[MrHappy0]

Hi Chem folks,

I am answering some critical thinking questions for a lab. The Beer's Law is fresh is new to me and the math associated with it is challenging. Here are the questions and my thoughts. If you have any advice or anything helpful to add that would be great. Thanks!

Summary of the lab: Note: Analyte Fe was complexed with orthophenanthroline-

Absorbance was measure at max 508.12nm for known concentrations of Fe for 5 different samples. The plot produced gave me a nice straight line. I got an R^2 value and a trendline equation.

Absorbance was measure at max 508.12 nm for unknown concentrations of Fe for 4 samples (approx same concentration).  I calculated the concentration with following formula: Absorbance/slope of trendline. Something tells me I should have just used the trendline equation and solved for x. Which one is right? I am graded on accuracy of my results. 

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 before use. Why is it not necessary to standardize this analytical reagent before use in this experiment? Explain.

A:Unless I am missing something obvious I feel like the answer is a needle in a haystack for this one. My thoughts are that orthophenanthroline complexes only with Fe but a wikipedia search didn't confirm this.
I also know that Fe(II) is much less prone to be oxidized by air.

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

A:The Beer’s Law is when the absorbance for a monochromatic radiation beam of a solution of an absorbing analyte is proportional to the concentration of the analyte. Thus, if the polychromatic radiation cause deviation from the Beer’s Law. Another deviation would be cause from stray radiation. These two deviations would cause the non-linearity

[Borek]

Absorbance was measure at max 508.12 nm for unknown concentrations of Fe for 4 samples (approx same concentration).  I calculated the concentration with following formula: Absorbance/slope of trendline. Something tells me I should have just used the trendline equation and solved for x. Which one is right? I am graded on accuracy of my results. 

I would go for the full trendline equation. In theory zero concentration should give zero signal, but if there are any interfering impurities, they will show as a non-zero intercept.

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 before use. Why is it not necessary to standardize this analytical reagent before use in this experiment? Explain.

What was the full procedure? Did it contain a step that could help you answer this question?

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

Linearity is also lost for higher concentrations.