Determination of Copper in Brass

Learning Goals:

    1. Prepare standard solutions containing known concentrations of copper.
    2. Use a UV-vis spectrometer to measure the absorbance of known copper solutions.
    3. Construct a Beer's Law Plot from the known copper solutions.
    4. Use a Beer’s Law Plot to calculate the percentage of copper in brass.

Abstract:

Brass is a metal alloy usually composed of copper and zinc. The amount of copper in a brass sample can be experimentally determined by using UV-visible spectroscopy. First solutions of known copper concentrations are prepared and their spectra obtained. A Beer's law graph (standard curve of the known concentrations vs. absorbance) is then plotted and used to calculate the percentage of the copper in the brass sample.

UV-VIS Spectroscopy and Beer’s Law

UV-VIS spectroscopy provides useful qualitative and quantitative information about a chemical sample. Since electromagnetic radiation in the ultraviolet and visible range causes electrons to be promoted between energy levels, the shape and wavelength of maximum absorbance of a UV-VIS spectrum can help chemists identify a particular chemical. UV-VIS spectra can also provide quantitative information about a particular chemical through a relationship called Beer’s Law (also referred to as the Beer’s – Lambert Law). Beer’s Law states that the amount of light absorbed by a chemical is directly related to the concentration of the chemical in a solution. See equation 1.

A = e bc

Equation 1

In this equation, the absorbance (A) of the solution is directly proportional to the concentration (c) in moles/Liter, the pathlength (b) in cm-1 and the molar absorptivity (e ) in Lcm-1mol-1. Under the conditions of this experiment, the molar absorptivity and the path length are constant. Thus, the absorbance of a solution is directly proportional to the concentration of solution. This concentration is expressed in Molarity or moles/Liter.

A little more about Beer’s-Law: Be sure you are working with a consistent set of units.

e - molar absorptivity. This value is constant for a given molecule in a given solvent at a given wavelength. That is for blue food coloring dissolved in water at a wavelength of 600 nm, there is one and only one value of e. Molar absorptivity is found by dividing the slope of the absorbance – concentration (standard curve) line by the pathlength. Once you have determined the molar absorptivity of a solution, you only need measure the absorbance of an unknown and divide by e at a given wavelength. This is how many commercial spectrometer’s calculate concentration.

b – pathlength is the distance the light travels through the solution between the source and the detector. It is related to the amount of stuff the light beam (photons) has to go through. Pathlength is also represented by l.  in some texts.

c – solution concentration. This is simply the Molarity (M) of the solution upon which you are running the spectra.

This equation and relationship between absorbance and concentration can be used to create a Beer’s Law Plot. By plotting the known concentrations of several samples of a particular chemical on the x-axis and the corresponding absorbance at a given wavelength (typically lmax) on the y-axis, a Beer’s Law Plot is obtained. 

The Plot can then be used to determine an unknown concentration of the same chemical at the chosen wavelength (typically lmax). At dilute concentrations, many chemical solutions obey Beer’s Law, resulting in a straight line plot with the general equation for a straight line y = mx + b.

Creating a Beer's Law Plot with the ICN software

To create a Beer's Law Plot of your data, use the Plotter under the "tools" menu in the toolbox. Enter the concentrations of solutions onto the x-values column and the corresponding absorbance values onto the y-values column. Include a set of zero values for the "blank," (zero concentration and zero absorbance.) Click on the Plot Options button and pick Grid Lines and Least Squares fit. Label the axes and title the plot. Print your Beer's Law Plot. This plot is also called a standard curve.

To find the unknown concentration of a solution, use the equation for a straight line and solve for x.

y = mx + b

Sample calculation: Using the Beer's Law Plot above, determine the unknown concentration of a solution with an absorbance reading of .600.

In this example, the slope (m) is 6.414 and the intercept ( b) is -7.99 x 10 -3. Solving for x,

.600 = 6.414 x - 0.00799

x = 0.095 M

Creating a Beer's Law Plot with Excel

 

Excel is a popular and powerful spreadsheet program that will allow you to plot and fit experimental data to a variety of mathematical models. The instructions given below will allow you to plot data in an X-Y chart and determine the best-fit linear equation. We have chosen to use menu commands for this description; however other methods can attain the same results. Please open this demonstration workbook as you work through the instructions below.

