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PurposeAbsorbance Single AnalyteAbsorbance Multiple AnalyteSolving unknown concentrationsProcedureSafety ConcernsWasteWhat to turn inExperiment 10 reminder. Outline. The purpose of this lab is to demonstrate the additive property of absorbance.The molar absorptivity (e) values for both
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1. Simultaneous Spectrophotometric Quantitation of Cu(II) and Ni(II) Experiment 9
2. Purpose
Absorbance Single Analyte
Absorbance Multiple Analyte
Solving unknown concentrations
Procedure
Safety Concerns
Waste
What to turn in
Experiment 10 reminder Outline
3. The purpose of this lab is to demonstrate the additive property of absorbance.
The molar absorptivity (e) 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. Purpose
4. For a single analyte, we can use the Beer-Lambert Law:
Abs = e b c to determine any one of the variables, given that three of them are already known.
Since molar absorptivity (e ) is the same for a given compound (x), regardless of concentration, at a given wavelength, we can clarify Beers Law terms:
A?1 = e?1x b [x]
Given that the path length (b) through our cuvets is 1.445 cm, we can define a new variable k?1x = e?1x b to simplify calculations and modify Beers Law accordingly: A?1 = k?1x [x]
After you make up your solutions today, you will simply use this modified Beers Law to determine the molar absorptivity (k?1x ) 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. Absorbance Beers Law
5. 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:
A?1 = e?1x b [x] + e?1y b [y];
Define k?1x = e?1x b and k?1y = e?1y b;
So A?1 = k?1x [x] + k?1y [y]
A?2 = e?2x b [x] + e?2y b [y];
Define k?2x = e?2x b and k?2y = e?2y b;
So A?2 = k?2x [x] + k?2y [y]
Absorbance is Additive
6. Beers Law states: A?1 = k?1x [x] + k?1y [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+] = At 395 nm, only Ni2+ absorbs
