Chemical Safety – Know Your pH Correctants

Carbon Dioxide (CO2)

CO2 is a non-toxic and non-flammable gas, colourless and odourless but with a characteristic taste and pungency at higher concentrations. The normal concentration of CO2 in the air that we breathe is approximately 400 ppm (0.04% by volume). If its concentration in the ambient air is increased, the pulmonary gas exchange in the lungs is compromised. In simple terms, as its concentration in the ambient air increases, lower quantities of CO2 leave the body and so there is less room for oxygen (O2). Without sufficient O2 one cannot live. This effect is called intoxication.


Carbon Dioxide Cylinders
Carbon Dioxide Cylinders


CO2 is easier and safer to handle than other acids – no direct contact. It can be supplied in cylinders or bulk tank. Unlike other acids, it is not possible to mix CO2 with sodium or calcium hypochlorite (in liquid form) through spillage in bunds or operator error when acids are mixed in day or main tanks with hypochlorite. This means no possibility of accidental production of chlorine gas – a significant hazard in swimming pool installations.

Hydrochloric Acid

Hydrochloric acid is supplied in liquid form of varying strengths. It can be used for pH control when the pool water is treated with sodium or calcium hypochlorite. It is a colourless, odourless, non-fuming liquid when supplied as a 10 or 5% solution. It is a safer acid to handle than sulphuric acid (discussed later).


Hydrochloric Acid
Hydrochloric Acid


Stronger solutions (up to 35% concentration) are pungent and fuming and present a greater hazard during handling. Good seals are required when stronger acid strengths are utilised, to prevent the fumes escaping into the atmosphere. Fumes are a threat to the fabric of the building as well as people’s health.

It is corrosive substance and can cause skin burns and eye damage on contact; ingestion can burn the mouth, throat and stomach. It is irritating to respiratory system and may cause respiratory failure at acute doses. Chronic exposure may cause asthma

The quantities required for public pools may present a storage problem. As with any acidic liquid it produces chlorine gas when mixed with hypochlorites.

Sodium Bisulphate (Dry Acid)

Sodium bisulphate, (sometimes termed dry acid) is supplied in crystal/powder form, generally white. A powder as opposed to liquid is thought to reduce mixing accidents.


Sodium Bisulphate
Sodium Bisulphate


It needs to be put into solution before use. As with any acidic liquid it produces chlorine gas when mixed with hypochlorites. Reaction can lead to exposure to a sulphuric acid mist, which over a long period of time is thought to be carcinogenic (may cause cancer). It causes irritation on contact with skin. There is risk of damage to the eyes.

Sulphuric Acid

Sulphuric acid is supplied as a liquid in various strengths. PWTAG has for some time been concerned about the use of concentrated sulphuric acid (48%) for pH correction in swimming and spa pool water.

It is a highly dangerous, corrosive acid; the greater the strength the greater the hazard. COSHH (Control of Substances Hazardous to Health) regulations place on employers the responsibility to use the ‘least hazardous’ chemical that gives satisfactory performance. As with any acidic liquid it produces chlorine gas when mixed with hypochlorites. Great care is required when handling.


Sulphuric Acid
Sulphuric Acid


Exposure to a sulphuric acid mist over a long period of time is thought to be carcinogenic (may cause cancer). It causes serious skin burns and eye damage on contact with skin, damage to the gastrointestinal tract if ingested, and lung damage if inhaled.

When mixed with water, concentrated sulphuric acid has a vigorous exothermic reaction (i.e., producing heat) and produces significant fumes over and above the background fuming of hydrochloric acid.  When dosed via a lance direct from the canister, a residue is left in the bottom of the drum which has to be removed prior to the return of the canister.

Sulphuric acid, when used undiluted direct from the delivered canister or drum, is of higher concentration compared with other acids and therefore very effective. Canisters can be fitted directly with suction lances, so only the canister needs to be handled, reducing exposure to the operator.

If there is no bulk storage facility, sulphuric acid should not be dosed via a day tank so that operators do not have to transfer the liquid from the drum that it is delivered in. All of the dosing equipment should be fitted with suction lances to fit the drum of which the acid is delivered. This means that, as with hydrochloric acid, exposure of this acid to the operator is minimised.

If it is used, it should not be as concentrated acid (48%), but diluted to 25% or lower by the supplier. Personal protection equipment should be worn when handling it (as for all of these chemicals).

Dosing needs to be monitored initially very carefully and dosed as required to adjust the pH slowly. Dosing should be undertaken only using an automatic dosing device (control of dosage and pH with pH electrode).

If pools are using a calcium hypochlorite circulation feeder whose nozzles need cleaning with acids, hydrochloric acid is the 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 acid is fed again.

Dealing with Spillages

Small spillages can be neutralised by containing the spill and using a spill kit to neutralise the acid. Larger spillages should be dealt with by calling the Fire & Rescue Service.

Bulk Delivery Warning

A key risk is the potential for mixing if the delivery person connects to the wrong bulk tank. Although different size nozzle adapters are installed at design, some delivery drivers carry delivery nozzle adaptors so that they can deliver into any tank, whatever the site tank connector size.