College of Science
Determination of Chloride Ion Concentration by Titration
(Mohr’s Method)
Safety
Solutions Needed
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.
Silver nitrate solution: (0.1 mol L−1) If possible, dry 5 g of
AgNO3 for 2 hours at 100°C and allow to cool. Accurately
weigh about 4.25 g of solid AgNO3 and dissolve it in
250 mL of distilled water in a conical flask. Store the
solution in a brown bottle.
Determination of Chloride
Ionchromate indicator solution: (approximately
Potassium
0.25 molL ) Dissolve 1 g of K CrO in 20 mL distilled
Concentration by Titration
(Mohr’s Method)
water.
-1
2
4
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.This method determines the chloride ion concentration
of a solution by titration with silver nitrate. As the silver
nitrate
added, a precipitate of silver
Ag+(aq)solution
+ Cl–is slowly
→ AgCl
(s)
chloride
forms. (aq)
Burette containing
silver nitrate solution
The end point of the titration +occurs–when all the
Ag (aq) + Cl (aq) → AgCl(s)
chloride ions are precipitated. Then
additional silver
point of the titration occurs when all the
ions react withThe
theend
chromate
ions of the indicator,
chloride ions are precipitated. Then additional chloride
potassium chromate,
form
red-brown
precipitate
of
ions reactto
with
the achromate
ions of
the indicator,
silver chromate.
potassium chromate, to form a red-brown precipitate of
silver chromate.
2 Ag+(aq) + CrO42–+ (aq) → Ag
CrO4(s)
2– 2
2 Ag (aq) + CrO4 (aq) → Ag2CrO4(s)
This method can
be
used
to determine the chloride ion
This method can be used to determine the chloride ion
concentration of
water
samples
many
concentration of waterfrom
samples
fromsources
many sources
such as seawater,
water,
riverwater,
water
and
estuary
suchstream
as seawater,
stream
river
water
and estuary
Seawater
is used
as the example
water. Seawaterwater.
is used
as the
example
here. here.
The pH ofsolutions
the sampleshould
solutionsbe
should
be between
The pH of the sample
between
6.5 and 10. (Refer to the additional notes (3) for the
6.5 and 10. (Refer
to the additional
notes
for the
thegravimetric
explanation).
If the solutions
are(3)
acidic,
explanation). Ifmethod
the solutions
are
acidic,
the
gravimetric
or Volhard’s method should be used.
method or Volhard’s method should be used.
Equipment Needed
Equipmentburette
Needed
and stand
10 and 20 mL pipettes
burette and stand
100 mL volumetric flask
10 and 20 mL pipettes
250 mL
conical flasks
100 mL volumetric
flask
mL and 100 mL measuring cylinders
250 mL conical10flasks
10 mL and 100 mL measuring cylinders
Solutions Needed
Silver nitrate solution:(0.1 mol L−1) If possible, dry 5 g of
AgNO3 for 2 hours at 100°C and allow to cool. Accurately
Conical flask
Seawater and chromate
indicator
asSample
sand orPreparation
seaweed, it must be filtered before use.
Otherwise they will end up being weighed along with
If the
seawater
contains
traces of solid matter such
the
silver
chloride
precipitate.
as sand or seaweed, it must be filtered before use.
Otherwise they will end up being weighed along with
Titration
Method
the silver chloride precipitate.
1. Dilute seawater by pipetting a 20 mL sample into
Sample
Preparation
aTitration
100 mL volumetric flask and making it up to the mark
Ifwith
thedistilled
seawaterwater.
contains traces of solid matter such as
1. orDilute
seawater
by pipetting
mL sample
sand
seaweed,
it must
be filtereda 20
before
use. into
2.a 100Pipette
a
10
mL
aliquot
of
diluted
seawater
into
a
mL volumetric flask and making it up to the
mark
Titration
conical
flask and
add about 50 mL distilled water and 1
with distilled
water.
mL
of
chromate
indicator.
1.2.Dilute
seawater
by aliquot
pipetting
a 20 mLseawater
sample into
Pipette
a 10 mL
of diluted
into aa
−1
100Titrate
mLflask
volumetric
and
it upwater
to theand
mark
nitrate
3.conical
the
sample
with
0.1
moldistilled
L silver
and
addflask
about
50making
mL
1
with
distilled
water.
solution.
