry, Microscal of London (UK) developed the Flow Microcalorimeter (FMC). Our FMC allows the holding of a sample of powder or granular solid in its cell whilst a gas, vapour, or liquid stream percolates through it. The conditions in the FMC cell, and hence their influence on the contents, may be altered to facilitate a variety of thermal measurements – vacuum (heats of evacuation), saturation (integral heats of adsorption), desaturation (integral heats of desorption), injection (pulse adsorption), pressure cycling (pressure swing technology), etc. Most significantly, heats of adsorption from and desorption into a flowing carrier medium (gaseous or liquid) may be measured as and when the composition of the carrier medium passing through the sample is changed in accordance with the planned experimental sequence or as modified by the resulting surface chemical interactions which take place within the FMC's cell.
Thus evacuating the cell, saturating the sample, starting a flow of liquid or gas through the sample, and measuring the heat evolved or absorbed in each of these procedures may start a typical experimental sequence. With the sample in a stable flow of fluid, the researcher can make changes to the composition of the fluid stream and measure the heat that is produced. A major advantage of the instrument's design is the option to feed the cell effluent to an appropriate concentration monitor. Liquid phase monitors are normally in–line variable wavelength ultraviolet spectrophotometers (UV) or refractive index detectors (RI). In gas phase systems, thermal conductivity detectors (TCD), flame ionisation detectors (FID) or photo ionisation detectors (PID) can all be used as concentration monitors.
Other measurements such as liquid–liquid mixing, and many thermal reactions for which static microcalorimeters are suitable have been added to the FMC's repertoire via the Static Microcalorimeter (SMC) adaption of the flow–through instrument. The extra cost of the SMC adaption is modest, being only a few percent of the main instrument's cost.
We have included examples of some studies using the FMC, a flow diagram of the instrument, literature references, and information on our "standard" models in this internet site.
So please look around and leave us your details so that we can discuss your applications in detail.
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