On Chemical Burns, Vinegar and Liquefied Chicken Feet

“This is a chemical burn.” - Fight Club.

So says Tyler Durden, after he pours lye over the hand of Tyler Durden to teach Tyler Durden that he needs to lose control and place his life in the hands of Tyler Durden. Then, Tyler Durden pours vinegar over Tyler Durden’s hand to neutralise the burn.

(If the phrase “Tyler Durden” no longer means anything to you after reading that paragraph, you’re experiencing a phenomenon called semantic satiation.)

Does lye burn like that? Should you really pour vinegar over it? Can a chemical burn actually induce visions of Helena Bonham Carter in an ice cave?

Lye is another name for the chemical sodium hydroxide, or NaOH. You might have used it in the form of Drano, because it will quite happily dissolve whatever gunk is clogging up your sink. It is, indeed, used in soap making as an agent to saponify fat. It’s also used for disposing of larger objects such as animal carcasses and drug cartel enemies. Put simply, NaOH likes to eat things.

In its powdered form, as shown in the movie, sodium hydroxide will just sit on your skin until you get rained on, or start sweating, or find some other way to introduce moisture to it. It’s hygroscopic, which means it likes water so much it will eventually just start taking moisture from the atmosphere. When it does get wet it turns into a strong alkali, capable of turning your hand and any other body parts into a gross, soapy mess. This is why Tyler Durden kisses the back of Tyler Durden’s hand before he pours the lye over it - to leave moisture for the burn to occur. 

The burn is caused by a chemical reaction between the hydroxide (OH) part of the sodium hydroxide, and the glycerine and fatty acids (triglycerides) in the skin. The end products are soap and glycerine, which is used in the food and pharmaceutical industry (and yes, Tyler was correct when he stated "with enough soap, you can blow up anything" - glycerine is an ingredient in dynamite). 

If you spill a weak concentration of sodium hydroxide solution on your hand, you’ll experience itching and stinging, as well as the uniquely unpleasant sensation of being able to lubricate your hands with your own slowly-dissolving skin as you wash it off. Higher concentrations will cause deep burns and nerve damage, as well as blindness if you're unlucky enough to get it in your eyes, so kudos to Brad Pitt for wearing his safety goggles.

Here's a video of sodium hydroxide being used to break down a chicken foot. This is also an example of how NOT to create a sodium hydroxide solution - more on that later.

For a personal touch, below is a photo of my own foot. The little red mark is where I dropped a 20% solution of sodium hydroxide on it about six months ago. It's not particularly impressive, but it’s worth noting that at the time I spilled it on my foot, I was wearing both shoes and socks. (I'd like to pretend I did this as a public service to demonstrate the importance of safety in the lab, but actually I just broke the bottom off a test tube by smacking it against a benchtop.) Yes, it hurt; no, I did not experience delusional visions of Helena Bonham Carter.

So, imagine you’re at home one day, and your underground soap-making business is going so well that it’s time to render another pile of fat (human or otherwise). You spill sodium hydroxide on your hand, and it’s just damp enough that it starts to react with your skin. Is it time to break out some vinegar to neutralise that burn?

It’s true that the reaction between an acid and a base is a neutralisation reaction: acids have free hydrogen ions (H⁺) , and bases have hydoxides (the “OH” in NaOH - to be accurate, it's OH^- and has a negative charge). Together, they form neutral H₂O: water!

And heat. Did I mention heat? Because this reaction is exothermic, which is to say, it releases energy in the form of heat. So in addition to the chemical burn, you're also heating up your skin. In fact, the chicken-dissolving video at the beginning of this post actually demonstrated a really ill-advised method of creating a sodium hydroxide solution: adding a small amount of water to a large quantity of sodium hydroxide can cause it to boil or explode.

Warning: this next section gets ridiculously detailed, but there is a summary at the end if you'd prefer not to read 800 words about enthalpy.

If you consult the material safety data sheet (MSDS) for NaOH, you’ll see that the recommended course of action after skin contact is a visit to the safety shower - ideally, you’re going to be flushing the burn with water for at least fifteen minutes. That’s because, even though the reaction between NaOH and water is also exothermic, you’re going to have a much larger quantity of water available compared to vinegar (and the vinegar is mostly water, anyhow). Rather than just neutralising the alkali, it’s better to use a large amount of water to dilute and wash it all off.

