The pOH Scale and Its Relationship with pH
- The pOH scale, like the pH scale, is logarithmic.
- However, instead of measuring the concentration of hydrogen ions $ [H^+] $, it measures the concentration of hydroxide ions $ [OH^-] $.
- The pOH of a solution is defined by the formula: $$\text{pOH} = -\log_{10}[\text{OH}^-]$$
- Here, $ [OH^-] $ represents the molar concentration of hydroxide ions.
This formula allows us to express very small concentrations of $ [OH^-] $ as manageable numbers.
ExampleA solution with $ [OH^-] = 1 \times 10^{-3} \, \text{mol dm}^{-3} $ has a pOH of:
$$\text{pOH} = -\log_{10}(1 \times 10^{-3}) = 3$$
Tip- The pOH scale is inversely proportional to $ [OH^-] $.
- As $ [OH^-] $ increases, the pOH decreases.
The pH-pOH Relationship
- The pOH scale is closely linked to the pH scale through the ionic product of water ($ K_w $), which describes the equilibrium constant for water's self-ionization: $$\text{K}_w = [H^+][OH^-]$$
- At 298 K (25°C), $ K_w = 1.00 \times 10^{-14} \, \text{mol}^2 \, \text{dm}^{-6} $. Taking the negative logarithm of both sides gives: $$\text{pH} + \text{pOH} = 14 \quad \text{(at 298 K)}$$
This formula allows you to convert between pH and pOH:
- If you know the pH: $$\text{pOH} = 14 - \text{pH}$$
- If you know the pOH: $$\text{pH} = 14 - \text{pOH}$$
- A solution has a pH of 11. What is its pOH?
- Using the relationship $ \text{pH} + \text{pOH} = 14 $: $$\text{pOH} = 14 - 11 = 3$$
- This low pOH indicates the solution is strongly basic.
