Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

172 EXPERIMENTAL METHODS FOR FAST REACTIONSare observed. The latter forms no significant concentration of any compound, butlithium produces a large amount of the stable gaseous hydroxide, LiOHThis equilibrium, for which the equilibrium constant K can be estimated, is setup comparatively rapidly. If the total amounts of sodium and lithium in the flameare equal, then at a given position the free Na concentration is equal to the freeLi concentration (both estimated from the intensity of emission) plus the concentrationof LiOH. By using the equilibrium constant for equation (10) the concentrationof hydrogen atoms may be calculated. In the second method, Fenimore andJones'og added small amounts of D,O, which reacts with hydrogen atoms at aknown rateH+D,O + HD+OD.Samples are withdrawn at different points and the concentration of HD measuredmass spectrometrically. This method which, incidentally, only works with richflames (where the HD is not consumed by an excess of oxygen) gave results whichwere in excellent agreement with those obtained by the first method. It should benoted that in both cases the indicator concentration was very much less than theconcentration of the hydrogen atoms.9. Molecular beamsThe successful use of molecular beams for investigating physical properties ofgas molecules has been discussed by several '; more recently, chemicalapplications have aIso been disc~ssed"~~"~. A molecular beam differs from a gasBCObservation chamberI