What a titration does

Definition

Titration

A method in which one solution is added gradually from a burette to a measured volume of another solution until the reaction between them is just complete.

A titration finds exactly what volume of one solution reacts completely with a fixed volume of another.
In an acid-base titration, an acid reacts with an alkali in a neutralisation reaction to form a salt and water.
1. For example, hydrochloric acid reacts with aqueous sodium hydroxide:
  $\mathrm{HCl(aq) + NaOH(aq) \rightarrow NaCl(aq) + H_2O(l)}$
Both of these solutions are colourless, so you cannot see the moment at which the reaction is complete without help.
A few drops of an indicator show that moment as a sudden colour change.
The volume measured in a titration can be used to work out an unknown concentration, or to make a salt solution with no excess acid or alkali left over.

Titration apparatus

Definition

Burette

A long, graduated glass tube with a tap at the bottom, used to deliver a measured, variable volume of liquid.

Definition

Volumetric pipette

A glass pipette marked with a single line that transfers one fixed volume of liquid very accurately.

Volumetric pipette: transfers a fixed volume of one solution, usually $25.0 cm325.0\ \text{cm}^3$ $25.0 cm^{3}$ , into a conical flask.
1. A pipette filler is used to draw the liquid up, because liquid must never be sucked into a pipette by mouth.
2. The pipette is filled until the bottom of the meniscus sits exactly on the line on its neck.
Burette: holds the other solution and adds it slowly through a tap, while its scale shows how much has been added.
1. A standard burette holds $cm350.0\ \text{cm}^3$ , with $cm30.00\ \text{cm}^3$ at the top of its scale and readings increasing downwards.
2. It is held upright in a clamp on a stand, above the conical flask.
Conical flask: holds the measured solution, and its sloping sides let you swirl the contents without spilling them.
White tile: sits under the flask so that the colour change is easy to see.
Small funnel: used only to fill the burette and then removed, because drips from it would change the reading.
The burette is rinsed first with the solution it will hold, and the pipette with the solution it will measure, so that leftover water does not dilute either solution.
1. The conical flask is rinsed with distilled water only, because the amount of alkali in it is fixed by the pipette and extra water does not change that amount.

Indicators and the end-point

Definition

Indicator

A substance that shows one colour in an acidic solution and a different colour in an alkaline solution.

Definition

End-point

The point in a titration at which the indicator changes colour, showing that the reaction is just complete.

A suitable indicator for a titration changes colour sharply, within a single drop, at the point where the acid and alkali have exactly reacted.
Methyl orange is red in acidic solution and yellow in alkaline solution.
1. When acid is added from the burette to alkali containing methyl orange, the end-point is reached when the yellow solution turns orange.
Thymolphthalein is colourless in acidic solution and blue in alkaline solution.
1. When alkali is added from the burette to acid containing thymolphthalein, the end-point is reached when the colourless solution turns blue.
Universal indicator is not suitable, because it passes gradually through many colours and gives no single sharp change.
Only two or three drops of indicator are added, which is enough to give a clear colour.
Near the end-point, the new colour appears briefly where each drop lands and disappears again when you swirl the flask.
1. The end-point has been reached when the new colour stays after swirling.

Practical

Titrating an alkali with an acid

Aim: to find the volume of dilute hydrochloric acid that exactly neutralises $cm325.0\ \text{cm}^3$ of aqueous sodium hydroxide.
Apparatus: a $cm350\ \text{cm}^3$ burette, a clamp and stand, a small funnel, a $cm325.0\ \text{cm}^3$ volumetric pipette with a pipette filler, a $cm3250\ \text{cm}^3$ conical flask, a white tile, a wash bottle of distilled water, methyl orange indicator, dilute hydrochloric acid and aqueous sodium hydroxide.
Method, preparing the burette:
- Rinse the burette with a little of the acid, let it run out through the tap and pour the rinsings away.
- Clamp the burette upright with its top below eye level, close the tap and fill it with acid using the funnel.
- Remove the funnel, then open the tap briefly so that acid fills the narrow tip below the tap and no air bubble is left there.
- Read the bottom of the meniscus at eye level and record the initial reading.
Method, preparing the flask:
- Rinse the pipette with a little of the sodium hydroxide solution.
- Use the pipette filler to draw up sodium hydroxide solution until the bottom of the meniscus sits on the line, then run it into the conical flask.
- Let the pipette drain, then touch its tip against the inside of the flask, but do not blow out the last drop.
- Add three drops of methyl orange, so that the solution turns yellow, and stand the flask on the white tile under the burette.
Method, titrating:
- Run in acid a little at a time while swirling the flask until the colour changes, which gives a rough titre.
- Refill the burette, prepare a fresh flask of sodium hydroxide and indicator, and run in acid quickly until you are about $cm32\ \text{cm}^3$ short of the rough titre.
- Add the acid drop by drop, swirling after each drop, until one drop turns the solution orange and it stays orange.
- Record the final reading and repeat the accurate titration until you have concordant results.
Results: record the initial reading, the final reading and the titre for each titration in a table, with every reading to at least one decimal place.
Conclusion: the mean of the concordant titres is the volume of acid that exactly neutralises $cm325.0\ \text{cm}^3$ of the sodium hydroxide solution.
Watch out: an air bubble left in the tip of the burette makes the reading larger than the volume of acid actually added, so remove it before taking the initial reading.
Safety: wear eye protection, because sodium hydroxide solution damages eyes, and fill the burette below eye level so that acid cannot splash into your face.

