Post-Lab Questions You have 10.0 mL of an unknown sample and your transmittance
ID: 712374 • Letter: P
Question
Post-Lab Questions You have 10.0 mL of an unknown sample and your transmittance reading of it was very low, corresponding to a very high absorbance. Such high absorbance values are unreliable. What could you do to improve the accuracy of your transmittance/absorbance measurement, and your determination of the concentration? 1. How would the measured absorbance of a dye unkno to determine if the absorbance measured would be high, low, or unaffected, relative to the expected outcome, and tell briefly why. 2. by the mistakes below? Use logic The wavelength of light passing through the cuvette was different than 635 nm. (The 635 nm wavelength corresponds to the wavelength of maximum extinction coefficient.) a. Gertrude, a student, forgot to blank with water at the beginning of the lab. How would that affect the reading of the % transmittance? b.Explanation / Answer
1. The Beers Law break down at high concentrations due to deviations in absorptivity coefficients at high concentrations (>0.01M) due to electrostatic interactions between molecules in close proximity.If one's samples showed a higher absorbance, one should dilute the solution to a lower concentration, or used a lower concentration of the chromogenic substrate in the reaction being monitored.
Better alternative -Some spectrophotometers do correct for the high concentration effect to some extent and therefore can produce a linear standard curve up to 2.0 Abs units or more i.e. we have to just increase the maximum absorptivity from 1.0 to 2.0.
A = C l
A = absorbance (no units)
= molar absorptivity coefficient (units = L/mol-cm)
C = concentration of absorbing species (units = mol/L)
l = path length (units = cm)
A , , l are known C can be calculated. A= 200%
2. a.
The absorption spectrum is a plot of the absorbance of a sample as a function of wavelength.For dilute solutions, the amount of light absorbed at a specific wavelength is directly proportional to the concentration of the solution.
2. b.
We take generally blank first and then tare the readings for the spectrometer. If one forgets to take blank at the begining, it can be taken at the last and the reading so obtained for the blank should be made zero .
If a negative reading comes, add same interger in positive to all the readings obtained including the blank.
If a positive reading comes, add same interger in negative to all the readings obtained including the blank.
The blank must be used to adjust the spectrophotometer every time the wavelength is changed. If you are working with a single wavelength, the blank should be placed in the spectrophotometer periodically to check for drift.