Why can you drink vinegar but you cannot drink a dilute solution of battery acid?

Answer 1

Acetic Acid vs. Sulphuric AcidThe pH of vinegar is typically around 2.5 while the pH of battery acid is usually around 0.8. The concentration of hydrogen ions in battery acid is hence much higher than in vinegar. This lower pH is more unnatural for the body and can have harmful effects. Also vinegar's acidity is caused by acetic acid while battery acid is sulphuric acid. It is likely that the body is more tolerable to acetic acid because it is a weak acid. Although you can drink vinegar straight, but I wouldn't drink too much of it -- it will upset your stomach if you drink a lot, and like anything else, will kill you if consumed in large excess! Weak vs. Strong AcidThe pH of vinegar and a diluted solution of battery acid (which is sulfuric acid, or H2SO4) can be exactly the same. The key word is "diluted." Yes, pure battery acid is at a very low pH, but if sufficiently diluted, it can have a pH greater than that of vinegar! The pH of an acid solution is determined by two factors: the strength of the acid (typically characterized by it's Ka or equivalently its pKa) and the concentration of the acid (typically characterized by its molarity).

Clearly drinking pure battery acid would kill you very quickly because of the extremely low pH (it would severely burn your mouth and esophagus, among other things)! But drinking even a fairly dilute solution of sulphuric acid would be very bad for you. This is because sulphuric acid is a strong acid, while acetic acid is a weak acid. That means that sulphuric acid dissociates completely in water to form H3O+ and HSO4-. Acetic acid will not dissociate completely and an equilibrium will exist between its protonated and deprotonated form.

What does this mean in practice. Vinegar contains about 5-10% acetic acid. This is approximately 1 molar (1 M). The pH of this solution is ~2.5, and this is not harmful.

Now if you have the same 1 M concentration, but now of sulfuric acid, what would the pH be? For a 1 M solution of H2SO4, the pH is 0. Yes, zero. The concentration of pure battery acid is about 30-35% sulfuric acid, which is about 6 M. That has a pH of -0.78! Yes, it's negative. So, to get battery acid to have the same pH as vinegar (a pH of 2.5), you'd need it to have a concentration of 0.0032 M, so you'd have to dilute the battery acid by a factor of 2000! That'd be VERY dilute! But you know, I wouldn't want to drink anything, dilute or not, that came out of my car... but that's just me. But really, don't.

NOTE: I've found some discrepancy about the pH of battery acid. I've seen the pH of battery acid reported to be about 1, but I've also seen it reported that the concentration of battery acid is 30-35% sulphuric acid. If that percentage is correct, than a such a concentration solution of sulphuric acid would have a negative pH. I'm not sure which is correct, the reported pH of 1 or the concentration of 30%. The two numbers seem to me to be inconsistent.

Answer to Note: I would suspect that the reporting of a pH of 1 for that concentration of Sulfuric is based on the limitation of the testing equipment. Most common pH probes are not designed to read that low with any accuracy.

It doesn't change the important point of the answer above -- that a solution of sulphuric acid is still MUCH more acidic than the same concentration of acetic acid, even if both are fairly dilute. All that being said, if the sulphuric acid is very very dilute, and I mean VERY dilute, there is nothing dangerous about! But don't try this at home boys and girls...

ANOTHER TIDBIT

Sulphuric acid also breaks down the proteins in your tissues. So not only will the low pH burn, the tissues of your body will coagulate. Think cooked egg white or the lack of strength in fork tender meat.