Sulphuric Acid Technical Note

PWTAG Technical notes are updates or new material for the standards and guidance given in the PWTG book, Swimming Pool Water and the PWTAG Code of practice and should be read in association with these publications.

  • Subject: Sulphuric Acid
  • Date: January 2011

Sulphuric acid is sometimes considered for use as an alternative to carbon dioxide, hydrochloric acid or sodium bisuphate (dry acid) to reduce pH when using hypochlorite disinfectant.

PWTAG does not recommend sulphuric acid for this, as it is a more hazardous material at high strengths than the alternatives. COSHH regulations place on employers the responsibility to use the ‘least hazardous’ chemical that gives satisfactory performance.

When mixed with water, concentrated sulphuric acid has a vigorous exothermic reaction (ie producing heat) and produces significant fumes. It can be fatal if ingested and causes serious burns on contact with the skin. If it is used, it should not be as concentrated acid, but diluted to perhaps 25% or lower by the supplier. Personal protection equipment should be worn when handling it.

Like hydrochloric acid and sodium busulphate, it will produce chlorine gas when mixed with hypochlorites. Another byproduct is sulphates, so additional dilution of the pool water may be required to maintain the sulphate concentration below 360mg/l to preserve the integrity of the pool fabric.

If pools are using calcium hypochlorite circulation feeder whose nozzles need cleaning with acids, hydrochloric is a safer choice. In any case, the acid used must be stored appropriately, the manufacturers’ instructions followed, and the acid flushed out to waste before any hypochlorite is fed again.