Dilution and Spectroscopy Lab

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Dilution Background• Acids and bases usually come in concentrated

form, as “concentrates,” but they are rarely used in this form.

• A dilution is prepared by adding a specific amount of a concentrate to water to obtain a new volume and concentration.

• In order to calculate dilutions of solutions the equation:

M1V1 = M2V2

where M is concentration in molarity (mol solute/L solution) and V is volume (L), is used

Dilution Examples

• Dilutions by a factor of 2:– Take 3 mL of “acid” and dilute with 3 mL of

water– 1 part concentrate, 1 part water

• Dilutions by a factor of 4:– 1 part concentrate, 3 parts water

• Dilutions by a factor of 10:– 1 part concentrate, 9 parts water

Pipet Instructions

1. Press the A button and squeeze the bulb to suck air out

2. Place the pipet into the solution3. Press the S button to suck the liquid

into the pipet4. Take the pipet out of the solution and

place it into the next cuvette5. Press the E button to empty the liquid

into the new cuvette

AE

S

Spectroscopy Introduction• Spectroscopy is one method of determining

the concentration of an unknown solution. • By measuring the respective absorbance

values for solutions with known concentrations, a calibration curve can be constructed.

• The absorbance of an unknown can be used to determine its concentration through use of this calibration curve.

Absorbance vs. Transmittance

• Transmittance - the fraction of light that is transmitted through a sample

• Absorbance - The fraction of light that is absorbed by a sample

100% Transmittance = 0% Absorbance

Spec-20 Instrument

Sample holdercover

Amplifiercontrol knob

Light controlknob

Wavelength control knob

Schematic representation of a spectrophotometer

Detector

Sample-holder

Meter

Zero adjustand power control

Wavelength scale

Wavelength control

Light control

Monochromator

LightSource

(1)

Samplecell

(3)(2) (4) (5)

Io I

Meter

A spectrophotometer is an instrument that measures the fraction of an incident beam of light which is transmitted or absorbed by a sample at a particular wavelength.

Spec-20 Absorbance

l (wavelength)

Absorbance

100 % Absorbance 0 % Transmittance

0 % Absorbance100 % Transmittance

Insert Photograph

Spec-20

Sample-holder

Meter

Zero adjustand power control

Wavelength scale

Wavelength control

Light control

1.5

1.2

0.9

0.6

0.3

0.0300 400 500 600 700 800

Wavelength (nm)

Abs

orb

ance

Absorbance of Chlorophyll

1026 1024 1022 1020 1018 1016 1014 1010 108 106 104 102 1

10-8 10-6 10-4 10-2 1 102 104 106 108 1010 1012 1014 1016

Frequency (Hz)

Wavelength (nm)

cosmicrays

gammarays

x-rays ultra-violet

infra-red

radio(microwave)

radar tele-vision

radio powertransmission

Violet Blue Green Yellow Orange RedUV

NearInfrared

400 nm 500 nm 600 nm 700 nm

Kelter, Carr, Scott, , Chemistry: A World of Choices 1999, page 480

Am

ou

nt a

bso

rbe

d

663 nm

Wavelength (nm)400 500 600 700

Calibration Curve

Concentration

Absorbance

100 % Absorbance 0 % Transmittance

0 % Absorbance100 % Transmittance

out of linear range(too concentrated)

x 2

(fixed wavelength)

?

Dilute sample with water 50:50. Run sample, read concentration.

Dilution of Solutions

KeysKeys

Dilution of Solutions  Dilution of Solutions

Dilution of Solutions  Dilution of Solutions

http://www.unit5.org/chemistry/Solutions.html