Dabbling about with orange juice and stomach acid is all very well and good, but what happens when pH is negative? It is, in fact, physically possible to have a negative pH, since pH = -log[H+]. In other words, pH is the power to which the number ten must be raised to equal the concentration of hydronium (the ion brought about by an acid reacting with water) in a solution. A pH of 0, then, means that the concentration of hydronium is 10^0, or 1 mole per liter of solution.
It's rare, but possible, to have more than one mole of hydronium in a liter of solution. The lowest recorded pH I could find is -3.6, from some very acidic water in a mine in California. Since this pH is on a logarithmic scale, this is crazily acidic-- 10,000 times more acidic than battery acid. It almost seems preposterous, but these numbers come from the US Geological Survey. They must have done the measurement right. Right?
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