Determination of Chloride Ion Concentration by ... - Science Outreach

College of Science
Determination of Chloride Ion Concentration by Titration
(Mohr’s Method)
Safety
Lab coats, safety glasses and enclosed footwear must
be worn at all times in the laboratory.
The chromate solution needs to be prepared and used
with care as chromate is a known carcinogen.
Silver nitrate solution causes staining of skin and
fabric (chemical burns). Any spills should be rinsed
with water immediately.
Introduction
This method determines the chloride ion concentration
of a solution by titration with silver nitrate. As the silver
Introduction
nitrate solution is slowly added, a precipitate of silver
chloride forms.
Ag+ + Cl– → AgCl
chloride (aq) forms. (aq) (s)
The end point of the titration occurs when all the
Ag
chloride ions are precipitated. Then additional silver
ions react with the chromate ions of the indicator,
potassium chromate, to form a red-brown precipitate of
silver chromate.
+ + Cl–
(aq) (aq) → AgCl (s)
The end point of the titration occurs when all the
ions react with the chromate ions of the indicator,
2–
2 Ag+ + CrO (aq) 4 (aq) → Ag2CrO4(s) 2 Ag
This method can be used to determine the chloride ion
concentration of water samples from many sources
such as seawater, stream water, river water and estuary
water. Seawater is used as the example here.
+
(aq) + CrO 2–
4 (aq) → Ag2CrO4(s) water. Seawater is used as the example here.
The pH of the sample solutions should be between
6.5 and 10. (Refer to the additional notes (3) for the
6.5 and 10. (Refer to the additional notes (3) for the
explanation). If method the solutions or Volhard’s are method acidic, should the gravimetric
be used.
method or Volhard’s method should be used.
Equipment Needed
Equipment Needed
burette and stand
10 and 20 mL pipettes 100 mL volumetric flask
100 mL volumetric 250 mL flask conical flasks
250 mL conical flasks
10 mL and 100 mL measuring cylinders
Solutions Needed
Solutions Needed
Silver nitrate solution: (0.1 mol L −1 ) If possible, dry 5 g of
AgNO 3 for 2 hours at 100°C and allow to cool. Accurately
weigh about 4.25 g of solid AgNO 3 and dissolve it in
250 mL of distilled water in a conical flask. Store the
solution in a brown bottle.
Determination of Chloride Ion
Concentration by Titration (Mohr’s Method)
This method determines the chloride ion concentration
of a solution by titration with silver nitrate. As the silver
nitrate solution is slowly added, a precipitate of silver
chloride ions are precipitated. Then additional chloride
potassium chromate, to form a red-brown precipitate of
silver chromate.
This method can be used to determine the chloride ion
concentration of water samples from many sources
such as seawater, stream water, river water and estuary
The pH of the sample solutions should be between
explanation). If the solutions are acidic, the gravimetric
burette and stand
10 and 20 mL pipettes
10 mL and 100 mL measuring cylinders
Silver nitrate solution:(0.1 mol L −1 ) If possible, dry 5 g of
AgNO 3 for 2 hours at 100°C and allow to cool. Accurately
Potassium chromate indicator solution: (approximately
0.25 molL-1 ) Dissolve 1 g of K CrO in 20 mL distilled
2 4
water.
Conical flask
Seawater and chromate
indicator
Burette containing
silver nitrate solution

Sample Preparation
If the seawater contains traces of solid matter such as
sand or seaweed, it must be filtered before use.
Titration
1. Dilute seawater by pipetting a 20 mL sample into a
100 mL volumetric flask and making it up to the mark
with distilled water.
2. Pipette a 10 mL aliquot of diluted seawater into a
conical flask and add about 50 mL distilled water and
1 mL of chromate indicator.
3. Titrate the sample with 0.1 mol L−1 as sand or seaweed, it must be filtered before use.
Otherwise they will end up being weighed along with
the silver chloride precipitate.
1. Dilute seawater by pipetting a 20 mL sample into
a 100 mL volumetric flask and making it up to the mark
with distilled water.
2. Pipette a 10 mL aliquot of diluted seawater into a
conical flask and add about 50 mL distilled water and 1
mL of chromate indicator.
