Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

162 EXPERIMENTAL METHODS FOR FAST REACTIONSferrous (D) at potentials at which H,O, is virtually unaffected. In the presence ofan excess of hydrogen peroxide, Fe" is reoxidized to Fe"; thus the current for thereduction of ferric ion is increased above its normal value. The difference betweenthe limiting currents in the presence and absence of the oxidant is the catalytic currenti, and expressions have been derived" for the ratio i,/id.Polarography and the electrochemical methods in general have been used to studythe kinetics of many systems, and the results usually agree with those obtainedfrom other fast reaction techniques. However, there are some data from acid-basereactions in particular which do not agree. Sometimes the rate is too low, and thismight be explained in terms of a reaction preceding the main reaction. Sometimesthe rate coefficient is larger than that calculated for a diffusion-controlled reactionand so must be regarded with a great deal of suspicion. But, provided that the possiblecomplications are taken into account' the electrochemical methods can beexpected to give reliable data.7. Flowmetbods7.1 FLOW METHODS IN SOLUTIONThe application of flow methods to the study of kinetics in solution has beendiscussed in detai188*89*15; it stemsg0 from the original work by Hartridge andRoughton on the reaction of oxygen with haemoglobin. An excellent survey hasbeen given by Roughton and Millikangl which discusses the general aspects of flowmethods.Fig. 21 shows the general experimental arrangement. Two solutions, A and B,are allowed to flow into a mixing chamber and the mixed solution flows with avelocity of u cm.sec.-' along a single tube past an observation position x cm fromthe mixer. A suitable function of the concentration of reactants or products, suchas optical density at a given wavelength or conductivity, is monitored electronicallyand the extent of the reaction in a small section of the reaction mixture correspondingto a known the x/o sec after the start of the reaction thus determined.By varying certain functions, the reaction profile can be constructed, and exactlyhow this is done depends on whether the method is continuous-flow, accelerated-y--J ! -VelocityI , ,, v crn sec"-c Solution 8+xcm -c(Fig. 21. Generalized scheme of flow-mix system, showing mixing chamber M and observationposition P.