Originally Posted by Mac120
Unfortunately, my titration iron test kit is still able to measure Fe-EDTA, Fe-Gluconate, Fe-Citrate and some parts Fe-HEDDTA - but, it failures on Fe-DTPA, Fe-EDDHA.
So, I am searching for a better test kit.
lol - and this is your problem. A hobby grade test kit is going to rely on the formation of a chelated compound to form an Fe-chelated compound that absorbs in the UV-Vis spectrum. The presence of stronger, multi-dentate ligands like DPTA or EDDHA means that it is thermodynamically and/or kinetically unlikely (i can't remember which) for those ligands to dissociate in order to form a compound with the test reagent chelate like you want.
I checked the Hach Water Analysis handbook for Fe analysis methods and there are two methods that are not inhibited by chelaters: The FerroVer Method (using 1,10-phenanthroline (which is a bidentate ligand, i think)) or the TPTZ Method (i have no idea what TPTZ stands for...). In either case, both methods clearly that that you need to digest your samples first in order to destroy the chelaters.
The standard methods handbook states the following for Fe Analysis (SM 3500-Fe B):
If noticeable amounts of color or organic matter are present (Read - chelators), it may be necessary to evaporate the sample, gently ash the residue, and redissolve in acid.... The presence of excessive amounts of organic matter may necessitate digestion before use of the extraction procedure.
Here's my take on this: The UV-Vis based test kits you have are fine. The problem is that you need to digest the samples prior to analysis. (And for the record, i would not recommend digesting samples in your house without VERY good ventilation - nothing like concentrated acid vapors to do wonders to your stainless steel appliances...) Otherwise, i would suggest sending your samples to a certified laboratory where they will digest the samples and analyze them by ICP-OES.