Bromine, electrogenerated

Bromine electrogeneration was carried out from tetrabutylammonium bromide in 0,1 M NaClO acetonitrile solution with 100 % current yield. 

In this article we shall describe the development of new elec-troanalytical techniques for the determination of three different classes of compounds of interest to biologists and clinicians. We shall first describe a novel detector using a ring-disc electrode[l] for the estimation and identification of proteins, in which the protein is titrated with bromine electrogenerated on the disc electrode and the unreacted bromine is measured on the ring electrode. 

The SECM can also be used as a tool for modification of surfaces. For example, metals or semiconductors can be etched or metals can be deposited on a surface by passing the tip close to the surface and carrying out an appropriate electrochemical reaction. Two different modes are possible. In the direct mode, the tip acts as the counter electrode and the desired electrochemical reaction occurs on the substrate for example, the etching of Cu occurs on the Cu substrate. Spatial resolution is determined by the current density distribution between tip and substrate. In the feedback mode, a reactant is generated at the tip that promotes the reaction on the substrate. For example, Cu can be etched by bromine electrogenerated at the tip. In this case, resolution is determined by the lateral (x-y) diffusion of reactant as it diffuses from tip to substrate. Details of fabrication using SECM are covered in Chapter 15. 

Coulometric determination of Cu(II) and Ni(II) was canied out applying such reagents as 8-hydroxyquinoline and 8-mercaptoquinoline. Electrogenerated bromine was used as a titrant. 

Furan was dimethoxylated to give 2,5-dihydro-2,5-dimethoxyfuran, using electrogenerated bromine molecules generated from bromide salts in electrolyte solutions [71]. This reaction was characterized in classical electrochemical reactors such as pump cells, packed bipolar cells and solid polymer electrolyte cells. In the last type of reactor, no bromide salt or electrolyte was used rather, the furan was oxidized directly at the anode. H owever, high consumption of the order of 5-9 kWh kg (at 8-20 V cell voltage) was needed to reach a current efficiency of 75%. 

FIG. 25 Typical DPSC data for the oxidation of 10 mM bromide to bromine (forward step upper solid curve) and the collection of electrogenerated Br2 (reverse step lower solid curve) at a 25 pm diameter disk UME in aqueous 0.5 M sulfuric acid, at a distance of 2.8 pm from the interface with DCE. The period of the initial (generation) potential step was 10 ms. The upper dashed line is the theoretical response for the forward step at the defined tip-interface separation, with a diffusion coefficient for Br of 1.8 x 10 cm s . The remaining dashed lines are the reverse transients for irreversible transfer of Br2 (diffusion coefficient 9.4 x 10 cm s ) with various interfacial first-order rate constants, k, marked on the plot. (Reprinted from Ref. 34. Copyright 1997 American Chemical Society.)... 

Holland, L.A. and Lunte, S.M., Postcolumn reaction detection with dualelectrode capillary electrophoresis-electrochemistry and electrogenerated bromine, Anal. Chem. 71, 407, 1999. 

The return, reverse peak is smaller than the forward peak if the CV of the Brj, Br couple is run with allyl alcohol in solution I pc a [bromine], where pc = peak, cathodic . The fact that Ipc is smaller than Ipo implies that some fraction of the electrogenerated bromine has been consumed by the chemical reaction between Br2 and allyl alcohol the CV thus contains evidence of an EC reaction. Figure 6.19 below shows the cyclic voltammogram for the EC reaction between bromine and allyl alcohol, while Table 6.5 lists a series of simple diagnostic tests for an EC reaction. 

Conversely, if v is slow (e.g. 10 mV s ) then the electrogenerated bromine stays in solution for quite some time before being re-reduced to bromide (so X is long). The scope for chemical reaction is seen to be greater at 10 mV s than at 500 mV s . We see therefore that the /pc//pa ratio will be less than 1 because some of the bromine has been removed from solution by the reaction with allyl alcohol. 

