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After reading this article you will learn about the principles and Ficks’s law for measuring Oxygen Diffusion Rate (ODR).
Principles of Measuring Oxygen Diffusion Rate (ODR):
When a certain electric potential is applied between a platinum electrode inserted into the soil and a reference electrode, oxygen (O2) is reduced at the platinum electrode surface. The general reaction taking place at the platinum micro electrode surface in the reduction of O2 is in two steps involving two electrons in each step.
The reactions in two different media are:
An electric current flows between the two electrodes and is proportional to the rate of O2 reduction. The rate of O2 reduction is in turn related to the rate at which it diffuses to the electrode. The increase of current with potential continues until it is limited by the rate at which O2 can diffuse from the soil environment to the electrode.
So at the higher potentials, the reaction rate is limited by an extrinsic factor and the current becomes somewhat independent of the potential.
A second reaction starts at a potential of 0.7 or 0.8 volt. The second rise is caused by the reduction of ionic hydrogen to molecular hydrogen. Because the current in the plateau region is limited by the maximal rate at which O2 can diffuse to the platinum micro-electrode surface resulting difficulty in measuring the movement of O2.
Since the general procedure in using the platinum electrode to characterise soil O2 status consists of measuring a current which is proportional to the O2 diffusing to the electrode, it is necessary to consider diffusion theory in order to complete understand, what is actually being measured?
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Potential between 0.55 and 0.75 volt can be used for standard recommendation size of the electrode, however, is also an important factor and so 25 gauge electrode usually gives accurate measurement of ODR value over a wide range of soil moisture.
Theoretically, ODR increases with decreasing soil moisture. It has been also found that an increase in ODR as moisture decreased to a point beyond which ODR decreases with decreasing moisture content. This may be due to incomplete electrode wetting. Excess concentration of sulphur in the soil very often showed poisoning and that may be avoided by removing and reinstalling the electrode in the soil.
The current is related to the O2 flux, qa by the relation,
it× 10-6 = nF Aq0
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where, it = current in microamperes at time, t seconds,
(Oxygen diffusion rate) ODR =it × 10-6 × 60 × 32 × 106/4 × 96,500 × A µg cm-2 mm-1
= No. of electrons required to reduce one molecule of O2 (Æž = 4), F = Faraday constant (96,500 coulombs)
q0 = O2 flux at the electrode surface at time t (mol-1 cm-2),
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A = area of the electrode surface (cm2).
It is one of the criteria generally used to determine the O2 concentration in the soil pore space. It consisted of allowing the free diffusion of O2 into a diffusion chamber that was inserted into the soil. The calculated partial pressure of O2 at 10 min interval was used to evaluate the diffusion rate.
This determines the rate at which oxygen can be replenished if it is used by respiring plant roots or replaced by water. The ODR characterizes the soil O2 conditions.
There are various factors that can influence the ODR namely depth of the soil, temperature, moisture content, soil texture, etc. The ODR decreases with the depth of the soil indicating that with an increase in depth the concentration of O2 decreases.
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Temperature affects the ODR by increasing the rate of respiration and the diffusion co-efficient of O2 in water; the solubility of O2 in the liquid phase is decreased. The ODR decreases as the moisture content increases because of filling pore spaces with moisture resulting no spaces for gaseous exchange.
The ODR has some practical implications relationships between plant roots and soil, soil moisture and aeration status and biological activities in the soil etc. which ultimately influence the plant growth by affecting various physical and chemical properties of the soil. It has been found that the root growth ceased when the ODR dropped to about 20 g × 10-8/cm2/min.
Fick’s Law for Measuring Oxygen Diffusion Rate (ODR):
According to Fick’s law, diffusion is a function of the concentration gradient, the diffusion co-efficient of the medium, and the cross-sectional area
dQ = DA (dc/dx) dt
where dQ is the mass flow (moles) during the time at across area A (sq cm), dc/dx the concentration gradient [moles/c.c. (cm)], and D the proportionality constant or diffusion coefficient (sq cm/sec).
D depends upon the property of the medium as well as the gas. It varies directly with the square of the absolute temperature and inversely with the total pressure.
The diffusion coefficient of O2 is about 1.25 times that of CO2. The rate of diffusion of CO2 and O2 in air is nearly 10,000 times greater than in water. The greater solubility of CO2 in water increases its concentration gradient and will be transferred in water at a higher rate than O2.
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