SPE: Spectrophotometric Determination of Copper
Purpose: Spectrophotometry is the study of the interaction of matter with electromagnetic radiation (light). The radiant energy of a source (such as a tungsten filament lamp) produces photons of different wavelengths that may interact with a substance. Some of these wavelengths will be absorbed by the substance producing a change in the energy of the substance. For example, if photons in the visible region of the spectra are absorbed, an electron moves to a higher energy level. This is called an electronic transition and is responsible for producing the color of a substance. If photons in the infrared region of the spectra are absorbed, a change in the vibrational energy of the molecule will result.
The energy levels within a substance (electronic and vibrational) are quantized, that is, only certain energy levels are possible. As such, each substance absorbs a unique set of wavelengths. Other wavelengths of unabsorbed light are simply transmitted through the sample. A plot of the wavelengths of light that are absorbed by a substance is called a spectrum. This plot is produced using a device called a spectrophotometer. A spectrophotometer contains a light source, a prism or grating for selecting desired wavelengths, detector, and various optics and electronics.
Finally, if a wavelength of light is chosen where a substance absorbs, the amount of radiant energy absorbed is directly proportional to the amount of substance present in a mixture (Beer's Law). The amount of radiant energy absorbed by a substance at a given wavelength is called the absorbance. If solutions of a substance are made to known, but different concentrations (called standards) and the absorbance of each standard is measured, a linear plot will be produced (called a Beer's Law plot). Such a plot may be used to determine the concentration of a solution with unknown concentration, by measuring the absorbance of the unknown solution. The substance being determined is referred to as the analyte.
This lab will familiarize you with the following:
Background reading:
Cooperative
Chemistry Laboratory Manual
Page 64 To Make up a Solution of Known Concentration
Page 65 Dilution of Solutions
Page 87-90 The Spectronic 20
Chemistry & Chemical Reactivity
Section 1.8 Mathematics of Chemistry, Graphing
Section 5.8 Measuring Concentration of Compounds in Solution
Chemical Principles, The Quest for Insight
Pages F55-F59 Molarity, Dilution
Equipment
and Materials:
You will work in-groups of four for this experiment for data collection. Reports, calculations and graphs are to be done individually. Each person in the group will prepare a standard of different concentration. Each group will need the following:
*Cuvettes look like test tubes but are much more
expensive. You do not want to scratch the bottom of a cuvette. Therefore, they
are stored in a beaker with a kim wipe in the bottom of the beaker to protect
the bottom of the cuvettes. Do not get them mixed up with test tubes. Anyone
found using a cuvette as a test tube at any point during the semester will be
subjected to a five-point penalty.
Safety Precautions:
Wear safety goggles at all time. You will be using small amounts of concentrated aqueous ammonia. In addition, the copper standard solution contains a reasonably high concentration of aqueous ammonia. Use these solutions in your fume hood only. Keep standards stoppered when not in use. If a solution comes into contact with your skin, wash the area with water. Place all waste in the specially marked waste containers in the front hood.
Overview
You will need to complete the background reading assignment to fully understand this lab.
The goal of this lab is to determine the concentration of Cu2+ in an unknown solution. This is accomplished through a series of steps. The first step is to measure the absorption spectrum of a copper solution. An absorption spectrum shows the relationship between the amount of light absorbed by a sample as a function of wavelength. In order to generate the absorption spectrum of the copper solution, you will need to determine the amount of light that is absorbed by the solution at various wavelengths. You will use this data to generate a plot of absorbance vs. wavelength in order to determine the wavelength at which the copper solution absorbs the most light. This wavelength is referred to as the wavelength of maximum absorption.
The second step of this lab is to generate a Beer's Law plot. A Beer's Law plot shows the relationship between absorbance and concentration at the wavelength of maximum absorption. You will find that absorbance is directly proportional to concentration i.e. a linear relationship. As you increase the amount of copper in a solution (increases the concentration), the absorbance increases. Therefore, in this part of the experiment, you will prepare copper solutions of different concentrations. Next, using the wavelength of maximum absorbance, you will determine the absorbance of each diluted standard solution. Next, a plot of absorbance vs. concentration is generated. Now, it is possible to find the concentration of an unknown copper solution by using the absorbance of the unknown solution and the Beer's Law plot.
Experimental Procedure:
Note: We will be using the experimental procedure in this handout. You do not need to be concerned with the procedure section in the laboratory manual.
Part B: Preparation of Copper Standards
Part C: Preparation of Beer's Law Plot for Copper
Part
D: Determination of an Unknown Copper Solution Concentration