The pH scale in cleaning

The pH scale runs from 0 to 14: 0 is strong acid, 7 is neutral, and 14 is strong base. Where a cleaning agent sits on the scale determines which contaminants it can dissolve — and which surfaces it is gentle on. But pH is only half the story: a cleaning agent is a formula of several components, where surfactants often do the actual work.

The higher the pH, the more powerfully grease-dissolving — and the more aggressive. Strongly acidic agents must NOT be used on acid-sensitive surfaces such as natural stone, limestone and marble, where they eat away the surface.

Surfactants (surface-active substances) are the core of most cleaning products. A surfactant molecule has a water-loving (hydrophilic) end and a fat-loving (hydrophobic) end. This does two things: it lowers the surface tension of water, so the washing better penetrates into and under the dirt, and it emulsifies fat — wraps fat droplets in small spheres (micelles) that can be rinsed away with water. This is why a neutral product with good surfactants often can handle fatty dirt, which pH alone would not loosen.

In addition to surfactants, professional cleaners often contain builders (that bind lime and keep soil in suspension), chelating agents, possibly enzymes against protein/fat, and — in sanitation cleaning — an acid against lime. The composition, not just the pH value, determines what the product can do.

Danish water is in many places hard, i.e. rich in calcium and magnesium. Hard water 'uses' some of the surfactants to bind the lime, so there is less left to clean — and it leaves limescale on bright surfaces. Therefore many products contain softening / sequestering agents that bind calcium and magnesium, so the surfactants can work freely. This also explains why overdosing does not necessarily clean better: once the lime is bound, extra product makes no difference — it just leaves film and foam.

If you use an acidic product (e.g. sanitary cleaner) and then a basic product (e.g. degreaser) without rinsing in between, they neutralise each other on the surface — acid + base forms salt and water, and both products lose their effectiveness. Therefore, you rinse with clean water between two products with opposite pH. It is the same chemistry that lies behind the safety rule further down: the neutralisation can occur violently and release heat or gases.