Recording and using results

Definition

Titre

The volume of solution added from the burette to reach the end-point of a titration.

Definition

Concordant results

Repeated titres that are very close in value to each other.

The titre is found by subtracting the initial burette reading from the final burette reading.
The first, rough titration is done quickly to find roughly where the end-point is, so it usually overshoots and is not used in the mean.
Accurate titrations are then repeated until you have concordant results, titres that agree closely with each other.
The mean titre is calculated from the concordant titres only, leaving out the rough titre and any titre that does not agree with the others.
Results go in a table with headings such as $reading/cm3\text{final reading} / \text{cm}^3$ , and every reading is given to the same number of decimal places.

Example

Calculating a mean titre

A titration is carried out four times, and the titre for each is the final reading minus the initial reading.
Rough titration: final reading $cm324.60\ \text{cm}^3$ and initial reading $cm30.00\ \text{cm}^3$ , so the titre is $cm324.60\ \text{cm}^3$ .
Titration 1: final reading $cm324.05\ \text{cm}^3$ and initial reading $cm30.20\ \text{cm}^3$ , so the titre is $cm323.85\ \text{cm}^3$ .
Titration 2: final reading $cm347.80\ \text{cm}^3$ and initial reading $cm324.05\ \text{cm}^3$ , so the titre is $cm323.75\ \text{cm}^3$ .
Titration 3: final reading $cm323.90\ \text{cm}^3$ and initial reading $cm30.10\ \text{cm}^3$ , so the titre is $cm323.80\ \text{cm}^3$ .
The rough titre is left out, and titrations 1, 2 and 3 agree closely, so all three are used in the mean.

\text{mean titre} = \frac{23.85 + 23.75 + 23.80}{3} = 23.80\ \text{cm}^3

Exam technique

Describing a titration

Name both measuring instruments: the volumetric pipette measures the solution in the flask, and the burette adds the other solution.
Name a real indicator, such as methyl orange or thymolphthalein, and give the colour change at the end-point, such as yellow to orange.
Say that the solution is added drop by drop near the end-point while the flask is swirled.
If asked why a volumetric pipette is used instead of a measuring cylinder, say that it measures the volume more accurately.

Common Mistake

Do not use universal indicator in a titration, because it changes colour gradually.
Do not include the rough titre in the mean titre.
Do not rinse the conical flask with the alkali, because extra alkali left in the flask increases the titre.
Do not read the burette from above or below the meniscus, because the reading will be wrong.

Self review

What does a volumetric pipette do in a titration?
Why is the burette rinsed with the solution it will hold?
What colour change shows the end-point when acid is added to alkali containing methyl orange?
Why is universal indicator not used in titrations?
Which titres are used to calculate the mean titre?

What a titration does

Definition

Titration

A method in which one solution is added gradually from a burette to a measured volume of another solution until the reaction between them is just complete.

A titration finds exactly what volume of one solution reacts completely with a fixed volume of another.
In an acid-base titration, an acid reacts with an alkali in a neutralisation reaction to form a salt and water.
1. For example, hydrochloric acid reacts with aqueous sodium hydroxide:
  $\mathrm{HCl(aq) + NaOH(aq) \rightarrow NaCl(aq) + H_2O(l)}$
Both of these solutions are colourless, so you cannot see the moment at which the reaction is complete without help.
A few drops of an indicator show that moment as a sudden colour change.
The volume measured in a titration can be used to work out an unknown concentration, or to make a salt solution with no excess acid or alkali left over.