Although
the
silver
chloride
that
forms
is
a
mL of chromate indicator.
white
precipitate,
the
chromate
indicator
initially
2.3.Pipette
a 10the
mLsample
aliquot
of diluted
intogives
a
Titrate
with
0.1 molseawater
L−1 silver nitrate
theconical
cloudyflask
solution
a
faint
lemon-yellow
colour
(figure
and
add
about
50
mL
distilled
water
solution. Although the silver chloride that forms is aand
1). 1The
of the
titration is identified as the first
mLendpoint
of chromate
indicator.
white
precipitate,
the
chromate indicator initially gives
appearance of a red-brown colour of−1 silver chromate
cloudy
faint0.1
lemon-yellow
3.the
Titrate
thesolution
sampleawith
mol L silvercolour
nitrate(figure
(figure 2).
1).solution.
The endpoint
of thethe
titration
is identified
the first
Although
silver chloride
thatasforms
is
4.appearance
Repeat
the
further
aliquots
of
diluted
of atitration
red-brown
colour
ofindicator
silver
chromate
a white
precipitate,
thewith
chromate
initially
seawater
until
concordant
agreeing within
(figure
gives 2).
the
cloudy
solutionresults
a faint(titres
lemon-yellow
colour
0.1
mL)
are
obtained.
(figure
1).
4. Repeat the titration with further aliquots of diluted
incompletely titrated reference flask for comparison is a helpful way to
Result
identify theCalculations
first appearance of red-brown colouration.
1. Determine the average volume of silver nitrate
used
fromCalculations
your concordant titres.
Result
4. Repeat the titration with further aliquots of diluted
2.1. Calculate
the moles
of silver
nitratesilver
reacting.
Determine
average
volume
nitrate
seawater
untilthe
concordant
resultsof
(titres agreeing
used
from
your
concordant
titres.
3. within
Use the
following
reaction equation to determine
0.1 mL)
are obtained.
the
of chloride
ionsof
reacting.
2. moles
Calculate
the moles
silver nitrate reacting.
3. Use Calculations
the following
Ag+(aq) +reaction
Cl–(aq) →equation
AgCl(s) to determine
Result
the moles of chloride ions reacting.
4.
Calculatethe
theaverage
concentration
ions inused
the
1. Determine
volumeofofchloride
silver nitrate
+
–
diluted
seawater.
Ag (aq) + Cltitres.
→ AgCl(s)
from your concordant
(aq)
5.2.
Calculate
the
concentration
of
ions in the
the
moles
of silver nitrate
reacting.
4. Calculate
Calculate
the
concentration
of chloride
chloride
original
undiluted
seawater.
diluted
seawater.
3.
Use the
following
reaction equation to determine the
of chloride
ions reacting.
6.5. moles
Calculate
the
of
in
Calculate
theconcentration
concentration
of sodium
chloridechloride
ions in the
−1
−1
,
gL
and
g/100
mL
(%).
the
seawater
in
molL
original undiluted seawater.
Ag+(aq) + Cl–(aq) → AgCl(s)
6. Calculate the concentration of sodium chloride in
4. Calculate the concentration
of chloride ions in the
Additional
the seawater inNotes
molL−1, gL−1 and g/100 mL (%).
diluted seawater.
1.5. Calculate
Silver nitrate
solution will stain
clothesions
andin
skin.
the concentration
of chloride
the
Any
spills
should
be
rinsed
with
water
immediately.
Additional
Notes
original undiluted seawater.
2.
Residues
containing
ions
are
usually
1. Calculate
Silver nitrate
solutionsilver
will stain
clothes
andsaved
skin.
6.
the
concentration
of sodium
chloride
in the
later
recovery
of
silver
Check
this
with
your
Any
spills
should
be
with
water
immediately.
−1rinsed
−1metal.
seawater until concordant results (titres agreeing within for
seawater in molL , gL and g/100 mL (%).
teacher.
0.1 mL) are obtained.
2. Residues containing silver ions are usually saved
for later
of silver
metal.beCheck
this
with
your
3.Additional
The recovery
Mohr titration
should
carried
out
under
Notes
teacher.
conditions of pH 6.5 – 9. At higher pH silver ions may be
removed
precipitation
with hydroxide
and at
3. Theby
Mohr
titration should
be carriedions,
out under
low
pH chromate
ions– may
removed
by an
acid-base
1.conditions
Residues
ions
aresilver
usually
saved
ofcontaining
pH 6.5
9.silver
At be
higher
pH
ions
may for
be
reaction
to
form
hydrogen
chromate
ions
or
dichromate
later
recovery
of
silver
metal.
Check
this
with
your
removed by precipitation with hydroxide ions, and
at
ions,
affecting
thelaboratory
accuracy
ofsupervisor.
the
end point.
or the
lowteacher
pH
chromate
ions
may be
removed
by an acid-base
2.
