Titrations Involving Precipitation Reactions How They Work Titrations can be used to determine the concentration of a specific ion in a sample solution

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



Stopcock (Valve)

The standard solution can also be called the titrant.

Buret

0.100 M AgNO3



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



Cl– solution

Stopcock (Valve)

The solution in the flask is called the sample

Buret

0.100 M AgNO3



Stopcock (Valve)

Cl– solution

SampleUnknown

Concentration (Analyte)

It is the solution with an unknown concentration

Buret

0.100 M AgNO3



Stopcock (Valve)

Cl– solution

SampleUnknown


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



Stopcock (Valve)

Cl– solution

SampleUnknown


In this titration, a few drops of sodium chromate solution are added to the sample.

Buret

0.100 M AgNO3



Cl– solution

A few drops of

Na2CrO4(aq)

SampleUnknown


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



Cl– solution

A few drops of

Na2CrO4(aq)

SampleUnknown


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.


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.


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.



Ag

0.100 M AgNO3


Volume of AgNO3

solution needed to reach the endpoint.

And



Ag

0.100 M AgNO3


Volume of AgNO3


This volume will be needed for the calculations used to find the concentration of chloride ion



Ag

0.100 M AgNO3


Volume of AgNO3


This volume will be needed

for the calculations

used to find the concentration of chloride ion

In the original sample solution.



Ag

0.100 M AgNO3


Volume of AgNO3


This volume will be needed

for the calculations

used to find the concentration of chloride ion

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Titrations Involving Precipitation Reactions How They Work Titrations can be used to determine the concentration of a specific ion in a sample solution