Lab 8
The purpose of this lab is to demonstrate the
additive property of absorbance.
The molar absorptivity (ε) values for both Cu(II)
and Ni(II) will be found at two analytical
wavelengths by measuring the absorbance of
both ions in solutions of known
concentration.
The two analytical wavelengths will then be
utilized to find the concentrations of both
Cu(II) and Ni(II) in an unknown mixture.

For a single analyte, we can use the Beer-Lambert Law:

Abs = ε b c to determine any one of the variables, given
that three of them are already known.



Since molar absorptivity (ε ) is the same for a given
compound (x), regardless of concentration, at a given
wavelength, we can clarify Beer’s Law terms:
Aλ1 = ελ1x b [x]
Given that the path length (b) through our cuvets is 1.445
cm, we can define a new variable kλ1x = ελ1x b to simplify
calculations and modify Beer’s Law accordingly:
Aλ1 = kλ1x [x]
After you make up your solutions today, you will simply use
this modified Beer’s Law to determine the molar absorptivity
(kλ1x ) of each solution at a given wavelength. The average
k value for each set of solutions at each wavelength is
calculated and used to solve for unknown concentrations.



For multiple analytes we exploit the additive
property of absorbance: AbsT = Abs1 + Abs2 + ...
The same number of wavelengths are used as
number of analytes that are analyzed.
Therefore, in two analytes, we use:
Aλ1 = ελ1x b [x] + ελ1y b [y];
Define kλ1x = ελ1x b and kλ1y = ελ1y b;
So Aλ1 = kλ1x [x] + kλ1y [y]
Aλ2 = ελ2x b [x] + ελ2y b [y];
Define kλ2x = ελ2x b and kλ2y = ελ2y b;
So Aλ2 = kλ2x [x] + kλ2y [y]
Beer’s Law states:
Aλ1 = kλ1x [x] + kλ1y [y]
At 395 nm, this equation becomes:
A395 = k395Ni(II)[Ni2+] + k395Cu(II)[Cu2+] = k395 [Ni2+] + 0
To solve for [Ni2+] in our unknown solution:
[Ni2+]
A395
=
k 395Ni2 
Beer’s Law states:
Aλ2 = kλ2x [x] + kλ2y [y]
A 2  k  2x   x 
To solve for [y], [y] =
k  2y
To solve for [Cu2+] in our unknown solution:
[Cu(II)] =
Abs775  k 775,Ni(II)  Ni(II)
k775,Cu(II)




Make up 4 solutions of Cu2+(aq) and 4 solutions
of Ni2+(aq).
Determine the k value of each solution at the
indicated wavelengths.
Find the average k values for Cu2+(aq) at 775 nm
and for Ni2+(aq) at 775 nm and 395 nm.
Solve for the unknown concentrations of your
unknown solution using the derived equations.

Reagents:

Eye Contact:

Skin Contact:

Inhalation:

Ingestion:
◦ Cupric Sulfate
◦ Nickel Sulfate / Nickel Chloride
◦ Irritation, redness, pain, conjunctivitis, ulceration, clouding of
cornea
◦ Irritation, redness, rash and itching. Sensitizer.
◦ Coughing, sore throat, shortness of breath, ulceration and
perforation of the respiratory tract. Fumes from heating may
cause symptoms similar to a cold. May cause metallic taste in
mouth. Lung damage, allergy and asthma may occur.
◦ Burning of the mouth, esophagus, and stomach. Hemorrhagic
gastritis, nausea, vomiting, abdominal pain, giddiness, myocardial
weakness, metallic taste, and diarrhea. Systemic copper poisoning
with capillary damage, headache, cold sweat, weak pulse, kidney
and liver damage, CNS excitation and depression, jaundice,
convulsions, blood effects, paralysis, coma and death.


Copper and Nickel are both toxic.
Dispose of them in the appropriate
container(s) in the fume hood.

Lab 9 is next.