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:

  1. The proper method to make a solution of known concentration to a high degree of accuracy.
  2. The proper method to dilute a solution of known concentration to a new concentration.
  3. The use and care of a visible spectrophotometer to produce a Beer's Law plot and determine the unknown concentration of a substance in a mixture.
  4. Using excel to generate a graph

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:

  1. Four 25-mL volumetric flasks (1 per group member obtained from the common equipment area).
  2. Four 10-mL graduated pipets. (1 per group member obtained from the common equipment area).
  3. 2 cuvettes* (located at each spectrophotometer).
  4. 10-mL of the copper solution labeled “Ímax determination” for the determination of maximum absorbance wavelength
  5. 30-mL of the copper standard solution for preparing diluted standards. Note the concentration on the data sheet, Part B, and place immediately in your fume hood.

*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.

  1. Spectrum of Copper-Ammonia Complex
  1. First you will need a blank solution for the spectrophotometer. A blank solution contains everything but the analyte (Cu+2). Measure about 10 mL of distilled water into a beaker and add 20 drops of concentrated aqueous ammonia (NH4OH). CAUTION: This step should be performed in a hood. Fill 1 of the cuvettes 3/4 full with this blank solution and dispose of the remainder.
  2. Fill the other cuvette about 3/4 full with the copper solution labeled “copper solution for lmax determination”. Using the procedure described in the lab manual (pg. 87-90), take absorbance measurements in 20 nm increments over the range of 400-650 nm. Record absorbance measurements on the data page. Prepare a plot of the absorbance measurements vs. wavelength (nm) using Excel spreadsheet. Note: Absorbance is a unitless value. Determine the wavelength of maximum absorbance and record on the data page. Turn in spectrum along with data sheets.
  3. Set the spectrophotometer to the wavelength of maximum absorbance and proceed with part B.

Part B: Preparation of Copper Standards

  1. Using the copper standard solution, each group member will prepare a dilute copper standard solution. Add a volume (using a graduated pipet) of the copper standard solution between 2 and 8 mL to a clean 25-mL volumetric flask. Each group member should choose a different volume.
  2. Add 30 drops of concentrated aqueous ammonia to the volumetric flask and mix. Caution: This step should be performed in a hood. Add distilled water to the etch mark on the volumetric flask. A wash bottle filled with distilled water serves nicely for this step. Stopper the volumetric flask and mix.
  3. Calculate the diluted copper standard solution concentration (See page 65 in lab manual.) and record the volume pipeted and dilute copper standard molar concentration on the data page.

Part C: Preparation of Beer's Law Plot for Copper

  1. Using the blank solution prepared in Part A, measure the absorbance of each diluted copper standard solution at the wavelength of maximum absorbance  (l max) and record on the data sheet.
  2. Prepare a plot of Absorbance (A) vs. Molar Concentration (M) using Excel spreadsheet for your report. You must include the linear equation on your graph.  The linear equation of the line is in the form of y = mx + b. 

 

Part D: Determination of an Unknown Copper Solution Concentration

  1. Obtain an unknown copper solution from your instructor. Measure the absorbance using the blank prepared in Part A at l max found in Part B.
  2. Using the linear equation determined in Part C, calculate the concentration of the unknown copper solution. Record the unknown concentration on the Data Sheet.  The linear equation of the line is in the form of y = mx + b.  Now that you know the absorbance of the unknown (y), use the linear equation of the line from the Beer’s Law Plot to calculate the concentration of the unknown copper solution.  (i.e. solve the equation for x)
  3. Thoroughly rinse all pipets, volumetric flasks, and cuvettes before returning them to the common equipment area.

Part A, B Data Sheet

Part C, D Data Sheet