Every time a food scientist checks the acidity of a new batch of ketchup, they rely on a technique called a titration. This is an accurate experimental procedure used to find the exact concentration of an unknown acid or alkali.
To do this, you react the unknown liquid with a standard solution (a solution where the concentration is already precisely known). By carefully measuring the exact volume needed to completely neutralise the unknown sample, you can calculate its exact concentration.
During a titration, you need to know exactly when neutralisation has occurred. Universal indicator is completely unsuitable for this because it produces a gradual colour change over a range of pH values.
Instead, you must use an indicator that provides a sharp end-point, meaning the colour changes instantly with a single drop. Edexcel specifically requires you to know the colours for two indicators. Assuming the alkali is in the flask and the acid is being added from the burette, the colour changes are:
Thorough preparation is critical to avoid inaccurate readings. The primary measuring tools must be rinsed with deionised water to clean them, and then rinsed with the specific solution they will contain to ensure the concentration is not diluted.
First, prepare the volumetric pipette. Rinse it with deionised water, followed by the alkali. Attach a pipette filler to safely draw exactly of alkali into the pipette. Fill it until the bottom of the meniscus (the curved surface of the liquid) rests exactly on the calibration line.
Transfer this alkali into a conical flask. You must only ever rinse the conical flask with deionised water. If you rinse it with alkali, you will accidentally add extra moles of reactant, which will ruin the accuracy of your results. Touch the tip of the pipette to the surface of the liquid to ensure the final drop is dispensed.
Next, prepare the burette. Rinse it with deionised water, followed by the acid. Fill it using a funnel, but you must remove the funnel immediately after filling. If left in place, extra drops of acid could fall into the burette during the experiment and alter the volume.
Before starting, briefly open the tap to fill the jet space (the small area below the tap) with acid. If you leave an air bubble here, it will fill during the experiment, making it look like you used more acid than you actually did, resulting in an inaccurately high reading.
Place your conical flask on a white tile. This creates a bright background, making the sudden indicator colour change much easier to spot.
You should always perform a "rough" titration first. Open the burette tap and add the acid in small portions while swirling the flask vigorously. This gives you a rough idea of where the end-point is.
For your accurate titrations, add the acid quickly until you are about away from your rough value. From this point, add the acid dropwise while continuously swirling the flask. Stop the exact moment the indicator undergoes a permanent colour change.
Record the final volume from the burette. To avoid a parallax error, you must position yourself so your eyes are exactly level with the bottom of the meniscus. The total volume of acid added is known as the titre.
You must repeat the accurate titration process until you obtain concordant results. These are precise results where the measured titres are within of each other.
To find the true volume of acid needed, calculate the mean of your concordant results. You must completely ignore the rough titration and any non-concordant results when calculating this average.
Once you have your mean titre, you can find the unknown concentration using the mole equation. Remember that titration volumes must be converted from to by dividing by .
A student carries out a titration and records the following titres: Rough = , Titre 1 = , Titre 2 = , Titre 3 = . Calculate the mean titre.
Step 1: Identify the concordant results (those within of each other).
Step 2: Calculate the mean using only the concordant results.
Step 3: State the final answer to two decimal places.
Sodium hydroxide is an irritant, and at higher concentrations, it is highly corrosive. Always wear eye protection during this practical. To avoid dangerous chemical splashes to your face, place the burette on a lower surface (such as a stool or the floor) so that you are filling it below eye level.
Students often include the rough titration when calculating the mean titre. You must strictly exclude the rough titre and any non-concordant results.
In 6-mark procedure questions, examiners explicitly look for you to mention 'swirling the flask', 'adding dropwise near the end-point', and 'using a white tile'.
Always write burette readings to two decimal places, ensuring the final digit is either a 0 or a 5 (e.g., ). Writing '24' instead of '24.00' will lose you marks.
If an exam question asks what happens if you rinse the burette with only water, you must state that the acid becomes diluted, so a larger volume is needed, resulting in an inaccurately high titre.
Titration
An accurate experimental technique used to determine the concentration of an unknown solution by reacting it with a standard solution.
Standard solution
A solution of a precisely known concentration used in quantitative analysis.
Concordant results
Titre volumes that are within 0.10 cm³ of each other.
Meniscus
The curved upper surface of a liquid in a tube; volume readings must always be taken from the bottom of this curve.
Titre
The exact volume of solution added from the burette to reach the end-point of a titration.
End-point
The specific point during a titration when the indicator permanently changes colour, showing neutralisation is complete.
Parallax error
An inaccurate measurement caused by viewing a scale from an angle rather than precisely at eye level.
Phenolphthalein
A chemical indicator that is pink in alkaline solutions and colourless in acidic solutions.
Methyl orange
A chemical indicator that is yellow in alkaline solutions and red in acidic solutions.
Volumetric pipette
A piece of glassware used to accurately measure and transfer a fixed volume of liquid (usually 25.00 cm³).
Pipette filler
A rubber or electronic safety device used to safely draw liquids into a pipette.
Conical flask
Glassware with a narrow neck used to hold the reactant; the shape allows for vigorous swirling without spilling.
Burette
A long, graduated glass tube with a tap at the bottom, used to deliver accurately measured volumes of liquid.
Jet space
The volume of the burette below the tap; it must be filled with liquid before starting a titration to avoid volume errors.
White tile
A tile placed under the flask to provide a neutral background, making the indicator's colour change easier to see.
Sharp end-point
An immediate and distinct colour change that occurs with the addition of a single drop of reagent.
Put your knowledge into practice — try past paper questions for Chemistry
Titration
An accurate experimental technique used to determine the concentration of an unknown solution by reacting it with a standard solution.
Standard solution
A solution of a precisely known concentration used in quantitative analysis.
Concordant results
Titre volumes that are within 0.10 cm³ of each other.
Meniscus
The curved upper surface of a liquid in a tube; volume readings must always be taken from the bottom of this curve.
Titre
The exact volume of solution added from the burette to reach the end-point of a titration.
End-point
The specific point during a titration when the indicator permanently changes colour, showing neutralisation is complete.
Parallax error
An inaccurate measurement caused by viewing a scale from an angle rather than precisely at eye level.
Phenolphthalein
A chemical indicator that is pink in alkaline solutions and colourless in acidic solutions.
Methyl orange
A chemical indicator that is yellow in alkaline solutions and red in acidic solutions.
Volumetric pipette
A piece of glassware used to accurately measure and transfer a fixed volume of liquid (usually 25.00 cm³).
Pipette filler
A rubber or electronic safety device used to safely draw liquids into a pipette.
Conical flask
Glassware with a narrow neck used to hold the reactant; the shape allows for vigorous swirling without spilling.
Burette
A long, graduated glass tube with a tap at the bottom, used to deliver accurately measured volumes of liquid.
Jet space
The volume of the burette below the tap; it must be filled with liquid before starting a titration to avoid volume errors.
White tile
A tile placed under the flask to provide a neutral background, making the indicator's colour change easier to see.
Sharp end-point
An immediate and distinct colour change that occurs with the addition of a single drop of reagent.