Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

136 EXPERIMENTAL METHODS FOR FAST REACTIONSin elucidating particular types of kinetic patterns and reaction mechanisms. In mosttypes of system data have been collected with the help of several relaxation methods.Metal-complex formation is discussed in greater detail below, but for some metalsthe T-jump is a particularly suitable tool. For example, magnesium forms complexestwo or three orders of magnitude more slowly than does calcium; the T-jump hasbGen used in several cases for magne~ium~~.~~~36 but it is generally not possible touse this method with calcium. Metal-complex formation data have been collectedby Eigen and Wilkin~~~. If possible, changes in optical properties associated withthe ligand or metal are followed, but this is sometimes not practical. Such a case wasML(H20),+NH3 s ML(H,O),-,NH, +H20where M is a divalent metal ion (NiZ+ or CU”) and L is a chelating ligand. Thisreaction was coupled with the reactionsandNH, + H,O s NH,+ + OH-OH- + HInd f Ind- + H,Othe shift in relative concentrations of HInd and Ind- resulting in a colour change5’.It may be possible to select conditions such that these indicator reactions are fastand do not interfere with the relaxation under investigation (as was the case here).Often, however, this is not possible, and the overall relaxation expression containscontributions from the indicator reactions4’. The rate coefficients for these reactionsmust then be obtained separately.Usually proton-transfer reactions in aqueous solution are diffusion-controlledand are, consequently, too fast to be studied in this way. Sometimes structuralfeatures such as internal hydrogen-bonding slow them down sufficiently to maketheir investigation with the T-jump feasible. This type of reaction has been consideredfully5‘. An example of a system studied by this method is the indicator phenolred in water52. Because measurements must be made around pH 7 (the pKof phenolred is 7.8) in unbuffered solutions at small acid or base concentration, the errorlimits are quite high. A problem also arises which for most applications of the T-jump is not important. This is the long-term disturbance of the system due to polarizationeffects at the electrodes. In this case, at low indicator concentrations acolour gradient may be seen after the electrical discharge ranging from yellow atthe “cathode” (associated with the protonated species) to red at the “anode” (thedeprotonated species).If the enthalpy change AH of a reaction sequence, in which one step is a protonation,is too small to permit the application of the T-jump, it might be possible