Feasibility of Performing Concurrent Coulometric Titrations Using a Multicompartment Electrolysis Cell.

Feasibility of performing multiple coulometric titrations in a single course of electrolysis is presented. In these titrations, three pairs of cathode and anode compartments were connected with a network of electrodes and salt bridges. Passage of current through the cell caused concurrent electrolysis in cathode and anode compartments. Electrogenerated reagents produced in these compartments were used as titrants for quantifying the analyte samples. Endpoints of the titrations were determined from the visual color change of an indicator. The charge passing through the cell was monitored and Faraday's laws of electrolysis were applied to assess the quantitative relation between the charge and analyte concentration. Experimentally determined coulombs required to titrate aqueous potassium hydrogen phthalate, MnO4 -, OH-, and S2O3 2- were 0.100, 0.466, 0.103, and 0.0934 C, respectively. These results matched with estimated values of 0.0965, 0.482, 0.0965, and 0.0965 C, respectively. Agreement between the coulombs determined from experimental results and reaction stoichiometry suggests a feasible application of concurrent coulometric titrations. Efficacy of the method was tested for determining the active ingredients in household vinegar and vitamin C dietary supplement tablets. Quantities of acetic acid and ascorbic acid in these products were 5.1% and 980 mg, respectively, agreeing with the quantities determined from volumetric titrations (5.1% and 990 mg) and manufacturer's label (5.0% and 1000 mg).

Feasibility of Using an Electrolysis Cell for Quantification of the Electrolytic Products of Water from Gravimetric Measurement.

A gravimetric method for the quantitative assessment of the products of electrolysis of water is presented. In this approach, the electrolysis cell was directly powered by 9 V batteries. Prior to electrolysis, a known amount of potassium hydrogen phthalate (KHP) was added to the cathode compartment, and an excess amount of KHCO3 was added to the anode compartment electrolyte. During electrolysis, cathode and anode compartments produced OH-(aq) and H+(aq) ions, respectively. Electrolytically produced OH-(aq) neutralized the KHP, and the completion of this neutralization was detected by a visual indicator color change. Electrolytically produced H+(aq) reacted with HCO3-(aq) liberating CO2(g) from the anode compartment. Concurrent liberation of H2(g) and O2(g) at the cathode and anode, respectively, resulted in a decrease in the mass of the cell. Mass of the electrolysis cell was monitored. Liberation of CO2(g) resulted in a pronounced effect of a decrease in mass. Experimentally determined decrease in mass (53.7 g/Faraday) agreed with that predicted from Faraday's laws of electrolysis (53.0 g/Faraday). The efficacy of the cell was tested to quantify the acid content in household vinegar samples. Accurate results were obtained for vinegar analysis with a precision better than 5% in most cases. The cell offers the advantages of coulometric method and additionally simplifies the circuitry by eliminating the use of a constant current power source or a coulometer.

Measurements of electron-transfer rates of charge-storage molecular monolayers on Si(100). Toward hybrid molecular/semiconductor information storage devices.

Redox kinetics were measured for two electroactive molecules attached to Si(100) surfaces, a ferrocene (Fc-BzOH) and a Zn(II) trimesitylporphyrin (Por-BzOH). Each molecule was derivatized with a benzyl alcohol linker for attachment to the Si surface via the formation of a Si-O bond. A complete protocol was developed for the preparation of stable Si(100) surfaces derivatized with the electroactive molecules. The redox-kinetic measurements were performed on the resulting Fc-BzOH and Por-BzOH monolayers to probe (1) the rate of electron transfer (k0) for oxidation in the presence of applied potentials and (2) the rate of charge dissipation after the applied potential is disconnected (in the form of a charge-retention half-life t1/2). The k0 values for the two types of monolayers were found to be similar to one another as were the t1/2 values. Perhaps more importantly, the electron-transfer rates for both the Fc-BzOH and the Por-BzOH monolayers differ from the charge-dissipation rates by approximately 6 orders of magnitude and are strongly dependent on the surface concentration of the electroactive species. For the Por-BzOH monolayers on Si(100), the k0 and t1/2 values and their trends as a function of surface coverage were determined to be similar to those previously measured for the analogous thiol-derivatized molecule assembled on Au(111). In contrast, the Fc-BzOH monolayers on Si(100) were found to exhibit much slower electron-transfer and charge-dissipation rates than those in the corresponding thiol-Au(111) case. Two alternative hypotheses are advanced to explain both the diminution in rates with increased surface coverage and the contrasting behavior with the analogous thiols on Au, one based on space-charge effects at the monolayer-solution interface, and a second relying on changes in distance of the redox centers from the surface as modulated by the orientation of the linking chains. Collectively, the ability to prepare and study stable, electroactive molecular media on Si(100) is likely to be key in the development of hybrid molecular/semiconductor devices.