acid-base titration

As acid drains into alkali the pH changes. This can be monitored using a pH sensor to produce an instant graph of pH against volume. The easy way of doing this is to not actually measure the acid, but to assume that the acid drip rate is constant. The other way is to get the software to record the volume of acid using the keyboard. The experiment can be repeated using different combinations of strong and weak acid. In this way, the characteristic shapes of these graphs can be compared.

what you need

Burette, stand, magnetic stirrer, indicator solution., pH electrode, pH buffer solution, 200cm3 0.1 M sodium hydroxide NaOH, 50cm3 0.5M hydrochloric acid HCl, 200cm3 0.1M ethanoic acid CH3COOH, 200cm3 beakers, interface, pH sensor/electrode.

setting up

Set up a beaker with 20 cm3 alkali and a few drops of indicator. Place this on the stirrer and fill a burette with acid. Connect a pH electrode to the pH sensor. Connect the pH sensor to the interface. Place the pH electrode in the beaker of alkali. Start your sensing software, it may recognise the pH sensor and you may also be able to calibrate the pH sensor to read the correct pH in buffer solution. If your software allows you to enter the volume of acid via the keyboard, set it up for this. When you start recording you should be prompted to enter a volume at the keyboard. With no acid added, type 0. Add 1 cm3 acid from the burette, then type in 1 for the volume. Otherwise, start the stirrer, start recording and allow the acid to drip in at a steady rate.

questions

• When does the pH change most slowly? Is this at the beginning, the middle or the end of the titration?
• When does the pH change most rapidly?
• What does the graph tell you about the change in pH during a titration?

teacher question

If you were merely comparing strong and weak acids, how important would it be to record the volume in this titration?

Activities in this section adapted from The IT in Science book of Data logging and Control. © IT in Science and may be reproduced as needed for use within school.