RAI Training

Chemistry: Cleaning

Cleaning is the removal of unwanted material (‘soil’) such as visible dirt, food residues, grease or other contamination from any surface. When presented with a cleaning problem, there are five things to consider before deciding the appropriate products and method of use:

  • The type of surface: Hard, soft, porous etc
  • The type of soiling: Chemical composition, solid or liquid etc
  • Cleaning methods: Manual or automatic? What ancillaries are required?
  • Water: Temperature and hardness of available water
  • Time: How much is available?

All these questions need to be considered before you decide on the appropriate course of action and necessary products. Then it’s about methods. Manual or automatic? How much time is available? Then the question is…

[important]Organic vs Inorganic Soiling[/important]

Whether the soiling is organic or inorganic pretty much dictates how you shift it. Organic soils derive mainly from animal or plant matter. Examples are oils, fats, grease, proteins, sugars, starches, carbohydrates and cellulose. Inorganic soils include grit, salt, rust and limescale. The Golden Rule is:

Acids Shift Scale. Alkalis Shift Shite

In general, alkaline detergents work best on organic soiling, and acid detergents work best on inorganic soiling. In a kitchen pretty much all soiling is organic: fats, oils, greases, food residues etc. Pretty much the only inorganic one will be water scale on surfaces, floors, dish wash machines etc in a hard water area. Some soils are a mixture of both: a detergent is needed which attacks one of the constituents such that the whole soil can be removed.

[important]The Importance of pH[/important]

pH is the ‘power of hydrogen’ in a solution and is a measure of acidity or alkalinity. It is quoted on a numeric scale between 1 and 14.

pH 1 is very acidic – i.e. lots of hydrogen (H+) ions in solution

pH 14 is very alkaline with lots of hydroxyl ions (OH) in solution

pH 7 is neutral as the H+ and OH neutralise each other to form H-OH or water – more commonly written H2O

The intermediate numbers have varying degrees of acidity or alkalinity. With regard to cleaning, acidic solutions are generally used to remove inorganic soils such as rust and limescale. The acid dissolves these types of materials – many are carbonates so you see the carbon dioxide (CO2) gas fizzing off. Or at least you will with a decent product. Examples are toilet cleaners and kettle or boiler descalers.

Alkalis are generally used to remove organic soils such as fats and grease. The alkali will break down the fats making the residue soluble or dispersible in water. It’s called saponification: alkalis turn fats into soap which is why a greasy floor gets as slippery as a bag of arseholes when you put an alkali on it. While we rely on thermal disinfection in dish washers the fact is the alkali in a proper machine wash turns microbial cell walls into soap.

Alkali based cleaners are used for example to clean meat processing areas and food preparation surfaces.


You can’t use acids on metals especially aluminium, brass, many natural stone surfaces (notably marble and granite) and strong, mineral acids can corrode stainless steel. Alkalis also knacker aluminium – and don’t forget most professional cleaning products designed to shift shite are very strong alkalis. Read the bleedin’ label.

[important]Other Considerations[/important]

pH is just one factor to take into account when selecting a suitable cleaner. The wetting and emulsifying nature of surfactants and sequestration of hardness and soils by builders are also important factors. Solvent cleaners are neither acidic nor alkaline. They do not respond to pH measurement because they are not aqueous (i.e. dissolved in water). They can be considered as neutral cleaners in terms of their corrosive (or more likely non-corrosive effect on metals). Care needs to be taken when using solvents on painted or plastic surfaces as there can be some solvent attack on certain materials.