Beer's Law

background [prelab assignment (20 points); due at the beginning of lab]

1.  briefly, describe the following [6 points; 2 points / item]

Beer's law; include relationship between absorption and % transmission

standard or calibration curve, and its application / use

spectrophotometer

2.  run Beer's law simulation; include screen shot of completed simulation (i.e. your screen shot shows the filled-in standard curve, intensity of the unknown sample, & your calculations to determine [unknown])  [5 points]

3. sketch a data table (i.e. not calculations) for this lab containing entries for all required experimental measurements (i.e. review purpose & content of lab report) [4 points]

4.  MSDS data on copper (II) nitrate [5 points]

5.  review below "methods" section prior to lab; otherwise, you might be "lost" during the lab, since you won't know how to use the colorimeter; because, this issue won't be addressed during the prelab session (due to shorter block period) 

 

purpose

generate a standard (or calibration) curve

compare the determination of the concentration of Cu²⁺ in an unknown sample (range: 25 - 250 mM) using an algebraic- versus graphical- method in conjunction with the preceding calibration curve.

materials

aqueous solution of copper (II) nitrate

• 250 mM; dilute to generate known [Cu(NO₃ )₂ ] - to be used to generate the calibration curve
• unknown [Cu(NO₃ )₂ ]

colorimeter (a "cheap" spectrophotometer)

water; beakers; test tubes; micropipets; tissue paper

methods - use of probe device - record in your lab notebook, since you will use the colorimeter in a subsequent lab [see vodcast; may use your smart phone during lab to access the link]

1. turn-on Xplorer data collection device
2. insert colorimeter into Xplorer device
3. open lid, then insert small glass vial (cuvette) filled with water into colorimeter
4. close lid; should display %T ~ 100 or Abs ~ 0 for all wavelengths;  press "+" on Xplorer - to cycle thru various setting (red, green, blue, & orange, where reading % T or Abs
5. if %T not ~ 100% or ABS not ~ 0 , then press green button on colorimeter / wait till lights goes off  else goto next step
6. replace cuvette with samples; press "+" on Xplorer - to cycle thru various setting; record data; repeat with other samples
7. DO NOT PRESS ANY OTHER BUTTON DURING YOUR MEASUREMENT

data analysis

student design

content of the lab report [35 points]

data analysis [15 points]

figure 1:  standard curves using Excel:  include all wavelengths, i.e. colors, and equation & R² value of the "best" color to use in the analysis

figure 2:  based on preceding graph, using the "best" color (to analyze the data) and graph paper, do a "free-hand" drawing of the graph

determination of the [Cu²⁺ ] in your sample - show your work using both methods

• algebraic method:  use results of Excel output in figure 1
• graphical method:  use graph in figure 2 and draw imaginary lines to connect the value of the absorbance (on the vertical axis) to the graph then to the value of the concentration (on the horizontal axis).

statistical analysis (include interpretation of p-value) of class data

discussion [10 points]

is there a difference in the determination of [Cu²⁺ ] using both methods described in the purpose ?  Support your conclusion based on statistics

what is the rationale of choosing a specific wavelength of light for your determination of [Cu²⁺ ] ?  [hint:  see prelab question 1 in the below cdf based simulation]

rationalize why blue light has the lowest absorbance by the Cu²⁺ solution

error analysis (in regards to a cuvette containing the sample) [10 points]

how could the presence of fingerprints on the cuvette affect your determination of [Cu(NO₃)₂] ?  elaborate.

when would the volume of the sample in the cuvette affect your determination of [Cu(NO₃)₂] ?  elaborate.

 

resources:

Beer's law:  derivation / regression analysis  [calculus based presentation]

for Excel 2007 tutorials to analyze the calibration curve, see vodcast (@ lab / stats tab)

spectrophotometry 

simulations (requires cdf player plugin  or mathematica (nb)):  guided inquiry into exploring the considerations to select the wavelength to use in the Beer's law lab.