Acids and Bases

Finding pH Values

So pH is defined as the negative log of the concentration of hydrogen ions. So what does that mean? It sounds really complicated, right?

Let's start with this right here first. So this part of the equation (H+) is just defining concentration of hydrogen ions. A hydrogen ion, if you recall, is basically just a hydrogen atom which has lost one of its electrons. We'll come back to this in a minute and talk a little bit more specifically about how this relates.

Logarithmic Equations

This part of the equation then is going to tell us what kind of relationship the pieces of data have between each other. What we have when we talk about a log scale is we have a non-linear relationship between the data, and every time I move a unit on the x scale, I am moving ten times as much as the previous y unit.

The difference between y values increases exponentially - or logarithmically - every time I increase the x value by one.

What exactly is this part of the log part of the equation doing? What does that mean? To put this into terms of a scale that you might be familiar with that's a log scale, think about the Richter Scale. The Richter Scale measures the intensity of earthquakes. If I have an earthquake that is 4.0 on the Richter Scale, that has a certain power that's associated with it. If I talk about an earthquake that's 5.0, it's not that it was just one unit stronger than the 4.0 earthquake, it's actually 10 times more powerful than the 4.0 earthquake. And then if we talk about something that's 6.0, that's 10 times more than the 5.0 earthquake and 100 times more than the 4.0 earthquake. You can see then that by using a log scale, it allows us to measure a wider range of values.

Illustration of a log scale

So for instance, water has a concentration of hydrogen ions of 10^-7.

So you can see here that we're going to be measuring very small values, and because we're usually dealing with concentration values that are less than one, we want to use this negative sign to make everything positive.

Defining an Acid

Let's go back to our pH scale here, and by using this equation, what that means is that water is going to translate to a pH of 7. Anything that is more than 7, we're going to call a base; anything that is less than 7, we're going to call an acid. We define these numbers because of the way these types of molecules behave in terms of the hydrogen ion concentration. So let's revisit this hydrogen ion concentration concept a little more closely.

Let's talk about acids first. An acid is defined as a molecule which donates hydrogen ions. Let's take an example of a strong acid known as hydrogen chloride. Hydrogen chloride, because it's a strong acid, will completely dissociate and it's going to form one molecule of hydrogen ion and also one chlorine ion. Because it donates into solution this hydrogen ion, this molecule, hydrogen chloride, is known as an acid.

Because it's donating this hydrogen ion, it's increasing the hydrogen ion concentration of the solution, right? If you recall, we said that the concentration of the hydrogen ions in water is 10^-7. So if I add more hydrogen ions to that solution, I'm going to increase the concentration. If I increase the concentration and say now the concentration is 10^-5. Any value that's less than 7 is defined as an acid because it has a higher concentration of hydrogen ions than water.

Defining a Base

Logarithmic equation for water

So let's talk about the other side of the scale now. A base is any sort of molecule that can accept a hydrogen ion. Let's take an example again of something that's strong because it'll be easier for us to think about and conceptualize. Sodium hydroxide is an example of a strong base. Sodium hydroxide is also known as lye. Because it's a strong base, just like when we had our strong acid, a strong base is going to completely dissociate into a sodium ion and also a hydroxide ion, or OH-. Now this OH- is something that can react with a hydrogen ion; H+ and OH- can actually produce a more stable compound, which we know more commonly as water - H20.

If a base then is essentially removing hydrogen ions from solution, it's lowering the concentration of H+ ions. If we stick these concentration values into our pH formula, what you'll see is we end up with a pH of 7 in this case, we end up with a pH of 9 in this case. So anything that has a value that's greater than 7 is defined to be a base.

Lesson Summary

So we've learned today that a pH scale is a logarithmic measure of the concentration of hydrogen ions in a solution. Solutions with a pH of 7 are considered neutral. Any solution that has a pH lower than 7 is considered acidic. Any solution that has a pH greater than 7 is considered basic.

To put that into molecular terms, an acid is any sort of molecule which will donate a hydrogen ion to the solution. A strong acid is an acid which will dissociate completely to form an anion and a hydrogen ion.

In contrast, a base is any type of molecule which can accept hydrogen ions from the solution. A strong base dissociates completely to form a cation and a hydroxide ion.

You'll see in a later lesson how weak acids and bases differ from these examples of strong acids and bases that we've talked about.