Electroanalytical and optical methods in the study of analytically important reactions

Abstract

The use of polarography and spectrophotometry in the study of analytically important reactions is demonstrated on two examples: determination of arginine based on reaction with 1,2-dicarbonyl compounds, and determination of bile acids in strongly acidic media. Guanidine and arginine react at 25 °C with biacetyl in three reaction steps of considerably different rates, none of them involving the formation of a heterocycle. Only the decrease of biacetyl concentration can be followed by polarography, but neither this method nor spectrophotometry enabled following of intermediates or products. Therefore attention has been paid to the reaction of the guanidino grouping with 9,10-phenanthrenequinone (I), which behaves like an orthoquinone rather than a 1,2-diketone. Compound I reacts with two hydroxide ions and forms a stable geminal diol dianion. Reaction of I with a third hydroxide ion results in a trianion which undergoes a benzilic acid rearrangement. The nature of the species reacting with the guanidino grouping is being studied. Bile acids can be determined based on absorption at 310 and 380 nm in 70–80% H 2SO 4. The chromophores are allylic and dienylic carbocations formed in a reaction of a carbocation and olefin. The latter results from dissociation of the carbocation. In the study of the dependence of kinetics of this reaction on acidity, the problem of reaction of ethanol (used as co-solvent) with sulfuric acid was encountered. Equilibrium constants for the reaction of ethanol and methanol with sulfuric acid were measured. From their dependence on acidity it follows that ROH reacts with HSO 4 − to yield ROH +SO 3 −. For ROH 2 + ⇌ ROH + H + approximate values of K ROH = 5 × 10 3 liters mol −1 for ethanol and 1.8 × 10 4 liters mol −1 for methanol were found.