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The chemistry behind Gaviscon (1 of 3)

Gaviscon works quite differently from a simple antacid. It contains an ingredient, sodium alginate, which forms a gel that floats to the top of the stomach contents. This acts as a barrier to stop acid refluxing into the oesophagus while leaving the acidity of the stomach relatively unchanged. Hence the phrase ‘keeping the acid where it works, not where it hurts’.

The main ingredients of Gaviscon are:

  • sodium alginate;
  • sodium bicarbonate (sodium hydrogencarbonate); and
  • calcium carbonate.

The alginate raft

The key ingredient of Gaviscon Liquid is sodium alginate, which is derived from seaweed.

Active Ingredients
Sodium alginate Ph Eur 500 mg
sodium bicarbonate Ph Eur 267 mg
calcium carbonate Ph Eur 160 mg per 10 mL dose
methyl and propyl hydroxybenzoates
sodium saccharin.

The active ingredients of Gaviscon (Ph Eur stands for European Pharmacopoeia, a compendium of pharmaceutical ingredients used in Europe).

The structure of sodium alginate is shown in Figure 4.


Figure 4: Three sections of sodium alginate polymer molecules showing the two different monomers, ‘G’ and ‘M’.

It is a linear polymer made from two sugar-like monomers (which we shall call ‘G’ and ‘M’) each of which has a −COO- group. For each −COO- group there is a Na+ ion to balance the charges.

G stands for (1 → 4) α-L-guluronate and M for (1 → 4) β-D-mannuronate. These are both sugars, so sodium alginate is a polysaccharide.

You will not know (or be expected to know) any of the chemistry of sodium alginate, but you can make some predictions based on what you know about more familiar compounds. In particular, you should be able to spot the sugar monomers (which have a ring of five carbons and one oxygen) in the structures in Figure 4.


Question 3

Although it is a large molecule, sodium alginate is quite soluble in water. Explain why you would expect this.

Answer 3

It is an ionic compound as each molecule has a large number of −COO- groups (and the same number of Na+ ions). It also has a number of polar groups such as −OH and −COO- which can form hydrogen bonds with water molecules.