4. Measuring pH

• Acid-Base Indicators

Most acid-base indicators are rather complex organic weak acids. The condition in order to be able to be used as an indicator is that the molecule (non-ionized) has a different color than the ion (conjugate base). Adding some drops of an acid-base indicator to a solution, the color depends on the pH of the solution. As the pH of the solution changes, the color also will change at a certain pH-area, specific for the indicator (color change area of the indicator). For instance, bromthymol blue has a yellow color in the pH-area 0 - 6.0. It turns blue from pH 7.6 on. In the color change area of bromthymolblue (6.0 - 7.6) it produces a greenish tint.

Acid-base indicators are often used to indicate the stoichiometric point during a titration (see next topic). Mixtures of these indicators can be used to determine the pH of a solution, though in a rather inaccurate way.

• Electrical pH-measurement

pH-meter with combined glass electrode	For accurate measurements, a (combined) glass electrode is used to determine the pH. This electrode is made up of two electrodes: a standard electrode of known constant potential (Ag/AgCl-electrode) and a special glass electrode, whose potential depends on the concentration of H+ in the solution into which it is dipped. The electrode is connected to a potentiometer that measures the potential difference between the two electrodes. That potential difference is automatically converted to a pH reading. Before using the glass electrode, the system must be callibrated by using a least one, better two buffer solutions with a well-known constant pH.
When the glass electrode is not used, it is kept in a 3 molar KCl-solution.

Summary

Measuring pH
Acid-Base Indicators color changes at a certain pH-area often used in titrations
pH-meter with glass electrode accurate pH measuring