Conductividad

Electrochemical Analysis LaboratoryDepartment of Chemical Metrology and Biometry

Calibration and Maintenance of Conductivity Meter

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Outline

What is Conductivity?

How is Conductivity Measured?

What is Conductive solution?

Conductivity Meter

Traceability

Standard method

Rules for reliable measurement

Calibration


3What is Conductivity?Conductivity is the ability of a solution to pass an

electric current. The current is carried by cations andanions.

RG

1

The electrical conductance(G, Siemens) is thereciprocal of the resistance(R, Ohm).


4What is Conductivity?

A

lK

K is the cell constant (m-1)

l

A

l

AG

1

A is the electrode area (m2)l is the electrode gap (m) is the specific conductivity (S/m) is the specific resistance (m)


5What is Conductivity?

Solution conducts electricity depends on

Concentration

Mobility of ions

Valence of ions

Temperature


6How is Conductivity Measured?

Conductivity may be measured by applying an

alternating electrical current (I) to two electrodes

immersed in a solution and measuring the resulting

voltage (V).


7What is Conductive solution?

Conductivity is measured in aqueous solutions of

electrolytes.

Electrolytes are substances containing ions or

compounds that ionise in solution.

Strong electrolytes: fully ionised in solution

Weak electrolytes: not fully ionised in solution


8What is Conductive solution?

nCC

is molar conductance (S cm2 mol-1) is conductivity (S/m)C is concentrationn is charge of ion

is equivalent conductivity of ion


9What is Conductive solution?Equivalent conductivity of various ions at 25 oC


10

Conductivity value of solution


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What is Conductive solution?

At the low electrolytic conductivity, the

relationship between conductivity and concentration is

linear.


12

How is Conductivity Measured?

The cell constant (K) cannot be determined by

measuring the electrode gap and area.

The cell constant is determined using a calibration

solution with known conductivity (usually 0.01 mol/l

KCl solution)


13

Conductivity Meter


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Conductivity Meter

The conductivity

meter uses the conductance

and cell constant to display

the conductivity

Conductivity = cell constant x conductance


15

Conductivity Cell

2 pole cell

3 pole cell

4 pole cell

Platinised cell

Flow through cell


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Conductivity Cell

2 pole cell : universal used, interfere with

polarization

3 pole cell: better reproducibility


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Conductivity Cell

4 pole cell: should measure with outer tube

to minimize volume effect


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Conductivity Cell

Platinised cell: coated electrode

with platinum black to increase

surface area and decrease

polarization effect.


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Conductivity Cell

Flow through cell: designed for flow

system or small volume sample,

suitable for measurement of

conductivity of pure water (avoid

carbon dioxide)


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Storage

Before storing the cell, rinse with deionised water

Short-term : deionised water

Long-term : deionised water or store dry

After long-term storage, condition the cell for 8 hours in

deionised water before use.


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Selection of Conductivity Meter

Factor affecting measurement accuracy

Formats and units of reports results

Suitability for the measuring environment

features assisting ease of use


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Selection of Conductivity Cell

Advantages Disadvantages

- Easy to maintain - Field effect cell must be positioned in the middle of measuring vessel

- Economic - Polarization in high conductivity sample

- Recommended for viscous sample with suspension

2 pole cell


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Selection of Conductivity Cell

Advantages Disadvantages

- Linear over a very large conductivity range

- Unsuitable for micro samples

- Calibration and measurement in different range

- Ideal for high conductivity measurement

4 pole cell


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25

Conductivity Measurement

The accuracy of conductivity measurements can be influenced by

Polarization

Geometry

Temperature


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Polarization

Polarization occurs on the electrode surface, which leads to errors on result.


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Preventing polarization

increasing the electrode area: coating platinum black on platinum electrode

Selecting 4 pole conductivity cell


28


Geometry related errors field effect

Field linesThese field lines can effect the

measurement if something

interferes with the field lines

such as the sample vessel

wall.

This effect can be minimized

by using 4-pole conductivity

cell


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Temperature

Conductivity measurements are the temperature

dependent, varying from 1.5 to 5.5 %/oC

Temperature increasing - conductivity increasing

Conductivity meter measures the actual

conductivity and temperature and then converts

it to reference temperature using temperature

correction function


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Temperature Temperature correction function

Linear function

Non-linear function

User-Definable temperature compensation


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Temperature correction function

Linear function


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Determination of the temperature coefficient

The temperature coefficients of some electrolytes


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Temperature correction function

Non-linear function

Non-linear

compensation data for

variety of solutions is

widely available such

as ISO 7888-1985

(natural waters)


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Traceability = G K

Primary cell(defined dimension)

Primary reference solution

Secondary cell

Secondary reference solution

Electrolytic conductivity meter


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Standard method for Conductivity Measurement of Water

ASTM D1195 95 (Reapproved 1999) Standard

Test Methods for Electrical Conductivity and

Resistivity of Water

ISO 7888 1985 Water quality Determination of

electrolytic conductivity

OIML R 68 Calibration method for conductivity

cells


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Rules for reliable measurements

Calibrate frequently: cell constant is an important

factor for conductivity measurement. Calibration

interval for conductivity cell will depend on the

application, sample and operating condition.

Temperature and stirring conditions: For accurate

measurement, recommended to use thermostat bath


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Position of conductivity cell: All the pole of

conductivity cell are covered by the sample and stand

at the center of sample vessel.

Low conductivity measurement:

use a flow through cell, to avoid CO2 from air

use non-platinised and low cell constant (< 1 cm-1)


38


High conductivity measurement:

use platinised cells to prevent polarization error

use high cell constant (> 1 cm-1)

do not dilute sample (conductivity is not

proportional to concentration at high levels)


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General Condition

The conductivity of an electrolytic solution may be determined from the measured resistance of the cell, when it contains the solution, and the constant K of the cell

The value of the cell constant K depends onthe construction of the cellthe temperature and concentration of the

solutionthe measuring frequencyetc.


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Preparation for Calibration

check the rigidity of the fixing of the electrode and

the state of their surface

the cell shall be degreased with the appropriate

solvent according to the materials of which the cell

and electrode are made and then wash with distilled

water

the standard solution shall be prepared in quantity

sufficient for calibration


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Calibration

the cell shall be washed with standard solution at

least 5 times

the solution of the last washing shall be left in the

cell for 10 minutes

the cell containing the solution shall be brought in

thermostat bath at 25 0C and maintain at thermal

equilibrium


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Calibration

the cell shall be washed with standard solution at

least 5 times

the solution of the last washing shall be left in

the cell for 10 minutes

the cell containing the solution shall be brought

in thermostat bath at 25 0C and maintain at thermal

equilibrium


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