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HEALTH AND SAFETY NOTE

Issued by: SEPS
Reference Number: 98/019
Issue Date: February 1998

Perchloric acid is extremely hazardous, and every effort must be made to find safer alternatives, its use should be avoided when possible. Perchloric acid is bought as a 70-72% solution (the dihydrate) and in this form it is simply an extremely corrosive liquid. Its major hazard, however, is the variety of ways it can be involved in explosions and fires. When the water concentration falls (due to distillation or addition of a dehydrating agent), the acid becomes an extremely strong oxidising agent and can react explosively with organic materials. The acid itself can explode: organic perchlorates can explode: some inorganic perchlorates can explode. The materials can be sensitive to heat, friction or shock, and even the most trivial of mishandling can have catastrophic results.

The following is an extract from the C.R.C. Handbook of Chemical Safety *

" Considerable interest has been taken in the explosive hazards to be encountered in the use of perchloric acid since a mixture of perchloric acid and acetic acid exploded in a Los Angeles factory in 1947, killing 15, injuring 400, and causing $2 million damage. On a smaller scale, Robinson reported a detonation of 3g of a perchlorate salt of a rhodium-polyamine complex undergoing an evaporation step in a rotary evaporator. A violent explosion destroyed the evaporator, smashed a lab jack, cracked the bench top and chipped walls over 15 feet away. Fortunately, this happened in an empty laboratory."

(25 other incidents are listed)

We therefore feel that perchloric acid and its compounds should not be used without first having the process vetted by Safety and Environmental Protection Services. Some tried and tested analytical procedures are probably quite safe but equally some processes from research papers, especially from older publications, may be suspect.

If it is still felt essential to use these substances then take the following steps :-

1. Carry out a risk assessment ( based on Appendix 1 of "Effective Safety Management") and work out what you want to do and how you will minimise the risks.
2. Discuss the procedure with SEPS to have the assessment and procedure validated.
3. When an assessment and procedure have been validated, the procedure should be carried out as agreed and without modification.

* The Chemical Rubber Company Handbook of Chemical Safety, third edition, (1990) pp 273-284

When the use of perchloric acid or a perchlorate is essential to the process, the following arrangements should be made :-* The substance must be stored safely, ideally in an enamel tray of sand, away from organic materials or dehydrating agents such as sulphuric acid.

• Permission should be sought from the Head of Department before contemplating the use of perchloric acid or perchlorates in a process.
• Any process involving perchloric acid and organic materials should be viewed as high risk.
• Formulate emergency plans for dealing with an explosion or spillage during use.
• The amount of material used should be as small as possible.
• Any process using the acid or perchlorates involving heat or dehydrating agents must be carried out behind blast shields.
• Heating should be by use of a sand bath, a water bath or heating mantle, never an oil bath.
• Perchloric acid must not be used on wooden benches nor in wooden fume cupboards
• Vessels must not be sealed with corks or rubber bungs, nor with greased glass stoppers.

When perchloric acid digestions are to be carried out this should be done in a fume cupboard fitted with a scrubber. There have been several incidents where perchloric acid fumes and organic materials such as dust have interacted in fume cupboard ducts causing explosions. There have also been several reports of old wooden fume cupboards exploding after the use of perchloric acid. During a digestion it is strongly recommended that the sample is treated first with concentrated nitric acid to remove any easily oxidised material before adding the perchloric acid. The fume cupboard used must be kept clear of any other materials.

The use of "Anhydrone"(magnesium perchlorate) for drying organic solvents should be considered as a high risk operation. A suitable alternative drying agent should be sought.

Dismantling of fume cupboards used for perchloric acid The dismantling or relocating of fume cupboards or their ducts after perchloric acid has been used is extremely hazardous. Suitable procedures should be set up after discussion with the Safety and Environmental Protection Services.

Spillage of perchloric acid. When dealing with a spillage of perchloric acid appropriate personal protective equipment must be worn. After perchloric acid has been spilled on the floor or bench it should not be mopped up nor soaked up with dry combustible materials such as sawdust or paper towels. It should first be neutralised with sodium bicarbonate and then mopped up. The spill area should then be thoroughly washed down. The mop should then be thoroughly rinsed out and while still wet, sealed in a plastic bag and disposed of as flammable waste.

The use of perchloric acid will only be countenanced after all reasonable alternatives have been considered and discarded as not being reasonably practicable. A full risk assessment for the process must be carried out. This exercise will cover the storage, use transportation, disposal and emergency arrangements appropriate to the work. SEPS will only validate the use of perchloric acid on receipt of this information and agreement with the conclusions.

Director:
Dr. Andrew D. Rankine

Contact:
safety@gla.ac.uk

Pearce Lodge, University Avenue, University of Glasgow, G12 8QQ

TEL: 0141 330 5532 FAX: 0141 330 3545

Page Editor:
Jo McNally