[swiftxstuff]

So I've completed an investigation into the iron concentration in different parts of a spinach leaf.

After doing a lot of calculations, the final result for the tip of the leaf says I have about 26 mg of iron per 100g of spinach. This is supposed to be about 2.71 mg!

My calculations are below. Please look through them and tell me if there are any mistakes, I can find none.

Working through calculations for ‘Tip’ sample:
Mass of sample before burning = 12.10g
Volume of solution = 60 mL + 5 mL + 1 mL = 66 mL
Density of 2 mol L-1 HCl is 1.033 g/mL
Density of KSCN is 1.886 g/mL
Density of 20% KSCN solution = 1.886*0.2 + 0.8*1.000 = 1.1772 g/mL
Transmittance of sample = 26%
Molar mass of Fe3+ = 55.845
Mass of solution (assuming mass of ash is approximately 0.5g):
60*1.033 + 5*1.000 + 1*1.1772 + 0.5 g = 68.1572 g
68.2 g
Iron concentration of sample:
Equation from iron standards graph:
y = -828736x + 94.256
y = transmittance = 26%
26 = -828736x + 94.256
x = (26 - 94.256) / -828736
x = 0.000082362 %

0.000082362 g/100mL
0.000082362/100 = c(Fe3+)/68.1572
c(Fe3+) = 0.00005613534 mol L-1
n = cV
n = 0.00005613534 * 68.1572
n = 0.00382602760 mol

n = m/M
m = nM
m = 0.00382602760 * 55.845
m = 0.21366451132 g
0.214 g of Iron in 12.10 g spinach sample
(12.10 / 100) * 0.214 = 0.02589

25.89 mg of iron per 100 g of spinach tips

Spinach is expected to contain 2.71 mg of Iron per 100 g

Please post a reply if you went through it and found no errors! 

[Hunter2]

I think the whole calculation is strange.

Normally you dissolve the organic material (ash) and then you do a determination of iron.
Example you dissolve 0.5 g ash and get a value. This value you can recalculate to the spinach.

The last calculation is wrong.  You found 0,214 g Fe³⁺ in 12,1 g spinach. This means

0,214 g/12,1 g = x g/100g  x = 1,76 g But this value is to high.