Spectrophotometry: Iron in Vitamin tablets and Absorbance

[MitsuShai]

This is the experiment that I did in class, please read the whole thing before posting:
http://www.chem.uic.edu/fetzer/chem222/Exp_19.pdf

How do I calculate the molarity of Fe(o-phenanthroline) in each solution?

So I was thinking of doing this:
According to the pdf there are 2.5 grams of o-phenanthroline dissolved in 100 mL of ethanol and 900 mL of water was added, but I don't know the moles of o-phenantroline.

For the standard Fe: .04 mg/mL, I can find the mg of Fe of each solution
.04 x 10 ml= .4mg
5mL= .2 mg
2 mL= .08 mg
1 mL= .04 mg
and from the periodic table table we can find the moles of Fe
So I'm wondering if I can find the molarity of Fe(o-phenanthroline) from the absorbance because I don't see how else to find it.

Here's my data:
Micrograms of Fe Standard (mg)
0
0.4
0.2
0.08
0.04

Background Corrected Average ABS (A)
0
0.79
0.39
0.15
0.08

 

[Borek]

You can ignore concentration of phenantroline assuming complex stability constant is so high all iron is complexed.

 

[symbolipoint]

The principle is that you can as Borek said, ignore the concentration of the ligand, but you want to look for a linear relationship between Fe concentration and absorbance.

 

[MitsuShai]

You can ignore concentration of phenantroline assuming complex stability constant is so high all iron is complexed.

Are you sure because on step 8 of the PDF it says, "Calculate the molarity of
Fe(o-phenanthroline) in each solution and find the average molar absorptivity (ε in Beer's
law) from the four absorbances."

 

[Borek]

That just supports what I said - calculate concentration of complex assuming it equals concentration of iron.

 

[MitsuShai]

That just supports what I said - calculate concentration of complex assuming it equals concentration of iron.

ok so for the 10mL of iron I do this:

M1V1=M2V2
(10mL)(.04mg/mL)=M2(100mL)
= .004mg/mL
is that right?
and I don't have to worry about the 3.00 mL of o-phenanthroline solution, right?

 

[Borek]

Looks like.

 

[MitsuShai]

Looks like.

Should I put the molarity in mg/L or should I convert to moles/L or can I convert to moles/L?

4 mg/L * (1mole/ 55847 mg)= 7.16 E -5 moles/L, I'm not sure if doing this is a good idea


to find the average molar absorptivity I was told to ignore the length the light travels, so it's just A/c= e
when doing the molarity should I leave it in mg/L or moles/L to calculate the molar absorptivity?

 

[MitsuShai]

something in this experiment is sensitive to air, but I forgot which, does anyone know?

 

[chemisttree]

Hydroquinone is an oxygen reactive species as is Fe+2. Not sure if the complexed Fe+2 ion is somewhat more stable to air.