Analysis of ammonia using the Nessler technique relies on absorbance measurement

Question

Analysis of ammonia using the Nessler technique relies on absorbance measurements of a yellow compound from ammonia in a sample. Using a 2.00 cm cell, the absorbance of a reagent blank is 0.292, the absorbance of a 0.020 mM standard is 0.333, and the absorbance of an unknown sample is 0.314.

A) What is the molar absorptivity of the yellow compound?

B) What is the concentration of ammonia in the sample?

C) Although increasing the cell pathlength would increase the sensitivity of the analysis, your instructor suggest taht this would be a mistake due to the instrument you are using. What problem might you encounter with a 10 cm cell? (and yes it does fit in the instrument)

Explanation / Answer

Ans. #A. Beer-Lambert’s Law, A = e C L             - equation 1,              

where,

                       A = Absorbance

                       e = molar absorptivity at specified wavelength (M-1cm-1)

                        L = path length (in cm)

                        C = Molar concentration of the solute

Corrected absorbance of 0.02 mM standard sample =

= (Abs of standard – Abs of sample blank)

                                                            = 0.333 – 0.292 = 0.041

Putting the values in equation 1-

            0.041 = e x 0.02 mM x 2.0 cm       

or, e = 0.041 / (0.04 cm mM) = 1.025 mM-1 cm-1 = 1.025 x 10-3 M-1 cm-1

Thus, molar absorptivity of yellow compound = 1.025 mM-1 cm-1

#B. Corrected Abs of unknow sample = Abs of sample – Abs of sample blank

                                                            = 0.314 – 0.292 = 0.022

Putting the values in equation 1-

            0.022 = (1.025 mM-1 cm-1) x C x 2.0 cm   

or, C = 0.022 / (2.05 mM-1) = 0.0107 mM

Thus, concentration of unknown sample = 0.0107 mM

#C. Every instrument has an intrinsic detection limit due to the type of detector, light source and other components used in it.

Using a path length of 10 cm would give an abnormally high absorbance value due to limited transmittance through the 10 cm- cuvette. Under such circumstance, the absorbance of the sample may not be proportional to concentration. That is, Beer-Lambert law would no longer be obeyed.

Therefore, deviation from Beer-Lambert law for a 10-cm cell would give non-concordant results. So, the method won’t give reproducible results any longer. Furthermore, the method is of no use for quantification of NH3 in sample.

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