Compleximetric methods

It is important to recognize that the following analytical methods essentially determine EO-PO ratio ( H NMR, IR, cleavage methods) or even simply alkylene oxide content (compleximetric methods) of the analyte, and as such are not specific quantitative or qualitative methods for poloxamers, since EO-PO copolymers of a different structure (for instance, random copolymers, or PO-EO-PO block copolymers) may respond to the methods in a way indistinguishable from poloxamers. The principal technique that permits definitive identification of a sample as a poloxamer is C NMR, which allows structural details, such as the distribution of EO and PO units along the polymer chain, to be elucidated [10]. 

Poloxamers are used primarily in aqueous solution and may be quantified in the aqueous phase by the use of compleximetric methods. However, a major limitation is that these techniques are essentially only capable of quantifying alkylene oxide groups and are by no means selective for poloxamers. The basis of these methods is the formation of a complex between a metal ion and the oxygen atoms that form the ether linkages. Reaction of this complex with an anion leads to the formation of a salt that, after precipitation or extraction, may be used for quantitation. A method reported to be rapid, simple, and consistently reproducible [18] involves a two-phase titration, which eliminates interferences from anionic surfactants. The poloxamer is complexed with potassium ions in an alkaline aqueous solution and extracted into dichloromethane as an ion pair with the titrant, tet-rakis (4-fluorophenyl) borate. The end point is defined by a color change resulting from the complexation of the indicator, Victoria Blue B, with excess titrant. The Wickbold [19] method, widely used to determine nonionic surfactants, has been applied to poloxamer type surfactants 120]. Essentially the method involves the formation in the presence of barium ions of a complex be-... 

Kosheleva et al. have reported a compleximetric method for the determination of procaine in the presence of its hydrolysis products [91]. 

Other methods have involved compleximetric titration, nephelom-etry, potentiometric titration, and gravimetric methods. In 1981, a paper by Mayanna and Jayaram11 outlined the determination of caffeine in a wide variety of products including pharmaceuticals and food products using sodium N-chloro-p-toluene-sulphonamide (chloramine-T) in a titri-metric procedure. 

Finally, a method for the determination of total salt content of natural waters, compleximetric and precipitation titrations monitoring were reported by Yao et al (119,120). Analytes at concentrations down to 10M were determined. Several metal ions were titrated with EDTA in the absence of buffer and competing ligands, in buffered media, in acidic solutions and in presence of competing ligands. 

The method of invariant points was used. A third component was added to eutectic systems until a new solid appeared. The solid and liquid phases were each analyzed. Chloride ion content was determined by the Volhard method. H2P0 " ions were precipitated as NH MgP04 6H2O, the excess of Mg " " being titrated compleximetrically (1). NH ions were removed and the excess of base used was titrated with 0.1 M HCl. 

Equilibrium was reached isothermally during the course of 1 to 3 days. The chloride content was determined by the Volhard method, the dihydrogenphosphate was precipitated as NH MgPO, the excess of Mg was titrated compleximetrically. The index of refraction was measured with an IRF-22 ref ractometer. 

The chloride content was determined by the Volhard method, the H2P0 content was determined alkalimetrically after ion exchange calcium content was determined by a compleximetric titration. Where the amount of phosphate ion was small, it was determined gravimetrlcally as ammonium phosphomolybdate. 

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