According to the Office of National Statistics, in 2008 almost 40% of homes in the UK had a dishwasher, up from around 25% in 1998. It is tempting to think of machine dishwashing as comparable to hand washing of dishes but in fact it is a much more complex process and involves a great deal more chemistry.
Hand dishwashing relies mostly on mechanical effort - continuing to brush or sponge (with the help of hot water and a detergent), until the dishes appear clean. With machine dishwashing, mechanical effort counts for less, and the chemistry has to do the cleaning. However, the conditions for machine washing can be both hotter and more alkaline that could be withstood by human hands.
Here we will look at some of the chemistry that goes on inside the dishwasher and relate it to chemistry that you already know. The science of dishwasher products is complex and we can only cover the basics here.
Did you know?
Josephine Cochrane invented the modern dishwasher in 1886.
Cochrane was a wealthy socialite whose servants kept chipping her fine china while hand washing it. She developed a rack and water jet system that was first demonstrated at the 1893 Chicago World Fair.
Figure 1: A product from the Finish range
How does a dishwasher work? (1 of 2)
Before we consider the chemistry, we need to understand the mechanics of a dishwasher, Figure 2. This is essentially very simple.
Figure 2: The basic layout of a dishwasher (courtesy of howstuffworks.com)
A pump draws water through a water softener unit / ion exchanger and into the base of the machine. Here a heater heats the water to a suitable temperature and the dishwasher product dispenser opens so that the dishwasher tablet or powder containing a mix of chemicals falls into the water after approximately 5-10 minutes, when the water is still cold.
A pump then forces the water through two rotating spray arms which spray the water onto the dishes arranged in two baskets, Figure 3.
Figure 3: The washing cycle (courtesy of howstuffworks.com)
The dirty water is removed from the base of the machine, Figure 4.
Figure 4: The draining cycle (courtesy of howstuffworks.com)
This is then refilled with clean water which is sprayed on the dishes to rinse them. You can find more detail from the links below.
How does a dishwasher work? (2 of 2)
At various points in the cycle, the water is heated to a suitable temperature for the process taking place. The profile of a typical washing cycle is shown in Figure 5 in which temperature is plotted vertically and time horizontally.
Hot water is shown in red, cold (ie room temperature) in blue and sections involving the
ion exchange resin in white.
Figure 5: A typical dishwasher cleaning cycle. Only intensive programs use prewash cycles. Normal or Eco programs immediately start with the main cycle. The detergent is only dosed in the main
Softening the water
The first chemical process in dishwashing is softening the water. Water in most areas of
2+2+3+the UK contains significant levels of Ca and Mg (and in some cases Fe) ions.
These arise from the fact that in many areas water soaks through limestone rocks and dissolves some compounds containing these ions. This is assisted by the fact that rainwater is naturally acidic due to dissolved carbon dioxide. So tap water contains dissolved compounds such as calcium hydrogencarbonate and magnesium sulfate.
-3Figure 6: Hardness of water in the UK. The units are mg/l, ie mg dm, of calcium carbonate
equivalent (Courtesy of Caterchem UK)
The map, Figure 6, indicates generally the hardness of water in various parts of the UK. You can find the level in your area by typing your postcode into the following site: http://www.homesolutionsnews.com/waterhardness/uk/popup1.jsp
These metal ions in hard water can interfere with a number of the chemical processes involved in dishwashing, which are explained below, and it is therefore better to remove them. They may also cause limescale to build up inside the machine. There are more details of the chemistry of hard water and its effect on cleaning processes in Calgon.
Dishwashers soften water by passing it through an ion exchange resin before it enters
the machine itself. The resin is well-named – it literally exchanges doubly and triply
positively-charged metal ions for singly positively-charged sodium ions which do not affect the washing processes.
Explain why metals always form positive ions.
This cannot go on forever, of course - the resin eventually runs out of sodium ions to exchange. To prevent this situation the resin is regenerated by flushing a concentrated solution of sodium chloride through it. This replaces the calcium and magnesium ions on the resin for sodium ions again and the calcium and magnesium ions are flushed away. From time to time, pure salt (sodium chloride) must be added to the reservoir in the dishwasher where the salt solution is made. Table salt should not be used, as most brands of this contain magnesium sulfate as an anti-caking agent - the last thing required in a water softener.
This process is essentially a reversible reaction and can be represented by an equation, where R represents the resin:
++2+2++RNaNa + Ca RCa + 2Na (s)(aq)(s)(aq)
Although it is a reversible reaction, it never reaches equilibrium as in both softening and regeneration the water is constantly passing over the solid resin.
What does a dishwasher cleaning agent contain?
The purpose of a dishwasher cleaning agent is to remove dirt (often called ‘soil’) from
the articles inside by dissolving it or making it into a suspension in the washing water. It must also prevent its re-deposition on the articles so that the soil can be carried away by the washing water.
It must be chemically compatible with all the materials being washed and also with the materials from which the dishwasher itself is made.