Titration apparatus

Definition

Burette

A long, graduated glass tube with a tap at the bottom, used to deliver a measured, variable volume of liquid.

Definition

Volumetric pipette

A glass pipette marked with a single line that transfers one fixed volume of liquid very accurately.

Volumetric pipette: transfers a fixed volume of one solution, usually $25.0 cm325.0\ \text{cm}^3$ $25.0 cm^{3}$ , into a conical flask.
1. A pipette filler is used to draw the liquid up, because liquid must never be sucked into a pipette by mouth.
2. The pipette is filled until the bottom of the meniscus sits exactly on the line on its neck.
Burette: holds the other solution and adds it slowly through a tap, while its scale shows how much has been added.
1. A standard burette holds $cm350.0\ \text{cm}^3$ , with $cm30.00\ \text{cm}^3$ at the top of its scale and readings increasing downwards.
2. It is held upright in a clamp on a stand, above the conical flask.
Conical flask: holds the measured solution, and its sloping sides let you swirl the contents without spilling them.
White tile: sits under the flask so that the colour change is easy to see.
Small funnel: used only to fill the burette and then removed, because drips from it would change the reading.
The burette is rinsed first with the solution it will hold, and the pipette with the solution it will measure, so that leftover water does not dilute either solution.
1. The conical flask is rinsed with distilled water only, because the amount of alkali in it is fixed by the pipette and extra water does not change that amount.

Indicators and the end-point

Definition

Indicator

A substance that shows one colour in an acidic solution and a different colour in an alkaline solution.

Definition

End-point

The point in a titration at which the indicator changes colour, showing that the reaction is just complete.

A suitable indicator for a titration changes colour sharply, within a single drop, at the point where the acid and alkali have exactly reacted.
Methyl orange is red in acidic solution and yellow in alkaline solution.
1. When acid is added from the burette to alkali containing methyl orange, the end-point is reached when the yellow solution turns orange.
Thymolphthalein is colourless in acidic solution and blue in alkaline solution.
1. When alkali is added from the burette to acid containing thymolphthalein, the end-point is reached when the colourless solution turns blue.
Universal indicator is not suitable, because it passes gradually through many colours and gives no single sharp change.
Only two or three drops of indicator are added, which is enough to give a clear colour.
Near the end-point, the new colour appears briefly where each drop lands and disappears again when you swirl the flask.
1. The end-point has been reached when the new colour stays after swirling.

Practical

Titrating an alkali with an acid

Aim: to find the volume of dilute hydrochloric acid that exactly neutralises $cm325.0\ \text{cm}^3$ of aqueous sodium hydroxide.
Apparatus: a $cm350\ \text{cm}^3$ burette, a clamp and stand, a small funnel, a $cm325.0\ \text{cm}^3$ volumetric pipette with a pipette filler, a $cm3250\ \text{cm}^3$ conical flask, a white tile, a wash bottle of distilled water, methyl orange indicator, dilute hydrochloric acid and aqueous sodium hydroxide.
Method, preparing the burette:
- Rinse the burette with a little of the acid, let it run out through the tap and pour the rinsings away.
- Clamp the burette upright with its top below eye level, close the tap and fill it with acid using the funnel.
- Remove the funnel, then open the tap briefly so that acid fills the narrow tip below the tap and no air bubble is left there.
- Read the bottom of the meniscus at eye level and record the initial reading.
Method, preparing the flask:
- Rinse the pipette with a little of the sodium hydroxide solution.
- Use the pipette filler to draw up sodium hydroxide solution until the bottom of the meniscus sits on the line, then run it into the conical flask.
- Let the pipette drain, then touch its tip against the inside of the flask, but do not blow out the last drop.
- Add three drops of methyl orange, so that the solution turns yellow, and stand the flask on the white tile under the burette.
Method, titrating:
- Run in acid a little at a time while swirling the flask until the colour changes, which gives a rough titre.
- Refill the burette, prepare a fresh flask of sodium hydroxide and indicator, and run in acid quickly until you are about $cm32\ \text{cm}^3$ short of the rough titre.
- Add the acid drop by drop, swirling after each drop, until one drop turns the solution orange and it stays orange.
- Record the final reading and repeat the accurate titration until you have concordant results.
Results: record the initial reading, the final reading and the titre for each titration in a table, with every reading to at least one decimal place.
Conclusion: the mean of the concordant titres is the volume of acid that exactly neutralises $cm325.0\ \text{cm}^3$ of the sodium hydroxide solution.
Watch out: an air bubble left in the tip of the burette makes the reading larger than the volume of acid actually added, so remove it before taking the initial reading.
Safety: wear eye protection, because sodium hydroxide solution damages eyes, and fill the burette below eye level so that acid cannot splash into your face.

