pH measurement

Principle

The course, reaction rate and equilibrium position of many chemical reactions are strongly influenced by the concentration or more accurately, the activity of hydrogen ions in solutions α_H+. Rapid and accurate determinations of hydrogen ion activity are thus of great importance. Since α_H+ can vary over many orders of magnitude, it has proved convenient to introduce the pH scale (pH from the Latin "pondus hydrognii" meaning "amount of hydrogen"). The most important and common method used to determine the pH value is to measure the potential of the electrode which is sensitive to hydrogen ion activity. In certain practical situations, however, a simpler and more direct method o determining pH is required, and use is often made of pH indicators.

Tasks

Calibrate the following pH-sensitive electrodes in buffer solutions of known pH:

• the glass electrode
• the antimony electrode
• the quinhydrone electrode.

Using these calibrated electrodes, measure the pH of an unknown solution. Compare and contrast the results obtained with the three pH-sensitive electrode.

Use the glass electrode to determine the pH range in which the following indicators change colour:

• methyl orange
• bromothymol blue
• phenolphthalein

Compare the suitability of the three indicators for different types of acid-base titrations.

What you can learn about

• Potentiometric determination of pH
• Glass electrode
• pH indicators
• Acid-base titrations

Literature for this article als follows

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