The
titration
be carried
out
reaction
form
hydrogen
or under
dichromate
4.
It Mohr
is atogood
idea
toshould
firstchromate
carry
outions
a “rough”
titration
conditions
ofthe
pHaccuracy
6.5 – 9. with
At
pH
silver
ionsatmay
affecting
of higher
the
end
point.
inions,
order
to become
familiar
the
colour
change
be
removed
by
precipitation
with
hydroxide
the
end
point.
4. It is a good idea to first carry out a “rough” ions,
titration
Figure 1 Before the addition of any silver nitrate the chromate indicator
at low
pH chromate
ions
be removed
Thethe
endpoint
of the
titration colour.
is identified as the first 5.in and
gives
clear solution
a lemon-yellow
order
become
familiar
with may
thetocolour
changeby
atan
Theto
Mohr
titration
is sensitive
the presence
acid-base
reaction
to
form
hydrogen
chromate
ions
appearance of a red-brown colour of silver chromate of
the
endchloride
point. and bromide ions in solution and
both
or dichromate ions, affecting the accuracy of the
(figure 2).
will
not
be
too accurate
there to
is athe
significant
5. endThe
Mohr
titrationwhen
is sensitive
presence
point.
concentration
of
bromide
present
as
well
as
the
of both chloride and bromide ions in solution
and
3. It is a good
ideain
tomost
first carry
out
a “rough”
titration
chloride.
However,
cases,
such
as
seawater,
will not be too accurate when there is a significant
in
order
to
become
familiar
with
the
colour
the
bromide concentration
will be negligible.
Forchange
this at
concentration
of bromide present
as well as the
the end
point.
reason,
the
methodincan
alsocases,
be used
toas
determine
chloride.
However,
most
such
seawater,
either
the
total
concentration
of
chloride
and
4.
The
Mohr
titration
is
sensitive
to
the
presence
of both
the bromide concentration will be negligible. bromide
For this
in
solution,
or
the
concentration
of
bromide
when
thebe
chloride
bromide
ionsbe
in used
solution
and will not
reason,
theand
method
can also
to determine
Figure 2 Left flask: before the titration endpoint, addition of Ag+ ions
chloride
concentration
is known
to
be negligible.
too accurate
when there
is aofsignificant
concentration
either
the
total concentration
chloride
and
bromide
leads to formation of silver chloride precipitate, making the solution
of
bromide
present
as
well
as
the
chloride.
However,
in
solution,
or
the
concentration
of
bromide
when
the
cloudy. The chromate indicator gives a faint lemon-yellow colour.
+of Ag+
Figure
22 Left
beforethe
the
titration
endpoint,
addition
Figureflask:
Leftatflask:
flask:
before
endpoint,
addition
of Ag
ions
have precipitated.
The
Centre
the endpoint,
alltitration
the
Cl− ions
in
most
cases,
such
as
seawater,
the
bromide
chloride concentration is known to be negligible.
ions
leads
to formation
ofchloride
silverwith
chloride
precipitate,
leads
toexcess
formation
silver
precipitate,
making
themaking
solution
the chromate
indicator
giving a
slightest
of Agof+ precipitates
concentration will be negligible. For this reason, the
cloudy.
The chromate
indicator
gives
aindicator
faint
lemon-yellow
the
solution
cloudy.
The
chromate
gives
a +faint
lemoniscolour.
continued
slight
red-brown
colouration.
Right
flask:
If addition
of Ag
−
method can also be used to determine either the total
−
ions
haveallprecipitated.
The
Centre
flask:
atCentre
thefurther
endpoint,
allchromate
the Cl
yellow
flask:
at
the
endpoint,
the
Cl ions
past
thecolour.
endpoint,
silver
precipitate
is formed
+
with theofchromate
indicator giving
slightest
excess of Ag
concentration of chloride and bromide in solution,
have
precipitated.
Theprecipitates
slightest excess
Ag+ precipitates
with a
+
is
continued
slight
red-brown
colouration.
Right
flask:
If
addition
of
Ag
or the concentration of bromide when the chloride
indicator giving a slight red-brown colouration.
2 thepastchromate
the endpoint, further silver
+ chromate precipitate is formed
Right flask: If addition of Ag is continued past the endpoint,
concentration is known to be negligible.
Figure 11Before
addition
of any
silversilver
nitrate
the chromate
indicator
Figure
Beforethe
the
addition
of any
nitrate
the chromate
gives the clear
a lemon-yellow
colour.
indicator
givessolution
the clear
solution a lemon-yellow
colour.
silver chromate precipitate is formed and a stronger
2further
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.