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Unit 1How do we analyze

a system?

Separating Substances

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The Challenge

Imagine you are a researcher interested in identifying and quantifying the FD&C food dyes present in commercial

candies for a study examining a possible link between such

dyes and attention disorders in children.

Your task is to devise an efficient strategy to characterize FD&C food dyes.

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Your Tools

To face your challenge, you will use the following analytical techniques to separate

and identify FD&C food dyes:

Paper Chromatography

Absorbance Spectroscopy

In this session you will develop skills with Absorbance Spectroscopy

Centrifugation & Filtration

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Your ChallengeYour task is to use absorbance

spectroscopy to quantify the mass of dye in commercial candy

How much dye is present per gram of candy?

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Let’s Think

The resulting NaNO3

solution is clear & colorless

NaNO3 is a white solid

Added to water NaNO3 dissolves

with stirring

If you were to measure the visible light absorbed by a

NaNO3 solution, what would the absorbance spectrum look like? 400 500 600 800

Wavelength (nm)

Abs

orba

nce

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Absorbance SpectrumWhat do you predict

for a:

Green solution?

Blue solution?

Why is knowledge of the absorbance

spectrum useful?Very small

or no absorbance

in this range.

400 500 600 800

Wavelength (nm)

Abs

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Generate the absorbance spectrum of four reference food dyes.

Determine their max.

Initial Exploration

You have 20 minutes

Available resources: Seven FD&C food dye stock solutions; USB650 absorbance spectrometer; Equipment in your locker.

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Your Initial PlansTo begin this part of the project, come up with a plan to QUALITATIVELY generate:

The absorbance spectrum of four different reference food dye and determine its max.

The absorbance spectrum for the dyes present in two different candies that you have been analyzing, as well as their associated max.

You need to carefully discuss how you plan to prepare each candy sample for analysis.

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Present your results to the class. Decide as a class which data should be

shared to facilitate subsequent analyses.

Claims and Evidence

Food Dye Literature max Observed max

Blue 2 608 nm

Blue 1 630 nm

Green 3 625 nm

Yellow 5 426 nm

Yellow 6 480 nm

Red 3 530 nm

Red 40 507 nm

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C

Absorbance spectra can be used to determine the concentration of different substances in a system.

Quantitation

How would you find the

effect of concentration

(C) on absorbance

(A)?

Compare A at a given for different C

max

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To find the mathematical relationship between A and C, A = f(C), for a given substance we need to

build a “calibration plot” using standard solutions of known concentration.

References

How many data points

do we need?

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As you run the experiment, record and reflect on the observed max and make sure it is reasonable given the color of your dye.

Color Wavelength Interval

red ~ 630–700 nm

orange ~ 590–630 nm

yellow ~ 560–590 nm

green ~ 490–560 nm

blue ~ 450–490 nm

violet ~ 400–450 nm

max

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DilutionsYou will need to prepare solutions of

different concentration. To accomplish this, you can dilute serially or in parallel.

More Concentrated,Less Dilute

Less Concentrated,More dilute

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Parallel Dilutions

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Design a plan to: 1) generate calibration plots for at least four different food colorings, and 2), then find the mathematical relationship between A and C for each of the four dyes.

Your Plan

Make sure your data points are well

distributed in the absorbance range

from 0.1 to 1.

A = 0.85 C

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Individually, build a calibration plot for one food dye and find A = f(C) for this

substance at a specific max.

Available resources: Stock food dye solutions of known concentration; USB650 absorption spectrometer; Plastic transfer pipettes; Equipment in your locker.

Your Challenge

You have 90 minutes

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Present your results to the class. Decide as a class which data should be

shared to facilitate subsequent analyses.

Claims and Evidence

Food Dye Literature max Observed max A = f(C)

Blue 2 608 nm

Blue 1 630 nm

Green 3 625 nm

Yellow 5 426 nm

Yellow 6 480 nm

Red 3 530 nm

Red 40 507 nm

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Further AnalysisFor many materials, the absorbance (A) at a given is proportional to the concentration (C) of the

absorbing species (number of moles or molecules per unit volume).

A α C

b

(path length)

A α b

How would you expect A to change with b?

How can we convert this proportionality into a

mathematical equation?

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Putting this all together we come up with the Beer – Lambert Law: For any particular wavelength,

A bC

Beer’s Law

AbsorbanceMolar absorptivity

L/(mol∙cm)

Path length (cm)(cuvette path

length is 1.00 cm)

Concentration

(mol/L)

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Use your data, together with Beer’s Law, to derive the value of the molar absorptivity for each

substance.

More Claims and Evidence

Food Dye Literature max

Observed max

A = f(C)

Blue 2 608 nm

Blue 1 630 nm

Green 3 625 nm

Yellow 5 426 nm

Yellow 6 480 nm

Red 3 530 nm

Red 40 507 nm

A bC

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Your PlanUsing your data:

q

Your next task is to identify the FD&C food dyes present in at least two commercial candy samples

and to quantify the total amount of each dye per candy piece. Your group is free to select the

samples to analyze.

In your groups, build an experimental plan to complete this task

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Identify the food dyes present in at least two commercial candies and quantify the

total dye amount per piece of candy.

Available resources: 19 commercial candy samples; Seven FD&C food dye stock solutions; USB650 absorbance spectrometer; Sample preparation resources; Equipment in your locker.

Your Final Challenge

You have 20 minutes

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Present your results to the class. Decide as a group which data should be

shared to facilitate the analysis.

Claims and Evidence

What dyes did you find in each candy? How much food dye is present in each piece of candy? How do you know?

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What did you learn from doing your experiment? How would you improve what you did? How have your ideas changed as a result of this lab? What do you not completely understand? What new questions do you have?

Final Reflections

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Central goals (2 p): What questions guided your explorations?

Experimental Procedures (4 p): What experiments did you do to answer your questions?

Experimental Data and Calculations (10 p): What observations did you make?What data did you collect?What calculations and representations

helped you make sense of the data?

Your Report

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Claims (4 p): What can you claim to answer your questions?

Evidence and Analysis (12 p):How did you interpret your results to support

your claims? Discussion, Reflections and Implications (8 p):

What did you learn?What do you not completely understand? How have your ideas changed as a result of this lab?What new questions do you have?

How would you improve what you did?

Your Report

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Project 3

How can we explore chemical changes?

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Your ChallengeYour core task is to use emission

spectroscopy to characterize and control a changes in a chemiluminescent system:

GLOW STICK CHEMISTRY

Don’t forget to submit your proposal!