Relationship between %Transmittance and light path length and concentration . To get from the Absorbance to the concentration you solve the equation of the. Students will study the relationship between transmittance, absorbance, and concentration of one type of solution . If you have one type of solution of different. If some of the light is absorbed, the reflected or transmitted light has the cell reduces the transmittance to 49%(Abs = ) doubling the absorbance as is.
And let's say solution number 2 has more of the solute. 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?
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.
Spectrophotometry introduction (video) | Khan Academy
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.
- How to relate the transmittance to the absorbance, concentration, and molar absorptivity?
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. 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.
Spectrophotometry - Chemistry LibreTexts
And as we see, this one right here will be a lower number. I2 is lower than I1.
So this will have a lower transmittance than number 1. So let's call this transmittance 2. This is transmittance 1. And transmittance 3 is the light that comes out, that gets through, over the light that goes in. And this is the smallest number, followed by that, followed by that. So this will have the least transmittance-- it's the most opaque-- followed by that, followed by that. Now another definition-- which was really kind of a derivative of the-- not in the calculus sense, this is just derived from transmittance and we'll see it has pretty neat properties-- is the notion of absorbance.
And so here, we're trying to measure how good is it at absorbing? This is measuring how good are you at transmitting? A higher number says your transmitting a lot.
Visible spectrophotometers, in practice, use a prism to narrow down a certain range of wavelength to filter out other wavelengths so that the particular beam of light is passed through a solution sample. Devices and mechanism Figure 1 illustrates the basic structure of spectrophotometers. Detailed mechanism is described below. First a collimator lens transmits a straight beam of light photons that passes through a monochromator prism to split it into several component wavelengths spectrum.
A single wavelenth spectrophotometer You need a spectrometer to produce a variety of wavelengths because different compounds absorb best at different wavelengths. For example, p-nitrophenol acid form has the maximum absorbance at approximately nm and p-nitrophenolate basic form absorb best at nm, as shown in Figure 3. An isosbestic point is the wavelength in which the absorbance of two or more species are the same.
The appearance of an isosbestic point in a reaction demonstrates that an intermediate is NOT required to form a product from a reactant. An example of isosbestic point Referring back to Figure 1 and Figure 5the amount of photons that goes through the cuvette and into the detector is dependent on the length of the cuvette and the concentration of the sample.
Once you know the intensity of light after it passes through the cuvette, you can relate it to transmittance T. Transmittance is the fraction of light that passes through the sample. This can be calculated using the equation: Transmittance is related to absorption by the expression: With the amount of absorbance known from the above equation, you can determine the unknown concentration of the sample by using Beer-Lambert Law. Transmittance illustrated by Heesung Shim Beer-Lambert Law Beer-Lambert Law also known as Beer's Law states that there is a linear relationship between the absorbance and the concentration of a sample.
For this reason, Beer's Law can only be applied when there is a linear relationship.