SpectroscopySpectroscopy

In addition to this presentation, before coming to lab or In addition to this presentation, before coming to lab or

attempting the prelab quiz you must also:attempting the prelab quiz you must also:

Read the introduction to the lab in the coursepackRead the introduction to the lab in the coursepack

Review preparation of solutions Review preparation of solutions

* solutions of known molarity (pg 146, Brown et al.) * solutions of known molarity (pg 146, Brown et al.)

* solutions by dilution (pp 148-150 Brown et al.)* solutions by dilution (pp 148-150 Brown et al.)

Familiarize yourself with spectrometers Familiarize yourself with spectrometers

* read pp. 65-67 in Stanitski et al.* read pp. 65-67 in Stanitski et al.

What’s the point?What’s the point?

• Review the relationship between color and wavelengthReview the relationship between color and wavelength

• Learn an application of spectroscopy to chemical Learn an application of spectroscopy to chemical

analysisanalysis

• Become familiar with Beer’s Law: the relationship Become familiar with Beer’s Law: the relationship

between light absorption and solution concentrationbetween light absorption and solution concentration

• Gain practice in accurate solution preparationGain practice in accurate solution preparation

When light passes through a solution, some of it can be lost to When light passes through a solution, some of it can be lost to absorption. This decreases the power of the transmitted beam absorption. This decreases the power of the transmitted beam (P(PTT) relative to the incident beam (P) relative to the incident beam (P00).).

The amount of absorption usually depends on the wavelength The amount of absorption usually depends on the wavelength of the light: some colors are transmitted and others aren’t.of the light: some colors are transmitted and others aren’t.

BackgroundBackground

sample PTPo

detectordetector

sample PTPo

detectordetector

Abs

orb

ance

Spectrum and SpectrometersSpectrum and SpectrometersA plot of absorbance versus A plot of absorbance versus is an absorbance is an absorbance spectrumspectrum• each substance has a unique absorbance spectrumeach substance has a unique absorbance spectrum

This is a spectrum of methylene blue, This is a spectrum of methylene blue, a dye used in microscopy. a dye used in microscopy.

IRIRUVUV VisibleVisible

The light absorbed below 400 nm The light absorbed below 400 nm does not affect the color, because we does not affect the color, because we can’t see those wavelengths.can’t see those wavelengths.

It looks blue because red light is It looks blue because red light is absorbed between 550 and 700 absorbed between 550 and 700 nm. nm.

Red and yellow are absorbed

Blue is transmitted through the solution

SpectrometersSpectrometers

displaydetector

selector

sample electronics

• Spectrometers are used to measure spectraSpectrometers are used to measure spectra

• Spectrometers performs two functionsSpectrometers performs two functions

• allow wavelength selection of the lightallow wavelength selection of the light

• detect & report the light intensitydetect & report the light intensity

Genesys 10 SpectrometerGenesys 10 Spectrometer

Our spectrometers look like this.Our spectrometers look like this.

The sample, contained The sample, contained in a cuvette, goes in in a cuvette, goes in this compartmentthis compartment

The spectrum can be The spectrum can be viewed here as a plot viewed here as a plot or a table of numbersor a table of numbers

TransmittanceTransmittance

sample PTPo

detectordetector

sample PTPo

detectordetector

Spectometers measure the transmittance (T)Spectometers measure the transmittance (T)

T = (PT = (PTT / P / P00))

This is a fraction. It is often reported as a %This is a fraction. It is often reported as a %

% T = T x 100%% T = T x 100%

The absorbance is calculated from TThe absorbance is calculated from T

A = -log(T)A = -log(T) why bother with this?why bother with this?

PP00 = incident light intensity = incident light intensity

PPTT = transmitted light intensity = transmitted light intensity

Beer’s LawBeer’s Law• Also called the Beer-Lambert lawAlso called the Beer-Lambert law

• Relates absorbance and concentrationRelates absorbance and concentration

A = a b CA = a b C

• a = absorption coefficienta = absorption coefficient physical property, unique to each substancephysical property, unique to each substancedetermines the intensity of absorbancedetermines the intensity of absorbance magnitude magnitude depends on wavelengthdepends on wavelength

• b = sample optical pathlength (width of cuvette)b = sample optical pathlength (width of cuvette)• C = solution concentrationC = solution concentration

