Zone 2 lighting is usually used in the area surrounding hazardous substances like petrol and other potentially explosive chemicals – so why might it be necessary near dirty water?

A study in the Journal of Hazardous Materials carried out at the National Institute of Advanced Industrial Science and Technology in Japan helps to answer this question.

The researchers modelled an incident that occurred back in March 2005, when five litres of sodium hypochlorite aqueous solution – used in processes including water purification – was poured into 300 litres of liquid waste which contained ammonium.

Just two minutes later, the solution exploded, breaking through the tank cover, the air duct and the “fluorescent lamp” illuminating the area.

The study’s abstract does not say whether this lamp provided the source of ignition, but in the presence of potentially explosive materials, Zone 2 lighting is a sensible safety precaution.

In their research, the scientists found that the presence of platinum black can increase the risk of explosions in solutions of ammonium sulphate and sodium hypochlorite, due to the formation of nitrogen trichloride.

To reduce this risk – aside from sensible precautions such as hazardous area lighting – the study recommends NH4 at a concentration below 3 mol/L, hypochlorite below 0.6 mol/L at room temperature, and pH higher than 6.