Showing papers on "Polarography published in 1972"

Electrochemical Studies of Zinc Tetraphenylporphin

Abstract: The reduction of zinc (II) tetraphenylporphin (ZnTPP) in dimethylformamide (DMF) was investigated. The products of electron transfer and associated chemical reactions were studied by polarography, cyclic voltammetry, thin‐layer controlled potential coulometry and spectroscopy, and epr. Reduction proceeds initially as a series of two reversible one‐electron steps. After the second and subsequent steps, however, proton abstraction from the solvent medium occurs which, in turn, determines the reaction path. Characteristics of the porphyrin ring reactions are studied and compared with those of porphyrin free bases.

Adsorption of single-stranded and double-helical polynucleotides on the mercury electrode.

Abstract: The adsorption of single-stranded polynucleotides and double-helical DNA on the dropping mercury electrode has been studied with the aid of Breyer's alternating current (a.c.) polarography. Our results indicate that all three constituents of polynucleotides (residues of bases, sugar, and phosphoric acid) are involved in the adsorption. At neutral pH their participation in adsorption depends on the ionic strength, the potential of the electrode, and the conformation of the polynucleotide in the solution. At an ionic strength of about 0.1, double-helical DNA is adsorbed electrostatically on a positively charged electrode surface by inadequately masked negative charges of the phosphate groups. At a higher ionic srength (about 0.5), this electrostatic adsorption is no longer detectable by using a.c. polarography; under these conditions it is probable that native DNA is adsorbed around the potential of the electrocapillary maximum with the aid of sugar residues and a few bases. Single-stranded polynucleotides, on the other hand, are primarily adsorbed by means of the bases. Desorption of double-helical DNA occurs around a potential of −1.2 V against SCE. At this potential, the helical regions of single-stranded polynucleotides are also desorbed. Desorption of the disordered regions of single-stranded polynucleotides occurs at more negative potentials. Adsorption and desorption of a small number of bases released from double-helical DNA was evident in the a.c. polarograms only at elevated temperature, or at room temperature after degradation of DNA by sonication.

The Mechanism of the Zinc(II)‐Zinc Amalgam Electrode Reaction in Alkaline Media As Studied by Chronocoulometric and Voltammetric Techniques

Abstract: The reduction of zinc (II) and the oxidation of zinc amalgam was studied in 0.18-4.0M KOH solutions. The pr imary electrochemical technique employed was potential step chronocoulometry with data acquisition on a small digital computer. The kinetic parameters were obtained by fitting the exper imental charget ime curves to the complete theoretical equation for a stepwise elect ron t ransfer mechanism. The sytem was also studied by d-c and a-c polarography and l inear scan voltammetry. The results of this study were compared to those of previous workers and shown to be consistent with a mechanism based on stepwise electron t ransfer Zn(OH)42~ Zn(OH)~ + 2 O H Zn (OH) 2 + e ~--Zn (OH)=Z n ( O H ) 2 ~Zn(OH) + O H Zn(OH) ~u Hg -te ~=~Zn(Hg) -b O H One of the most significant features of the study of electrochemical kinetics during the last 20 years has been the evolution of theoretical and exper imental procedures which are useful in uncovering the different stages in the complex series of processes which comprise an electrode reaction. An investigation of the kinetics of an electrochemical reaction would ideally lead to the elucidation of the na ture of the various steps and a determinat ion of their rate constants, the identification of the intermediates and products, and the determinat ion of the adsorption isotherm for all adsorbed species. Usually, because of exper imental and theoretical limitations, only par t of the informat ion is accessible for a given system. Although characterization of intermediates is of prime importance, direct observation of these may not be possible if their concentrat ions are too low or their lifetimes are too short. However, analysis of the cur ren t -poten t ia l relat ionship obtained by several nonsteady-sta te methods can yield fur ther informat ion on complex mechanisms and faster processes. Of part icular interest in this work is the study of electrode processes involving more than one electron t ransfer step (1). Vetter (2-4), Hurd (5), and Mohilner (6) have calculated the steady-state polarization curves for a set of two or more consecutive single elect ron transfer steps. Some reactions for which consecutive electron transfer mechanisms have been proposed on the basis of steady s ta te-measurements are T I ( I I I ) / TI(I) (7) and qu inone /hydroquinone (8). The theory of consecutive electron transfer mechanisms for electrochemical techniques involving potential steps, current steps, and a-c polarography has also been given (9-15). The establishment of the mechanisms of an electrode reaction often requires acquiring large amounts of accurate exper imental data and fitting this data to complicated theoretical equations. Digital data acquisit ion systems (16-21), especially those based on general * Elect rochemical Society Act ive Member . 1 Presen t address : Tennessee E as tman Company, KingsDort , Tennessee 37660.