(Before you get to dumping water on your hand, it would also be a good idea to turn that hand upside down so that any excess powder on the skin falls off. The less powder on the skin, the less powder will be reacting with water, and the less heat will be liberated.)

Let’s assume that the amount of NaOH left on Tyler Durden’s hand was around a quarter of a cup (so, just short of 70 cm³). Powdered NaOH weighs about 1 gram per cm³, so that’s about 70 g of NaOH. The molar mass - the mass of 6 x 10²³ molecules of NaOH - is around 40 g, so 70 g contains almost 2 moles of NaOH. This means that we need 2 moles of vinegar (acetic acid) to neutralise it.

Vinegar is usually 5% acetic acid by weight. Since the molar mass of acetic acid is around 60 g, that means we need 120 g, which is the amount of acid in 2.5 litres of a 5% solution. You have to aim around 2.5 litres of vinegar squarely at the burn to neutralise all of the NaOH. The amount of energy given off by this reaction is on the order of 50 kilojoules per mole, which is 100 kilojoules in total. That's enough energy to heat the entire 2.5 litres of vinegar by about 9 degrees Celcius. 

If you used a smaller volume of acid (for instance, if you reasoned that one cupful of an 8-mole-per-litre solution of acid contained the 2 moles necessary for neutralisation) the temperature change would be greater. If you were to use only one cupful of liquid, it would actually boil on contact with the sodium hydroxide. (This is why the common suggestion to wash the burn with a little bit of vinegar before rinsing with water is such a bad idea.)

These calculations assume you're taking your hand and dunking it in a tub of vinegar, rather than pouring it over the skin. In reality, much of the benefit would come from the action of using those 2.5 litres of vinegar to physically wash the sodium hydroxide from the skin. Alternatively, you could stand under a shower for fifteen minutes, which would dispense almost 150 litres of water. Dissolving 2 moles of sodium hydroxide in water also liberates almost 100 kilojoules of energy, however, in such a large volume of water the temperature change is negligible.

So, in short: you can fumble your way through opening a jug of vinegar and pouring it over your hand (which would be especially tricky if it's your dominant hand that's burned), or you can turn on a tap and dispense a much greater amount of water to rinse the skin. Either way, your best course of action is focusing on removing the sodium hydroxide powder from your hand, rather than trying to neutralise it. 

And unless you're applying water with an eye-dropper, Tyler Durden's assertion that "you can run water over your hand and make it worse" is just not true. Don't stand around with your hand burning because you're scared to put water on it! The greater the volume of water (or vinegar) you use, the more heat from the dissolution or neutralisation of the sodium hydroxide will be absorbed by the water rather than by your skin.

Obviously, nobody is actually taking chemical safety advice from Brad Pitt movies ...well, except for all these different soapmaking websites, seriously, every soapmaking website ever. One of them actually suggests pouring milk in your eye if you get some lye in there. Unless you have enough milk to irrigate your eye for fifteen minutes, this is not a good idea. I would also advise against accidentally swallowing lye and then chasing it with citrus juice, as suggested here.

Okay, so this turned out way longer than I'd planned. Here's the tl;dr version: when you dissolve NaOH in water, the energy that was holding the molecules together is released. It's called the enthalpy of dissolution. Similarly, when you neutralise NaOH with acid, it releases an amount of energy called the enthalpy of neutralisation. 

In the case of NaOH, these two enthalpies are actually pretty similar: about 50 kJ per mole of NaOH. Because the temperature change during the dissolution/neutralisation is a function of the volume of liquid used, you're better off dousing an NaOH burn in water. This is because you will have a lot more water available than you will have vinegar - to run a shower to flush the skin for fifteen minutes will use around 150 litres of water. 

Either way, the worst thing you can do is using too little water or vinegar - some soapmaking websites suggest "rubbing a splash of vinegar" on the burn. This will actually cause the NaOH to boil on your skin, creating a thermal burn on top of your chemical burn. So: don't do that.