Above a critical scan rate, /pc//pa always has a value of 1 because v is so fast that there is no time for a chemical reaction to occur at all, and the electrogenerated bromine is retrieved quantitatively. Below another critical scan rate, /pc//pa is always 0 because v is so slow that all of the bromine has reacted. 

Between these two critical scan rates, I pc/I pa varies between 1 at the fast extreme to 0 at the slower extreme. Figure 6.20 show a plot of /pc//pa against the logarithm of the time-scale r. From this figure, we can again see that fractions of the electrogenerated bromine are retrieved as a function of the scan rate. 

From the following data, what is /(pseudo for the homogeneous reaction between electrogenerated bromine and styrene in DMF solution Assume that the potential differences, (Ep(,o ard> - Ex) and (Ep(back)-Ex) add up to 1.0 V. 

Note that the discussion here is much simplified, with a fuller treatment being given in reference [1].) The collection efficiency is No if no reaction occurs between the electrogenerated intermediate (such as bromine) and the material in solution. 

The formation of epoxides is synthetically a very important transformation. The indirect epoxidation of olefins (see Eq. 7) in the presence of electrogenerated chlorine (or bromine) [95] is a commercial process in which chlorine is recycled and not part of the product. The products such as propylene oxide are key intermediates in many further chemical processes. 

The use of electrogenerated bromine for the analytical determination of pharmaceuticals has been reported by Abdullin... 

Electrogenerated bromine can also be used as a mediator in catalytic electrolysis processes. The regioselective formation of epoxides is of high synthetic value and has been reported [131] (Eq. 10)... 

Miscellaneous Methods and Applications Involving Electrogenerated Bromine... 

Bromine has also been suggested for an indirect detection process for the determination of tetracyclines in pharmaceutical formulations [155]. A bromine/hydrogen peroxide-based electrogenerated chemiluminescence reaction is shown to be enhanced by tetracycline derivatives and detection levels down to the pg dm level are reported on the basis of this enhancement effect. 

Bromide and iodide ions are not only effective as redox catalyst in the indirect electrochemical formation of carbon-hetero atom bonds but also of hetero-hetero atom bonds. Thus, the nitrogen-sulfur bond in sulfenimides (Eq. (53) Table 4, No. 33) and sulfenimines (Eq. (54) Table 4, No. 34) can be generated. The electrogenerated active bromine species forms a positively polarized sulfenyl intermediate from disulfides. 

Seven phenothiazines have been titrated with electrogenerated bromine in the range 0.1-3 mg with an average error of less than 1% [43]. The assay utilizes the oxidation of the phenothiazine in two one-electron steps to a cation-free radical and sulfoxide, respectively. 

The uses of constant-current coulometry for the determination of drugs in biological fluids are few, basically due to sensitivity restriction. Monforte and Purdy [46] have reported an assay for two allylic barbituric acid derivatives, sodium seconal and sodium sandoptal, with electrogenerated bromine as the titrant and biamperometry for endpoint detection. Quantitative bromination required an excess of bromine hence back titration with standard arsenite was performed. The assay required the formation of a protein-free filtrate of serum with tungstic acid, extraction into chloroform, and sample cleanup by back extraction, followed by coulometric titration with electrogenerated bromine. The protein precipitation step resulted in losses of compound due to coprecipitation. The recoveries of sodium seconal and sodium sandoptal carried through the serum assay were approximately 81 and 88%, respectively. Samples in the concentration range 7.5-50 pg/mL serum were analyzed by this procedure. 

The activity of immobilized alcohol dehydrogenase was probed and modified by changing the local pH [133], The tip was positioned close to the enzyme film and used to increase the local pH by reducing water and producing hydroxide ions. A significant increase in enzyme activity was observed at higher pH. The opposite effect, that is, local inactivation of the immobilized enzyme (diaphorase) by chlorine or bromine species electrogenerated at the tip, also has been reported [134]. 

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