Think of pH as "water balance."

    Water is H2O.  You know that.  Well, sometimes water breaks apart into two halves:  a hydrogen ion (H+) and a hydroxide ion (OH-).  Notice that if you put the hydrogen and hydroxide ions back together, you will restore H2O (2 hydrogen atoms and one oxygen atom).  If you go looking around in web pages, you will see that the hydrogen ion is also often referred to as a proton.  Since the hydrogen ion often attaches to another water molecule, it often makes H3O+, called a hydronium ion... I only mention this in case you see it on the web.

   When pure water existswatersep.gif (15489 bytes), its water molecules are in a constant state of splitting and rejoining.   However, whenever a water molecule splits, the number of hydrogen ions and hydroxide ions will always remain the same.  In this animation, I drew three water molecules.  During the course of the animation, each of the water molecules breaks.   But, even when all are broken, the concentration (represented by brackets, [ ]) of hydrogen ions remains the same as the concentration of hydroxide ions.  In this way, the water is balanced in the number of hydrogen and hydroxide ions.  When this balance exists, as in pure water, the pH of the solution is neutral.

    Please note that in addition to a neutral pH, the solution carries no electrical charge.  I only bring this up because for some reason, students get electrical neutrality confused with pH neutrality.  They are different.  But in all of the examples I will be giving you on this page, the solution carries absolutely no electrical charge.  Even the strong acids and bases I show you will be electrically neutral.  OK?

    Getting back to pH... what I mean by "water balance" is that the concentration of hydrogen ions is the same as the concentration of hydroxide ions ([H+] = [OH-]).  Whenever this balance is tipped, the pH is no longer neutral.  Take a look at the typical pH scale:

pHscale.jpg (6252 bytes)

A solution with a pH from 0 to 6.9 is an acid, while a solution with a pH from 7.1 to 14 is a base (can also be called an "alkaline" solution).  Acids and bases do not have an even balance of hydrogen ions with hydroxide ions.  Acids have more hydrogen ions, while bases have more hydroxide ions.  This has nothing to do with electrical charges!  Please note, that when a solution becomes more acidic, its pH decreases, while when it becomes more basic, its pH increases; the terms "increase" and "decrease" when used with pH only refer to pH number, and the more acidic the pH, the lower the number.

    Let me summarize in this table:

Neutral pH Acidic pH Basic pH
[H+] = [OH-] [H+] > [OH-] [H+] < [OH-]
pH = 7 low pH number high pH number

Every step of the pH scale is a tenfold change in concentration!

    The pH scale is odd.  Who ever heard of a scale going from 0 to 14?  Why is 7 neutral?  The reason it is so odd is because the concentration of these ions is measured on a logarithmic scale, not a regular scale.  I'm not sure how much you know about logarithmic scales, probably very little... and you don't need to understand it just to understand pH... so, to skip past the details, you will just need to understand that every step of the pH scale represents a tenfold change in ion concentration.  That is, a pH of 5 has ten times as many hydrogen ions in it as a pH of 6.

    There's a neat little JAVA applet at a website where you can Play in the Acid-Base Pool.  There you can see that if you double or triple the concentration of hydrogen ions, you still won't get the pH to change a full step.   Try out that applet just to make this little idea sink in.

What is the normal pH of our body fluids?

    Well, the one solution for which pH is defined in our book is blood.   The pH of blood should always be 7.4.  When the pH changes from 7.4 at all, we start to have terrible troubles.  If the pH of blood gets more acidic, that is called acidosis.  Acidosis is when blood pH drops below 7.35.   However, if blood pH drops below 6.8, the person with acidosis dies.  Our bodies cannot tolerate blood pH lower than 6.8.  And if our blood pH gets more basic, or alkaline, that is called alkalosis.  Alkalosis is when the blood pH rises above 7.45.  But if it rises above 8.0, the affected person dies.

    You see, our bodies are extremely sensitive to blood pH.  Any blood pH more acidic than 6.8 or more basic than 8.0 causes death.  You can imagine, I hope, that if a blood pH of 6.75 is deadly, it cannot feel too good to have a blood pH of 7.0.  Right?  It also cannot feel too good ot have a blood pH of 7.8.   Right?  We are so sensitive to blood pH that we have immediate symptoms with either acidosis or alkalosis.

bulletSome acidosis symptoms:
bulletSome alkalosis symptoms:
bullettwitchy movements

How can a solution become more acidic or basic (without becoming charged)?acid.gif (38574 bytes)

    Certain molecules give off hydrogen ions or hydroxide ions when put into solution.  For example, hydrochloric acid (HCl) tends to give off hydrogen ions.   I have shown this in the animation here.  HCl dissociates in water into hydrogen ions and chloride ions.  For every hydrogen ion there is one chloride ion, so there is no electrical charge build-up.  But, suddenly, the water isn't balanced anymore.  Instead, there are a lot more hydrogen ions than hydroxide ions.  This larger concentration of hydrogen ions is what makes this solution an acid.

base.gif (42170 bytes)    To the left is a similar animation, but this time, I have made a base.  You see, if you add sodium hydroxide (NaOH) to a solution, it dissociates.  Upon dissociation, NaOH splits into a sodium ion and a hydroxide ion.   When this happens, for every sodium ion there is a hydroxide ion, so there is no electrical charge generated.  But, the increase in hydroxide ions causes there to be more hydroxide ions than hydrogen ions.  For this reason, this solution is a base.

    There are many molecules that can give off hydrogen ions or hydroxide ions.  There are also molecules that make acids or bases by sucking up hydrogen ions or hydroxide ions... Let me try to explain.

    If you add a molecule that tends to suck up hydrogen ions into a solution, what is going to happen to the pH of the solution?  To answer this, first consider what will happen to the hydrogen ion concentration... will it increase or decrease?  If hydrogen ions are sucked up by the newly added molecule, the hydrogen ion concentration will decrease, because fewer hydrogen ions can be floating around.   If the hydrogen ion concentration decreases, that will tend to make the solution more basic.