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Control measure

Dilution: Corrosive materials

Control measure knowledge

This control measure should be read in conjunction with National Operational Guidance: Environmental Protection - Dilution

Dilution is a chemical method by which a water-soluble corrosive solution release is made larger but less corrosive by adding large volumes of water. This should be regarded as a last resort if no other method can be used. Specialist advice should be taken to ensure the corrosive can be safely and effectively diluted. Consultation with environment agencies and site operators must take place prior to any dilution.

These important criteria must be considered before dilution:

  • Concentration
  • pH
  • Water reactivity
  • Potential to generate heat and/or toxic vapours on contact with water
  • Potential to form any kind of solid or precipitate
  • Water solubility

As a general rule, dilution should only be attempted on liquid and solid corrosives, and only when all other reasonable methods of mitigation and removal have proven unacceptable.

In emergency situations, water sprays or 'curtains' may be used for controlling the spread of corrosive vapours to populated areas. This tactic will lead to a degree of 'scrubbing' of the vapour or gas cloud if the corrosive is water soluble. This means that the water run-off may itself become corrosive. This side effect may cause environmental damage unless it is controlled.

Dilution can be effective for small corrosive spills of up to one litre but environment agencies, water authorities and landowners should be consulted if the products of dilution are to be dispersed on-site.

The major disadvantage to dilution is that it is not well understood by emergency responders. It is not a straight, linear, one-to-one process. It is important to recognise that dilution is actually a logarithmic process (on a one-to-ten scale). The viability of the tactic will depend on the resulting pH that can be achieved.

Table 1: The resulting pH when diluting a one-litre spill of acid with increasing amounts of water
One-litre spill of acid with pH of 0
Water needed to dilute it 1 pH level Resulting pH
10 litres 1
100 litres 2
1000 litres 3
10,000 litres 4
100,000 litres 5
1000,000 litres 6
10,000,000 litres 7 neutral

However, partial dilution to reduce the level of corrosivity and therefore reduce the hazard may be a suitable tactic in certain situations following specialist advice.

See National Operational Guidance: Environmental protection

Strategic actions

Fire and rescue services should:
  • Have procedures in place to enable responders to safely dilute corrosive materials

  • Ensure that responders understand the potential risks associated with dilution

Tactical actions

Incident commanders should:
  • Consider dilution and dispersal of corrosive materials as a last resort following consultation with relevant environmental agency

  • Confirm the suitability of the corrosive materials for dilution, in terms of physical and chemical properties

  • Consider using water sprays and 'curtains' to reduce and control corrosive vapours when they are threatening populated areas (but be aware that the run-off will be corrosive)