Chemistry in your cupboard

Suspension and regrowth inhibition

Another important ingredient of Calgon® is polycarboxylate. This is a polymer based on the monomer whose non-systematic name is acrylic acid. Its skeletal formula is shown in Figure 12.

Figure 12: Structure of acrylic acid

A short section of a polyacrylate (polycarboxylate) molecule is shown in Figure 13.

Figure 13: Structure of polyacrylate

This polymer can also complex metal ions in the same way as can citrate ions.

Notice that in solution COOH groups of the polymer exist as COO^- ions. These ions can form a coating around any calcium carbonate that has formed in the wash mixture and hold it in suspension. This stops it depositing on parts of the machine, such as the heater, or on fibres of clothing.

This is similar to the complexing effect of citrate and phosphate ions on Ca²⁺, but as the polymer molecule is larger, it can hold larger aggregates of limescale in suspension. Figure 14 shows this schematically.

Figure 14: Suspension inhibition. The polymer coats the limescale particle and prevents its deposition

Regrowth inhibition

This works in the same way as discussed above. Calcium carbonate will not deposit on parts of the machine that become coated with polyacrylate.