The chemistry of a dishwasher

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Huge piles of washing up can be daunting – but luckily, dishwashers can make light work of it. What’s in the colourful dishwasher tablets that helps them get your dishes sparkling and clean? This graphic takes a look at some of the key components.

Before the dishwasher even starts cleaning dishes, it needs to resolve chemical issues with the water it will use to wash them. In many locations, water is referred to as ‘hard’. In chemical terms, this means that it contains significant levels of dissolved calcium and magnesium ions. These can lead to the formation of limescale, which is bad news for your dishwasher. We looked at how limescale forms (and how it can be removed) in a previous graphic.

Dishwashers use an ion exchange resin to remove calcium and magnesium ions from water. The ion exchange resin contains sodium ions. As the name suggests, as water passes through it calcium and magnesium ions swap places with sodium ions, softening the water. The supply of sodium ions is finite, so from time to time it needs to be topped up. This is why you have to add salt to the reservoir in the dishwasher. Adding salt leads to more sodium ions replacing the calcium and magnesium ions, which then wash away.
 
Dishwasher detergents and tablets often containing compounds referred to as builders. These continue to remove calcium and magnesium ions during the washing process. Phosphates, citrates, and polycarbonates bind and remove the calcium and magnesium ions. The use of phosphate compounds for this purpose has lessened in recent years due to ecological concerns. Namely, they can cause algal blooms in waterways, which can have further impacts on ecosystems.
 
After the water coming into the dishwasher has been softened, dishwashing can begin. Dishwasher tablets contain a range of components to help them clean. Some of the most important are surfactants. These are a class of molecules found in many other cleaning products including shampoos and laundry detergents. One end of surfactant molecules dissolves in water, while the other dissolves in oil and grease, helping to wash it away. Nonionic surfactants are often used in dishwasher detergents, as they foam less than other types.
 
The alkaline compounds in dishwasher detergents are also important for cleaning. They can react with grease and break it down into soluble compounds, which wash away. Alkalis also prevent the metal parts of the dishwasher from acid corrosion. In addition, they ensuring optimum alkalinity for other components in the dishwasher detergent to do their work.
 
Alkalis do a good job of breaking down fatty deposits on plates, but we need enzymes to remove some other stains. Amylases break down starches, and proteases break down proteins. Because enzymes stop working at high temperatures, they do their work early in the dishwashing cycle. After this, the temperature is increased, and it’s the turn of other cleaning components.
 
Bleaches are next in line. They oxidise coloured compounds, breaking down colour-causing sections of chemical structures. Peroxide-releasing compounds, such as sodium peroxycarbonate, are often used in this role. Activators are also present in the dishwasher detergent. These compounds react with the hydrogen peroxide to form more efficient bleaching agents which work better at lower temperatures. Tetraacetylethylenediamine (TAED) is the main activator used in Europe. In the U.S., sodium nonanoyloxybenzoylsulfate (NOBS) is used.
 
At the end of the wash cycle, there’s still some chemistry needed to make sure that your dishes dry quickly. This is where rinse aid comes in. Rinse aid contains surfactants which help to lower the surface tension of water. With a lower surface tension, water forms thin sheets on the surface of dishes and glasses instead of droplets. This means the water evaporates more quickly and doesn’t leave water marks on glasses.
In summary, your dishwasher might save you some work, but it’s clear it’s doing a fair amount of chemical work itself to get your dishes sparkling. Something to think about next time you pop a dishwasher tablet into your dishwasher and start it up!