Year 12 HSC Chemistry Titrations

Acid-base indicators are one of the more colourful aspects of Chemistry. Year 12 HSC students recently investigated how natural substances such as beetroot can be used as acid-base indicators. Other natural indicators include the juice from red cabbage and hydrangeas.

A week later, our students were using phenolphthalein as they learnt to set up and conduct acid-base titration experiments. Running a titration involves physical manipulation skills and an observational challenge. The students need to observe closely and question ‘Will that next drop change the solution from colourless to pink or not?’ at the same time as adjusting the burette tap and swirling the conical flask. Data obtained from the titration is then used to calculate the concentration of one of the solutions which means that the students need to take care and conduct repeat trials for reliable data.

The Year 12 girls donned their lab coats and rose to the challenge of finding the light pink endpoint. The students discovered that the indicator changes colour as the base drips into the conical flask and it is possible to see how quickly the acid and base react. The dark pink quickly swirls away, but over time this takes longer – one of the clues that there’s not many more drops needed. Then finally – sometimes even with just half a drop washed in off the tip of the burette – the colour of the solution changes to a permanent light pink. Time to read and record the volume of titrant used.

– Felicity Cassidy
Science Teacher