Turn on Spec 20’s
*** NOW! ***
Absorption Spectroscopy
Today’s Lab 11 :O :O !!!
Lab 12 – VSEPR Lab (informal in class)
 Last day – NO LAB FINAL

You are required to come – will clean up a
certain portion of the lab (ie glassware)
 After clean up, I will host lecture exam studying
based upon the ACS standardized exam for
your lecture final

your textbook, ACS books, and questions – I
will stay in the lab answering questions until lab is
over.
 Bring
Unknowns
Unknown
Ni
Co
Cu
Color
Green
Red
Blue
λ
395 nm
525 nm
625 nm
Possible unknowns and their wavelength of
maximum absorption (nm)
Procedure
Get unknown assignment from Lab
Instructor.
 Fill test tube with stock solution(in back
hood) of similar color, green (Ni), red (Co),
or blue (Cu)
 Record absorbances from 400 to 700 nm at
25 nm intervals, determine the wavelength of
maximum absorbance, double check with
lab instructor.

Finding Maximum
Wavelength
Procedure
Set Spec 20 to appropriate wavelength
 Record absorbance (or %T for more
significant figures) of four standard solutions:

10 mL water
 3 & 7 mL diluted solutions (detailed on next slide)
 10 mL stock solution

Plot absorbance vs. concentration (EXCEL)
 Use calibration curve to ID unknown
concentration!

Calibration with your blank
solution


With empty, closed sample compartment, turn the
left-hand knob to obtain a reading of 0% T.
Use the mirror behind the needle to avoid parallax
error.
Calibration
•Fill your tube ~ 1/2 to ¾ of the way with your blank
solution (NaOH)
•Wipe the cuvette with a dry Kimwipe to remove drops of
solution or finger prints.
•Insert in the compartment, and close the cover
Calibration

Turn the right-hand knob to obtain a reading
of 100% T.
Using Spectrometer
To analyze your sample, insert sample cuvette and read the
Absorbance value on the scale. Use the mirror behind the
needle to avoid parallax error
Source: http://www.dartmouth.edu/~chemlab/techniques/spectrometer.html
Procedure- Calibration Curve

Prepare two standard solutions with known
concentrations



Use graduated cylinder to deliver 3 mL and 7 mL DI
H2O to separate test tubes, record actual volume
Deliver 7 mL and 3 mL DI H2O, record actual volume
Calculate concentrations of the two solutions
Vconc M conc  Vdil M dil
Tables p. 47-48
SKIP PART 1 (Adjust your wavelength to the appropriate nm based on
previous slide and the color of your unknown!!)
Part 2
Sample Number Vol Stock Vol H2 O Diluted Vol Conc (M) Absorbance
1 (pure water
0 mL
10 mL
10 mL
2
3 mL
7 mL
10 mL
CALC
RECORD
3
7 mL
3 mL
10 mL
THESE
THESE
4 (stock soln)
10 mL
0 mL
10 mL
↑ MAKE THESE ↑
Go Home and plot
Convert %T to absorbance!

Convert from %T to absorbance:
Absorbance = -log(%T/100)
In Class

Show me a rough graph of your absorption
spectrum (page 40 example) and indicate
where your maximum absorption you found
to be.
Formal Lab
Data Table –raw individual data (as per page
48)
 Calculated data: show your slope, intercept
and the unknown concentration found


Graph – calibration curve of abs vs. conc
based upon your own individual raw data
Calibration curve (on
Excel)
Calibration Curve of Iron
Absorbance
0.4
0.3
0.2
y = 1.5x + 0.005
R² = 0.9981
0.1
0
0
0.05
0.1
0.15
0.2
Concentration (M)
0.25
Calculations

Calculations – show your trendline equation found
from calibration curve y=mx+b



Show how you determine your unknown concentration
(plug in absorbance and solve for conc!)
Show also your % error and how you found your dil conc
Discussion – formal discussion 1-2 pages




Discuss the maximum wavelength you found in part A
versus the actual, and the reasoning
Discuss the concentration you found
Discuss the calibration curve, was it precise (r^2)
Error analysis