3. Titrate the sample with 0.1 mol L
silver nitrate
solution. Although the silver chloride that forms is
a white precipitate, the chromate indicator initially
gives the cloudy solution a faint lemon-yellow colour
(figure 1).
−1 Sample Preparation
If the seawater contains traces of solid matter such
as sand or seaweed, it must be filtered before use.
Otherwise they will end up being weighed along with
the silver chloride precipitate.
Titration
1. Dilute seawater by pipetting a 20 mL sample into
a 100 mL volumetric flask and making it up to the mark
with distilled water.
2. Pipette a 10 mL aliquot of diluted seawater into a
conical flask and add about 50 mL distilled silver water nitrate and 1
solution. mL of chromate Although indicator. the silver chloride that forms is a
white precipitate, the chromate indicator initially gives
3. Titrate the sample with 0.1 mol L
the cloudy solution a faint lemon-yellow colour (figure
1). The endpoint of the titration is identified as the first
appearance of a red-brown colour of silver chromate
(figure 2).
4. Repeat the titration with further aliquots of diluted
seawater until concordant results (titres agreeing within
0.1 mL) are obtained.
−1 silver nitrate
solution. Although the silver chloride that forms is a
white precipitate, the chromate indicator initially gives
the cloudy solution a faint lemon-yellow colour (figure
1). The endpoint of the titration is identified as the first
appearance of a red-brown colour of silver chromate
(figure 2).
4. Repeat the titration with further aliquots of diluted
seawater until concordant results (titres agreeing within
0.1 mL) are obtained.
Method Titration
Figure 1 1 Before the the addition of any of any silver silver nitrate nitrate the chromate the chromate indicator
indicator gives the clear gives solution the clear a lemon-yellow solution a lemon-yellow colour. colour.
The endpoint of the titration is identified as the first
appearance of a red-brown colour of silver chromate
(figure 2).
Figure 2 Left flask: before the titration endpoint, addition of Ag +
ions leads to formation of silver chloride precipitate, making
the solution cloudy. The chromate indicator gives a faint lemonyellow
colour. Centre flask: at the endpoint, all the Cl− ions
have precipitated. The slightest excess of Ag + precipitates with
the chromate indicator giving a slight red-brown colouration.
Right flask: If addition of Ag + Figure 2 Left flask: before the titration endpoint, addition of Ag
is continued past the endpoint,
further silver chromate precipitate is formed and a stronger
red-brown colour results. NB: The titration should be stopped
when the first trace of red-brown colour is observed. Using an
incompletely titrated reference flask for comparison is a helpful
way to identify the first appearance of red-brown colouration.
+ ions
leads to formation of silver chloride precipitate, making the solution
cloudy. The chromate indicator gives a faint lemon-yellow colour.
Centre flask: at the endpoint, all the Cl− ions have precipitated. The
slightest excess of Ag + precipitates with the chromate indicator giving a
slight red-brown colouration. Right flask: If addition of Ag + Figure 2 Left flask: before the titration endpoint, addition of Ag
is continued
past the endpoint, further silver chromate precipitate is formed
2
+ ions
leads to formation of silver chloride precipitate, making the solution
cloudy. The chromate indicator gives a faint lemon-yellow colour.
Centre flask: at the endpoint, all the Cl− ions have precipitated. The
slightest excess of Ag + precipitates with the chromate indicator giving a
slight red-brown colouration. Right flask: If addition of Ag + is continued
past the endpoint, further silver chromate precipitate is formed
2
Figure 1 Before the addition of any silver nitrate the chromate indicator
gives the clear solution a lemon-yellow colour.
incompletely titrated reference flask for comparison is a helpful way to
identify the first appearance of red-brown colouration.
Result Calculations
4. Repeat the titration with further aliquots of diluted
seawater until concordant results (titres agreeing
within 0.1 mL) are obtained.
Result Calculations
1. Determine the average volume of silver nitrate used
from your concordant titres.
2. Calculate the moles of silver nitrate reacting.
3. Use the following reaction equation to determine the
moles of chloride ions reacting.
Ag+ + Cl– → AgCl (aq) (aq) (s)
4. Calculate the concentration of chloride ions in the
diluted seawater.
