pH and buffers
/ pee-AYCH / BUH-ferz /
Taste lemon juice and it puckers your mouth; taste baking soda and it is bitter and slippery. The lemon is acidic, the baking soda basic. pH is the number that places any watery solution somewhere on this scale, and buffers are the safety system that keeps a cell from sliding too far either way.
pH measures how many free hydrogen ions are floating in a solution — loosely, how many spare positive charges water has shed. The scale runs from 0 to 14: low numbers mean lots of free hydrogen ions and a strongly acidic solution, high numbers mean very few and a basic (alkaline) solution, and 7 is neutral, like pure water. The scale is logarithmic, so each step of one unit is a tenfold change; a pH of 4 is ten times more acidic than a pH of 5. A buffer is a chemical pair that mops up extra hydrogen ions when acid is added and releases them when base is added, so the pH barely budges.
This matters because the molecules of life are exquisitely sensitive to pH. Proteins, especially enzymes, work only within a narrow pH window and lose their shape outside it. Your blood is buffered near pH 7.4, and a swing of even a few tenths can be life-threatening. Buffers are the unsung heroes that keep the cell's chemistry within livable limits.
When you hold your breath, carbon dioxide builds up and would acidify your blood, but the bicarbonate buffer system absorbs the change so your pH stays close to 7.4 until you breathe out.
Blood's bicarbonate buffer guards a pH window so narrow that survival depends on it.
Because the pH scale is logarithmic, do not treat the numbers as ordinary distances: pH 3 is not a little more acidic than pH 6, it is a thousand times more acidic.