 Welcome to the series, Photographic Chemistry, presented by the Foundation of the American Institute for Conservation of Historic and Artistic Works. This program was made possible by grants from the National Endowment for the Humanities and the Andrew W. Mellon Foundation. Each program in this series is presented as a short video. Depending on your video viewer, you should be able to pause, return to a previous section, or skip ahead to a later section by using a scroll bar or on-screen icons. You will find an outline of the course and short quizzes to test your understanding on the course webpage. So what's going on when we talk about a silver halide salt precipitating or crashing out of solution when I combine the soluble forms of each salt in a single or double jet experiment? To be able to describe what's going on, I have to invoke the concept of salt solubility. Now, we have an intuitive idea of what solubility means. When something is very soluble, we can get a lot more of it to dissolve in a water-based solution. The concepts of salt solubility are going to be very much rooted in two chemical concepts. One is concentration, oftentimes referred to as molarity, which might require a review on what the mole is. So we're going to require a knowledge of molarity or concentration. But the second is an equilibrium. And so what's going on is nature is always striving in all of our processes to achieve an equilibrium state. And the same is true in the solubility of any salt or any chemical for that matter in solution. The equation in the middle of the slide shows what's going on and how we're going to think about the silver halide formation process. We start out with a silver halide solid. Now, silver halide is oftentimes abbreviated A-G-X. A-G is the atomic symbol for silver. And X refers to any halide, chlorine, bromine, iodide, or their combination. So this is a very generic form of the chemical equation, which when we read it from left to right, says I have a solid silver halide in equilibrium with, and that means double arrows. In other words, we can go to the left or we can go to the right to form a soluble form of silver ion and halide anion. So when we drop this silver halide grain into water, we're establishing this equilibrium. If the salt is very soluble, it disappears. In other words, we haven't reached the condition known as saturation. But if the solid is sitting at the bottom of our container or it's sitting on our photographic film and paper, then we're pretty confident that the solution is holding as much ion as possible. And the solution is referred to as being saturated in that salt or that chemical. So what we're after is an understanding of the equilibrium. On which side of this equilibrium are we in a particular investigation with a salt? Are we on the left-hand side where we have the solid or the right-hand side where we have the dissolved salt? How can we depict that? Well, if we want to know the magnitude, which side is predominant, we just need to know what the concentrations of the ions are relative to the solid. And so we follow this with what's known as an equilibrium constant or the equilibrium equation, which by definition in general chemistry is products over reactants. If I have more products than I have reactants, then I have a lot more ions than I do solid material in my solution. The equilibrium constant is a number that tells me that that value is very high. In other words, a numerator is much higher than a denominator. I have a higher value, which means I have more products. And then the reverse is true. In a silver halide grain, we know that these materials are very insoluble because they come crashing out of solution when we mix silver nitrate with silver halide. And so we expect the equilibrium constant to have a lot more reactant than products. And so the equilibrium constant is going to be a very small value, a low value. So the value of the equilibrium constant tells us on which side of this equilibrium process we reside. Now oftentimes, I describe the solubility of silver halide as being very insoluble. And it's hard to be able to wrap your head around what I mean by very insoluble. But it would be easy to understand the following. If I took a grain of salt and dropped it in a swimming pool, you would never find it. It dissolves almost immediately. But if I took a grain of silver halide and dropped it in a swimming pool, I bet you're going to find it at the bottom of the pool. And that says something about its solubility. Silver halide is very insoluble because of its chemical and physical properties. So we have very, very small equilibrium constants. OK, so that's one idea. What that means is that most in our equilibrium equation, most of the form of our silver halide is on the left-hand side. It's the silver halide held together as a solid. So if I write that equilibrium equation as shown on the slide, products over reactants, the concentration of silver halide, or AGX, is a constant. Meaning it's not going to change. We're already saturated. So the concentration of silver halide solid is not going to change in a saturated solution condition. Now, we can express a new form of this equilibrium equation by just taking that constant and combining constants. This is something we used to do in algebra all the time. Take all your constants and throw them on one side of the equation. We can do the same thing here. So we're going to take the concentration of silver halide, which is a solid, remaining constant in a saturated condition. And we're going to move it to the left-hand side of the mathematical equation. Because my constants are on the left, what these two constants together indicate is how much ion I can possibly have in a saturated solution condition. This is known or the combination of the equilibrium constant with the concentration of the salt in a saturated condition is known as the solubility product, or the KSP. In reading about silver halide emulsification and photographic chemistry, the KSP is the value you see most often to describe processes. The KSP will describe why silver halide is the best choice for light-sensitive materials. The KSP describes why the emulsification process occurs. The KSP describes why we tone. The KSP describes why we fix. And with what do we fix with? Are all described by the KSP. Now, what does the KSP mean? What does it say? And oftentimes, when I am teaching chemistry of any form, I like to instruct my students of this very important point, that math is a language. Chemical equations tell a story, red, left, to right. And the KSP is no different. This is not some scary thing. It is a conceptual concept. KSP tells you how much. So KSP is equal to. KSP tells you how much ion, silver and halide ion, you can have dissolved in a solution. Or, said in a math way now, the product of the concentrations of the soluble ions will be the value of the KSP. What this means is, if the KSP is really, really small, then the concentrations of ions are going to be really, really small. Remember, that's the equation, the quality sign in an equation, is your balancing point. It's where you make your decisions. And what this says is, when the KSP is very, very small, that must mean the concentration of ions is really, really small. And the reverse. So when I say I can take a silver halide grain and I can drop it in water and still see the grain at the bottom, this means we have a saturated solution. Some of it does dissolve. How much of it, and here's the point, how much of it is controlled by the magnitude or the value of the KSP, the solubility product? So let's look at how these values are used in photographic chemistry by looking at the KSP's of well-known silver halide materials. And you'll get a sense on just how small the solubility of silver halides are and why we use this to our advantage in using silver halides, both as a light sensitive capture material and in the processes we use to develop up the silver in producing photographic images. You have completed this unit. Depending on your video viewer, you should be able to scroll back to any point in the video as desired. The short quiz found in the course materials on the website may help you confirm your understanding of the concepts introduced here. Many thanks to the instructor, production editor, coordinator, and the collaborative workshops in Photograph Conservation Committee for their work to make this program possible.