Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

CHEMICAL RELAXATION METHODS 135Fig. 8. Oscillogram showing relaxation behaviour for Mg2’ -adenosine-5’-triphosphate (ATP4-)complex (5 x M) and 8-hydroxyquinoline buffered at pH 8.2.small. Secondly, the photomultiplier/oscilloscope trace measures transmittancechanges whereas the concentration is proportional to the absorbance. The relationbetween these is approximately linear only for small total changes. Typically, thecell contains about 5 ml of solution and the optical path length is 1 cm. For mostsensitive recording the optical density at the chosen wavelength should be quitehigh: 0.8-0.9. For a given system to be suitable for T-jump study, three propertiesmust have suitable quantities-the overall effective equilibrium constant, the enthalpychange of at least one of the reaction steps, and the optical density change ata suitable wavelength.An alternative approach to the problem of suddenly raising the temperature is touse a pulse of microwave^^^. The advzntage of this method is that non-conductingsystems may be used. A disadvantage of the microwave T-jump is that only arelatively small temperature rise can be obtained, generally less than 1”. Recently,the heating effect of a flash lamp has been used43; a laser has been employed asthe heat source in surface-T-jump experiments and attempts are being made to useit for heating solution^^^,^^. One difficulty is that at present the wavelengths ofavailable lasers are not very suitable, since some species (such as whichinterferes with many reactions of potential interest) must be present to absorb theenergy. The T-jump technique is also being applied to reactions of species withshort life-times, an extension which will be of special interest in enzyme-chemistry.By combining the flow and T-jump techniques, reactions of species in quasi-equilibriumare being studied (Section 7)46.3.2.2 Applications of T-jumpIt is difficult to say that a given relaxation technique has been especially usefulReJerences pp. I76-179