The pH Scale: A Measure of Acidity and Basicity
What is pH?
Definition
pH scale
The pH scale is a logarithmic measure of the concentration of hydrogen ions ($ [H^+] $) in an aqueous solution.
It is defined mathematically as:
$$ \text{pH} = -\log_{10} [H^+] $$
Here:
- $ [H^+] $ represents the concentration of hydrogen ions in moles per cubic decimeter ($ \text{mol dm}^{-3} $).
- The negative logarithm ensures that pH values are positive, as $ [H^+] $ in aqueous solutions typically ranges from $ 1 \, \text{mol dm}^{-3} $ to $ 10^{-14} \, \text{mol dm}^{-3} $.
If the $ [H^+] $ of a solution is $ 1.0 \times 10^{-3} \, \text{mol dm}^{-3} $ at 298 K, its pH is: $$ \text{pH} = -\log_{10} [H^+] = -\log_{10} (1.0 \times 10^{-3}) = 3.00 $$
This indicates the solution is acidic.
HintIf you know the pH, you can reverse the process to calculate the $ [H^+] $ concentration using the equation:
$$ [H^+] = 10^{-\text{pH}} $$
ExampleIf the pH of a solution is $ 5.0 $ at 298 K, the $ [H^+] $ is:
$$ [H^+] = 10^{-\text{pH}} = 10^{-5} = 1.0 \times 10^{-5} \, \text{mol dm}^{-3} $$
- pH is temperature-dependent and is typically quoted at 298 K (25°C).
- Changes in temperature can affect the ionization of water, altering the $[H^+]$ concentration and thus shifting the neutral pH value slightly from 7 at standard conditions.
The pH Scale: Ranges and Interpretations
- The pH scale generally ranges from $ 0 $ to $ 14 $, though values outside this range are possible for very strong acids or bases.
- Here’s how to interpret pH values:
- Acidic solutions: $pH< 7$ ($ [H^+] >[OH^-] $)
- Neutral solutions: $pH = 7$ ($ [H^+] = [OH^-] $)
