etc. are maintained within very narrow ranges. Deviations in the solution and the less light will pass through that solution. relationship between the concentration and light absorbance (i.e. comparing its light absorbance or transmittance with a that of . When glucose reacts with o-toluidine, it forms a greenish complex. absorbance and transmittance spectra have a "common data base", you can display any Only there you can use the Lambert-Beer relationship. function in Spekwin32 proto type hardware control program, and I was curious to know if I set. Describe the relationship between Absorbance and Transmittance and the The transmittance of a sample is the ratio of the intensity of the light that has.
So let's say we have two solutions that contain some type of solute. So that is solution one, and then this is solution two. And let's just assume that our beakers have the same width. Now let's say solution let me put it right here, number 1, and number 2. Now let's say that solution 1 has less of the solute in it.
So that's the water line right there. So this guy has less of it. And let's say it's yellow or to our eyes it looks yellow. So this has less of it. Actually, let me do it this way. Let me shade it in like this. So it has less of it. And let's say solution number 2 has more of the solute.
Chem - Experiment II
So I'll just kind of represent that as more closely packed lines. So the concentration of the solute is higher here. So let me write higher concentration. And let's say this is a lower concentration.
Now let's think about what will happen if we shine some light through each of these beakers. And let's just assume that we are shining at a wavelength of light that is specifically sensitive to the solute that we have dissolved in here.
I'll just leave that pretty general right now. So let's say I have some light here of some intensity. So let's just call that the incident intensity. I'll say that's I0. So it's some intensity.
What's going to happen as the light exits the other side of this beaker right here?Beer-Lambert's law with derivation - Absorbance & Transmittance - Spectrophotometer cell
Well, some of it is going to be at absorbed. Some of this light, at certain frequencies, is going to be absorbed by our little molecules inside the beaker. And so you're actually going to have less light coming out from the other side. Especially less of those specific frequencies that these molecules in here like to absorb. So your're going to have less light come out the other side. I'll call this I1. Now in this situation, if we shine the same amount of light-- so I that's supposed to be an arrow there, but my arrow is kind of degrading.
If we shined the same amount of light into this beaker-- so it's the same number, that and that is the same-- the same intensity of light, what's going to happen? Well more of those specific frequencies of light are going to be absorbed as the light travels through this beaker.
It's just going to bump into more molecules because it's a higher concentration here. So the light that comes out when you have a higher concentration-- I'll call the intensity I this is going to have a lower intensity of light that's being transmitted than this one over here. In this case, I2 is going to have a lower intensity, is going to be less than I1.
And hopefully, that makes intuitive sense. These light, if you imagine, photons are just going to bump into more molecules. They're going to be absorbed by more molecules. So there'll be fewer that make it through than these right here, because here it is less concentrated.
It's also the case if the beaker was thicker. Let me draw another beaker. If you have another beaker that is maybe twice as wide, and let's say it has the same concentration as number 1. We'll call this one number 3. It has the same concentration as number 2, so I'll try to make it look fairly similar to this. And you were to shine some light in here. Generally you want to focus on the frequencies that this is the best at absorbing.
But let's say you shine the same light in here. And you have some light that makes it through, that exits. And this is actually what your eyes would see. So this is I3 right there, what do you think is going to happen? Well it's the same concentration, but this light has to travel a further distance to that concentration.
So once again, it's going to bump into more molecules and more of it will be absorbed. And so less light will be transmitted. So I2 is less than I1, and I3 is actually going to be the least. And if you were looking at these, this has the least light, this has a little bit more light being transmitted, this has the most light being transmitted.
So if you were to look at this, if you placed your eyeball right here-- those are eyelashes-- this one right here would have the lightest color. You're getting the most light into your eye. This would be a slightly darker color, and this would be the darkest color. That makes complete sense. If you dissolve something, if you dissolve a little bit of something in water, it will still be pretty transparent.
If you dissolve a lot of something in water, it'll be more opaque. And if the cup that you're dissolving in, or the beaker that you're in gets even longer, it'll get even more opaque.
So hopefully that gives you the intuition behind spectrophotometry. And so the next question is, well what is it even good for? Why would I even care? Well you could actually use this information. You could see how much light is transmitted versus how much you put in to actually figure out the concentration of a solution. That's why we're even talking about it in a chemistry context. So before we do that-- and I'll show you an example of that in the next video-- let me just define some terms of ways of measuring how concentrated this is.
Beer's Law - Theoretical Principles
Or ways of measuring how much light is transmitted versus how much was put in. So the first thing I will define is transmittance. And so when the people who defined it said, well you know, what we care about is how much is transmitted versus how much went in. So let's just define transmittance as that ratio, the amount that gets through. So in this example, the transmittance of number 1 would be the amount that got through over the amount that you put in.
Over here, the transmittance would be the amount that you got out over the amount that you put in. Now for the fun part! Using the calibration plot that YOU made from the data two pages ago. We are going to determing the concentration of an unknown solution. Make sure you have your plot ready, because here we go!
Here's a typical problem. You take 3mL of your unknown sample and 7mL water and mix them together. The dilluted sample gives an absorbance of 0. What is the concentration of the initial unknown? Where do you begin?! You have an absorbance, and you have a straight line equation that relates absorbance to concentration. This is the line of best fit through your data. Remember you dilluted it once, so you can use the Dilution Equation Ready to try one on your own?
Here are a few more problems.