If an alien were to come down from space and ask you to describe what the term pH means, you would probably state one of the following:

1. It measures the acidity of a solution.

2. It is a measure of the hydrogen concentration within a solution.


I recently taught a class on acid base interpretation utilzing the Stewart approach. The students seem to like my explanation of pH and said: "I have never heard it explained like that before" (this could be a good or bad thing).


I felt like a blog post may be helpful as you tackle the monster that is acid base interpretation.

Let's Start With Water

Water is made up of one oxygen molecule and two hydrogen molecules, hence the name H2O. 

Because oxygen has such a strong passion for hydrogen, occasionally the selfish little jerk steals a hydrogen ion from another H20 molecule. This turns the greedy oxygen molecule into hydronium (H30) and the oxygen molecule that lost one of his hydrogens into hydroxide.

This game of "hydrogen-hot-potato" between oxygen molecules happens very fast and the rate and frequency in which it occurs determines the pH. Here is an example:

In this illustration above there is one hydonium molecule for every 10,000,000 moles of H2O. If we count the zeroes in 10,000,000 we end up with 7. That means that the pH of this concentration is 7. 

Let's try another example:


If there is one hydronium molecule for every 1000 moles of H2O, what would my pH be?

....Scroll down for the answer.....





Bingo!  The pH would be 3. There are three zeroes in 1000.

Obviously 1 out of 1000 is a lot more concentrated than 1 out of 10,000,000. This is why the lower the pH is, the more acidic a solution is (this finally makes sense to me). You will hear this described as the "negative logarithm blah blah blah," bascially all this is saying is that we count the zeroes in the concentration. There are some people that think pH stands for the power of hydrogen and others that think it stands for the power of hydronium. The transit phase of hydrogen from one oxygen molecule to another happens so fast that it probably doesn't matter.

A Snap Shot In Time

I was sitting in a stadium for Disney On Ice (with my children) and noticed that at any one time there was always someone getting up from their seat and someone else returning to theirs. The rate at which they were disassociating their butt from the seat was about equal to the rate at which others were returning to their seat. This reminded me of the hydrogen hot-potatoe that occurs in water. 

If I were to take a picture of the stadium and count how many people were standing, it would be like evaluating the pH of the audience.

The Butt-to-seat disassociation rate of this stadium is 2.. just in case you were wondering. You could adjust this ratio by announcing a sale on beer (yes at a Disney show), or by having a climatic event in the show that nobody wants to miss.

There are many variables that will change the rate at which this hydrogen shuffle occurs, such as temperature and the introduction of strong ions. However, the rate at which this H20 to H30 occurs remains a DEPENDENT variable. We will explore what the indepedent variables are in future posts.

PS. Watch out for this new show on ABC.


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