Titrations Involving Precipitation Reactions How They Work Titrations 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 molar silver nitrate solution. Buret M AgNO 3 Stopcock (Valve)

The solution in the burette is called the standard solution Buret M AgNO 3 Standard Solution Known Concentration (Titrant) Stopcock (Valve)

The standard solution has a known concentration. In this case it’s molar AgNO3 Buret M AgNO 3 Standard Solution Known Concentration (Titrant) Stopcock (Valve)

The standard solution can also be called the titrant. Buret M AgNO 3 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 M AgNO 3 Standard Solution Known Concentration (Titrant) Cl – solution Stopcock (Valve)

The solution in the flask is called the sample Buret M AgNO 3 Standard Solution Known Concentration (Titrant) Stopcock (Valve) Cl – solution Sample Unknown Concentration (Analyte)

It is the solution with an unknown concentration Buret M AgNO 3 Standard Solution Known Concentration (Titrant) Stopcock (Valve) Cl – solution Sample Unknown 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 M AgNO 3 Standard Solution Known Concentration (Titrant) Stopcock (Valve) Cl – solution Sample Unknown Concentration (Analyte)

In this titration, a few drops of sodium chromate solution are added to the sample. Buret M AgNO 3 Standard Solution Known Concentration (Titrant) Cl – solution A few drops of Na 2 CrO 4(aq) Sample Unknown 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 M AgNO 3 Standard Solution Known Concentration (Titrant) Cl – solution A few drops of Na 2 CrO 4(aq) Sample Unknown Concentration (Analyte) Stopcock (Valve) An indicator

We’ll focus on the solutions M AgNO 3 Cl – solution A few drops of Na 2 CrO 4(aq)

We’ll dissociate the AgNO3 into its individual ions M AgNO 3 Cl – solution dissociate A few drops of Na 2 CrO 4(aq)

Which are Ag+ and nitrate, or NO3 minus ions M Ag + NO 3 – Cl – solution A few drops of Na 2 CrO 4(aq)

The nitrate ion does not form any precipitates. It is a spectator ion here. So we’ll just delete it from our discussion M Ag + NO 3 – Cl – solution spectator A few drops of Na 2 CrO 4(aq)

The nitrate ion does not form any precipitates. It is a spectator ion here. So we’ll just delete it from our discussion M Ag + NO 3 – Cl – solution A few drops of Na 2 CrO 4(aq)

And tidy up a bit M Ag + Cl – solution A few drops of Na 2 CrO 4(aq)

So we can think of the solution in the burette as a source of Ag+ or silver ions M Ag + Cl – solution A few drops of Na 2 CrO 4(aq) Ag +

In a titration, we briefly open the stopcock M Ag + Cl – solution A few drops of Na 2 CrO 4(aq) Ag +

The solution in the burette drips into the flask (click) bringing Ag+ ions with it M Ag + Cl – solution A few drops of Na 2 CrO 4(aq) Ag +

Let’s take a closer look at what happens in the flask as silver ions are added to it M Ag + Cl – solution A few drops of Na 2 CrO 4(aq) Ag +

Now we’ve zoomed in to the flask M Ag +

Silver ions preferentially bond to chloride ions (click) rather than chromate ions M Ag +

This forms the precipitate silver chloride. Because silver chloride is white (click), the solution turns to a milky yellow colour.

As silver ions are added, some will temporarily (click) bond to chromate ions.

They will form the precipitate Ag2CrO4 or silver chromate. Silver chromate is reddish brown, so the solution (click) will turn a slightly reddish colour.

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

And the reddish colour will go away.

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

Added silver ions will (click) continue to bond with the remaining chloride ions.

At a certain point, all of the available chloride ions have bonded with silver ions.

Since there are no chloride ions left, any silver ions that are added will have to bond (click) to the chromate ions

The formation of the silver chromate precipitate will cause (click) the solution to turn red again.

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.

We say the solution has turned a slight permanent reddish colour. 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. 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. 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. 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. STOP adding silver ions to the flask Slight permanent reddish colour The Endpoint

This is because we want to know exactly what volume of M AgNO 3 solution was needed to JUST react with all the Cl – ions that were in the sample. We want to know exactly what volume of M AgNO 3 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 M AgNO 3

Then we begin the titration, adding drops very slowly (click) while swirling the flask. Initial burette reading Cl – sample solution during titration M AgNO 3

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 Cl – sample solution at the Endpoint M AgNO 3

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 M AgNO 3 Cl – sample solution at the Endpoint Volume of AgNO 3 solution needed to reach the endpoint.

And Initial burette reading Final burette reading M AgNO 3 Cl – sample solution at the Endpoint Volume of AgNO 3 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 M AgNO 3 Cl – sample solution at the Endpoint Volume of AgNO 3 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 M AgNO 3 Cl – sample solution at the Endpoint Volume of AgNO 3 solution needed to reach the endpoint. This volume will be needed for the calculations used to find the concentration of chloride ion