Validation of Flow-Through Microcalorimeters

This has been achieved, for solution-phase flow calorimeters, through the use of a chemical test and reference reaction the base-catalysed hydrolysis of methyl 4-hydroxy-benzoate (methyl paraben). 

Note that for reactions that are rapid (with respect to the time constant of the instrument), this validation is not necessary since the reaction is initiated inside the calorimetric cell, and has no measurable kinetic dependence i.e. instantaneous) and the required thermodynamic parameters can be calculated through the use of a calibration constant. 

Studies using the base-catalysed hydrolysis of methyl paraben (BCHMP) test and reference reaction have been conducted with a variety of different solution-phase flow calorimeters. The results obtained from these studies have shown that the flow rate dependency of the thermal volume is different for each of the instruments used and indeed for each experimental arrangement e.g. sample and reference cell set-up). The determined value for can differ by as much as 15% (over a range of experimental flow rates) from the nominal engineered volume (typically approximately 1ml). This effect can be minimised by careful design of the flow cell and also by careful consideration of the sample and reference cell arrangements (more details can be found in ref. 22 and references therein). 

Knowledge of how thermal volume varies with flow rate allows the physical characteristics of the flow cell to be investigated. For example, it is possible to simulate 

It should be noted that control is achievable over the flow rate, the concentration and to a lesser extent over the rate constant, the latter by changing the temperature. However, it is not possible to alter the enthalpy, assuming that it has no temperature dependence (it is possible, in some circumstances, to magnify the enthalpy by careful choice of an appropriate buffer system). The changes in the concentration and the rate constant must also satisfy the requirement that the mechanism of reaction is not affected as a consequence. Changing the flow rate will have no impact on the mechanism since it is purely a physical change to the system. 

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