Acid-base titrations
In a nutshell
Acid-base titrations are used to find the concentration of unknown solutions. For a standard solution the concentration is known.
Hazards and risks
Definitions
A hazard is something that can potentially cause harm to someone or something. A risk is the probability of somebody being harmed when exposed to a hazard.
Before carrying out any experiment, be sure to write a risk assessment which identifies all of the hazards and risks in the experiment.
Standard solutions
Definition
A standard solution is a solution with a known concentration.
To carry out a titration it is important that you make up a standard solution:
procedure
| 1.
| First, calculate the number of moles of solute using:
n=c×V(dm3)
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| 2.
| Next, calculate the mass of solute using:
m=n×M
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| 3.
| Weigh out the mass of solute using a balance. Be sure to weigh the container first, then add the required mass to this.
Note: The balance must have a minimum precision of 2 d.p.
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| 4.
| Add the solid to a beaker of 100cm3 of distilled water and stir to dissolve.
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| 5.
| Reweigh the 'empty' container. Take this away from the total measured mass at the start to find the actual mass placed in the beaker.
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| 6.
| Use the actual mass and calculate the number of moles using:
n=Mm
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| 7.
| Calculate the concentration of the standard solution using:
c=V(dm3)n
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| 8.
| Place the solution in a volumetric flask.
Note: Use a funnel to ensure all of the solution is added.
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| 9.
| Rinse the beaker and stirring rod with distilled water and add to the volumetric flask.
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| 10.
| Ensure the volumetric flask is filled to the top (250cm3) with distilled water.
Note: Make sure the bottom of the meniscus reaches the line on the flask.
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| 11.
| Add a stopper and invert the flask a few times to ensure proper mixing.
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Titrations
Once you have prepared your standard solution, you can carry out a titration.
procedure
| 1.
| Measure out some of the alkali with an unknown concentration (not the standard solution of acid) using a pipette and add to a conical flask.
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| 2.
| Add an indicator to the conical flask.
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| 3.
| Rinse the burette with some of the standard solution.
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| 4. | Fill the burette with the standard solution.
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| 5.
| Take an initial reading from the burette to see how much acid there is initially.
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| 6.
| Complete a rough titration to find the end point. Add the acid from the burette to the alkali in the flask, swirling the flask constantly. Stop adding the acid once there is a colour change. |
| 7.
| Record the final reading of acid from the burette. |
| 8.
| Do an accurate titration, running the acid within 2cm3 from the end point. Then begin to add the acid dropwise slowly.
|
| 9.
| Calculate the amount of acid needed to neutralise the alkali using a difference in the readings:
Final reading−Inital reading=Titre
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| 10.
| Repeat the experiment until you achieve a set of concordant results. Concordant results are when recordings are within 0.1cm3 of each other.
Note: Wash the conical flask between each titration to remove the acid and alkali present.
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| 11.
| Calculate the mean titre. Be sure to only use your concordant results.
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Indicators
Indicators change colour and indicate the end point of a titration due to the change in pH. Two main indicators are used in titrations:
- Methyl orange - turns yellow to red upon addition of an acid to alkali
- Phenolphthalein - turns red to colourless upon addition of an acid to alkali
Tip: To ensure you are seeing a clear colour change during a titration, place the flask on a white tile.
Concentrations
Unknown concentrations can be calculated using titration reactions.
Note: Always make sure you are carrying out your calculations with the units for volume in dm3 unless the question specifically asks for a volume in cm3.
Concentration of an unknown alkali solution
procedure
| 1.
| Find the number of moles of the standard solution (acid solution) using:
n=c×V(dm3)
|
| 3.
| Write the balanced equation for the reaction to establish the information you know and what you need to work out.
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| 4.
| Use the molar ratios to work out the number of moles of the alkali solution.
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| 5.
| Calculate the concentration of the alkali solution using:
c=V(dm3)n
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Concentration of an unknown acid solution
procedure
| 1.
| Find the number of moles of the alkali dissolved in the solution:
n=Mm
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| 2.
| Find the concentration of the alkali solution using:
c=V(dm3)n
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| 3.
| Use the concentration of the alkali solution along with the mean titre volume to work out the number of moles of the alkali solution using:
n=c×V(dm3)
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| 4. | Write the balanced equation for the reaction to establish the information you know and what you need to work out.
|
| 5.
| Use the molar ratios to work out the number of moles of the acid solution. |
| 6.
| Calculate the concentration of the acid solution using:
c=V(dm3)n
|
Volumes
Similar to unknown concentrations, unknown volumes can be also be calculated.
procedure
| 1.
| First, calculate the number of moles of the standard solution using:
n=c×V(dm3) |
| 2. | Write the balanced equation for the reaction to establish the information you know and what you need to work out.
|
| 3.
| Use the molar ratios to work out the number of moles of the solution with the unknown volume. |
| 4.
| Use the number of moles and the concentration to calculate the unknown volume using:
V(dm3)=cn
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