Siemens ET 78X Series Manual de usuario descargar pdf (Pagina 6)

Technical description

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Siemens PA 20 · 1999

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Measuring procedure for conductivity measurements

The conductivity of liquids is based on the electrolytic

dissociation of acids, bases or salts in water into electrically

charged particles (ions).

The magnitude of the electrical conductivity κ is the reciprocal

value of the electric resistance of the solution.

The dimension for electric conductivity is S/m (Siemens per

meter).

A differentiation is made between strong and weak electrolytes

depending on the degree of dissociation.

Strong electrolytes are dissociated into ions completely or at

least by more than 80% when dissolved in water. These

electrolytes include many salts (NaCl, KCl), strong bases

(KOH, NaOH) and strong acids (HCl, HNO

, HClO

Weak electrolytes (H

,CH

COOH, H

S,NH

OH) are only

dissociated to a small percentage. Pure water is also a weak

electrolyte with a basic conductivity of 0.048 µS/cm, resulting

from the selfĆdissociation of the H

O molecules

O+H

O↔H

+OH

In general the electrolytic dissociation of a dissolved material

increases with the dilution of the solution, the magnitude of the

dielectric constant of the solvent and the temperature of the

solution.

The conductivity of a dilute electrolyte solution depends on:

 The number of ions in the solution, i.e. its concentration

 The number of unit charges capable of being transported by

each ion, i.e. the ion charge number

 The migration velocity or mobility of the ions.

The conductivity of an electrolyte is a linear function of the

concentration at a constant temperature since the valency -

and also the ion mobility in dilute aqueous solutions - remain

constant.

Examples of the dependence of conductivity on the

concentration are shown in Fig. 1/5 and Fig. 1/6.

The total conductivity of a solution is the sum of the

conductivities of all ion pairs present in the solution and is thus

usually a nonĆspecific variable..

Salt content mg NaCl/l

Conductivity

µS/cm

Fig. 1/5 Dependence of conductivity of water on the NaCl content

Concentration

Conductivity

Fig. 1/6 Conductivity of dilute solutions at 18 ºC

1 2 3 4 5 6 7 8 9 10 11 ... 37 38

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Siemens ET 78X Series Manual de usuario Pagina 6

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