Module # 3 Photometry

Question 1

Photometry

Answer 1

The measure of light through a sample to see how much is absorbed.

Question 2

A blue solution (for example)

Answer 2

All other colours of certain wavelengths are absorbed and only blue is allow to pass through.

Question 3

Spectral transmittance/Spectral absorbance

Answer 3

Is the result of a wavelength scan that can be plotted and analyzed.

Question 4

Spectral transmittance curve

Answer 4

Is a plot of all the wavelengths used versus the observed transmittance of the solution.

Question 5

Spectral absorbance curve

Answer 5

Is a plot of all the wavelengths used versus the observed absorbance of the solution.

Question 6

Absorbance max

Answer 6

Is the wavelength that is absorbed the most.

Question 7

The higher the concentration the (more/less) light that will be absorbed, therefore (more/less) light is transmitted

Answer 7

More, less

Question 8

The lower the concentration the (more/less) light that will be absorbed, therefore (more/less) light is transmitted.

Answer 8

Less, more

Question 9

% transmittance equation

Answer 9

% transmittance = transmitted radiant energy/original intensity.

Question 10

If the concentration of a solution increases the transmittance (increases/decreases)

Answer 10

Decreases.

Question 11

If the concentration of a solution decreases the transmittance (increases/decreases)

Answer 11

Increases.

Question 12

Absorbance and transmittance are ___________ and ____________ related.

Answer 12

Inveresly, logarithmically.

Question 13

Absorbance is __________ related to concentration.

Answer 13

Directly.

Question 14

Equation for absorbance

Answer 14

A = 2-log%T

Question 15

Absorbance

Answer 15

The ability of a solution to absorb light. Is often reported to three decimal places. Most photometers display both %T and absorbance.

Question 16

Lambert’s law

Answer 16

Relates the absorbance of light to the thickness (or depth) of the absorbing material. The law states that absorbance is directly proportional path length.

Question 17

According the lambert’s law, the greater the path length, the (greater/less) light that is absorbed.

Answer 17

Greater

Question 18

According the lambert’s law, the greater the path length, the (greater/less) light that is absorbed.

Answer 18

Less (more light is able to pass through, greater transmittance.)

Question 19

According the lambert’s law, a cuvette with a 2cm diameter compared to one with a 1cm diameter will absorb (more/less) light.

Answer 19

More. Less is transmitted.

Question 20

Beer’s law

Answer 20

States that the amount of radiant energy of a particular wavelength varies directly with the concentration of absorbing molecules. Basically, the greater the concentration of the absorbing molecules, the more light that will be absorbed.

Question 21

Beer-lambert law (combined)

Answer 21

Absorbance (A) of a solution is proportional to the concentration (c) times the depth of solution the light must travel (b) times a constant (a). A = a x b x c. If the light path and absorptivity remain constant, absorbance will vary with concentration only. Equation can then be simplified to A = c.

Question 22

Molar absorptivity

Answer 22

Is sometimes used in place of absorptivity, represented by the greek letter epsilon. Defined as the absorbance of a solute in a concentration of 1 mole/L of solution measured in a lcm light path at a specified wavelength, temperature, pH and solvent.

Question 23

Conditions affecting Beer’s law

Answer 23

Incorrect wavelength, high concentration of solution, light path not constant, failure to set 0 %T and 100%T accurately, failure to maintain temperature and pH, impurity or turbidity of solution, presence of an interfering substance that absorbs the same wavelength.

Question 24

Applications of Beer’s law

Answer 24

1. Calculate the concentration of the unknown, 2. Compose a calibration curve of absorbance vs. concentration. This can be used to calculate the concentration of the unknown.

Question 25

Conditions of Beer’s law

Answer 25

1. Law is obeyed, 2. Using A (not %T), 3. If the blank is set at 0A, 4. The standard is close in concentration of the unknown, 5. The standard was analyzed at the same time as the unknown.

Question 26

Equation to find the concentration of the unknown:

Answer 26

Au/As = Cu/Cs ; where Au = absorbance of the unknown, As = absorbance of the standard, Cu = concentration of the unknown and Cs is the concentration of the standard.

Question 27

Rearrangement of equation to find concentration of the unknown sample:

Answer 27

Cu = Au/As x Cs