Highly selective spectrophotometric flow-injection determination of trace amounts of bromide by catalytic effect on the oxidation of m-cresolsulfonephthalein by periodate

Kinetics and mechanism of the oxidation of bromide by periodate in aqueous acidic solution

The periodate–bromide reaction has been studied spectrophotometrically mainly in excess of bromide ion, monitoring the formation of the total amount of bromine at 450 nm at acidic buffered conditions and at a constant ionic strength in the presence of a phosphoric acid/dihydrogen phosphate buffer. The stoichiometry of the reaction was established to be strictly IO4(–) + 2Br(–) + 2H(+) → Br2 + IO3(–) + H2O. The formal kinetic order of the reactants was found to be perfectly one and two in the cases of periodate and bromide, respectively, but that of the hydrogen ion lies between one and two. We have also provided experimental evidence that dihydrogen phosphate accelerates the formation of bromine, suggesting the appearance of strong buffer assistance. On the basis of the experiments, a simple two-step kinetic model is proposed involving BrIO3 as a key intermediate that perfectly explains all of the experimental findings. Furthermore, we have also shown that in huge excess of bromide, the apparent rate coefficient obtained from the individual curve fitting method of the absorbance–time series is necessarily independent of the initial periodate concentration that may falsely be interpreted as the rate of bromine formation is also independent of the concentration of periodate.

Determination of trace amounts of thiocyanate by a new kinetic procedure based on an induction period

A new, simple and sensitive kinetic spectrophotometric method with no need for removing of interfering substances is proposed for the determination of thiocyanate ion in biological and water samples. The procedure is based on the inhibiting effect of thiocyanate on the sodium periodate-potassium bromide-meta cresol purple (MCP) system in acidic media. The induction period of the reaction is proportional to the SCN- concentration. The decolorization of meta cresol purple by the reaction products was used to monitor the reaction spectrophotometrically at 525 nm. Under optimum conditions, thiocyanate can be determined in the range of 0.02-0.8 microg ml(-1) with a 3sigma detection limit of 5 ng ml(-1). The relative standard deviations for 10 replicate determinations of 0.060, 0.10 and 0.50 microg ml(-1) thiocyanate are 3.7, 2.4 and 1.0%, respectively. This method has been successfully used to the determination of thiocyanate content in smokers and non-smokers saliva and spiked water sample.