Chemistry in your cupboard

Alkalis

Many cleaning products act best in somewhat alkaline solution.

This is because alkalis

• emulsify grease by reacting with insoluble fatty acids to form ionic salts which are soluble
• protect the metal of washing machines and dishwashers from acid corrosion
• help to reduce re-deposition of dirt that has been removed, by coating particles of it with negatively charged hydroxide ions - this means the dirt particles repel each other and remain in suspension rather than clumping together to form large aggregates which would tend to precipitate out onto dishes
• percarbonate-based bleaches work best in somewhat alkaline solution

The dishwasher water is kept at about pH 10 - significantly alkaline. At the high temperature of the wash, hydroxide ions react with the molecules of grease and break them up into salts of fatty acids and glycerol. This is the main mechanism for removing grease.

The reaction is shown in Figure 14 and is the same as that for making soap from fats and oils - saponification.

Figure 14: The saponification of grease (click to enlarge)

Sodium carbonate, Na₂CO₃, is used in the dishwasher product to make the solution alkaline. It is the salt of a strong alkali (sodium hydroxide) and a weak acid (carbonic acid) and is therefore alkaline. It dissociates in solution to form carbonate ions, CO₃^2-, which help to maintain the pH of the washing water at around 10.