Expert: George Maxwell - 1/4/2008

Question
I know that hydrofluoric acid can be very dangerous. It was prohibited from use in industrial cleaning a few years back. I understand that it has some kind of free ions that combine with sodium if exposed to human flesh. I did some research and found that a special neutralizing solution is available --at great cost. Would not sodium hydroxide serve as a neutralizing solution in the proper proportions?   Thanks.

Answer
Hi, and thanks for your question.

You're quite right; Hydrofluoric acid is extremely dangerous. However, there are several reasons that Sodium Hydroxide is never used as a neutralising agent for this (or any other) compound.

1) Neutralisation reactions produce a lot of heat - adding sodium hydroxide to an aqueous acid usually causes the mix to boil, splashing corrosive liquid everywhere. On human skin, neutralisation can produce severe thermal burns - I found that out the hard way when, early in my career, I tried to neutralise a sulfuric acid spill on my arm with baking soda solution - the heat of the reaction gave me a permanent reminder of why neutralisation isn't a good response to a spill! With hydrofluoric acid, the reaction would be even more dangerous.

Whatever the chemical, the normal procdure in any uncontrolled spill is to dilute with water - this reduces the concentration of the spilled liquid, whilst at the same time cooling it. Neutralisation is only carried out in the case of precisely measured amounts of chemical waste.

2) Soium hydroxide itself is very caustic; it produces deep burns on the skin and dissolves the cornea of the eye.

3) One of the main problems of hydrofluoric acid is its toxicity, not just its corrosive nature. Once it has burned past an outer layer of skin, it enters the body (particularly bone tissue) and poisons the patient. Neutralising the liquid on top of the skin wouldn't deal with this poisioning. The gel that is used for hydrofluoric acid (calcium glucanoate) reacts with the fluoride ion to stop its spead in to the body and allows it to be removed.

In my time with the fire service, I have responded to two major incidents involving hydrofluoric acid spills, and in both cases, we diluted the spill and treated the waste water with glucanoate to prevent the spread of the toxic ions. It is expensive, but is also the best way of dealing with hydrofluoric acid spills that we currently have.

Note that the reaction between hydrofluoric acid and water is very violent - when using this method to tackle a spill, we use special equipment to drench the whole area, quelling the reaction in an excess of water.

Hope this information helps.