Question

Question to answer after EXPERIMENT OF SPECTROPHOTOMETRIC DETERMINATION OF TRACE Fe

Principles The purpose of this experiment is to determine the concentration of iron present as a trace constituent (ppm) in an aqueous sample by a spectrophotometric method. One of the most sensitive methods for the determination of iron involves the formation of the orange-red iron (II) orthophenanthroline complex. This complex has an absorption maximum at 508 nm, is stable over a long period of time, and follows Beer's Law quite faithfully.

The chemistry involved is straightforward. Hydroxylamine hydrochloride is used to reduce all Fe3+ present to Fe2+. Since orthophenanthroline (abbreviated as phen) is a weak base and an acid medium must be used in order to prevent precipitation of the iron as Fe(OH)2, the principal complexing species is the phenanthrolium ion (phenH+ ). The complex formation reaction:

Fe2+ + 3phenH+  Fe(phen)3 2+ + 3H+

has an equilibrium constant of 2.5 x 106 , so complex formation may be considered quantitative. Although pH control is not essential, too high of an H+ concentration (pH<2) can inhibit formation of the complex, while too high of a pH can cause the loss of iron by precipitation. Thus, the solution is buffered to be to about a pH of 3.5.

Since the system obeys Beer's Law, above, a plot of absorbance at 508 nm versus iron concentration yields a straight line. Thus, one can prepare a set of standard solutions of known concentrations, establish a straight line, and prepare the unknown solution in exactly the same way. If the absorbance of the unknown solution is measured, the unknown concentration may be determined from the analytical curve generated with the standard solutions.

Questions:

1. The ferrous ammonium sulfate used as a standard in this experiment is not of primary standard quality, yet you were not instructed to standardize this reagent 81 before use. Why is it not necessary to standardize this analytical reagent before use in this experiment? Explain.

2. Describe two reasons why deviations to Beer's Law (i.e. nonlinearity) can occur.

3. The sensitivity of spectrophotometric methods can greatly exceed sensitivities attainable with classical wet chemical methods. How do the Fe concentrations analyzed in this spectrophotometric experiment compare with the Fe concentrations used in the redox titration experiment? Be quantitative!

Answer 1

1. Ferrous ammonium sulfate does not decompose upon standing or absorb moisture. So the weight measured for it corressponds exactly to the amount taken. That is 1 mole of Ferrous ammonium sulfate will have 1 mole of Fe in it. The salt is stable under all working conditions and no change in concentration of it is seen and thus it is not necessary to stardardize this reagent before use.

2. As the precipitation is quantitative, the concentration of Fe in solution may be too high to apply Beer's law prperly. Remember Beer's law works for dilute solutions only. As concentration goes up it deviates from Beer's law. If the pH is not controlled well we may end up with very low concentration of Fe in solution. Again at extremely low concentrations Beer's law does not obey and we would see deviation from Beer's law.

3. By spectometric method the Fe concentration analyzed can be relatively low and accurate. Since even a small change in concentration would result in high change in absorbance values. On the other hand in redox titrations they tend to vary with every titration reading done. Also comparatievely the amount or concentration detetcted in redox is higher than in spectrometric experiments.