Conductivity (electrolytic)

Electrolytic conductivity is a measure of the electrical conductivity in electrolyte solutions.

Introduction

In an electrolyte ions is preferably move when an electric field into or against the field direction, thereby causing a charge transport and current flow. Accordingly, a dependence of the current on the following factors are expected to:

Theoretical Background

For the electricity transport within the solution, the empirically well confirmed Ohm's law applies:

Here are the geometrical factors and the contributions of the above last three points in the member, the conduction or reciprocal resistance, combined.

However, the Ohm's law is to determine the conductivity of the electrolyte only in accordance with certain precautions applicable. Applying a voltage of the connection is required to a external circuit, two metallic conductor (electrode ). As a result of current flow through the interface electrolyte / electrode now play at this interface from certain reactions which have the appearance of additional stresses result. We call this process as polarization. This undesirable effect can be avoided as follows:

The resistance of any conductor depends on two parameters: the resistivity (or conductivity) and a geometric factor. When this factor electrolyte cell constant is called. We then have:

At a uniform current-carrying, cylindrical conductor, whereby the length and the cross section of the conductor are.

In the general case performs a calibration of the measuring cell by measuring the resistance of a solution of known and it gets the cell constant in a simple way.

Ion transport

The ion transport in the electrolyte solution occurs in the following manner: an ion with the charge in the electric field experiences a force:

And is consequently accelerated the move. Due to the speed-proportional hydrodynamic friction force

Does this accelerated motion but after a very short start-up time ( s ) in a movement with stationary drift velocity over so that is. This results in:

The object relating to the field strength of 1 V / m velocity is called the mobility of the ion.

M / (Vs)

From the migration rates of the electrical conductivity are obtained according to the following consideration: A solution with cations and anions per the valences or are in a tube of length and cross-section. When a voltage at both ends of the tube in the interior to the effect that there is a field strength of the cations and anions in the opposite direction by the velocities and move. Due to the cross-section then pass per second cations and anions in the opposite direction, \ ie there are a total of

Charge units per second carried by the cross-section. However, this is equivalent to the current flowing through the electrolyte current. With the equations is obtained:

Since also apply according to Ohm's law:

Results: and

Thus, the specific conductivity is uniquely determined by concentration, quality and rate of migration of the individual ions.

Molar values

A conversion of molar quantities of the connection and taking into account the insertion of the electrochemical value obtained by the number of positive and negative ions in a molecule is dissociated; = Concentration in.

After Kohlrausch we group sizes (1 eq = 1 mol / z) in / val usually together, calling them equivalente ion mobilities. Kohlrausch also introduced the concept of the equivalent conductivity into val, the equivalent concentration.

According to the above equations, the equivalent conductivity is additively composed of the ion mobility and is independent of the ion concentration. In reality, this is only true for infinite dilution; at higher concentrations always observed a decrease of, which is due to the influence of the Dissoziationsgewichts and to the influence of inter -ionic interaction forces.

The influence of the dissociation equilibrium

Incomplete dissociation depends on the degree of dissociation:

Since at infinite dilution has its maximum value () and with increasing concentration is getting smaller, a decrease of the equivalent conductivity is readily understandable, what was also recognized early ( ostwald cal dilution law ).

The influence of the inter -ionic interaction forces

The motion of the ions does not take place freely. Rather, occurs as a result of far-reaching electrostatic forces a mutual obstruction of the migrating ions. An ion is surrounded as a result of its electrostatic effect in the middle of more opposite than similarly charged particles. This " ion cloud " clenches with increasing concentration, more and more together and has the following effects:

Both effects increase with the concentration.

The built on this model theory of Debye, Hückel and Onsager supplies for small concentrations the expression:

This result was found much earlier by Kohlrausch experimentally.