The Mobilities of Ions in Dry and Moist Air

Abstract

With the method recently described the distribution of mobilities of aged ions in dry air has been determined and the values for moist air remeasured because of a neglected correction in the previous results. In air dried by passage through metal tubes and filters immersed in liquid air the negative ions were found to consist of two main groups of which the less numerous group comprises about one third of the total number of ions and has a peak mobility only about 60 percent of that of the main group. The number of ions in the lesser group was relatively smaller in air dried by calcium chloride alone and their presence in still smaller numbers in air having a water content of 2 mg per liter accounts for the dissymmetry in the distribution curves of negative ions in moist air reported in the previous paper. The distribution curves for positive ions in dry air also show some indications of two groups of ions only here the ions of the less numerous group, which does not appear in moist air, have a peak mobility about 40 percent higher than those of the main group. The absolute values of the peak mobilities found for the main group of negative ions in the driest air used for a pressure of 76 cm and a temperature of 20\ifmmode^\circ\else\textdegree\fi{}C was 2.45 cm/sec. This diminished to 2.37 cm/sec for air dried by calcium chloride, and to 2.08 cm/sec for air containing 2 mg of water per liter. Under the same conditions the mobilities of the positive ions increased in succession from 1.05 cm/sec to 1.10 cm/sec to 1.36 cm/sec. These large opposite changes produced by a small fraction of one percent of water molecules can only be explained on the supposition that aged ions in air consist of molecular clusters whose structure is affected by the presence of water molecules. The second group of negative ions cannot consist of ions having multiple electronic charges, because the mobility of these ions is smaller than that of the main ion group. It cannot arise from the presence of impurities condensable at liquid air temperatures. Since electrons do not readily become attached to nitrogen molecules, the central molecule of each of these ions is probably an oxygen molecule and the two cluster groups may arise either from a difference caused by a different point of attachment of the electron to the central molecule or from the nature of the molecule that first becomes attached to this central molecule in the process of cluster formation.