Question

The dissolved oxygen in test solution shows interferring nuisance in polarographic analysis due to reactions of____​

Answer 1

Answer:

The measurement of dissolved oxygen is one of the most frequently used and the most important of all chemical methods available for the investigation of the aquatic environment. Dissolved oxygen provides valuable information about the biological and biochemical reactions going on in waters; it is a measure of one of the important environmental factors affecting aquatic life and of the capacity of water to receive organic matter without causing nuisance. Oxygen gas dissolves freely in fresh waters. Oxygen may be added to the water from the atmosphere or as a by-product of photosynthesis from aquatic plants and is utilized by many respiratory biochemical, as well as by inorganic chemical, reactions. The concentration of dissolved oxygen in water depends also on temperature, pressure, and concentrations of various ions [cf., Hutchinson (1957), Wetzel (1999)]. To be successful, a method for measuring dissolved oxygen must meet two requirements. First, owing to the small amount of substance to be determined (a few mg/l), it must be exact; second, it must be done with apparatus suited for field operation. The method least subject to chemical errors, and probably the first to be proposed, is that of Bunsen, in which the gases are boiled out under either atmospheric pressure or diminished pressure. The amount of gas collected then is measured by absorption methods. However, the Bunsen method is too cumbersome for field work and requires considerable skill for accurate manipulation. A few colorimetric methods have been proposed, but most have been found to be quite inaccurate, particularly at low concentrations. Although a number of chemical methods have been employed for dissolved oxygen measurement, the Winkler method, or some modification of it, is the most frequently used in limnology. In recent times, major advances have occurred in the development and application of oxygen-sensitive electrodes for the rapid and sensitive measurement of dissolved oxygen. The Clark-type Polarographic oxygen sensors often consist of platinum anode and a gold-plated cathode, encased in an electrolyte-filled housing and separated from the water by an oxygen-permeable membrane. Oxygen must diffuse through the membrane and electrolytic solution to the electrodes. The quantity of oxygen reduced per unit time is directly proportional to the oxygen concentration in the water, and the resulting electrical current is measured with a meter [cf., Gnaiger and Forstner (1983)]. Oxygen electrodes have the advantages of speed of measurement and the potential for continuous measurement in remote places. Commerically available macroelectrodes (ca. 1–5-mm diameter) require a rapid flow of water across the membrane; without such exchange, measurements are inaccurate and unreliable. Simple up and down movements of the electrodes in the water are insufficient to provide the conditions necessary for accurate measurements. Nearly all macroelectrodes are unreliable at dissolved oxygen concentrations between 0 and 1 mg/1. This low range is critical for many major chemical transformations and dissociation reactions, as well as crucial for microbial metabolism. Macroelectrodes are not satisfactory for studies of oxygen distribution near interfaces, particularly in zones of steep oxygen gradients, as at the sediment-water interface. Problems of slow diffusion rates and inaccuracy at low oxygen concentrations are circumvented to a significant extent with oxygen microelectrodes [less than 100-μm diameter; cf., Revsbech and Jorgensen (1986)]. The advantages of small size, where sensing surfaces are only a few micrometers in diameter, and rapid response times are counterbalanced by the difficulty of construction and fragility. Nonetheless, microelectrodes are providing unprecedented understanding about the distribution and dynamics of oxygen microgradients and about the ecology of the organisms generating these gradients [e.g., Carlton and Wetzel (1987, 1988)]. Recently fiber-optic oxygen microsensors based on chemical quenching of luminescence allow measurements with great stability without chemical consumption of oxygen and no need for flow about the sensing tip (Klimant et al., 1995). These optical electrodes circumvent many of the problems associated with the electrochemical oxygen sensors. Replicated calibration of oxygen sensors by chemical methods of analysis is required with solutions containing known quantities of dissolved oxygen; calibration in air is not satisfactory. Thus, although oxygen sensors are being improved constantly and will dominate measurements of dissolved oxygen in the future, the need still exists for chemical methods of measuring dissolved oxygen.

Answer 2

Concept: At an applied voltage, chemical species (ions or molecules) undergo oxidation (loss of electrons) or reduction (gain of electrons) at the surface of a dropping mercury electrode (DME). Only the DME is affected by polarography.

Answer: Polarography is a technique for detecting and determining electroactive chemicals that is extremely sensitive. At ambient temperature and pressure, oxygen can dissolve in aqueous solutions to the point of generating a one millimolar solution, and the polarographic reduction of oxygen hinders the polarographic determination of other electroactive compounds. The electrochemical reactivity of oxygen is advantageous for those who need to determine dissolved oxygen in a range of media (natural waters, physiological fluids, sewage, and so on), but not for those who need to electroanalyze compounds other than oxygen. Because oxygen can be found in large amounts in solutions and is electroactive, it must be eliminated from the analyte before polarographic analysis.

Problems emerge from both the voltammetric behaviour of oxygen and the chemical reactions that occur as a result.

At the lowering mercury electrode, oxygen is decreased in two steps. The reduction of oxygen to hydrogen peroxide and/or hydroxide ion is the initial step.

$O_{2} + 2H^{+} +2e^{-}$ →$H_{2} O_{2}$ (acid media)

$O_{2} + 2H_{2}O +2e^{-}$ → $H_{2} O_{2} + 2OH^{-}$

The reduction of oxygen to hydroxide ion or water is the second stage.

$O_2 + 4H^+ + 2e^-$ → $2H_2O$

$O_2 + 2H_2O + 4e^-$ → $4OH^-$

It is clear that if oxygen is not eliminated, two types of difficulties can occur. Because hydrogen peroxide can serve as both an oxidising and a reducing agent, it can have harmful effects on other electroactive species in solution.

Hence, the dissolved oxygen in test solution shows interferring nuisance in polarographic analysis due to reactions of hydrogen and oxygen.

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