Strength of acid or base

You should have gained an understanding on the pH page that anything that gives off hydrogen ions makes something more acidic. And anything that gives off hydroxide ions makes something more basic. Right? OK... well, it turns out that in our bodies, we really only have to deal with hydrogen ions. We don't have chemicals that tend to give off hydroxide ions in our bodies. So, only chemists and pharmacists really have to deal with hydroxide ions.

But that doesn't mean that we can't have very basic body fluids, either. You see, if something in our bodies sucks up lots of hydrogen ions, we will have a low concentration of hydrogen ions. That will make our bodies more basic. Therefore, we have to finely control our hydrogen ion concentration.

To understand how our hydrogen ion concentration is controlled, you have to know about what chemicals in our bodies lead to pH changes. And those chemicals can either lead to large pH changes or small pH changes. That depends on the strength of those chemicals as acids or bases.

What is a strong acid or a strong base?

A strong acid is one that tends to lose the hydrogen ions from all of its molecules. For example, when HCl is put in water, it entirely dissociates into hydrogen ions and chloride ions. By "entirely," I mean that every molecule of HCl will dissociate. Not just some. A strong base, like NaOH, will contribute a hydroxide ion from every one of its molecules; alternately, it will accept a hydrogen ion onto every one of its molecules.

What is a weak acid or a weak base?

Most biological chemicals are weak acids or weak bases. A weak acid is one that when you put a lot of it in a solution, only some of the molecules give off their hydrogen ions... the weaker the acid, the fewer the molecules of the chemical that give off their hydrogen ions. A weak base only sucks up some of the hydrogen ions in its environment (or only gives off some of its hydroxide ions). I have tried to show you what I mean by this in the table below...

Strength of Acid:	Very Weak Not-so-weak			Strong
Approximate Percentage of Molecules giving off a H⁺	10%	20 - 40%	50 - 75%	90 - 100%

Weak acids only tend to give off their hydrogen ions if the solution they are in is not so acidic already. So if a weak acid is put into a very acidic environment, it doesn't usually give off its hydrogen ions. That makes weak acids and weak bases rather good buffers. I'll explain that more on the buffer page.