 All right. Good morning. We're talking this morning about something called pH. All right. And we want to know what pH is. Many of you have probably heard about this a lot. It's often we see it in commercials where they're talking about cosmetics and lotions and acids and things like that. So it's a pretty commonly used term. And we want to talk about just exactly what pH is. And we're doing this in the context of acids and bases. And this is probably among the most important relationships in chemistry. Very, very critical. And even within that, we're talking specifically about aqueous solutions, which are solutions involving water. It's very simple, aqua, water. So pH, acids and bases, aqueous solution. So given that, let's start out talking about our old friend the water molecule. We commonly talk about this as H2O. So we have two hydrogens and an oxygen. And if you do a picture of this, it might look like this. We have an oxygen atom and then we have a couple of hydrogen atoms like this. And these hydrogens have positive and oxygen has got a negative charge on it. And interestingly, this gives our water molecule something called polarity, which means it's got a positive side and a negative side. So in this fabulous thing for water, I mean, it gives us all kinds of properties. So certainly life on earth would not be possible if you didn't have water and it wasn't polar like this. And a lot of things have water is very unusual. It can, at earth temperatures and pressures, it can exist as a vapor, as a gas, as a liquid, of course, and as a solid. So very, very, very unusual molecule. Now, given this, if we have two of these guys, so let's say we have two, two of these water molecules. And we might draw them like this. Okay, we got the plus here and plus here and the minus here. And we can dress it up a little bit. And we have another one, another guy over here. And we combine those two and they interact. We get some interesting things going on. And actually, this interaction can go both ways. We get something that looks like this. We have, they can bump into each other and actually one of them can steal a hydrogen from the other. And then the other one, of course, has got only one hydrogen on it. We'll give him a frowning face like that. And this happens, this is called, let's get another picture of this. So we have two H2O. And we have one of these guys and another guy like this. And you can do this. This is called autodissociation. Autodissociation. So these guys can bump around, say, in a glass of water. And as they bump into each other, sometimes, very, very quickly, one of them can steal a hydrogen from another one. And then, very, very quickly, they give it back. So it goes back and forth like that, very, very fast. And this is called the hydronium, hydronium ion. And this, of course, is the hydroxide. So we get this autodissociation going on. We get the hydronium ion and the hydroxide ion. And going back and forth, and it goes very, very quickly. And it's a very, very small fraction of the water. In fact, if we look at that, we have our, we have a pure water, our hydronium, hydronium ion, which is often written as H3O, like that. It's got an extra positive charge on it. And then we talk about the concentration of that, which we indicate by brackets. So we have a hydronium ion has a concentration of 10 to the minus 7 moles. And I remember a mole, of course, is equal to 6.022 times 10 to the 23rd. That's, you remember, that's Avogadro's number. So this is a really big number here. And minus 7, of course, is very, very small. So we're talking about this autodissociation thing going on in a very, very, very minute fraction of any volume of water. And it's going on continuously back and forth, back and forth. All right? Well, it's this hydronium ion that we're really interested in. Because it turns out that this concentration of hydronium ion is really what indicates whether something is acidic or basic. All right? So if this hydronium ion is a concentration equal to 10 to the minus 7, we call that neutral. And it's got an equal balancing amount of hydroxide. So that's what makes it neutral. If the concentration goes higher, we have a higher concentration, H3O, is higher. Okay? That's going to be, let's remember, well, take a little deviation. If we have a, we have a number line here, zero here. And we have a 10 here and a minus 10 here. And we have our minus 7 here, which is what this is. If we add something to it, we're going to actually move this way on the number line. So if we add hydronium ion to a solution, we're actually going to go, say, to a, to a 5. So we're going this direction, which is the positive, positive direction on the number line. So we might have a 10 to the minus 5th. And that would be termed acidic. All right? And then if we, but of course if we go the other way, if we have an H3O and it has a concentration of 10 to the, let's say, minus 9, that would be considered basic. So here and we have our definitions of acids and bases. They're based on the concentration of the hydronium ion. This constant, this guy here. And this definition, actually a guy that came up with this, his name was Arrhenius. And this is the Arrhenius definition of acids and bases. And just, just for reference, there are two other ways of looking at, one of them looks at electrons, the other one looks at protons. But we're looking at this hydronium ion. And what you're asking yourself at this point is, does all this have to do with pH? Okay. So we have this concentration of hydronium ion and draw the brackets to indicate concentration. We know that at neutral we're at 10 to the minus 7. And of course always looking for an easier way to do this. This kind of a pain in the butt to, to write all this out all the time. So we're going to take a short hand. We're going to say let's let H stand for the hydronium ion. And let's let P stand for something. We know that instead of dealing with exponents here, this minus 7, 10 to the raise to the minus 7th power, we can use logarithms to bring this exponent down to a regular number. So we have 10, I'm sorry, erase that. There we go. If we have the log to the base 10 of 10 to the minus 7, that equals 7. All right. And if we make this negative, we can negative and a negative makes that a positive. So if this expression here stands for P there, now we have a nice little shorthand. What this gives us is this. We can have an acid, which is a concentration of H3O, which is greater than 10 to the minus 7, which has a pH then, which is less than 7. Or we can have a base, which has an H3O concentration less than 10 to the minus 7, gives us a pH greater than 7. Or we can have a neutral solution, aqueous solution, where we have the hydronium ion concentration equal to 10 to the minus 7, or a pH equals 7. So that's basically the pH story. So pH then is essentially a measurement. It's a measurement. It's a measurement of hydronium ion concentration in a solution. It's a measurement just like we use interest in a ruler to measure distance. We're using this pH measurement to measure the hydronium concentration. Okay, so now that we have a basic understanding of what pH is, how do we use this? pH, it turns out, is very useful in chemistry as we talk about perhaps doing titrations where we have a known solution with a known acidity and we want to test the acidity of an unknown solution. We can add that and monitor the pH changes to identify the pH of the unknown solution. Another area that it's used is in the concept of buffering. And this happens all the time. For example, the pH of our blood is about 7.4. And it's very important that our body maintains a pH of 7.4 for our blood. And so there's a lot of chemistry that goes on behind the scenes, so to speak, in our bodies to buffer if it tends to go higher or lower to make adjustments to the pH of our blood. Third thing, of course, is in the environmental world. We want to make sure that our water systems and ecology is healthy. And all that relates to pH. Some plants require a higher pH, down around the 5.5 or so range, such as grape growing, important here in North Carolina. And others require more of a, you know, sort of a 6.5 to 7 to produce a healthy crop. So all that is important in considering this. pH is measured with instruments, primarily in two ways. One is a pH meter, which you simply have a little probe and a digital readout and you stick the probe in the solution and it takes a reading. That's pretty straightforward. The quote-unquote old fashioned way, of course, is to use color metric or color indicators, certain substances that will change color based on the pH of the solution they're introduced to. Sometimes you can combine a number of these. And you've heard of litmus paper, which is an indicator for pH, a color metric indicator. So that's the other principle way that pH is actually measured. Thank you very much for your attention and I appreciate it. Take care.