Spectrophotometry

Question 1

What is spectrophotometry?

Answer 1

the process that determines the amount of light absorbed by colored compounds (AKA colorimetry)

Question 2

Why does absorption occur?

Answer 2

Background: Each molecule absorb light of specific wavelengths and this depend on the specific nature of the molecule. Specific spectral properties.

Explanation: it is due to the interaction of light with the electronic and vibrational modes of molecules (their chemicals bonds)

Question 3

What are the two uses of spectrophotometry?

Answer 3

Background: Quantification of compounds is accomplished by comparing the amount of light transmitted through a blank sample to that transmitted through a colored sample. (you need a blank)

1).- Qualitative analysis. Identify and distinguish compounds by measuring the spectral properties of molecules and atoms. Spectral shapes and absorption
2).- Quantitative estimation of concentration of compounds

Question 4

Components of the spectrophotometer (in order by which light passes through) [6 components]

Answer 4

-Light Source: Visible Region: Tungsten Lamp; UV Region: Deuterium Lamp
-Monochromator: Select the particular wavelength (prism or grating and narrow
exit slit)
-Optical System
-Sample chamber: Hold the cuvette
-Detector: Are usually phototubes, photomultipliers, or photodiodes. When they receive light, they emit electrons that can be measured in the form of current by sensitive electronic devices
-Recorder/Printer

Question 5

What are the wavelengths for (the visible region, the uv region and the spectronic 20)

Answer 5

Visible Region: 350 - 800 nm.
UV Region: 200 - 350 nm.

“SPECTRONIC 20”
Limited Range: 340 - 650 nm.

Question 6

What is the beer-lambert equation?

Answer 6

A=-log10(T)= Ecl = OD (optical density)

E is the molar absortivity coefficient in units of (1/(Molarity)(concentration))

T= I/Io (Io is the initial intensity of light, but only a fraction of the total Io is transmitted)

Question 7

What are the relationships between (concentration and transmittance) and (concentration and absorbance)

Answer 7

There is an exponential decrease of T with respect to
concentration

There is a linear relationship between Absorbance and
concentration at low concentrations, at high concentration, there
is a deviation

Question 8

What pH does PNP (para nitro phenol) absorb at?

Answer 8

PNP does not absorb at acidic pH and only absorbs at basic ones

Question 9

Why can you extrapolate the pKa curve by plotting (absorption over various pH)

Answer 9

if deprotonated PNP is what contributes to absorption, then pH=pKa (when half is deprotonated and half is protonated) there should be an exhibition of the midpoint absorbance since only have the molecules of PNP are contributing to the absorbance

summary: pKa= pH, half deprotonated (basic), half contribute to absorbance

pH= pKa +log([A-]/[HA])

Question 10

Equation for dilutions

Answer 10

C1V1=C2V2