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Titrations Involving
Precipitation Reactions
How They WorkTitrations can be used to determine the concentration of a specific ion in a sample solution. Here we’ll see how titrations involving precipitation reactions work
Here is the set-up for a titration of a solution with an unknown concentration of Cl minus, or chloride ions.
This long tube is called a buret. This can also be spelled “burette”
Buret
This value, called a stopcock, is closed to keep the liquid in the burette. When it is opened, liquid will drip or flow out of the bottom of the burette.
Buret
Stopcock (Valve)
In this example, the burette is filled with 0.100 molar silver nitrate solution.
Buret
0.100 M AgNO3
Stopcock (Valve)
The solution in the burette is called the standard solution
Buret
0.100 M AgNO3
Standard Solution Known
Concentration(Titrant)
Stopcock (Valve)
The standard solution has a known concentration. In this case it’s 0.100 molar AgNO3
Buret
0.100 M AgNO3
Standard Solution Known
Concentration(Titrant)
Stopcock (Valve)
The standard solution can also be called the titrant.
Buret
0.100 M AgNO3
Standard Solution Known
Concentration(Titrant)
Stopcock (Valve)
In this titration, a solution containing chloride ions is added to an Erlenmeyer flask and placed under the burette.
Buret
0.100 M AgNO3
Standard Solution Known
Concentration(Titrant)
Cl– solution
Stopcock (Valve)
The solution in the flask is called the sample
Buret
0.100 M AgNO3
Standard Solution Known
Concentration(Titrant)
Stopcock (Valve)
Cl– solution
SampleUnknown
Concentration (Analyte)
It is the solution with an unknown concentration
Buret
0.100 M AgNO3
Standard Solution Known
Concentration(Titrant)
Stopcock (Valve)
Cl– solution
SampleUnknown
Concentration (Analyte)
It can also be called the analyte, because this solution is being analyzed to find out the concentration of chloride ions in it.
Buret
0.100 M AgNO3
Standard Solution Known
Concentration(Titrant)
Stopcock (Valve)
Cl– solution
SampleUnknown
Concentration (Analyte)
In this titration, a few drops of sodium chromate solution are added to the sample.
Buret
0.100 M AgNO3
Standard Solution Known
Concentration(Titrant)
Cl– solution
A few drops of
Na2CrO4(aq)
SampleUnknown
Concentration (Analyte)
Stopcock (Valve)
The sodium chromate solution is known as an indicator in this titration. It will change colour at what is called the endpoint of the titration. We’ll show you how all of this works.
Buret
0.100 M AgNO3
Standard Solution Known
Concentration(Titrant)
Cl– solution
A few drops of
Na2CrO4(aq)
SampleUnknown
Concentration (Analyte)
Stopcock (Valve)
An indicato
r
We’ll focus on the solutions.
0.100 M AgNO3
Cl– solution
A few drops of
Na2CrO4(aq)
We’ll dissociate the AgNO3 into its individual ions
0.100 M AgNO3
Cl– solution
dissociate
A few drops of
Na2CrO4(aq)
Which are Ag+ and nitrate, or NO3 minus ions
0.100 M Ag+ NO3
–
Cl– solution
A few drops of
Na2CrO4(aq)
The nitrate ion does not form any precipitates. It is a spectator ion here. So we’ll just delete it from our discussion.
0.100 M Ag+ NO3
–
Cl– solution
spectator
A few drops of
Na2CrO4(aq)
The nitrate ion does not form any precipitates. It is a spectator ion here. So we’ll just delete it from our discussion.
0.100 M Ag+ NO3
–
Cl– solution
A few drops of
Na2CrO4(aq)
And tidy up a bit.
0.100 M Ag+
Cl– solution
A few drops of
Na2CrO4(aq)
So we can think of the solution in the burette as a source of Ag+ or silver ions.
0.100 M Ag+
Cl– solution
A few drops of
Na2CrO4(aq)
Ag+
Ag+
Ag+
Ag+
Ag+
Ag+
In a titration, we briefly open the stopcock.
0.100 M Ag+
Cl– solution
A few drops of
Na2CrO4(aq)
Ag+
Ag+
Ag+
Ag+
Ag+
Ag+
The solution in the burette drips into the flask (click) bringing Ag+ ions with it.
0.100 M Ag+
Cl– solution
A few drops of
Na2CrO4(aq)
Ag+
Ag+
Ag+
Ag+
Ag+
Let’s take a closer look at what happens in the flask as silver ions are added to it.
0.100 M Ag+
Cl– solution
A few drops of
Na2CrO4(aq)
Ag+
Ag+
Ag+
Ag+
Ag+
Ag+
Now we’ve zoomed in to the flask
0.100 M Ag+
AgAgAgAgAg
Cl
Cl24CrO
Cl
Cl
Silver ions preferentially bond to chloride ions (click) rather than chromate ions.
0.100 M Ag+
AgAgAgAgAg
Cl
Cl24CrO
Cl
Cl
This forms the precipitate silver chloride. Because silver chloride is white (click), the solution turns to a milky yellow colour.
AgAgAgAg
Cl
Cl24CrO
Cl
AgCl
As silver ions are added, some will temporarily (click) bond to chromate ions.
