EUTECH INSTRUMENTS CYBERCOMM 5000 FOR CYBERSCAN 5000 Manuál s instrukcemi stáhnout pdf (Strana 169)

Instruction Manual CyberScan PC5500/ 5000

164

17.6 Conductivity Theory

Conductance is a quantity associated with the ability o primarily

aqueous solutions to carry an electrical current, I, between two

metallic electrodes when a voltage E is connected to them. Though

water itself is a rather poor conductor of electricity, the presence of

ions in the water increases its conductance considerably, the current

bring carried by the migration of the dissolved ions. This is a clear

distinction from the conduction of current through metal, which results

from electron transport.

The conductance of a solution is proportional to and a good, though

non-specific indicator of the concentration of ionic species present, as

well as their charge and mobility. It is intuitive that higher

concentrations of ions in a liquid will conduct more current.

Conductance derives from Ohms law, E= IR, and is defined as the

reciprocal of the electrical resistance of a solution.

C= 1/ R

C is conductance (siemens)

R is resistance (ohms)

One can combine Ohms law with the definition of conductance, and

the resulting relationship is:

C= I/ E

I is current (amps)

E is potential (volts)

In practice, conductivity measurements involve determining the

current through a small portion of solution between two parallel

electrode plates when an ac voltage is applied. Conductivity values

are related to the conductance (and thus the resistance) of a solution

by the physical dimensions- area and length- or the cell constant of

the measuring electrode. If the dimensions of the electrodes are such

that the area of the parallel plates is very large, it is reasonable that

more ions can reside between the plates, and more current can be

measured. The physical distance between the plates is also critical,

as it effects the strength of the electric field between the plates. If the

plates are close and the electric field is strong, ions will reach the

plates more quickly than if the plates are far apart and the electric

field is weak. By using cells with defined plate areas and separation

distances, it is possible to standardize or specify conductance

measurements.

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EUTECH INSTRUMENTS CYBERCOMM 5000 FOR CYBERSCAN 5000 Manuál s instrukcemi Strana 169