Measuring pH with Electrodes

Want voltage (potential difference, E) between electrodes to be a f(pH)

• pH sensitive (glass electrode) has E = f(pH of soln)
• reference electrode (calomel) has E_reference = pH independent = constant
• E = E_{pH electrode} - E_reference = f(pH of soln)

Why a glass electrode is pH sensitive

• inner glass surface has - charge
• originally, - charge is balanced by Na⁺ from the glass
• ion exchange occurs because of contact with HCl soln

H⁺(solution) + Na⁺Glass^-(s) Na⁺(solution) + H⁺Glass^-(s)

• extent of ion exchange is fixed since [H⁺] is constant

• outer glass surface also has - charge
• extent of ion exchange depends on sample soln [H⁺]
• difference in extent of ion exchange causes a potential across the membrane
• E_{pH electrode} = f(sample soln pH)

Ion exchange at outer glass surface can be slow

• pH meter display may stabilize slowly
• stirring sample soln helps