Oxygen polarography

M. J. Kaufman, Applications of oxygen polarography to drug stability testing and formulation development Solution-phase oxidation of hydroxymethylglutaryl coenzyme A (HMG-CoA) reductase inhibitors, Pharm. Res. 7, 289-292(1990). 

With sodium thiosulphate, (ti) 395 with Clark cell, (amp) 639 Oxygen, dissolved effect in polarography, 603, 618... 

Diffusion Currents. Half-wave Potentials. Characteristics of the DME. Quantitative Analysis. Modes of Operation Used in Polarography. The Dissolved Oxygen Electrode and Biochemical Enzyme Sensors. Amperometric Titrations. Applications of Polarography and Amperometric Titrations. 

An amperometric technique relies on the current passing through a polarizable electrode. The magnitude of the current is in direct proportion to the concentration of the electroanalyte, with the most common amperometric techniques being polarography and voltammetry. The apparatus needed for amperometric measurement tends to be more expensive than those used for potentiometric measurements alone. It should also be noted that amperometric measurements can be overly sensitive to impurities such as gaseous oxygen dissolved in the solution, and to capacitance effects at the electrode. Nevertheless, amperometry is a much more versatile tool than potentiometry. 

To appreciate that to minimize non-faradaic currents, the polarography solution should be purged of oxygen and contain a surfactant depolarizer in low concentration. 

The product of the one-electron reduction of O2, the superoxide ion, Oi, is highly unstable in acidic-aqueous solutions where its protonated form, the peroxyl radical H02(pA = 4.8), decomposes to ozone, oxygen, and hydrogen peroxide experiments have shown that the two latter compounds are produced almost quantitatively, when only traces of ozone are found. In alkaline solutions, the superoxide ion is more stable even if it decomposes spontaneously to O2 and H02 (AG-6-=—51.13 kj mol ), it has been studied by polarography in NaOH solutions, in the presence of compounds that adsorb at the surface of the electrode and slow down the protonation of 02 . From these electrochemical experiments... 

Figure 19.3—Polarographic cell and diffusion current. Dissolved oxygen, which leads to an interfering double wave, has to be removed from the sample solution by degassing. On the right features of the diffusion current are shown. These increase with time for every drop of mercury in a static (unstirred) solution. Direct polarography is a slow method of analysis. More than 100 droplets are needed to record the voltammogram.

GLC, atomic absorption and mass spectrophotometry, enzymatic, and specific colorimetric procedures seem to be the likely candidates for routine use in the future. Automation will certainly be common. GLC is now used to detect imitation muscat wines (127). Characteristic flavor byproducts of yeasts may be detected and measured. Multiple correlation of the amounts of the more influential major and minor constituents with wine quality is the goal of such research. A simple apparatus for the simultaneous determination of the redox potential (two platinum electrodes), pH, specific conductivity, oxygen, and carbon dioxide (ion-specific electrode) has been devised (128). Molecular oxygen in wines has been determined by several procedures—polarography (129) and GLC being the latest. 

Other traditional methods available for monitoring the extent of lipid oxidation include the Anisidine value, the Kreis test (Mehlenbacher, 1960), methods based on the carbonyl content of oxidized fats (Henick et al., 1954 Lillard and Day, 1961), and measurement of oxygen uptake either by manometry or polarography (Tappel, 1955 Hamilton and Tappel, 1963). 

Diffusion currents. Half-wave potentials. Characteristics of the DME. Quantitative analysis. Modes of operation used in polarography. The dissolved oxygen electrode and biochemical enzyme sensors. Amperometric titrations. Applications of polarography and ampero-metric titrations. 

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