Module # 3 Photometry Flashcards
Photometry
The measure of light through a sample to see how much is absorbed.
A blue solution (for example)
All other colours of certain wavelengths are absorbed and only blue is allow to pass through.
Spectral transmittance/Spectral absorbance
Is the result of a wavelength scan that can be plotted and analyzed.
Spectral transmittance curve
Is a plot of all the wavelengths used versus the observed transmittance of the solution.
Spectral absorbance curve
Is a plot of all the wavelengths used versus the observed absorbance of the solution.
Absorbance max
Is the wavelength that is absorbed the most.
The higher the concentration the (more/less) light that will be absorbed, therefore (more/less) light is transmitted
More, less
The lower the concentration the (more/less) light that will be absorbed, therefore (more/less) light is transmitted.
Less, more
% transmittance equation
% transmittance = transmitted radiant energy/original intensity.
If the concentration of a solution increases the transmittance (increases/decreases)
Decreases.
If the concentration of a solution decreases the transmittance (increases/decreases)
Increases.
Absorbance and transmittance are ___________ and ____________ related.
Inveresly, logarithmically.
Absorbance is __________ related to concentration.
Directly.
Equation for absorbance
A = 2-log%T
Absorbance
The ability of a solution to absorb light. Is often reported to three decimal places. Most photometers display both %T and absorbance.
Lambert’s law
Relates the absorbance of light to the thickness (or depth) of the absorbing material. The law states that absorbance is directly proportional path length.
According the lambert’s law, the greater the path length, the (greater/less) light that is absorbed.
Greater
According the lambert’s law, the greater the path length, the (greater/less) light that is absorbed.
Less (more light is able to pass through, greater transmittance.)
According the lambert’s law, a cuvette with a 2cm diameter compared to one with a 1cm diameter will absorb (more/less) light.
More. Less is transmitted.
Beer’s law
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.
Beer-lambert law (combined)
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.
Molar absorptivity
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.
Conditions affecting Beer’s law
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.
Applications of Beer’s law
- 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.
Conditions of Beer’s law
- 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.
Equation to find the concentration of the unknown:
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.
Rearrangement of equation to find concentration of the unknown sample:
Cu = Au/As x Cs