AgAgAg
Cl
Cl24CrO
Cl
AgCl
AgAg
They will form the precipitate Ag2CrO4 or silver chromate. Silver chromate is reddish brown, so the solution (click) will turn a slightly reddish colour.
AgAgAgAg
ClCl
Ag 24CrO Ag AgCl
Cl
But silver preferentially bonds with chloride, so as the flask is shaken, the silver ions will leave the chromate ion (click) and bond with available chloride ions
AgAgAgAg
ClCl
Ag 24CrO Ag AgCl
Cl
And the reddish colour will go away.
AgAgAgAg
ClAg
Cl
24CrO AgCl
ClAg
The solution will turn red momentarily as more silver is added, but as long as chloride is still present, shaking the flask will make the red colour disappear
AgAgAg
ClCl
24CrO AgCl
Ag
AgCl
Ag
Added silver ions will (click) continue to bond with the remaining chloride ions.
AgAgAg
ClCl
24CrO AgCl
Ag
AgCl
Ag
At a certain point, all of the available chloride ions have bonded with silver ions.
AgAgAg
Cl24CrO AgCl
Ag
AgClAgCl
Since there are no chloride ions left, any silver ions that are added will have to bond (click) to the chromate ions
AgAg
Cl24CrO AgCl
Ag
AgClAgCl
AgAg
The formation of the silver chromate precipitate will cause (click) the solution to turn red again.
AgAg
ClAgAg 2
4CrO AgAgCl
ClAg
AgCl
At this point, when the flask is shaken, the red colour will no longer disappear. There are no chloride ions available, so the silver will have to remain bonded with the chromate.
AgAg
ClAgAg 2
4CrO AgAgCl
ClAg
AgCl
We say the solution has turned a slight permanent reddish colour.
AgAg
ClAgAg 2
4CrO AgAgCl
ClAg
AgCl
Slight permanent
reddish colour
This is what is called the endpoint of the titration. A permanent colour change of the indicator signals the endpoint of the titration.
AgAg
ClAgAg 2
4CrO AgAgCl
ClAg
AgCl
Slight permanent
reddish colour
The Endpoint
The equivalence point or stoichiometric point, of this titration is the point where the moles of Ag+ added to the flask is equal to the moles of Cl minus that were in the original solution in the flask.
ClAgAg 2
4CrO AgAgCl
ClAg
AgCl
Slight permanent
reddish colour
The Endpoint
Equilvalence (Stoichiometric) Point: moles of Ag+
added = moles of Cl–in original solution
In most titrations if the proper indicator is used and the technique is good, the equivalence point and the endpoint are very close, and they can be assumed to be the same point.
ClAgAg 2
4CrO AgAgCl
ClAg
AgCl
Slight permanent
reddish colour
The Endpoint
Equilvalence (Stoichiometric) Point: moles of Ag+
added = moles of Cl–in original solution
Very close to the same point
Once we reach the endpoint we must stop adding silver ions to the flask.
AgAg
ClAgAg 2
4CrO AgAgCl
ClAg
AgCl
STOP adding
silver ions to the flask
Slight permanent
reddish colour
The Endpoint
This is because we want to know exactly what volume of 0.100 M AgNO3 solution was needed to JUST react with all the Cl– ions that were in the sample.
AgAg
ClAgAg 2
4CrO AgAgCl
ClAg
AgCl
We want to know exactly
what volume of 0.100 M AgNO3 solution was
needed to JUST react with all
the Cl– ions that were in the
sample.
STOP adding
silver ions to the flask
Slight permanent
reddish colour
The Endpoint
We record the initial reading of the AgNO3 solution in the burette before we start the titration.
Initial burette reading
Ag
0.100 M AgNO3
Then we begin the titration, adding drops very slowly (click) while swirling the flask.
Initial burette reading
Ag
Cl– sample solution during titration
0.100 M AgNO3
As soon a the endpoint is reached, we close the stopcock, stop the titration and record the final burette reading of AgNO3 solution.
Initial burette reading
Final burette reading
Ag
Cl– sample solution at the Endpoint
0.100 M AgNO3
The difference between the final burette reading and the initial burette reading will tell us the volume of AgNO3 solution required to reach the endpoint of this titration.
Initial burette reading
Final burette reading
Ag
0.100 M AgNO3
Cl– sample solution at the Endpoint
Volume of AgNO3
solution needed to reach the endpoint.
And
Initial burette reading
Final burette reading
Ag
0.100 M AgNO3
Cl– sample solution at the Endpoint
Volume of AgNO3
solution needed to reach the endpoint.
This volume will be needed for the calculations used to find the concentration of chloride ion
Initial burette reading
Final burette reading
Ag
0.100 M AgNO3
Cl– sample solution at the Endpoint
Volume of AgNO3
solution needed to reach the endpoint.
This volume will be needed
for the calculations
used to find the concentration of chloride ion
In the original sample solution.
Initial burette reading
Final burette reading
Ag
0.100 M AgNO3
Cl– sample solution at the Endpoint
Volume of AgNO3
solution needed to reach the endpoint.
This volume will be needed
for the calculations
used to find the concentration of chloride ion
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