The pH Scale
Quick Notes
- pH is a measure of the [H⁺] concentration in a solution.
- Defined as: pH = –log10[H⁺]
- You can calculate [H⁺] from pH: [H⁺] = 10−pH
- Low pH (< 7) = acidic, high [H⁺]
- High pH (> 7) = alkaline, low [H⁺]
- Neutral solution: pH = 7 and [H⁺] = 1.0 × 10⁻⁷ mol dm⁻³ (at 298K)
- pH can be measured:
- Approximately with a universal indicator
- Precisely with a pH meter or probe
Full Notes
What is pH?
The pH scale measures the concentration of hydrogen ions (H⁺) in solution, on a logarithmic scale. This allows us to express a wide range of H⁺ concentrations more simply.
![IB Chemistry expression showing pH = −log10[H⁺] with explanation of hydrogen ion concentration relationship.](images/phexpression.png)
A lower pH indicates a higher concentration of hydrogen ions (i.e. a more acidic solution).
To work backwards:
![IB Chemistry equation showing [H⁺] = 10^−pH, calculation of hydrogen ion concentration from pH.](images/h+ph.png)
This is useful when you’re given a pH value and need to calculate the hydrogen ion concentration, such as in weak acid problems.
Get comfortable with the equations – calculations involving pH and [H⁺] are commonly tested. Always ensure [H⁺] is in mol dm⁻³ before using equations and for quick multiple choice questions, remember pH is a logarithmic scale – change of pH by 1 = tenfold difference in [H⁺].
Measuring pH
The two main methods used to measure pH are indicators and pH probes.
Universal indicator: Gives an approximate pH through a colour change.
- Red = strongly acidic
- Green = neutral
- Purple = strongly alkaline
pH meter/probe: Provides a precise numerical value of pH, ideal for accurate measurements in experiments.
Graph of pH against [H⁺] Tool 1, 2, 3
The graph of pH versus hydrogen ion concentration [H⁺] is a decreasing curve.
![IB Chemistry sketch graph of pH vs hydrogen ion concentration [H⁺] showing logarithmic decrease.](images/pHH+.png)
It shows an inverse logarithmic relationship, because:
pH = –log10[H⁺]
- This means that as [H⁺] increases, the pH decreases, but not in a linear way.
- A tenfold increase in [H⁺] causes the pH to drop by 1 unit.
- At low [H⁺], the pH changes slowly.
- At high [H⁺], the pH drops more steeply.
Summary
- pH measures hydrogen ion concentration using a logarithmic scale.
- pH = –log10[H⁺] and [H⁺] = 10−pH.
- Universal indicators give approximate values; probes give precise measurements.
- Graph of pH vs [H⁺] shows a decreasing logarithmic curve.
Linked Course Question
When are digital sensors (e.g. pH probes) more suitable than analogue methods (e.g. pH paper/solution)?
Digital sensors such as pH probes are more suitable than analogue methods in situations where:
- Higher precision is required: pH probes provide values to 1 or 2 decimal places, whereas pH paper usually only gives a colour range.
- Continuous monitoring is needed: digital sensors can track pH changes over time and record data automatically.
- Data logging or graphing is important: electronic probes can interface with software to plot changes and export results.
- Subtle or rapid changes are being measured: such as in titrations or enzyme activity where small shifts in pH matter.