Question

a. List three advantages for recording absorbance at band A rather than band B during a quantitative analysis using Beer's law. b. Under what conditions one should consider setting the detection wavelength to band B instead of band A? Band A Band B Absorbance Wavelength

Answer 1

• This question is based on spectroscopy specially on absorbance.
• Firstly, we should know about absorbance.
• Absorbance is a process in which a substance absorbs light of a particular wavelength.
• Absorbance is equal to the negative logarithm of the transmittance.
• Plot between Absorbance and wavelength gives Gaussian curve which is presented as above in question.
• Absorbance spectrophotometer measures the absorbance. It is an instrument which measures the fractions of incident light which is transmitted through solution.

Answer (a) There are various advantages to measure absorbance at band A rather than band B.

(1) At band A, the absorbance is higher basically we can say that the absorbance is maximum at band A. As we konw, the higher the absorbance of light, the lower will be the percentage of transmittance. Hence, at Band A, The transmittance of light will be lower.

(2) At band A, The wavelength is also high. At this wavelength at which absorbance is highest, the solution will be most sensitive to changes of concentration, That's why it is useful to measure the absorbance at band A and this wavelength is called the maximum wavelength which is denoted by λ_max.

(3) At high concentrations molecular interactions takes place which causes the change in position and shape of absorption bands. Due to which we get a real absorbance spectrum which is a gaussian curve rather than a linear curve because According to the Beer's law, the absorbance is directly proportional to the concentration of matter in the given sample.

Answer (b)  According to Beer's  law, we have already mentioned that the absorbance and concentration are directly proportional and the equation is A = ε l c,

Where,

• ε = molar extinction coefficient or molar absorptivity constant ,
• l = path length of cell holder

and

• c = concentration of the solution.

If we increase the original concentration of sample then the absorbance will also increases and if we dilute the solution means decrease the original concentration of sample then the absorbance will be decreases in direct proportion.

One of the Limitation or deviation of Beer's law is that ; When the concentration of the sample in the solution is high, the sample starts to behave very differently due to some interactions with the solvent and other solute molecules or we can say that at high concentration we will get high absorbance value which will give us more error in our results which means the most of the light are absorbed by the sample and only small amount of the light detected by detector.

Hence, Band B will be preferred at lower concentration and lower wavelength.