 

  1. Open a blank Excel workbook

 

            To duplicate the chart at the end of these instructions, you can enter the following data:

                                    A                                             B

                                    0                                              0

                                    0.2                                           0.2

                                    0.4                                           0.4

                                    0.6                                           0.6

                                    0.8                                           0.8

                                    1.0                                           1.0

 

  1. Enter your independent variable in Column A (x-axis)
  2. Enter your dependent variable in Column B (y-axis)
  3. Highlight your data using the mouse.
  4. Select Insert from the menu bar.
  5. Select Chart from the dropdown menu.
  6. A chart Wizard will open up. Select XY (Scatter) from the list of charts.
  7. Click on Next
  8. A preview of what your chart will look like appears. Click Next
  9. This screen allows you to enter a chart title and appropriate axis titles. When you are done and like the way the preview looks, click on Next.
  10. This screen asks you if you want to place this chart in a new worksheet or as an object in the current worksheet. Select as “an object in” Sheet 1 and click Finish.
  11. You now have your chart and are ready to add the linear trendline.
  12. Click on the chart and select Chart from the menu bar.
  13. From the dropdown menu select Add Trendline
  14. Under Type select Linear
  15. Click on the Options Tab
  16. Check the box Set Intercept and make sure value is 0 (This forces the intercept to be 0 as in out case with Beer’s Law we know that the blank has 0 absorbance and 0 concentration)
  17. Check the box Display Equation on Chart
  18. Check the box Display R-Squared value on chart
  19. Click OK
  20. The equation of the best fit linear trend line and the r2 value will appear on the chart. The equation is in the form y=mX where m is the slope of the line. R2 values range from 0 to 1 and the higher the value the better the data fits the linear model.
  21. You should obtain a chart that looks like

 

Lecture Connections

Prelab Assignment

In your lab notebook, prepare the following information:

  1. View the video clips on Using the UV-visible spectrometer and Making Dilutions. You will need Quick Time video player to see them.
  2. A brief (2-3 sentence) introduction to the lab. 
  3. A table of safety information including the chemicals used in the lab and any safety handling precautions. This information can be obtained from the MSDS safety sheets. 

Give the information to your TA at the beginning of the lab. You will not be allowed to work in the lab without this information.

 
Chemicals Glassware and supplies Instruments

Standard copper (II) nitrate solution

Concentrated (16 M) Nitric Acid (HNO3)

Concentrated Ammonium hydroxide (NH4OH)

Brass wire

 5 mL, 10 mL, 20 and 25 mL volumetric pipettes

 Pipet filler

Plastic cuvets with holder

250 mL volumetric flask

 UV-vis spectrometer
 

Procedure:

A Flow Chart for Determination of Percentage Copper in a Brass Sample

 

Part I: Preparing a Standard Curve from Known Copper Concentrations

  1. Record the concentration of the standard copper (II) solution.
  2. Pour about 70 mL of the standard copper solution into a clean dry Erlenmeyer flask.
  3. Using a volumetric pipet, transfer 5.00 mL of copper (II) solution from the beaker into the 250 mL volumetric flask. Using a graduated cylinder, add 10 mL of concentrated ammonia solution (NH4 OH).  Fill the volumetric flask  EXACTLY to the mark with distilled water. Place the stopper into the flask and mix well by inverting several times. Pour the solution into a clean dry beaker. Label the beaker.

    What is the copper (II) concentration in molarity, M (moles/liter), of this first solution? See section 4.5 in your text and the example calculation. Enter this concentration into your lab notebook.

    4.
  4. Repeat the above procedure with  10.00,  20.00, and 25.00 mL of the copper (II) solution. 

    What is the copper (II) concentration in molarity of each of these solutions? Enter these concentrations into your lab notebook.

