Organic acids and bases

D. D. Perrin, B. Dempsey, and E. P. Serjeant, pK Prediction for Organic Acids and Bases, Chapman and Hall, London, 1981. 

Johansson, P.-A. Gustavii, K., Potentiometric titration of ionizable compounds in two phase systems. 2. Determination of partition coefficients of organic acids and bases, Acta Pharm. Suecica 13, 407 -20 (1976). 

Another method that may work is to change the pH. This works with weak organic acids and bases. In this case the material is adsorbed at some optimum pH. To desorb it the pH is changed and the adsorbed material is removed in a more... 

Other common examples are aluminium chloride, tin(rv) chloride, zinc chloride, etc. We shall, at this point, be concerned essentially with proton acids, and the effect of structure on the strength of a number of organic acids and bases will now be considered in turn. Compounds in which it is a C—H bond that is ionised will be considered subsequently (p. 270), however. 

ESCHER, B. I., SCHWARZENBACH, R. P., Westall, J. C., Evaluation of liposome-water partitioning of organic acids and bases. 2. Comparison of experimental determination methods, Environ. Sci. Technol. 2000, 34, 3962-3968. 

The semi-empirical Yasuda-Shedlovsky technique of extrapolating a series of apparent pKa values obtained in several ratios of water/solvent to obtain an aqueous value is well established [32, 33], but three or more experiments are required and this adds significantly to assay times. A method of calculating aqueous pKas for various classes of organic acids and bases from single apparent pKa values obtained in water/solvent mixtures has been reported [34], and shows promise as a means of further speeding pKa measurement. 

In the case of dissociating or ionizing organic chemicals such as organic acids and bases, e.g., phenols, carboxylic acids and amines, it is desirable to calculate the concentrations of ionic and non-ionic species, and correct for this effect. A number of authors have discussed and reviewed the effect of pH and ionic strength on the distribution of these chemicals in the environment, including Westall et al. (1985), Schwarzenbach et al. (1988), Jafvert et al. (1990), Johnson and Westall (1990) and the text by Schwarzenbach, Gschwend and Imboden (1993). 

HIOC is a term that refers to organic compounds that can be ionised in the environment and thus are present in two or more species. Basically, the group of HIOCs includes weak organic acids and bases and organometallic compounds, e.g. organotins. This latter group will be treated separately (see Section 2.2.4). 

FIGURE 2.6 Representative pKA values of organic acids and the conjugate acids of bases. The p values of organic acids and bases were obtained from Perrin et al. (1), of drugs from Foye et al. (2), and of amino acids from Merck Index (3). 

Perrin DD, Dempsey B, Serjeant EP. pK3 Pediction for Organic Acids and Bases. London Chapman and Hall 1981. 

Therefore, weak organic acids and bases are readily absorbed as uncharged lipid-soluble molecules, whereas ionized compounds are absorbed only with difficulty, and nonionized toxicants with poor lipid-solubility characteristics are absorbed slowly. Lipid-soluble acid molecules can be absorbed efficiently through the gastric mucosa, but bases are not absorbed in the stomach. 

Jones, H.P., Davey, R.J., Cox, B.G., 2005, Crystallization of a Salt of a Weak Organic Acid and Base Solubility Relations, Supersaturation Control and Polymorphic Behaviour, J. Phys. Chem. B, 109, 5273-5278. 

Generally, it has been found that the organic acids and bases do exist in aqueous solution as equilibrium mixtures of their respective neutral as well as ionic forms. Thus, these neutral and ionic forms may not have the same identical partition coefficients in a second solvent therefore, the quantity of a substance being extracted solely depends upon the position of the acid-base equilibrium and ultimately upon the pH of the resulting solution. Hence, extraction coefficient (E) may be defined as the ratio of the concentrations of the substance in all its forms in the two respective phases in the presence of equilibria and it can be expressed as follows ... 

Whether a toxic pollutant in a COM or a solid waste material (SWM) leachate carries a charge or exists as a neutral species will have a dramatic effect on its environmental chemodynamics. This is a possibility with weak organic acids and bases, and is a function of the pK of the particular organic compound and pH of the surrounding environment. For instance, the dissociation of any weak organic acid (proton donor) may be represented as... 

Extensive collections of pK values are available in the literature, e.g., [98-101]. It is also possible to predict pK values for a broad range of organic acids and bases using linear free energy relationships based on a systematic treatment of electronic (inductive, electrostatic, etc.) effects of substituents which modify the charge on the acidic and basic center. Quantitative treatment of these effects involves the use of the Hammett Equation which has been a real landmark in mechanistic organic chemistry. A Hammett parameter (a), defined as follows ... 

In addition, it is possible to extend the analysis to include an ortho substituent and the associated steric effects [98-101]. Thus it is possible by this procedure to predict with some accuracy the pKa and pKb of organic acids and bases leached from COMs. 

Wang, C., Xu, L., Xu, 0., Tian, L., and Zhang. Z. Determination of partition coefficients of organic acids and bases and the correlation of partition coefficients in different systems, HuanjingKexueXuebao, 9(4) 418-424, 1989. 

Crystalline salts of many organic acids and bases often have a maximum solubility in a mixture of water and water-miscible solvents. The ionic part of snch a molecule requires a strongly polar solvent, snch as water, to initiate dissociation. A mixture of water-miscible solvents hydrates and dissociates the ionic fraction of pollutants at a higher concentration than wonld either solvent alone. Therefore, from a practical point of view, the deliberate nse of a water-soluble solvent as a cosolvent in the formnlation of toxic organic chemicals can lead to an increased solnbility of hydrophobic organic contaminants in the aqueous phase and, conse-qnently, to a potential increase in their transport from land surface to groundwater. 

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