Dishwasher tablets and powders contain a surprising mix of chemicals, far more than just the detergent you might expect.
These are as follows:
； Surfactants (detergents) - these promote mixing between oil- and fat-based soil
； Alkalis - these emulsify grease and adjust the pH of the water to the optimum for
the other components to work
； Bleaches - these oxidise coloured substances to colourless ones
； Biosubstances - these are enzymes that break down starch- and protein-based
； Builders - these help to soften water and trap metal ions that would interfere
with the cleaning process and hold dirt in solution
； Auxiliaries - these include substances used to make and disintegrate the tablet
as well as colours and perfumes
One problem that may be encountered is getting all these different ingredients to work together when they have different optimum conditions. Enzymes, for example, work best at moderate temperatures around 50 ?C - see Figure 7. They are denatured and will no longer work if they have been exposed to temperatures much above this for any length of time.
Figure 7: The temperature dependence of enzyme activity. Proteases decompose proteins and
amylases decompose starch-based soils
On the other hand, grease removal will work best at high temperatures when fats melt to oils making them easier to remove. And, of course, all chemical reactions go faster at higher temperatures - twice as fast for every 10 ?C rise is a useful rule of thumb. There is a similar issue with pH. Cleaning takes place best at alkaline pHs but the optimum pH for most enzymes is neutral to mildly alkaline - they are denatured in strongly acidic or alkaline environments. The optimum pH for the bleach, however, is around 10.
One further complication for the designers of dishwashers and their associated cleaning products is to do with user behaviour; it is usual to wash together a large variety of materials e.g. glass, porcelain, plastics, and different metals. This is in contrast with clothes washing, where users seem prepared to separate out different types of material - wool, synthetics, cotton etc. The optimum conditions for many of the materials in the
dishwasher will differ.
Surfactants or detergents (1 of 2)
Greasy stains do not mix with water because the main interactions between water molecules are hydrogen bonding and those between molecules of oils and fats (which constitute grease) are van der Waals forces.
To get water and grease to mix we use molecules called surfactants or detergents. These two terms refer to essentially the same thing - molecules that are ‘tadpole shaped’ in
that they have a non-polar ‘tail’ and a polar or ionic ‘head’. The ‘tail’ can form van der Waals bonds with non-polar grease molecules whilst the ‘head’ can form hydrogen
bonds with water, Figure 8. This is an example of the ‘like dissolves like’ rule.
Figure 8: The non-polar tails (in yellow) of ‘tadpole shaped’ detergent molecules mix with grease,
while the polar heads (in red) mix with water, thus forcing the grease and water to mix There are essentially three types of surfactants – anionic, cationic and non-ionic.
Anionic surfactants have a negatively charged head. Common types include soaps, Figure 9, and alkylbenzene sulfonates, Figure 10.
Figure 9: Sodium stearate (a soap) – an anionic surfactant
Figure 10: Sodium dodecylbenzene sulfonate – an anionic surfactant
Cationic surfactants have a positively charged head. Common types include alkyl ammonium chlorides, Figure 11.
Figure 11: Trimethylhexadecyl ammonium chloride – a cationic surfactant
Non ionic surfactants have a polar, but uncharged, head. Common types include
polyethylene ethoxylates, Figure 12.
Figure 12: A polyethylene ethoxylate – a nonionic detergent
Surfactants or detergents (2 of 2)
Surfactants are probably the ingredients one expects in a dishwasher product - we often call the product simply a dishwasher detergent. However, they play a relatively minor role in the product. Non-ionic polyethylene ethoxylates are chosen as the main surfactants in Finish as they produce relatively little foam.
Their main function is to enable greasy soils to mix with water. Much of the soil on dishes is held there by grease. Greases and oils are chemically similar; they consist largely of esters of fatty acids - long chain carboxylic acids - with glycerol, see Figure 13. These contain long hydrocarbon chains which are essentially non-polar, and therefore do not form hydrogen bonds or dipole-dipole bonds with water molecules.
Figure 13: A typical fat or oil - which it is depends on the chain lengths of the three fatty acids and
whether or not their hydrocarbon chains are branched
When dissolved in water, surfactants tend to cluster at the surface (hence the name), so that their non-polar tails can stick out of the water. Surfactant molecules can form structures called micelles: these are small spheres made of surfactant that trap oil molecules and enable them to dissolve in the water.
The difference between fats and oils is one of melting point - at room temperature fats are solid and oils are liquid due to their shorter hydrocarbon chains. Solid greases adhere to surfaces better than liquid oils which just tend to roll off. So a high temperature which turns fats into oils is helpful for cleaning.
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
； 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
a) Explain why the products of the above reaction are more soluble than the original fat molecules.
b) Give the systematic name of glycerol.
Sodium carbonate, NaCO, is used in the dishwasher product to make the solution 23
alkaline. It is the salt of a strong alkali (sodium hydroxide) and a weak acid (carbonic
2-acid) and is therefore alkaline. It dissociates in solution to form carbonate ions, CO, 3
which help to maintain the pH of the washing water at around 10.