Chemistry

Understanding Acids Bases and the Ph Scale



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Despite the rumors out there, understanding acids, bases and pH is about as easy as, well, using a seesaw. Most people know what a seesaw is, right? A plank of wood balanced on a fulcrum, some pivoting point. A perfect seesaw is balanced, both sides the same height. That’s really all that acids, bases and ph is. It’s a balancing act between acids and bases. Acid is on one side, base is on the other, with the pH scale acts as the seesaw board that they’re sitting on.

The pH scale: easy as 1, 2, 3, …14!

The pH scale – our seesaw – has numbers running across it. The numbers go from 1 to 2 up to 14. That’s the scale. In the middle is the number 7. So if you looked at the seesaw from the side, 1 through 6 is on the left, 7 is in the middle, and 8 through 14 are on the right. There’s a lot of math that explains what these numbers are, but we’re trying to understand the idea, so we won’t worry about the math part right now.

A baseline pH.

Let’s look at something simple: water. Water is H2O. Water is made up of one oxygen and two hydrogens. And it can commonly be separated into two pieces: H+, OH-. The first one is a hydrogen ion and the second is a hydroxide ion. The plus charge and the minus charge cancel each other when they form water. (In reality they are what attract them to each other, but that’s another story.) The best part is that H+ is what makes things acidic, and OH- makes things basic.

Assume we have a jug of distilled water. That’s water that’s been evaporated and collected so that there are no impurities in it; no metal ions or chlorine ions. Just pure water. Oh, and we de-ionize it while we’re at it. Is the water acidic? Is it basic? It’s neither one. On our pH seesaw water would make it perfectly balanced. The seesaw isn’t tilting to either side.

So what does this say about water?

Water tends to be neutral – neither acidic or basic. Even the water that comes from your faucet is considered to be neutral. Neutral means there is as many H+ as there is OH- in the water. When the amount of H+ and OH- are equal, the pH is said to be 7. Mathematically speaking:

[H+] = [OH-] = pH7!

(The brackets around the H+ and OH- are the chemist’s shorthand for concentration. [H+] is read as “the concentration of hydrogen ions.”)

Imagine, then, our pH seesaw. Water would be sitting in the middle where the 7 is. Water has a pH of 7.

The difference between an acid and a base.

The difference between acids and bases is the concentration of hydrogen and hydroxide ions. Or, to be a bit more scientific, it’s the difference between [H+] and [OH-]. In a nutshell it’s like this:

If something has more [H+] than [OH-] it is acidic.

If something has more [OH-] than [H+] it is basic.

Scientifically we would make equations that look like this:

If [H+] > [OH-] = acidic

If [OH-] > [H+] = basic

Acidic stuff has a small pH value. That’s the numbers from 1 to 7. Basic stuff has a larger pH value. That’s the numbers from7 to 14. Seems weird, doesn’t it? You’d think that the more acidic something was, the larger the number. It’s a math thing don’t worry about it. For now…

Things acidic

Ever sucked a lemon? Sour! Sour things are acidic. But, please, don’t go tasting everything you think may be acidic. Some things can actually hurt you. Here’s a quick list of some things that are acidic:

Lemons

Tomatoes

Vinegar

Battery acid

Here they are with their pH, going from most to least acidic:

Battery acid    pH = 0.8

Lemons           pH = 2.2

Vinegar           pH = 3.0

Tomatoes         pH = 4.5

These numbers I’ve used are averages for each, because there are other factors, like dilution, that effect pH. Moving on…

Battery acid has the highest pH. What does that mean? That means it has a lot more [H+] than it does [OH-]. And because battery acid has a higher pH than lemons we can also say that the [H+] of battery acid is greater than the [H+] of lemons. There are more hydrogen ions, H+, floating around in battery acid than in a glass of lemon juice.

The smaller the pH, the larger the [H+].

See the seesaw.

Now, imagine the seesaw again. Water is in the middle, at pH of 7. Moving off to the left you would then have tomatoes, then vinegar, then lemons, and finally battery acid. The further you move to the left the more acidic things are. The [H+] becomes greater the further left you move.

This effects [OH-], too. Since [H+] is getting larger as we move to the left, [OH-] is getting smaller.

Remember that math I threatened you with?

Here’s an equation that shows the balance of [H+] and [OH-]: (Don’t be scared)

[H+][OH-] = kw = 10-14

Okay, maybe that’s a little bit scary. Let’s ignore the kw and the 10-14. Let’s replace them with just the number 14. Then, for simplicity of understanding:

[H+][OH-] = 14

That’s not so bad, now. And here’s what it’s meant to illustrate. The sum of the concentration of H+ and OH- must always = 14. Mathematically, it then requires that if one of the values, either [H+] or [OH-] goes up, the other must go down so that the answer is always 14.

Try this.

10 x 10 = 100

But what if I replace the first 10 with 20?

20 x ? = 100

What times 20 equals 100? Ah!

20 x 5 = 100

One value went up, so the other value had to go down to continue equaling 100! This is the same for pH. If [H+] increases then [OH-] must decrease.

We haven’t forgotten bases.

Bases appear on the right side of the seesaw. As we moved further left on the seesaw, what happened to the concentration of hydrogen ions? What happened to [H+]? It increased.

What do you think happens as we move to the right? It’d be the opposite of increase. It would decrease. Moving right on the pH scale means the concentration of hydrogen ions decreases. [H+] gets smaller.

And because [H+] x [OH-] = 14, what can we say, confidently, about [OH-]? What happens to the hydroxide ion concentration? It gets larger.

We know that water has equal concentrations: [H+] = [OH-]. For things that are basic, the concentration of hydroxide ions becomes larger than that of hydrogen ions:

[OH-] > [H+] and pH > 7

So what’s a base?

Here’s a list of basic items:

Baking soda

Ammonia

Blood

Each of these has more [OH-] than [H+]. Let’s put them in order of decreasing pH. Remember, the smaller the pH, the more acidic. So larger numbers of pH make things less acidic.

Blood              pH = 7.4

Baking Soda   pH = 8.3

Ammonia        pH = 11.0

And on our seesaw we would place these to the right of water with blood coming next, then baking soda, and then way to the right we’d have ammonia. (Ammonia is often found in household cleaners and is not to be touched. Wear gloves!)

We can say, without reservations, that ammonia has a lot more hydroxide ions, OH-, and it does hydrogen ions, H+. To put it in the words of a chemist:

[OH-] >> [H+]

Notice how we used the greater sign, >, twice? Remember that if the sum of the two must equal 14 and [OH-] is an 11, then the value of [H+] is very small.

In reflection.

The pH scale is like a seesaw. Anything on the left is going to have more H+ than OH-. That means it will be more acidic. The pH scale reflects this in smaller numbers. So greater [H+] is identified with a smaller number.

Anything on the right is going to have more OH- than H+. These are called basic. A smaller [H+] is identified with a larger number.

You see, there’s really nothing to fear when it comes to understanding acids, bases, and the pH scale.


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