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Peter
06-11-2012, 08:04 AM
So I was doing some redox titrations for a school assignment to find iron concentration levels in various supplements, for some reason unknown to my teacher as well as me, the endpoint could not be found. I was using potassium permanganate, the pink colour would come, but disappear 20-30seconds afterward. I took the reading and did the calculations neccesary to find we had surpassed the theoretical value by over 600%.

I was wondering is their a better method for determining Iron concentration?

riwu
06-11-2012, 08:15 AM
Try some displacement reactions? More reactive metals such as sodium, calcium etc will displace iron in solutions (eg. iron chloride).
Some other examples would include reaction with sulfuric acid/HCl to form a blue-green crystal. All these rely on color change which may not be accurate due to subjective judging.
More accurate experiments would be to react with oxygen & water (rusting), and measure the amount of oxygen/water used to fully react with the Fe metal.

Peter
06-11-2012, 08:34 AM
I was thinking of displacement or mass spec, but with those reactions, it wouldn't be possible to discover the mass (g) of the iron? Hmmm I like the sound of the rusting experiment, but wouldn't I first need to ionize the Fe?

riwu
06-11-2012, 08:40 AM
Are you trying to find the amount of Fe in an impure substance (and hence the concentration of Fe in the substance)? You will just have to measure the mass of the original substance, then calculate how many moles of reactants/product are used/produced, and through the chemical equation determine the mole ratio and u will be able to find the mass of Fe reacted, taking the mass of Fe reacted and divide by the original mass you will be able to find the concentration.

Peter
06-11-2012, 08:45 AM
Yeah, their iron supplements so they have a range of unknown substances in them. I've done all that, and have gotten reasonable values with the tests where there was a deffinative endpoint. However 3 out of 8 of them, no endpoint could be found for an unkown reason. I'm trying to see if their is a better procedure to test for the amount of Iron in an impure substance, mass spec could work, if you remove everything exept the iron, but tbh I am so lost :/

putonajonny
06-11-2012, 09:24 AM
You could use the variable oxidation states to convert the iron from Fe2+ to Fe3+ (or the other way) which has a distinct colour change. You would need an oxidising agent such as Potassium Dichromate (K2Cr2O7) or a reducing agent such as NaBH4 or LiAlH4 depending on the direction, this could give you moles, then you use mass = moles x Atomic Mass to get the mass in grams

Peter
06-11-2012, 11:20 AM
The problem is, it's an impure substance with a moshpit of other ions in solution. I was thinking of maybe using spectroscopy to first find out what is in the solution, then using some sort of precipitation reaction to remove the iron? I even have my teachers stumped -_-