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Acid-Base Equilibria in Organic-Solvent/Water Mixtures and Their Relevance to Gas/Particle Partitioning in the Atmosphere and in Tobacco Smoke

Acid-base equilibria in organic particulate matter (PM) are poorly understood, but have important implications for air quality and public health. First, acid-base reactions in organic particulate matter affect the gas/particle partitioning of organic compounds in the atmosphere, and these processes are not currently represented in atmospheric and climate change models. Second, the acid-base balance of tobacco smoke affects the amount of nicotine absorbed by the smoker, and a greater understanding of this balance would help to relate cigarette smoke composition to the addictive properties of cigarettes. This work presents data related to both air quality and tobacco smoke modeling.
The gas/particle partitioning behavior of organic acids and bases is highly dependent on acid-base equilibria and speciation between neutral and ionic forms, because ionic compounds do not volatilize. Descriptions of acid dissociation behavior in atmospheric PM have, to date, focused primarily on phases in which the solvent is water; however, atmospheric PM may include up to 90% organic matter. Data is presented here describing the acid dissociation behavior of organic acids and protonated amines in organic/aqueous mixtures (chosen to approximate the characteristics of organic PM) with varying levels of water content. In such mixtures, the preferential solvation of ions and neutral molecules (by the aqueous portion or the organic portion, respectively) affects the acid-base equilibria of the solutes. It is demonstrated that neutralization reactions between acids and bases that create ions are likely to have non-negligible effects on gas/particle partitioning under certain atmospheric conditions. Thus, including acid-base reactions in organic gas/particle partitioning models could result in a greater proportion of acidic and basic compounds partitioning to the particulate phase. In addition, the acid dissociation constants (pKa values) of atmospherically-relevant acids and bases vary with water content. Specifically, as water content increases, the pKa values of organic acids decrease dramatically, while the pKa values of protonated amines changes only slightly. This situation can result in drastically different speciations and partitioning behavior depending on water content.
This second part of this work reports some of the data needed to develop an acid-base balance for tobacco smoke PM using electroneutrality as a governing principle. Five brands of cigarettes were sampled and the smoke PM extracted. Cations (sodium, potassium, and ammonia) and anions (organic acids, nitrate, nitrite, and chloride) were measured using ion chromatography. Ammonia and organic acids were also re-measured after the acidification of the sample in order to determine whether "bound" forms of these compounds exist in cigarette PM. Weak acids were determined by acid-base titration to determine whether or not all of the weak acids (including organic acids) had been accounted for by the ion chromatography. Weak bases were also determined by acid-base titration, and the majority of weak base is expected to be accounted for by total nicotine (to be measured in a separate analysis). In terms of total acidic species and total basic species, two of the five cigarette brands measured were relatively basic, and three were relatively acidic. Between 50% and 89% of the titrated acids were accounted for by the anionic species measured in ion chromatography. Based on samples tested after sample acidification, about half of the potential ammonia in tobacco smoke PM exists in "bound" form. The speciation of weak acids and bases in tobacco smoke PM cannot be determined from this data alone, because the equilibrium constants of acid-base reactions are not understood in complex organic media. The data presented here, when combined with data from free-base and total nicotine analyses, represent a first step toward a predictive model of acid-base behavior in tobacco smoke PM.
Date11 March 2016
CreatorsDeGagne, Julia Lynn
Source SetsPortland State University
Detected LanguageEnglish
SourceDissertations and Theses

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