Formic acid aqueous-phase equilibrium

Reversed-phase high-performance liquid chromatography (RP-HPLC) is the usual method of choice for the separation of anthocyanins combined with an ultraviolet-visible (UV-Vis) or diode-array detector (DAD)(Hebrero et al., 1988 Hong et ah, 1990). With reversed-phase columns the elution pattern of anthocyanins is mainly dependent on the partition coefficients between the mobile phase and the Cjg stationary phase, and on the polarity of the analytes. The mobile phase consists normally of an aqueous solvent (water/carboxylic acid) and an organic solvent (methanol or acetonitrile/carboxylic acid). Typically the amount of carboxylic acid has been up to 10%, but with the addition of a mass spectrometer as a detector, the amount of acid has been decreased to as low as 1 % with a shift from trifluoroacetic acid to formic acid to prevent quenching of the ionization process that may occur with trifluoroacetic acid. The acidic media allows for the complete displacement of the equilibrium to the fiavylium cation, resulting in better resolution and a characteristic absorbance between 515 and 540 nm. HPLC separation methods, combined with electrochemical or DAD, are effective tools for anthocyanin analysis. The weakness of these detection methods is a lack of structural information and some nonspecificity leading to misattribution of peaks, particularly with electrochemical... 

Formic and Acetic Acids. The solubility equilibrium of these weak acids js treated as the two stage process described by Equations 2 3. The dissociation constants of both acids are well known and are given as functions of temperature in Table III. While there are several studies of the thermodynamics of aqueous acetic acid, e.g. (2 , and of formic acid (22), there are relatively few data for dilute aqueous solutions at 25 C (28-3Ik The chemistry of these acids is complicated by dimerisation (31-33) and higher association reactions (24) in both aqueous and gas phases. 

For pH values below 4 most of the formic acid remains in the cloud interstitial air and only a small fraction (less than 10%) dissolves in the cloud water. For pH values above 7 practically all the available formic acid is transferred into the aqueous phase and only traces remain in the interstitial air. In the intermediate pH regime from 4 to 7 the formic acid equilibrium partitioning varies considerably depending on the cloud liquid water content and pH. 

FIGURE 6.15 Equilibrium fraction of total formic acid in the aqueous phase as a function of pH and cloud liquid water content at 298 K. 

Given the long gas-phase lifetime for formic acid (1 month), it is clear that heterogeneous processes (wet and dry deposition) also must be considered as sinks for this species, particularly in the lower troposphere. An important parameter in determining the rate of wet deposition (uptake of a gas into a hydrometer, followed by removal via precipitation) is the solubility of the gas in aqueous solution, an equilibrium process described by the Henry s law constant, Hx = [Jflaq/pz see also section I-B-4. Here, Hx is the Henry s law constant for species X, [X]aq is its equilibrium concentration in the aqueous phase, and px is its partial pressure of the gas. For the organic acids under consideration here, increased solubility can result from ionization in solution, e.g. ... 

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