Photoelectric Methods of Measuring the Velocity of Rapid Reactions. I. General Principles and Controls

Until 1923, no direct, generally applicable method existed for measuring the velocity of chemical reactions with half-periods less than about 10 sec. In that year, however, Hartridge and Roughton extended the observable time range more than a thousandfold, by devising a method which could be used to follow reactions with half-periods ranging from 10 sec (or more) down to 0·002 sec. A rough list of the rapid reactions, 20 or so, which have since been measured in this way, has been given by Hartridge, Millikan, and Roughton in a recent discussion at the Royal Society. The principle of Hartridge and Roughton's method is as follows: the two solutions which are to react are placed in separate containers and driven thence into a special mixing chamber; the emerging mixed fluid passes down an observations tube, at various cross-sections of which the composition of the streaming fluid is determined by optical, thermal, electrical, or chemical analysis. If d is distance in cm from the mixing chamber of the cross-section examined, and v is the linear velocity in cm/sec of the streaming fluid, the time for which the reaction has proceeded before reaching the place of observations is simply d/v . Thus if the cross-section examined is 10 cm from the mixing chamber, and v = 100 cm/sec, the corresponding time = 10/100 = 0·1 sec. Observations at four or more cross-sections thus enable the usual type of concentration-time curve to be plotted, even when the time is measured in millisec (0·001 sec) as units.