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Author Topic: ppm Fe in parsley with AAS and spectroscopy  (Read 477 times)

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stickan1331

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ppm Fe in parsley with AAS and spectroscopy
« on: December 21, 2016, 02:16:35 AM »

Hi!
In school we did two different experiments to decide the amount of iron in parsley.

I have got two very different values(one is 61ppm and the other is 36ppm) and therefore I wonder If it's my calculations are correct or if it was something that went wrong during the lab.

First of all 0,1 liters  parsley solution were created that were used during both experiments .

First with Atomic absorption spectroscopy(AAS)
I made a function of how the absorption(abs.) was depending on the c[g/liter] Fe with 4 different calibration solutions.

abs. = k*c+m
k=12,15 and m=-0,0002

c = (abs.+0,0002)/12,15

and then to decide the mass of the parsley with c*V where V were 0,1 liters(The amount of parsley solution). This is correct right?
I think this is right because I got the amount of Iron to around 36ppm which is very close to what livsmedelsverket(Swedish food controllants) got it to.


SPECTROSCOPY


However with my spectroscopy lab something probably went wrong.

In a similar manor to the one in the last experiment a function was created this time with the concentration of Fe in [M].
abs. = 6395,1421*c - 0,02

In our 4 calibration solutions this time we had FeCl3 HCl and KSCN. (So it turned red)According to this picture

https://gyazo.com/05d69ca7188c27ea95830d307fa098a4

We also made one solution with nothing but 2,00cm3 distilled water and 1,00cm3 KSCN.
Lastly we took 2,00cm3 parsley solution and added 1,00cm3KSCN. (so it turned red).

We put 3,00cm3 of every solution in different cuvettes and the parsley solution abs.=0,169

Now I will try to explain how I calculated the mass of Fe.

c = (abs. - m)/k //this is c Fe in the cuvette.

n Fe = n FeSCN63- = c*V1 were V1 = 0,003(Total amount of solution in the cuvette) right?

then to get the concentration of Fe in the parsley solution, c1, you divide c*V1 with V2 were V2=0,002 (amount of parsley solution in the cuvette). //This and the step above is what I'm a little unsure about If you're acculy supposed to do them at all.

Since c1 is the concentration of Fe in the parsley solution you multiply it by the total volume of parsley solution(0,1liters) and then with Molar mass, M, of Fe (55,845).

c1*Vtot*M = mFe //this should be right

And then when I calculate the ppm of Fe in the same parsley I get 61ppm.

I think that something went wrong during the laboration but If someone understands my formulas, please try to see if I calculated something wrong or if something went wrong during the laboration.

Cheers!





 
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mjc123

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Re: ppm Fe in parsley with AAS and spectroscopy
« Reply #1 on: December 21, 2016, 02:39:17 AM »

Your procedure looks correct but it's hard to see if you've made a mistake in calculation because you haven't given us enough information.
Can you tell us how many g parsley were used to make your 0.1L solution?
Can you show us the absorption - concentration data for your standards and the absorption of your sample (both experiments, including the blank for exp. 2)?
Did you make just 3 mL of parsley-KSCN solution? Didn't you add any HCl? Why not? (Maybe it doesn't matter, I'm just wondering.)
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stickan1331

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Re: ppm Fe in parsley with AAS and spectroscopy
« Reply #2 on: December 21, 2016, 06:27:21 AM »

Well for the 0.1L solution 0.499grams of dried parsley was used (water% of 88.11%)

Spectroscopy lab data: https://gyazo.com/d1f6a202a106e27f7b49cdfeedccc193 along with my abs. of 0,169
AAS data: https://gyazo.com/e56a7ca858071a5e7c80fd3f44c53e3c and my abs. = 0,018

Yes, I only made 3 mL of parsley-KSCN solution and did not add any HCl I don't know why not but according to our instructions you shouldn't either so I guess it didn't matter.

What I'm most uncertain about Is if the step where I multiply by one volume and divide by another "then to get the concentration of Fe in the parsley solution, c1, you divide c*V1 with V2 were V2=0,002 (amount of parsley solution in the cuvette). //This and the step above is what I'm a little unsure about If you're acculy supposed to do them at all." because if that step is skipped I get a lot closer to 35ppm.

Thanks!

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mjc123

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Re: ppm Fe in parsley with AAS and spectroscopy
« Reply #3 on: December 21, 2016, 10:57:47 PM »

I don't quite see how you get your answers. Using your calibration data I get:
AAS: C = 0.00145 g/L, which in 0.1L is 145 µg. In 0.499 g that is 291 ppm.
UV-vis: C = 3.03E-5 M, corrected x3/2 = 4.55E-5M, in 0.1L is 4.55µmol or 254 µg, which is 509 ppm.
Have you multiplied by 0.1189 to take account of the "water % of 88.11%"? What does this mean? Does your "dried" parsley stil contain 88% water? Or did you have a mass of undried parsley which was 88% water, and after removing this water you had 0.499g left? In the latter case your procedure would be correct, but you have not described it clearly.
I still can't account for the different answers between the two methods. (You are sure of your standard concentrations, are you?)
And by the way, don't just say "spectroscopy". There are many kinds of spectroscopy, of which AAS is one, as the name implies. I assume you mean UV-visible spectroscopy, in which case say so.
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