CHEMISTRY UNIT 3 SAC COLORIMETRY

[Inhibition]

Hey guys, has anyone done a colorimetry sac for analyzing the percentage Iron in a fertilizer sample?
Heres the theory:
Iron contained in fertilizer is in the form of FeSO4 (Fe2+ ins). Some of the Fe2+ ions will oxidize to Fe3+ due to atmospheric Oxygen. However, since the intensity of both ions are too pale to analyse with a colorimeter, adding K3Fe(CN)6 Potassium Hexacyanoferrate containing the Fe(CN)6 [3-] ion the Fe2+ and Fe3+ ions will react with the Fe(CN)6 to give a Prussian Blue Colour - KFe[Fe(CN)6]

we have to make some standard solutions and then put the sample in and find its absorbance and hence, it's concentration.

So what Im getting at is, will the concentration be of the Fe2+ ion, Fe3+ ion, FeSO4 molecule, or the Hexacyanoferrate?

[jgoudie]

My understanding is this:  It really depends on how you made up your standard solutions.  My guess would be you added some Fe²⁺ then some Fe(CN)₆.  In this case you would be meassuring concentration of Fe²⁺.  The absorbance is still measured from the KFe[Fe(CN)₆] but this was made from the Fe²⁺.  Your standard solution would have been made in reagards to Fe²⁺, thus your calibration curve is [Fe²⁺] vs absorbency.  If you post how you made the standard solutions I might be able to help more.

[Alwin]

I just also like to point out that the point of adding Potassium Hexacyanoferrate is to form a complex ion with the iron present in the solution. This is a bit beyond the scope of year 12 I think, or just not a major part. Basically, it forms strong polar bonds around the iron ion, much like when salts ionise in water, except your Fe ions are surrounded by (complex) ligands. That's just for your.. erm.. general knowledge haha

But when used in colorimetry, complex ions are preferred because they often give off an intense distinctive colour. In your prac, you are adding the Hexacyanoferrate  to produce the purple colour, and in theory if you prepare your standards correctly, it will not interfere with your final results since the graph is all 'relative' to your reference cell.
You can consider the Hexacyanoferrate added as an impurity / contamination if you want, so if its present in all your standards, in the same amounts, your graph will be adjusted to it. A common example is that of a calibration curve not passing through the origin, because the element you are testing for is present in the solvent (eg heavy metals can be found in water). You can either adjust your equipment, or simply have a calibration curve that begins above the origin.

I'm only guessing at the method you intend to use for your sac, so I may be completely wrong but hope it makes sense.

Good Luck with your sac!

[Inhibition]

THANK YOU ALL YOU KIND PEOPLE FOR YOUR ADVICE!
I had my SAC yesterday, went better than expected =]
Some tricky worded questions but our colorimeter did most of the cal. curve for us.