     

    In this first part of the procedure, you have now prepared four solutions of known concentration by diluting a standard solution. Since absorbance is directly related to concentration, these solutions can be used to find the unknown copper concentrations in in an unknown solution. To use these solutions, you first need to use the ICN plotter program or another spread sheet program  to create a plot of absorbance versus concentration, called a Beer's Law Plot. The maximum absorbance for the copper solution should be around 600 nm. 

    Part 2: Determine the percentage of copper in a brass sample

  1.  Weigh a 0.1 g to 0.2 g piece of brass to the nearest 0.001g. Record the weight and place the piece of brass into a 25 mL Erlenmeyer flask. Under your hood, add 1.0 mL of distilled water and 1.0 mL (20 drops) of concentrated nitric acid(HNO3) to the flask. If the reaction stops before all of the brass is dissolved, add a few more drops of acid.
  2. After the brass has completely dissolved, rinse down the sides of the flask with distilled water and transfer the solution to a 150 mL beaker. Rinse several times and add the rinse water to the beaker. Add distilled water to a total volume of about 60 mL.
  3. Slowly add concentrated ammonia solution(NH4OH), until a white cloudy precipitate forms and remains upon mixing. Using a graduated cylinder, add 10 mL more of concentrated ammonia. The solution should now be clear and dark blue. Transfer the contents of the beaker to a clean 250 mL volumetric flask. Rinse the beaker several times to assure complete transfer. Fill to the mark with distilled water.

      

Part 3: Obtain UV-vis spectra for the solutions. 

  1. Prepare and label a cuvet for each standard solution, the unknown brass solution, and a blank containing distilled water.
  2. Using the UV-vis spectrometer, blank the instrument with the distilled water and scan a spectrum for each solution.
  3. Return all of your solutions to the laboratory and analyze your data. 
  4. Dispose of your solutions as directed by your instructor.

Post Lab Assignment

Include the following information in a lab report to give to your TA:

  1. Determine the concentration in Molarity (M),  for each of your known copper solutions. 
  2. The maximum peak for copper should be around 600 nm. From your spectra, find the absorbance for each known copper solution.
  3. Make a table of the known concentrations and their corresponding absorbances.
  4. Using one of the following software programs,

    (a.) the ICN View Spectra Program on your Progress Page

    (b.) the ICN Plotter Program on the computers in Room 421    

    (c.) a Spreadsheet Program such as Excel

    5. ®

    prepare a  Beer’s Law Plot by plotting the concentration of your known copper solutions on the x axis and absorbance on the y axis. Be sure to include zero absorbance for zero concentration of Copper. (This extra point is because the instrument was blanked with water that had no copper in it.) The Plotting program that you chose must be able to give you the slope and y-intercept for the best fit straight line using the least squares fit analysis to determine the best fit line for your data points. Print out your graph.
  5. Using the Beer’s Law Plot, the equation y=mx+b ( which describes the straight line) and the absorbance reading from your brass solution, determine the molar concentration of Copper in your unknown brass solution.
  6. Determine the percent of copper in your brass sample. (See Lecture Connections)
  7. Enter your percentage results on your ICN Progress Page and calculate your deviation from the class mean.
  8. When you are finished with the lab report you will be asked several Post-Lab Questions. they are Ranking Questions.  Here is an example of this type of question.

    The following exercise is called a ranking task.  You will be asked to rank the objects in the problem from greatest to least, and specify which, if any, are equal.

     

    Here is an example of how to do a ranking task problem:

    Rank the following areas from greatest to least.  Clearly specify which areas are equal.  Explain your ranking clearly.

     

    A.  circle, radius = 8cm

    B.  square, length = 15 cm

    C.  triangle, height 20 cm, width 20 cm

    D.  rectangle, height 10cm, length 20 cm

     

    Greatest 1. __B__  2. __C =___ 3. __D___ 4. ___A__

     

     

    In the example, B has the greatest area (225 cm²).  C and D both have areas of 200 cm², and A has an area of approximately 198 cm².

 

Copyright (c) 2006, the ICN Team.