Recording and using results

Definition

Titre

The volume of solution added from the burette to reach the end-point of a titration.

Definition

Concordant results

Repeated titres that are very close in value to each other.

The titre is found by subtracting the initial burette reading from the final burette reading.
The first, rough titration is done quickly to find roughly where the end-point is, so it usually overshoots and is not used in the mean.
Accurate titrations are then repeated until you have concordant results, titres that agree closely with each other.
The mean titre is calculated from the concordant titres only, leaving out the rough titre and any titre that does not agree with the others.
Results go in a table with headings such as $reading/cm3\text{final reading} / \text{cm}^3$ , and every reading is given to the same number of decimal places.

Example

Calculating a mean titre

A titration is carried out four times, and the titre for each is the final reading minus the initial reading.
Rough titration: final reading $cm324.60\ \text{cm}^3$ and initial reading $cm30.00\ \text{cm}^3$ , so the titre is $cm324.60\ \text{cm}^3$ .
Titration 1: final reading $cm324.05\ \text{cm}^3$ and initial reading $cm30.20\ \text{cm}^3$ , so the titre is $cm323.85\ \text{cm}^3$ .
Titration 2: final reading $cm347.80\ \text{cm}^3$ and initial reading $cm324.05\ \text{cm}^3$ , so the titre is $cm323.75\ \text{cm}^3$ .
Titration 3: final reading $cm323.90\ \text{cm}^3$ and initial reading $cm30.10\ \text{cm}^3$ , so the titre is $cm323.80\ \text{cm}^3$ .
The rough titre is left out, and titrations 1, 2 and 3 agree closely, so all three are used in the mean.

\text{mean titre} = \frac{23.85 + 23.75 + 23.80}{3} = 23.80\ \text{cm}^3

Exam technique

Describing a titration

Name both measuring instruments: the volumetric pipette measures the solution in the flask, and the burette adds the other solution.
Name a real indicator, such as methyl orange or thymolphthalein, and give the colour change at the end-point, such as yellow to orange.
Say that the solution is added drop by drop near the end-point while the flask is swirled.
If asked why a volumetric pipette is used instead of a measuring cylinder, say that it measures the volume more accurately.

Common Mistake

Do not use universal indicator in a titration, because it changes colour gradually.
Do not include the rough titre in the mean titre.
Do not rinse the conical flask with the alkali, because extra alkali left in the flask increases the titre.
Do not read the burette from above or below the meniscus, because the reading will be wrong.

Self review

What does a volumetric pipette do in a titration?
Why is the burette rinsed with the solution it will hold?
What colour change shows the end-point when acid is added to alkali containing methyl orange?
Why is universal indicator not used in titrations?
Which titres are used to calculate the mean titre?

12.2.1 Acid-base titrations

What a titration does

Titration apparatus

Indicators and the end-point

Titrating an alkali with an acid

Recording and using results

Calculating a mean titre

Describing a titration

Review 12.2.1 Acid-base titrations by teaching Genie

12.2.1 Acid-base titrations

What a titration does

Titration apparatus

Indicators and the end-point

Titrating an alkali with an acid

Recording and using results

Calculating a mean titre

Describing a titration

Review 12.2.1 Acid-base titrations by teaching Genie

12.2.1 Acid-base titrations

What a titration does

Titration apparatus

Indicators and the end-point

Titrating an alkali with an acid

Recording and using results

Calculating a mean titre

Describing a titration

Review 12.2.1 Acid-base titrations by teaching Genie

12.2.1 Acid-base titrations Revision Guide

12.2.1 Acid-base titrations

What a titration does

Titration apparatus

Indicators and the end-point

Titrating an alkali with an acid

Recording and using results

Calculating a mean titre

Describing a titration

Review 12.2.1 Acid-base titrations by teaching Genie

12.2.1 Acid-base titrations Revision Guide