Doing Titrations

Quantitative Chemistry

What is a titration?

A practical method used to measure the volumes of two solutions that react together and use the results to calculate the concentration of one of the solutions.

What is volumetric analysis?

A method of chemical analysis involving accurately measured volumes of solutions.

What is the equivalence point?

The point at which exactly the correct amounts of the two substances have reacted completely.

What is the end point?

The point at which the indicator just changes colour.

What should ideally be true about the equivalence point and end point?

They should coincide.

What is a titre?

The volume of solution added from the burette during a titration.

What are concordant titres?

Titres that are close together, usually within 0.20 cm³ of each other.

Why is an indicator needed in an acid-base titration?

The acid, base and products are usually colourless, so an indicator shows when the end point has been reached.

What apparatus is commonly required for a titration?

A conical flask, burette and stand, pipette and pipette filler, wash bottle of deionised water, small funnel and white tile.

What volume does a typical pipette deliver in a titration?

25.0 cm³.

What is the usual capacity of a burette?

50 cm³.

Why is a pipette used in a titration?

To accurately transfer a fixed volume of solution.

Why is a burette used in a titration?

To accurately measure the variable volume of solution required to reach the end point.

Why is a conical flask used in a titration?

It allows the solutions to be swirled and mixed without spilling.

Why is a white tile placed underneath the conical flask?

It provides a white background so the indicator colour change can be seen more clearly.

What should the conical flask be rinsed with before a titration?

Deionised water.

Why can the conical flask be rinsed with deionised water?

Extra water does not change the number of moles of reactant in the flask, so it does not affect the titre.

What should the pipette initially be rinsed with?

Deionised water.

What should the pipette be rinsed with after deionised water?

The solution that will be transferred using the pipette.

Why is the pipette rinsed with the solution it will contain?

To remove residual water that would otherwise dilute the solution.

How should liquid be drawn into a pipette?

Using a pipette filler.

Why should a pipette filler be used?

For safety; solutions should never be pipetted by mouth.

What should the burette initially be rinsed with?

Deionised water.

What should the burette be rinsed with after deionised water?

The solution that will be placed in the burette.

Why is the burette rinsed with the solution it will contain?

To prevent residual water from diluting the solution.

Why must the space between the burette tap and tip be filled with solution before starting?

If it contains air, some solution will initially fill this space rather than enter the flask, giving an inaccurate titre.

Why is a small funnel used when filling a burette?

To make filling easier and reduce the risk of spilling solution.

What should be done with the funnel after filling the burette?

Remove it before taking readings and carrying out the titration.

Why should the funnel be removed from the burette?

Drops remaining in the funnel could enter the burette and change the volume during the titration.

How should a burette reading be taken?

At eye level using the bottom of the meniscus.

Why should burette readings be taken at eye level?

To avoid parallax error.

Which part of the meniscus is normally read for a colourless solution?

The bottom of the meniscus.

To how many decimal places are burette readings normally recorded?

Two decimal places.

How is a titre calculated?

Final burette reading − initial burette reading.

What is added to the solution in the conical flask before titrating?

A few drops of a suitable indicator.

Approximately how much indicator should be added?

About 3 drops.

Why should only a few drops of indicator be used?

Indicators are weak acids or bases and too much could affect the end point and titre.

What should be done while solution is being added from the burette?

Continuously swirl the conical flask.

Why should the conical flask be swirled during a titration?

To ensure continuous and complete mixing of the two solutions.

How should solution be added when far from the end point?

It can be added relatively quickly while continuously swirling the flask.

How should solution be added when close to the end point?

Slowly, drop by drop.

Why is solution added drop by drop near the end point?

To reduce the chance of overshooting the end point.

When should addition from the burette be stopped?

When the indicator just permanently changes colour.

What should be done after completing a titration?

Calculate the titre and repeat the titration until concordant titres are obtained.

Why is a rough titration often carried out first?

To estimate approximately where the end point occurs so later titrations can approach it more carefully.

Should a rough titre normally be included when calculating the mean titre?

No, unless it is concordant with the accurate titres.

Which titres should be used to calculate the mean titre?

Concordant titres only.

Why are titrations repeated?

To improve reliability and obtain concordant results.

What indicates good precision in a titration?

Obtaining concordant titres.

What colour is methyl orange in acid?

Red.

What colour is methyl orange in alkali?

Yellow.

What colour is phenolphthalein in acid?

Colourless.

What colour is phenolphthalein in alkali?

Pink.

When is methyl orange a suitable indicator?

For strong acid–weak base and strong acid–strong base titrations.

When is phenolphthalein a suitable indicator?

For weak acid–strong base and strong acid–strong base titrations.

Give examples of strong acids.

Hydrochloric acid and nitric acid.

Give examples of strong bases.

Sodium hydroxide and potassium hydroxide.

What is a common weak base?

Ammonia.

What is a common weak acid?

Ethanoic acid.

Why must a suitable indicator be chosen for a titration?

Different indicators change colour over different pH ranges, so the indicator must change colour close to the equivalence point.

Why should the conical flask NOT be rinsed with the solution being measured using the pipette?

It would add an unknown extra amount of reactant to the flask and change the titre.

What happens if the conical flask is rinsed with sodium hydroxide before sodium hydroxide is pipetted into it?

Extra unknown moles of sodium hydroxide are present, so more acid is required and the titre is too high.

What happens if the pipette is rinsed only with water and not with the solution it will contain?

The solution is diluted, so fewer moles are transferred and the titre is too low.

What happens if the burette is rinsed only with water and not with the solution it will contain?

The solution in the burette is diluted, so a larger volume is required and the titre is too high.

What happens if the burette tip initially contains air?

Some solution fills the tip instead of entering the flask, so the measured titre is larger than the volume that actually reacted.

Why should all titrations use the same amount of indicator?

Different amounts of indicator could slightly affect the end point and reduce consistency.

Why does adding extra solution after the indicator has changed colour reduce accuracy?

The end point has been overshot, making the measured titre too large.

What is the key aim of good titration technique?

To determine the volume required to reach the end point as accurately and precisely as possible.