Photometric acid-base titrations in the presence of an immiscible solvent

Abstract

An apparatus is described which permits continuous monitoring of the UV absorbance of one phase in a vigorously agitated mixture of an aqueous and a water-immiscible organic solvent.Photometric titrations, using sodium hydroxide titrant, are performed in such a two-phase system on weak acid drug substances which show no spectral change upon deprotonation. Theoretical titration equations are derived and verified experimentally by titrating dextromethorphan hydrobromide.

Thermodynamics of Water-octanol and Water-cyclohexane Partitioning of some Aromatic Compounds

“…Cyclohexane and 1-octanol were of spectroscopic grade quality. Partition coefficients in the octanol-water and cyclohexane-water systems were measured over the temperature range 20-45 o C using the filter-probe method modified by Kinkel et al [2] from the original method developed by Cantwell and Mohammed [19]. It comprised a thermostatted mixing chamber whose contents could be vigorously stirred using a magnetic stirring bar; the output from the chamber was passed through either a hydrophilic (Whatman No.…”

Measurement of self-association and ion-pair dissociation constants by solvent extraction using a membrane phase separator

“…Measurement of the absorbance of the aqueous phase was made continuously at 25 f 0.l0C on an apparatus based on the two-phase titration device previously described (12). A threelayer paper membrane was employed.…”

“…The stirrer and pump were started and the 100%T set on the spectrophotometer with the aqueous phase circulating through the flow cell. Increments of sample stock solution were added from the gravimetric buret to the stirred two-phase mixture and, between each increment, the equilibrium absorbance value was recorded as previously described (12). Stock solutions of the three compounds studied and maximum weights of solution added were as follows: 0.1371 mmol/g of HBz in water-washed chloroform, up to 4.0g added; 0.212, mmol/g of Naloxone in water-washed chloroform, up to 2.3 g added; and 4.62 x mmol/g of TEA.P in methylene chloride, whose preparation is described above, up to 3.2g added.…”

“…Dissociation of ion-pairs in the organic phase is often negligible (1,(12)(13)(14)(15). However, the tendency toward dissociation is greater when the ions involved are less hydrophobic and when the solvent is more polar (1,2).…”

“…incrementally alter the composition of the system, such as in two-phase titrations and distribution isotherm measurements (12)(13)(14)(15)(16)(17). This characteristic makes the "filter-probe" an obvious device to use in solvent extraction studies designed to evaluate associationldissociation equilibria, because such studies are necessarily done as a function of changing sample concentration.…”