Use of Hydrofluoric Acid

Hazards: – CORROSIVE – TOXIC

• Causes severe burns.
• Very toxic by inhalation, in contact with the skin and if swallowed.

Hydrofluoric Acid has a number of properties which make handling particularly difficult.

• HF attacks glass, concrete, some metals and organic compounds.
• While HF gas is one of the most acidic gases known, aqueous HF is technically a weak acid. However the definition “weak” bears no relation to hydrofluoric acid’s ability to damage living tissue with fluoride ions rapidly absorbed through the skin and able to migrate through and destroy tissue until they are eventually sequestered in the bones. HF damage causes long- term excruciating pain and burns which are slow to heal. Burns around the finger- tips are reputed to be particularly painful and may require the surgical removal of finger nails.
• Fluoride ions are both acutely and chronically toxic so that even 1% solutions of HF (or metal fluorides) must be handled with care. However, the ability of HF to carry fluoride ions through intact skin increases greatly with increasing concentration. Thus, 5% (2.5M) HF can be handled with about the same level of care that is appropriate for handling 10M H2SO4. Above 10% (5M), the dangers of handling HF increases sharply and any contact with the skin for more than a few seconds can result in latent burns that may take hours before they start to cause pain. Manufacturers commonly supply HF as 48% (28M) solution and sometimes as 73% (44M). Handling HF of these concentrations is far more dangerous than handling any other common concentrated acids.

Precautions

• Hydrofluoric Acid of >10% (5M) concentration must be stored in a cool, well- ventilated area in a screw capped polyethylene (or equivalent) container. It is inadvisable to keep such acid at all in a laboratory unless an appropriate COSHH assessment covering its possible use by each of the workers in the laboratory has been made.
• Reference should be made to an up to date Material Safety Data Sheet or Laboratory Safety Sheet.
• A detailed COSHH Special Assessment must be completed and approved by the Safety Committee before any use is made of hydrofluoric acid. Even if only dilute (5%) acid is to be used, the assessment must define how the commercially available concentrated acid is to be diluted safely. (The main danger in this operation is spilling or splashing the concentrated acid or breathing HF fumes as only a little heat is evolved during dilution.)
• The appropriate First Aiders who are qualified in the treatment of hydrogen fluoride burns must be informed by the adviser of any group contemplating the use of >10% HF and adequate supplies of calcium gluconate gel made available.
• Procedures using hydrofluoric acid must never be attempted by an untrained person.
• Procedures using hydrofluoric acid must never be attempted out of normal working hours and it is strongly advised that procedures are restricted over the lunch period when trained First Aiders may not be available.
• Procedures using hydrofluoric acid must never be attempted by someone working alone and for larger scale operations, workers should operate in pairs.
• All procedures must be carried out in an appropriately rated fume hood.
• It is strongly advised that procedures that are new to the HF user should be practised as a “dry run” and written down as a protocol before involving the acid.
• Appropriate personal protective equipment must be worn i.e. safety glasses (or preferably a face shield), PVC or Neoprene gloves which are frequently and carefully checked for damage especially pin holes, a lab-coat and preferably a chemical proof apron.
• Washing hands and gloves frequently with water is wise when working with even dilute HF.
• Disposal:-unless large quantities (>10mole) of HF have been used, spent acid is best added slowly to a copious flow of water running down a drain. The flow of water must be continued for some time after to ensure that all of the acid has been cleared from the glass sink-trap.

Training

Training by a competent person is absolutely essential before this material is used

Level of Risk Remaining

Constant vigilance is required in the use of these materials but risks should be low if the procedures outlined above are followed.

Emergency Procedures

• Skin or Eye Exposure

  • Immediate washing with large amounts of water. Call for First Aid. Even if no pain is immediately apparent, affected areas must be treated with calcium gluconate gel. Seek medical attention for all exposure to HF.

• Spills

  • For large spills follow the procedure outlined for the Escape of Toxic Material.
  • Small spillages (100 ml or less of <10% solution) can be neutralised with sodium carbonate or sodium hydroxide solution. Gloves, lab-coat and eye protection must be worn.

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Adapted with permission from School of Chemistry, University of Bristol