A simple method for measuring positive ion concentrations in flames and the calibration of a nebulizer/atomizer

A simple and accurate (<+/-10%) method has been developed for measuring absolute concentrations of positive ions in a flame. The method involves only the measurement of a saturation current (similar tomuA) when collecting the total flux of cations in a flame when it impinges on a conducting plate or electrode perpendicular to the flame axis. A fixed negative bias (similar to50 or 100 V) is applied to this electrode with respect to the metallic burner; such a voltage is more than adequate to stop all negative charges from reaching the electrode. The required bias voltage for this separation of positive and negative charges varies roughly as the one-third power of the concentration of positive ions. One application of the method involves the accurate (<+/-10%) calibration of the delivery factor f for a nebulizer or pneumatic atomizer used to dope a flame with a metal (<5 x 10(-6) mole fraction); the metal is introduced by spraying an aerosol of an aqueous salt solution into the gas mixture feeding the burner. When a dilute solution (similar to10(-4) molar) of a Cs salt is sprayed into a fairly hot flame (>2300 K), the Cs added is almost completely ionized (>99%); accordingly, f can be determined by measuring the absolute ion density of Cs+. Calibration difficulties can be encountered if a more concentrated solution is employed, because Cs+ can, quite surprisingly, achieve a superequilibrium concentration early in the flame. However, the method was used to show that the delivery of a pneumatic atomizer is essentially linear when the strength of the salt solution varies by at least three orders of magnitude. Of great utility is the ease with which ion concentration profiles can be measured along a flat flame for studying the kinetics of reactions involving ions. Such ion profiles have revealed the very rapid production of Cs+ ions and free electrons in the reaction zone of a flame doped with Cs; two possible chemi-ionization reactions are discussed to explain this phenomenon. In addition, the rate constant of the recombination reaction H3O+ + e(-) H + H + OH is confirmed to be (3.2 +/- 0.3) x 10(-7) cm(3) ion(-1) s(-1) at 2400 K. Confirmation of this rate coefficient provides verification of the simple method presented here for the measurement of absolute concentrations of positive ions in flat flames. (C) 2003 The Combustion Institute. All rights reserved.