Interactive numerical educational resourcesfor the analysis of complex media
Hello, This video will show how to determine the conductivity of a solution
and at the same time how to use an electrical conductivity meter
To carry out this experiment, we will need
an electrical conductivity meter with its instruction manual
a conductivity cell connected to the electrical conductivity meter
a magnetic stirrer with a magnetic bar (if we want to homogenize the solution)
a potassium chloride solution at 0.1 mol/L that will be our calibrating solution
and of a solution to analyse (here we will choose tap water)
distilled water
and a thermometer to check the room temperature
The conductivity cell is made of two plates of platinized platinum
between which the solution will be placed
In this case, we will study the mobility of ions between the two plates
In order to know how to carry out a conductometry measurement
we will first try to understand the principle behind the measurement
For conductometry
the conductivity cell will determine the inverse of the resistivity of the solution between the two plates
The inverse of this resistance is called conductance
written G and expressed in Siemens, which correspond to Ohms-1
This conductance is related to the conductivity (written sigma) that we wish to determine
by the correlation factor k expressed in cm-1
The conductivity is expressed in Siemens per centimetre
therefore the relation between sigma and G is:
Sigma equals k times G
The objective is first to determine the cell constant k
and therefore to carry out a calibration of the electrical conductivity meter
To calibrate the electrical conductivity meter
we have to use a solution whose conductivity is known at several temperatures
We generally use a solution of potassium chloride at 0.1mol/L
and a thermometer to record the temperature
because this one can have an important influence on the conductivity value
If we look at the room temperature, we are at 21°C
I n order to know the conductivity value of the KCl solution at 0.1 mol/L at 21°C
we generally use the conductometer manual
but one can also look in the scientific literature
In this manual
the conductivity value of the KCl solution at 0.1 mol/L at 21°C
is fixed at 11.97 milliSiemens per centimetre
We will therefore try and find this value of 11.97 mS/cm on the screen of the electrical conductivity meter
for our calibration solution of KCl
To carry out this first step of calibration
we will activate the calibration mode of the conductometer by pressing the CELL button
It indicates the initial value for the cell constant, which is 0.930 cm-1
After pressing a second time on the CELL button
the electrical conductivity meter indicates to dip the cell in the calibrating solution
which we are going to do
We take the cell, and remove the cap filled with distilled water
We will first cautiously dry the cell
take the calibrating KCl solution at 0.1 mol/L
and then dip the cell in the KCl solution
Once this is done
the cell gives a conductivity value
here of 11.40 mS/cm
Our goal will be now to change the cell constant k
in order to find the value of 11.97 mS/cm
The conductivity value of a KCl solution at 0.1 mol/L at 21°C
This is what we will do here
We change the value of the constant k
and when doing a conductometry measurement
we must thoroughly wait for the value to be stable
Once the value is stabilized, here at 11.97 mS/cm
we can read the value for the constant k,here it is 0.937 cm-1
I can confirm this value for the cell constant
And before measuring with the solution to analyse
I will take the cell out of the calibration solution
dry it
and rinse it with distilled water
This calibration step will allow us to measure absolute values for the conductivity
It is not always necessary to calibrate
in particular when monitoring conductivity over time for a kinetic experiment
or when monitoring conductivity against a volume of base or acid added during an acid-base titration
However, this calibration step is interesting
in order to determine the cell constant
hich will inform us on the potential degradation of the cell
Indeed, the cell constant values are usually situated around the unit
If the value we get after a calibration is below 0.8 or over 1.2
it means the cell has been damaged and must be changed
We will now do the measurements on the solution to analyse: tap water
First we will put some of the tap water in another beaker
and rinse the conductivity cell with it
in order to remove impurities that may be present on the cell
and also any residual distilled water that could false the measurement
Then the solution to analyse is placed under the conductivity cell
the cell is dipped in the solution
and we can read the value
here we have 0.41 mS/cm
The last step is to remove the cell from the solution
to rinse it with distilled water
and place the cap with distilled water back on
It is important for the cell to be kept in aqueous environment
this is why distilled water is poured in the cap
The tap water we analyse here is not pure
considering that there are ionic species beside the water molecules
This has been demonstrated though the conductivity measurement we carried out
The value of 0.41 mS/cm
is largely superior to the value we would expect for pure water
Indeed for pure water
containing only very small amounts of H3O+ or HO- ions
10-7 mol/L
we would expect a conductivity of 0.055 microSiemens/cm
We are far from this value
which proves there are ionic species in solution
As we saw before, temperature influences a lot the value of conductivity
For chemical reactions where there can be a change in temperature
it is necessary to use a thermostatically-controlled beaker
like this one
A solution is flowing in the wall of the beaker
to control temperature
In certain cases, it is necessary to homogenize the solution
The solution to analyse is placed on a magnetic stirrer
a magnetic bar is added
and the solution is stirred
Once the solution is homogenized
the conductivity cell is dipped in it
The important point is not to carry any conductometry measurement while the solution is stirred
in order not to disturb the solution between the two platinized platinum plates
For an acid-base titration for example
it is necessary to leave the cell in the solution
and to homogenize each time we add a little volume of acid or base
This is how we proceed
we homogenize after each volume added
and then the magnetic agitator is turned off for the same reasons as before
to not disturb the solution between the platinized platinum plates
We read the conductivity value once it is stabilized
For a kinetic monitoring
we often cannot turn the agitation off
In this case we must proceed with a moderate agitation
and place the cell as far away as possible from the magnetic bar
in order to disturb the solution as less as possible
Interactive numerical educational resourcesfor the analysis of complex media