5. Calculate the concentration of chloride ions in the
original undiluted seawater.
6. Calculate the concentration of sodium chloride in the
seawater in molL−1 , gL−1 1. Determine the average volume of silver nitrate
used from your concordant titres.
2. Calculate the moles of silver nitrate reacting.
3. Use the following reaction equation to determine
the moles of chloride ions reacting.
Ag
and g/100 mL (%).
Additional Notes
1. Residues containing silver ions are usually saved for
later recovery of silver metal. Check this with your
teacher or the laboratory supervisor.
2. The Mohr titration should be carried out under
conditions of pH 6.5 – 9. At higher pH silver ions may
be removed by precipitation with hydroxide ions,
and at low pH chromate ions may be removed by an
acid-base reaction to form hydrogen chromate ions
or dichromate ions, affecting the accuracy of the
end point.
3. It is a good idea to first carry out a “rough” titration
in order to become familiar with the colour change at
the end point.
4. The Mohr titration is sensitive to the presence of both
chloride and bromide ions in solution and will not be
too accurate when there is a significant concentration
of bromide present as well as the chloride. However,
in most cases, such as seawater, the bromide
concentration will be negligible. For this reason, the
method can also be used to determine either the total
concentration of chloride and bromide in solution,
or the concentration of bromide when the chloride
concentration is known to be negligible.
+ + Cl–
(aq) (aq) → AgCl (s)
4. Calculate the concentration of chloride ions in the
diluted seawater.
5. Calculate the concentration of chloride ions in the
original undiluted seawater.
6. Calculate the concentration of sodium chloride in
the seawater in molL−1 , gL−1 Result Calculations
1. Determine the average volume of silver nitrate
used from your concordant titres.
2. Calculate the moles of silver nitrate reacting.
3. Use the following reaction equation to determine
the moles of chloride ions reacting.
Ag
and g/100 mL (%).
Additional Notes
1. Silver nitrate solution will stain clothes and skin.
Any spills should be rinsed with water immediately.
2. Residues containing silver ions are usually saved
for later recovery of silver metal. Check this with your
teacher.
3. The Mohr titration should be carried out under
conditions of pH 6.5 – 9. At higher pH silver ions may be
removed by precipitation with hydroxide ions, and at
low pH chromate ions may be removed by an acid-base
reaction to form hydrogen chromate ions or dichromate
ions, affecting the accuracy of the end point.
4. It is a good idea to first carry out a “rough” titration
in order to become familiar with the colour change at
the end point.
5. The Mohr titration is sensitive to the presence
of both chloride and bromide ions in solution and
will not be too accurate when there is a significant
concentration of bromide present as well as the
chloride. However, in most cases, such as seawater,
the bromide concentration will be negligible. For this
reason, the method can also be used to determine
either the total concentration of chloride and bromide
in solution, or the concentration of bromide when the
chloride concentration is known to be negligible.
+ + Cl–
(aq) (aq) → AgCl (s)
4. Calculate the concentration of chloride ions in the
diluted seawater.
5. Calculate the concentration of chloride ions in the
original undiluted seawater.
6. Calculate the concentration of sodium chloride in
the seawater in molL−1 , gL−1 and g/100 mL (%).
Additional Notes
1. Silver nitrate solution will stain clothes and skin.
Any spills should be rinsed with water immediately.
2. Residues containing silver ions are usually saved
for later recovery of silver metal. Check this with your
teacher.
3. The Mohr titration should be carried out under
conditions of pH 6.5 – 9. At higher pH silver ions may be
removed by precipitation with hydroxide ions, and at
low pH chromate ions may be removed by an acid-base
reaction to form hydrogen chromate ions or dichromate
ions, affecting the accuracy of the end point.
4. It is a good idea to first carry out a “rough” titration
in order to become familiar with the colour change at
the end point.
5. The Mohr titration is sensitive to the presence
of both chloride and bromide ions in solution and
will not be too accurate when there is a significant
concentration of bromide present as well as the
chloride. However, in most cases, such as seawater,
the bromide concentration will be negligible. For this
reason, the method can also be used to determine
either the total concentration of chloride and bromide
in solution, or the concentration of bromide when the
chloride concentration is known to be negligible.