Coffee needs to be “just right” and we can tell how strong it is by the color.
Coffee needs to be “just right” and we can tell how strong it is by the color (0:30). We can use the color to determine concentration also in Chemistry (1:09). The color we see is determined by the wavelength an object reflects, while absorbing all other colors (1:23).
The Beer-Lambert Law absorbance to molar absorptivity, path length and concentration (2:08). In AP Chemistry, the molar absorptivity and path length are held constant, therefore absorbance is directly proportional to concentration (2:28).
Experimentally, we measure absorbance using spectrophotometers (4:41). To determine the concentration of, for example, blue dye in a sports drink, we have to create a calibration curve using solutions with known concentration and measuring the absorbance (5:35). We can then use the graph and a measurement of absorbance of the sports drink to determine the concentration (6:16).
In which of the following examples could you use spectroscopy and the Beer-Lambert law to determine the concentration?
A. Determination of bilirubin in blood plasma samples. B. Determination of colorless zinc(II) nitrate in a sample. C. Determination of ethanol (drinking alcohol) in an alcoholic beverage. D. Determination of isopropyl in hand sanitizer.
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Hi and welcome to the APsolute Recap: Chemistry Edition. Today’s episode will recap the Beer-Lambert Law
Lets Zoom out:
Unit 3 - Intermolecular Forces and Properties
Topic - 3.13 Beer-Lambert Law
Big idea - Structure and Properties
I don’t know how many of you are coffee drinkers, but our author Sarah definitely needs a few cups a day! However, it’s important that the coffee is “just right” - not too strong, not too weak. But how can you tell how “concentrated” the coffee is? (and I hope you appreciate how I snuck in a chemistry term here)? You can use the color: the lighter the coffee, the weaker it is and the darker the color, the stronger. This does not only apply to coffee, but also other drinks, like with concentrated KoolAid mix or tea. As you can see, we are using the color of a solution to qualitatively determine the concentration, so we can, of course, also use this in Chemistry - and even take it a step further!
Let’s zoom in:
Let’s start by talking about color. Why do we see a certain color? It all ties back to the wavelength a substance is absorbing and reflecting. In a simplified version, we can say that if a substance is absorbing radiation with a wavelength of 600 nm, which is orange, then we will see its complementary color, blue, which is being reflected by the solution and reaches our eyes. Orange and blue are only one example of complementary colors. The other pairings are: violet - yellow, and green-red. Of course, white light contains all colors of the rainbow, so to see something as blue, the object itself has to absorb all colors but blue.
Now that we know what wavelengths are absorbed and reflected, what is the connection between concentration and color? This is where the Beer-Lambert Law is being used. Note: It has nothing to do with the alcoholic beverage, but with August Beer, a German physicist and chemist, who contributed to this law.
The Beer Lambert law relates the absorption of light by a solution to three variables: first, the molar absorptivity ε, which is a constant that is unique to each compound and describes with what intensity the atoms, ions and molecules are absorbing light. Second, the path length b of the solution container and third the concentration c of the solution. These variables are directly proportional to one another: The greater the concentration, the greater the absorbance and the greater the path length, the greater the absorbance - and vice versa. Therefore, the law is written as: absorbance = molar absorptivity constant times path length times concentration.
In the laboratory activities we are doing in AP Chemistry, path length - the container, and molar absorptivity are held constant. This allows us to directly relate absorbance and concentration: The greater the concentration, the greater the absorbance. We also know from our coffee example that the greater the concentration the darker the color. Therefore, we can say: the higher the concentration of molecules absorbing light, the darker the solution and the more visible light is being absorbed.
What does that look like experimentally? To measure absorbance, we use spectroscopy and spectrophotometers. Spectrophotometers can be set to have only a certain wavelength reach the solution in the cuvette - aka light of a certain color. Since we are using the Beer-Lambert Law, we are looking to measure absorbance. Therefore, we have to set the color that reaches the solution to the complementary color of that solution. For example: Imagine we want to measure how much blue dye is in a sports drink. The blue sports drink is absorbing red-orange light and reflecting blue light - that’s why we see it as blue. And so, we set our spectrophotometer to red-orange light, around 640 nm. We will then be able to read the absorbance, which is a unit-less value, on the output of the spectrophotometer.
But, sticking with our example, how do I know the concentration of blue dye? What can we compare it to? The absorbance alone doesn’t tell me the concentration. When using a spectrophotometer, we will often start with preparing a calibration curve, also known as standard curve. A calibration curve is a curve that is prepared by measuring the absorbance of samples of known blue dye concentration. For example: You prepare five solutions with concentrations of blue dye ranging from 0.7 µM to 7.0 µM. You measure the absorbance of each one of them, where your solutions with higher concentrations have a greater absorbance. You can then graph absorbance vs concentration and, using a line of best fit, you will get a calibration curve, which quantitatively relates absorbance and concentration. Then, you can measure the absorbance of your unknown, your sports drink. Using your calibration curve, you can now determine the concentration of blue dye that corresponds with your measured absorbance. Tada!
To recap:
The Beer Lambert law relates the absorption of light by a solution to three variables: the molar absorptivity, the path length and the concentration. Since molar absorptivity and path length are held constant, absorbance and concentration are directly proportional. For colored solutions that means that the higher the concentration of molecules absorbing light, the darker the solution and the more visible light is being absorbed. To determine the concentration we use spectroscopy and spectrophotometers. We create a calibration curve showing the relationship between absorbance and known concentrations. After measuring the absorbance, we can use the curve to determine the concentration of the unknown solution.
Coming up next on the APsolute RecAP Chemistry Edition: Unit 3 selected FRQs
Today’s Question of the day is about applications of the Beer-Lambert Law.
In which of the following examples could you use spectroscopy and the Beer-Lambert law to determine the concentration?
a. Determination of bilirubin in blood plasma samples.
b. Determination of colorless zinc(II) nitrate in a sample.
c. Determination of ethanol (drinking alcohol) in an alcoholic beverage.
d. Determination of isopropyl in hand sanitizer.