General chemistry texts usually devote a portion of one chapter to the solubility product principle (1). This is commonly in the form of K sp = f (concentrations of ions raised to some powers). For example, K sp = [Ag + ][Cl ᎑ ], where K sp is the solubility product constant for the slightly soluble substance silver chloride. [Ag + ] and [Cl ᎑ ] are the concentrations in moles per liter of the ions that are in equilibrium with the solid phase of AgCl. Tables of K sp values are given, and the usual two assumptions made are: the substances are strong electrolytes which ionize 100% in solution; and activities of the ions are close enough to the molar concentrations in these dilute solutions to allow defining the K sp in terms of concentrations. Reasoning from the equations, which presume that the only forward reaction is the production of the expected ions, the student is asked to calculate solubility from K sp and vice versa, to calculate the common ion effect on the solubility of substances, and to determine if slightly soluble solids will precipitate under certain concentration suppositions.This Journal contained an excellent article in 1966, by Meites, Pode, and Thomas called "Are Solubilities and Solubility Products Related?" (2). The authors showed clearly that, due to ion pair formation, hydrolysis, complex ion formation, and activity coefficient variations, there are only a few cases in which solubility and K sp are related in a simple way (Fig. 1). Meites et. al. concluded, "It would be better to confine illustrations of the solubility-product principle to 1-1 salts, like silver bromide and thallium iodide, in which the student's calculations will yield results close enough to the truth to permit him to feel it is worth his trouble to try to master what he is being taught."Ignoring the masculine pronoun common in the sixties, this advice is excellent. In the majority of cases solubilities are not simply related to K sp . Unfortunately, the authors of current general chemistry texts have largely ignored this advice and provide students with large K sp tables that include numerous slightly soluble substances. Further, many authors imply by example and by assigned exercises that K sp is a simple function of solubility. In effect they teach (3) that for A x B y (s) → xA z+ + yB z᎑ where solubility is S o , K sp = [xS o ] x [yS o ] y which results in the familiar K sp = S o 2 K sp = 4S o 3 K sp = 27S o 4 and K sp = 108S o 5
This paper, for chemistry teachers who have beginning students, computers, and spreadsheets, presents tables of titration data simulated using the program EQUIL. Instructors are invited to give students the titration data to enter into their spreadsheets, have them plot it in several ways, and produce an assortment of graphs. In the process, they will discover many things about pH,. the taking of derivatives, buffer capacity, and the way buffers behave upon dilution. These resulting plots show why there is pseudobuffering at high and low pH values, and show the equilibrium buffering maximum at the pKa or pKb of the electrolyte species. A convenient definition of buffer capacity with respect to dilution is beta dil, where beta dil = d(-log[conc]/d(pH). This definition has the advantage of being an intensive property of the solution, and also of being large for equilibrium buffer solutions undergoing dilution.
Generation of gaseous thiosulfonium ions (CHgSSCH3)R+, R = H, alkyl, and dithiosulfonium ions, R = alkylthio, has been achieved by ion cyclotron resonance techniques. Hitherto, dithiosulfonium ions have been inferred as transient intermediates in solution-phase reactions. It is here reported that these ions can be formed in the gas phase by displacement of methanethiol from protonated methyl disulfide with neutral disulfide. Related ions (CH3S)2CH3+ can also be formed in the gas phase by the methylation of methyl disulfide with CH3OCH2+ derived from methyl ethers on electron impact. Gaseous thioand dithiosulfonium ions have been shown to thiolate neutral sulfides and selected alkenes, as they do in condensed phase. The ion/molecule chemistry of methyl 1-phenylethyl sulfide has been studied and has been found to lead to thiosulfonium ions (CH3S)2R+, R = 1-phenylethyl, by methylthiolation of the neutral sulfide by the molecular ion. A degenerative rearrangement of (CH3S)2R+ which has been documented in solution was not evident in the behavior of the gaseous ion. The implication of this observation is discussed. Heats of formation of trimethylsulfonium and several thiosulfonium ions have been estimated.Thiosulfonium ions having the general structure RSS+R2 are highly reactive ions and have been isolated as stable salts in relatively few instances.1,2 The best characterized(1) (a) G.
Reactions of allylic compounds 3-methoxy-l-propene and 3-(methylthio)-1-propene with gaseous methoxymethyl cations generated from methyl ethers on electron impact have been investigated by ion cyclotron resonance techniques. Three modes of reaction have been identified for the allylic sulfide with CH3OCH2+ that correspond to the elimination of the elements of CHü-O, CH3OH, and CH3SH. The allylic ether and CH3OCH2+ reacted to eliminate CH3OH predominantly. Pathways for these reactions were studied by using isotopic labels. The results are interpreted in terms of attack of the reactant ion at the terminal carbon of the allyl group followed by hydride and/or proton transfers and elimination of CH20, CH3OH, or CH3SH. Oxygen lost as the neutral arose predominantly but not exclusively by cleavage of the methylene-oxygen bond of the reactant ion. Methylthiolation of 3-(methylthio)-1-propene also was achieved by reaction with CH3OCH2+ in the presence of methyl disulfide. The source of a small amount of label scrambling in deuterium-labeled reactant ions is discussed. mass spectrometry, respectively. Therefore, we are con-(1) (a) See P. D. Lawley in "Chemical Carcinogenesis", C. A. Searle,
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