Beer’s Law PlotsBeer’s Law PlotsA = a b CA = a b C

• Follows the equation of a straight lineFollows the equation of a straight liney is absorbance and x is y is absorbance and x is

concentrationconcentration The slope is (ab) and the The slope is (ab) and the y-intercept should be 0y-intercept should be 0 A plot of A versus C is A plot of A versus C is called a Beer’s Law plotcalled a Beer’s Law plot

Abs

orba

nce

Abs

orba

nce

concentrationconcentration

Slope = A / CSlope = A / C = ab= ab

Uses of Beer’s Law PlotsUses of Beer’s Law Plots• Used to find absorption coefficientsUsed to find absorption coefficients see see

previous slideprevious slide

• If you know a and b, can use a Beer’s Law plot to If you know a and b, can use a Beer’s Law plot to find C of an unknownfind C of an unknown

Abs

orba

nce

Abs

orba

nce

concentrationconcentration

1) Create B-L plot

2) Measure A for an unknown

3) Find C from plot

Summary of RelationshipsSummary of RelationshipsTransmittance and AbsorbanceTransmittance and Absorbance

T = (PT = (PTT / P / P00)) A = -log(T)A = -log(T)

TT AA %T%T

1.001.00 -log(1) = 0-log(1) = 0 100%100%

0.500.50 -log(0.50) = 0.30-log(0.50) = 0.30 50%50%

00 -log(0) = -log(0) = 0%0%

All the light is transmittedAll the light is transmitted

Half the light is transmittedHalf the light is transmitted

None of the light is transmittedNone of the light is transmitted

Absorbance and ConcentrationAbsorbance and Concentration

A = aA = a bb CC use to find a by measuring A when C is known use to find a by measuring A when C is known

use to find C by measuring A when a is knownuse to find C by measuring A when a is known

Sample CalculationsSample Calculations

Preparing Solutions of Known ConcentrationPreparing Solutions of Known Concentration

What mass of sodium chloride is required What mass of sodium chloride is required to make 50.0 mL of a 0.825 M solution?to make 50.0 mL of a 0.825 M solution?

0.825 mol0.825 mol x 0.0500 L = 0.0413 mol x 0.0500 L = 0.0413 mol 1 L1 L

Find required moles NaCl, then massFind required moles NaCl, then mass

0.0413 moles x 0.0413 moles x 58.44 g 58.44 g = = 2.41g NaCl2.41g NaCl 1 mol 1 mol

To prepare this solutionTo prepare this solution

• Place 2.41 g of NaCl in a 50 mL volumetric Place 2.41 g of NaCl in a 50 mL volumetric flaskflask

• Add some water to dissolve the NaClAdd some water to dissolve the NaCl

• Add DI to dilute to 50.00 mL, then mix Add DI to dilute to 50.00 mL, then mix thoroughlythoroughly

Sample CalculationSample CalculationPreparing Solutions by DilutionPreparing Solutions by Dilution

What volume of a 1.00 M solution is required What volume of a 1.00 M solution is required to prepare 50.0 mL of a 0.250 M solution?to prepare 50.0 mL of a 0.250 M solution?

How many moles are in 50.0 mL of a 0.25 M solution?How many moles are in 50.0 mL of a 0.25 M solution?(0.250 M)(0.0500 L) = 0.0125 mol(0.250 M)(0.0500 L) = 0.0125 mol

What volume of a 1.0 M solution will deliver this?What volume of a 1.0 M solution will deliver this?(0.0125 M) / (1.00 M) = 0.0125 L OR 12.5 mL(0.0125 M) / (1.00 M) = 0.0125 L OR 12.5 mL

This is the same as MThis is the same as M11VV11 = M = M22VV22::

MM11 = 1.00 M = 1.00 M V V11 = ? = ?

MM22 = 0.250 M = 0.250 M V V22 = 50.0 mL = 50.0 mLVV11 = =

(0.25 M)(0.0500 L)(0.25 M)(0.0500 L)

(1.0 M)(1.0 M)

Safety and PreparationSafety and Preparation• Carefully read the instructions for solution Carefully read the instructions for solution

preparation in the lab (part II). It is trickier than preparation in the lab (part II). It is trickier than you think, and you need to come prepared.you think, and you need to come prepared.

• Review the spectrometer instructions (pg. 40).Review the spectrometer instructions (pg. 40).

• Goggles and coat must be on at all times.Goggles and coat must be on at all times.

• Wash hands if solutions spill on you.Wash hands if solutions spill on you.

• Solutions can be disposed of down drain.Solutions can be disposed of down drain.