chemistry

Experiment #3b: Aluminum Content via

Colorimetry

Objective

To determine the aluminum content in Reynolds Wrap and Great Value foil using Spec-20 colorimeter.

Introduction

Visible light is a very small part of the electromagnetic spectrum that the naked eye can see. White light

contains a spectrum of different wavelengths and when it is passed through a prism, the light is broken

up into its different colors. Below is a table of different colors and the wavelengths associated with

them.

How we perceive color is due to how the different wavelengths are either absorbed or reflected

by a substance. The part of every material that absorbs the light is known as a chromophore. If

something appears red, it is because most of the other colors (wavelengths) are absorbed, except the

red light which is reflected.

One way to measure absorbance is with a Spec-20 colorimeter. A Spec-20 is a device that

measures the absorbance of particular wavelengths of light by a specific solution. The absorbance of a

specific compound can change with the concentration. The simplest way to think of concentration is

how much something is dissolved in something else. One is able to use a known concentration to find

the concentration of an unknown sample. This can be done using the formula M1V1 = M2V2, where M is

equal to Molarity and V is equal to the Volume.

In this lab, we will be looking at the absorbance of various solutions containing different concentrations

of aluminum.

The colors of the visible light spectrum

color wavelength interval

red ~ 700–635 nm

orange ~ 635–590 nm

yellow ~ 590–560 nm

green ~ 560–490 nm

blue ~ 490–450 nm

violet ~ 450–400 nm

Experimental Procedure

Weigh out approximately 50 mg of Reynolds Wrap aluminum foil into a 50 mL beaker. Record the exact

weight of the sample into your notebook. Dissolve the foil using 25 mL of 4 M hydrochloric acid and

transfer the solution to a 100 mL volumetric flask. Rinse the beaker with portions of deionized water

and add to the volumetric flask to ensure complete (quantitative) transfer of the solution. Dilute the

sample to the line with deionized water. This is your stock solution.

Obtain three 25 mL volumetric flasks and label them 1 – 3. Transfer 1.0 mL of the stock solution into

each flask followed by addition of 1 mL of the aluminon/buffer solution and 1 mL of the ascorbic acid

solution. Dilute the solutions to the line with deionized water and allow them to stand for 15 minutes.

Calculate the concentrations of the solutions in your notebook in mg/L (make sure to account for the

dilutions). Repeat the above procedure using Great Value aluminum foil (make sure you keep the two

trials Reynolds Wrap vs. Great Value separate from each other and organized).

While the solutions are standing, move to a lab station with the spectrometer setup. Each station will

contain a set of cuvettes, standard samples and a blank sample which will be used to construct a Beer’s

Law plot. Obtain a blank sample and fill one of the cuvettes approximately ¾ of the way to the top.

Make sure the wavelength on the instrument is set to 530 nm and insert your sample into the

compartment. Zero out the instrument. Next, obtain one of the standard sample solutions and record

the concentration into your notebook (see table below). Fill up a second cuvette ¾ of the way and insert

into the instrument. Close the compartment and record the absorbance value into your notebook.

Repeat for each additional known sample.

Sample Concentration (mg/L) Absorbance

Blank

Known 1

Known 2

Known 3

Known 4

Known 5

After your Reynolds Wrap and Great Value aluminum foil solutions have been sitting for 15 minutes, determine the absorbance values for each sample using the spectrometer. Record your values in your notebook.

Sample Concentration

(mg/L) Absorbance 1 Absorbance 2 Absorbance 3

Average Absorbance

Reynolds Wrap

Great Value

Once you have finished, dispose of the liquid material in the bottle labelled “Aqueous Waste”.

Part II: Data Analysis

Construct a standard curve using your values from the standard solutions by plotting the absorbance

values for each standard versus the concentration of the sample. Construct a best fit line (trendline) and

obtain the equation of the line and the R2 value. Using the equation of the line, determine the actual

mass of aluminum metal present in both Reynolds Wrap and Great Value aluminum foil.