Help your 16–18 students tackle pH, buffer solutions and the Henderson–Hasselbalch equation with this infographic and accompanying worksheet 

Changes in pH can have a big impact on natural systems, so it’s important that the hydrogen ion concentration stays relatively constant. Buffers minimise the change in pH that would otherwise happen if an acid or alkali were added to an aqueous solution.

A buffer is created when a weak acid and its salt are both present in solution. The buffer solution utilises the reversible reaction involving the loss of a proton from the weak acid.

An equation showing the reversible reaction in a buffer solution

The reversible dissociation of an acid (HA) into its conjugate base (A-) and a proton/hydrogen ion (H+)

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This infographic is designed to be displayed as a poster in the classroom, although it can also be displayed on a projector or printed as a handout.

Use the accompanying fact sheet and worksheet to get your students to apply their mathematical skills in different contexts.


The pH of a buffer solution can be calculated using the Henderson–Hasselbalch equation, which is derived from the equilibrium constant equation for dissociation of a weak acid with general formula HA.

The Henderson–Hasselbach equation

Covid-19 lateral flow test

The lateral flow test for Covid-19 contains antibodies to a protein that is produced by the virus. The binding of the antibody to the virus protein will only work if the pH is approximately 7.4.

The sample taken from the nose is mixed with a phosphate buffer solution to prevent any acids or alkalis in the sample from changing the pH too much.

  • H2PO4-(aq) ⇌ H+(aq) + HPO42-(aq)  pKa = 7.21 at 25°C

A cartoon of a hand dropping solution onto a small plastic lateral flow test

Source: © Dan Bright

Did you know …?

The pH of nasal mucus can vary from 5.5 to 7.0.

Healthy hair

The ideal pH for the scalp is around 5.5. Shampoo ingredients are often more alkaline than this, which could damage the scalp and change the properties of the hair.

In a pH balanced shampoo, a buffer is added to keep the pH at 5.5 or lower. Citric acid is often used as it can control the pH at any value from 3 to 7.

  • Dissociation of the first H+ ion: C6H8O7(aq) ⇌ H+(aq) + C6H7O7-(aq)  pKa = 3.13 at 25°C
  • Dissociation of the second H+ ion: C6H7O7-(aq) ⇌ H+(aq) + C6H6O72-(aq)  pKa = 4.76 at 25°C
  • Dissociation of the third H+ ion: C6H6O72-(aq) ⇌ H+(aq) + C6H5O73-(aq)  pKa = 6.39 at 25°C  

A cartoon of a bubbly bottle of shampoo with a label that says pH balanced

Source: © Dan Bright

Did you know …?

As the pH of hair increases, the fibres gain more negative charge. This causes repulsion, which makes hair more difficult to style.

Oceans in peril

The increased level of atmospheric carbon dioxide due to human activity is leading to a decrease in pH of the oceans. This is affecting marine ecosystems, in particular the survival of plankton, molluscs and coral that depend on dissolved carbonate ions to make their shells.

Dissolved carbon dioxide increases the ratio of hydrogen carbonate to carbonate in the natural buffer system that helps control ocean acidity.

  • HCO3-(aq) ⇌ H+(aq) + CO32-(aq)  pKa = 10.32 at 25°C

A cartoon of a coral reef with a fish and cone snail

Source: © Dan Bright

Did you know …?

Since the 1980s, over half the coral that makes up Australia’s Great Barrier Reef has died.

All illustrations © Dan Bright 

All equations © Royal Society of Chemistry