Bases organic, dissociation constant

The pKa or dissociation constant, is a measure of the strength of an acid or a base. The dissociation constant of an organic acid or base is defined by the following equilibria ... 

Since silica dissolves above about pH 10.7-11.0, silicates can be prepared with organic bases having dissociation constants greater than about 10 , although some silica will dissolve in aqueous solutions of weaker bases with constants as low as lO" or 10" (123-125). Merrill and Spencer (124) reported the preparation of a number of water-soluble quaternary ammonium silicates by grinding silica gel with a solution of the free base. However, the compounds all appear to have a ratio of 2 1 when expressed as SiOj (NR )jO by analogy with the alkali metal system. 

D.D. Perrin, Dissociation Constants of Organic Bases in Aqueous Solution, Butterworths, London, 1965 and Supplement 1972. 

The theory of titrations between weak acids and strong bases is dealt with in Section 10.13, and is usually applicable to both monoprotic and polyprotic acids (Section 10.16). But for determinations carried out in aqueous solutions it is not normally possible to differentiate easily between the end points for the individual carboxylic acid groups in diprotic acids, such as succinic acid, as the dissociation constants are too close together. In these cases the end points for titrations with sodium hydroxide correspond to neutralisation of all the acidic groups. As some organic acids can be obtained in very high states of purity, sufficiently sharp end points can be obtained to justify their use as standards, e.g. benzoic acid and succinic acid (Section 10.28). The titration procedure described in this section can be used to determine the relative molecular mass (R.M.M.) of a pure carboxylic acid (if the number of acidic groups is known) or the purity of an acid of known R.M.M. 

In gradient elution of weak acids or bases, gradients of organic solvent (acetonitrile, methanol, or tetrahydrofuran) in buffered aqueous-organic mobile phases are most frequently used. The solvent affects the retention in similar way as in RPC of nonionic compounds, except for some influence on the dissociation constants, but Equations 5.8 and 5.9 usually are accurate enough for calculations of gradient retention volumes and bandwidths, respectively. 

Tetracycline antibiotics are closely related derivatives of the polycyclic naphtha-cenecarboxamide. They are amphoteric compounds with characteristic dissociation constants corresponding to the acidic hydroxyl group at position 3 (pK about 3.3), die dimethylamino group at position 4 (pK, about 7.5), and the hydroxyl group at position 12 (pK about 9.4). In aqueous solutions of pH 4-7, tetracyclines exist as dipolar ions, but as the pH increases to 8-9 marked dissociation of the dimethylamine cation occurs. They are soluble in acids, bases, and alcohols but are quite insoluble in organic solvents such as chloroform. Their ultraviolet spectra show strong absorption at around 270 and 360 nm in neutral and acidic solutions. Tetracyclines are readily transformed into fluorescent products in the presence of metal ions or under alkaline conditions. 

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