Immersion of a well-defined Pt(lll) surface into an aqueous solution of KCN or KSCN formed an ordered double layer consisting of chemisorbed species (CNH or CN"; SCNH or SCN") and K+ counterions, on the basis of Auger spectroscopy and LEED following emersion from solution. Subsequent immersion of the Pt(lll) (2v/3X2v/3)R30°-KCN, Pt(lll) (V'13XV'13)R14°-KCN or Pt(lll) (2X2)-KSCN siifface into dilute aqueous CsCl or CaCl led to replacement of the K+ ions with Cs+ or Ca2+ without changing the structure or composition of the chemisorbed CN or SCN layer. Measurement by Auger spectroscopy of the amount of Cs present on the surface after emersion from CsCl solutions adjusted to various pH levels with HC1 or CsOH revealed that cation retention by CN or SCN adlattices increased with pH in multiple transitions analogous to a polybasic acid. The first stages of ionization occurred readily, as for a strong acid, and corresponded in packing densities to formation of lattices of anion-cation pairs. The final stage of ionization commenced above pH 10 and corresponded to formation of adjacent ion pairs. LEED patterns remaihed sharp throughout the neutralization process. When Ca2+ was the counterion, surface ionization was limited to a single transition below pH 3 due apparently to the larger size of the Ca2+ cation which remained substantially hydrated after adsorption, emersion, and evacuation. Experiments were also carried out in which the pH was held constant at 9.2 and the electrode potential was varied from -1 to +1 V vs. Ag/AgCl.Structure of the CN layer was potential dependent; reconstruction of the Pt(lll) (2v'3X2v/3)R30°adlattice occurred negative of-0.45 V with slight loss of CN from the surface to form a Pt(lll) (V'13Xv/13)R14°a dlattice. Cation packing density at these surfaces was virtually independent of electrode potential, apart from potentials where oxidation of the surface occurred.
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