Titration (OCR GCSE Chemistry A (Gateway)): Revision Note
Titration of a Strong Acid & Strong Alkali
Titration of a Strong Acid & Strong Alkali
Practical Skills Development
Use of appropriate apparatus and techniques for conducting and monitoring chemical reactions, including appropriate reagents and/or techniques for the measurement of pH in different situations
Use of appropriate qualitative reagents and techniques to analyse and identify unknown samples or products including gas tests, flame tests, precipitation reactions, and the determination of concentrations of strong acids and strong alkalis
Use of appropriate apparatus to make and record a range of measurements accurately, including mass, time, temperature, and volume of liquids and gases
Aim
To titrate a strong acid against a strong alkali and find the concentration of the acid using an appropriate indicator
Health & Safety Aspects
Hazard symbols to show substances that are corrosive and harmful to health
Dilute hydrochloric acid is not classified as hazardous at the concentrations typically used in this practical, however it may still cause harm to the eyes or the skin
Acids and alkalis are corrosive and should be handled with care
For both substances, avoid contact with the skin and use safety goggles
A pipette should always be used with a safety filler to avoid contact with corrosive liquids
Materials
25 cm3 volumetric pipette
Pipette filler
50 cm3 burette
250 cm3 conical flask
Small funnel
0.1 mol/dm3 sodium hydroxide solution
Sulfuric acid – concentration unknown
Phenolphthalein or methyl orange indicator
Retort stand, clamp, boss & white tile


Performing a titration
Practical Tip:
Make sure you remove the funnel after filling the burette as if left it can drop solution into the burette, leading to error
Method:
Use the pipette and pipette filler and place exactly 25 cm3 sodium hydroxide solution into the conical flask
Place the conical flask on a white tile so the tip of the burette is inside the flask
Add a few drops of a suitable indicator to the solution in the conical flask
Perform a rough titration by taking the burette reading and running in the solution in 1 – 3 cm3 portions, while swirling the flask vigorously, rinsing down the sides with distilled water after each addition
Quickly close the tap when the end-point is reached (sharp colour change) and record the volume, placing your eye level with the meniscus
Now repeat the titration with a fresh batch of sodium hydroxide
As the rough end-point volume is approached, add the solution from the burette one drop at a time until the indicator just changes colour
Record the volume to the nearest 0.05 cm3 and read from the bottom of the meniscus where possible
Repeat until you achieve two concordant results (two results that are within 0.1 cm3 of each other) to increase accuracy
Results
Record your results in a suitable table, e.g.:
Titration Results Table

(Only concordant results should be used to calculate a mean titre)
Conclusion:
The mean titre is calculated and used to calculate the concentration of the acid in mol/dm3, ignoring any anomalous results
Examiner Tips and Tricks
It doesn't matter whether you have the acid or alkali in the burette. The choice is usually determined by the colour change of the indicator to make as easy as possible to see the end-point. For example if you are using phenolphthalein you put the alkali in the burette as the colour change is form colourless to pink which is easier to see than going from pink to colourless.
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