Question

Okay so we did a series of dilutions using a standard solution of iron (40ug/mL), 2mL of hydroquinone, and 3mL of phenanthroline and then we took the absorbance. My results were:

1mL Fe solution= abs 0.059

2ml Fe solution= abs 0.124

5mL Fe solution= abs 0.271

7mL fe solution= abs 0.354

blank solution= 0mL=abs 0.000

I made a stadard curve using this and got a trend line equation of y=0.0505x+.0102.

We then dissolved a 45mg over the counter iron pill in 25mL of 6M HCl, boiled it, filtered it into a 100mL volumtetric flask and filled to the line. We then diluted it a second time by taking 5ml of that solution and putting it into another 100mL flask. 10mL of that solution was then taken to 100ml (third dilution?) and the hydroquinone and phenanthroline added. The abs was found to be 0.297.

How do I find the number of mg of Fe in the pill and the uncertainty in that number? Where does Beer's law come in?

Answer 1

First, the Beer's law is:

A = EbC

E is molar absorptivy (L/mol cm)

b is cell lenght (usually 1 cm)

C is concentration (mol/L)

Now the first plot you got, as y = 0.0505x + 0.102 is the linear equation, and I assume you plot Abs Vs Concentration of Fe Is that correct?. If that's the case, then the slope of that equation gives you the relation Eb. In other words:

A = EbC If you plot A vs C then:

y = Absorbance

Y intercept: is innitial absorbance = 0.102

Slope = Eb = 0.0505

x = Concentration

From the slope you can get the value of E (assuming b as 1 cm) = 0.0505 L/mol cm

Now, if absorbance obtained from the sample is 0.297, then all you have to do is replace this value into the linear equation of before:

0.297 = 0.102 + 0.0505C and solve for C:

0.297 - 0.102 / 0.0505 = C

C = 3.86 M

Now to get the original concentration use dillution factor:

3.86 M * (100/10) * (100/5) * (100/25) = 3088 M

With the molecular weight of iron, we can pass that concentration from mol to grams:

3088 mol/L * 55.8 g/mol = 172310.4 g/L or 172310400 mg/L

That should be the concentration in ppm.

However, Basing on this number, I think that the innitial standard curve is wrong. You have to plot absorbance vs concentration.

To calculate the concentration, you already have the volume of Fe, and the concentration so:

C = 40 ug/mL * (aliquot volume / final volume)

In this case, say what's the final volume (say 10 mL or 15 mL) and the aliquot volume would be 1, 2 5 and 7. Correct that, and then, use the procedure I put here.

Hope this helps