Learning Goals

1. Practice using volumetric flasks and graduated cylinders;
2. Prepare solutions from solids and calculate the concentration;
3. Prepare solutions from stock solutions and calculate the concentration;
4. Practice using a spectrophotometer and identify limitations to this instrument;
5. Discover Beer’s Law.

Introduction

In everyday life, you have probably noticed that color is often indicative of chemistry. For example, fruit often changes color when it ripens: a green banana contains complex starches (yuck), but a yellow banana contains simple sugars (yum). Color intensity is also indicative of chemistry, particularly in solutions. Today, you will use a spectrophotometer to quantify color hue and intensity for a relatively simple solution of Kool-Aid. This will allow you to derive the relationship between Kool-Aid concentration and color intensity, known as the Beer’s Law.

Background

Spectrophotometers contain a white light source, a diffraction grating, and a slit to select a specific wavelength. The selected wavelength of light passes through a sample, and any light that is absorbed is quantified as the absorbance. The absorbance range is 0 (no light absorbed) to 1 or more (lots of light absorbed). According to Beer’s law, absorbance and concentration are directionally proportional: samples with a high concentration of light absorbing particles have a high absorbance, and those with a low concentration of light absorbing particles have a low absorbance. In principle, any difference in absorbance can be measured with the spectrophotometer, but in practice, it is impossible to tell the difference between two samples if they are (1) too optically dense, meaning basically no light gets through, (2) too dilute, meaning practically all of the light gets through, giving both samples an absorbance of nearly 0, or (3) too similar, meaning there are not enough significant figures. This is analogous to a graduated cylinder, which cannot measure above the highest graduation line or below the lowest graduation line, and cannot differentiate between volumes that are within the uncertainty of the measurement.

Procedure

Prepare Kool-Aid Stock Solution

1. Select a packet of Kool-Aid. Record the flavor in your observations.
2. Tare a weigh boat, then pour approximately 1 g of Kool-Aid powder into the weigh boat. Record the exact mass of the Kool-Aid powder in Table 1.
3. Add the Kool-Aid powder to a 100 mL volumetric flask. Use a funnel to make sure no powder is lost. Rinse the weigh boat and funnel with DI water several times to ensure all of the powder gets into the flask. Fill to the mark with DI water (the water from the white faucet). Pro-tip: a highly concentrated solution that you will later dilute is called a “stock solution.”
4. Record the color of the stock solution in your observations. Predict the color that you expect it to absorb best (using the color wheel – it absorbs the opposite color that it appears). Record your prediction in your observations.

Measure Peak Absorption of Kool-Aid Solution

1. Prepare a blank cuvette by mostly filling a cuvette with DI water.
2. Prepare a sample cuvette by mostly filling a cuvette with the Kool-Aid solution.
3. Blank the Genesys 30 spectrophotometer, then put in the sample cuvette and scan the visible spectrum to determine the wavelength where the absorption is highest. Consult with your instructor or TA if there is not an obvious peak. Record this wavelength in Table 1. Does it correspond with your prediction?

Measure Absorbance of Solution at Various Concentrations

1. At the wavelength of peak absorption, blank the Genesys 10 spectrophotometer with the cuvette containing water. Record the blank measurement in Table 2.
2. Measure the absorbance for your solution cuvette. It may be out of range, which is fine, just write down the number that is displayed. Then, rinse your cuvette.
3. Measure out 50 mL of the solution. Pour it into the 100 mL volumetric flask (clean it first) and fill to the line with DI water. What is the new concentration in g/mL? Record this in Table 2.
4. Pour 1 mL of the diluted solution into the cleaned cuvette.
5. Repeat steps 1-4 with the diluted solution. Continue to dilute the solution by half until the absorbance measurement stops changing or goes to zero. Note: Table 2 may or may not have a row or two leftover. All waste can go down the drain.

Report

Fill out this worksheet. Graph may be submitted on Canvas or printed and attached to the worksheet.