Visible Spectrophotometer

white light

prism100 % adjustblankdistilled water

I0 I

I0 = I% transmittance = (I / I0) x 100 = 100

absorbance = 0

detector/recorder

Visible Spectrophotometer

100 % Adjust

white light

prismzero adjust(barrier inserted)

I0 I

I = 0% transmittance = 0

absorbance = maximum

detector/recorder

Visible Spectrophotometer

Zero Adjust

white light

prismsample solution

I0 I

I < I0

% transmittance = (I / I0) x 100 = 50%absorbance = 1.7

detector/recorder

Visible Spectrophotometer

Sample

white light

prismsample solution

I0 I

I0 = I% transmittance = (I0 / I) x 100 = 100

absorbance = 0

detector/recorder

Visible Spectrophotometer

In order for blue solution to absorb light, wavelength must be changed

to blue’s complement

white light

prismsample solution

I0 I

detector/recorder

Visible Spectrophotometer

Wavelength is changed by either rotating prism or position of slit

“Spec 20” Visible Spectrophotometer

digital analog

1. Adjust wavelength

2. Zero adjust with sample compartment empty : 0 % T

3. 100 % adjust with distilled water (blank) in sample compartment : 100% transmittance

4. Record sample absorbance (Be certain to switch MODE to absorbance)

Beer’s Law

• As the intensity of the color of the sample solution increases…

• the absorbance increases.

• A = b c

absorbance constant path length concentration

Determination of the Concentration of an Unknown

Solution• Prepare a set of standard solutions of known

concentrations

• Select the appropriate wavelength

• Determine the absorbance of each

• Determine the absorbance of the unknown solution

Sample Conc.(ppm) Absorbance

1 0.0 0

2 5.0 .0023

3 10.0 .0045

4 15.0 .0056

5 20.0 .0076

Unknown ? .0051

13 ppm

A = b c

y = mx + b

+ 0

.0051 = 0.00037 x C + 0.00030

C = 00037.0

0003.00051.0 = 13.0

Recall Beer’s Law: