pH curves made easy

Download the following files: Strong Acid/Strong Alkali, Weak Acid/Strong Base, Phosphoric Acid/Strong base.

Do your advanced students complain that using a pH Meter to construct pH curves is boring and time consuming? Use Datadisc and you will find that they can do 3 experiments in the time it used to take to do one. Datadisc will also print out the table of results and graphs too - if your students can't record results and draw graphs by now then there must be something wrong with your course structure.

Set up the normal equipment for this experiment; conical flask, magnetic stirrer, burette together with pH Sensor/Meter, with acid in the conical flask and alkali in the burette.
(If you are using a pH meter then you will have to connect the 0-1V output to your serial interface using a 'Blue Box' lead. In Datadisc, choose Calibrate on the Measure menu and then for the appropriate channel select 'Other Sensors' and enter the appropriate data. Normally 0V corresponds to a pH of 0 and 1V a pH of 10, but check your manual.)

The key to success is to calibrate a 'Keyboard Channel'. Choose Calibrate on the Measure menu and then one of the available keyboard channels. Enter appropriate name (Volume) and units (cm³), the lower and upper values will normally be 0 and 100 cm³. A resolution of 0.05 cm³ will be normal. As you are likely to repeat the experiment, why not use Save Calibration on the Measure menu.

As you are likely to repeat the experiment, why not use Save Calibration on the Measure menu? Next time you can load it in with a couple of mouse clicks:

Now choose Record on the Measure menu and you will find that the default recording is what you want - Volume on the x-axis and pH on the y-axis. Before you take the first reading, set the X+ value to 1.0cm³ or 0.5cm³. Click 'Record' for the initial reading, set the burette to add the alkali slowly (if you run it in too fast, mixing will be incomplete at the time of reading). Now simply, click 'Record' every time the indicated volume is reached. Near the end point, stop the burette and reduce the increment, increasing it again afterwards.

Finish the recording and manipulate your data in the usual way.

 Results

(The first two files were recorded on an earlier version of Datadisc, the Channel numbers are different.)

This is sodium hydroxide and hydrochloric acid.

This is sodium hydroxide and ethanoic acid

 By re-scaling the x and y axes, you can home in on the 'half-neutralisation' area and obtain an estimate for the pKa:

The final experiment using sodium hydroxide and tribasic phosphoric acid, shows only the first two end points, in my experience it is impossible to demonstrate the final one. Superimposed is the result of differentiating the data using Data menu: Calculate, which confirms the absence of a third end point.