Procedure for analyzing Iron in a Passivation solution

A. Carol. Due to the high concentration of chromium in your solution, iron titrations would be impossible. There is, however, a way to do it, but it's labor intensive. First, you would have to precipitate iron out of solution by reacting a measured sample with sodium hydroxide. Since the chromium (hopefully, most of it) is in it's sixth oxidation state, it won't drop out with the iron. You would have to filter the sample to extract iron hydroxide and wash with DI water several times, transferring iron into a flask using a wash bottle. Raise iron valence back up using nitric or printed circuit grade hydrochloric. Finally, the iron would be titrated against potassium permanganate [on eBay or Amazon]. The KMNO4 titration procedure is readily available using your search engine. This procedure may not be that accurate because iron reduces chromium and much of the in-solution Cr6 may have been reduced to Cr3. Let me know how it goes.

A. Oops! Carol, don't use nitric acid to resuspend iron hydroxide. Use hydrochloric.

A. The simplest analysis is gravimetric; titration with sodium hydroxide isn't quantitative since Fe(+3) hydroxide precipitates concurrently with the nitric acid neutralization. Precipitate at pH 4 to avoid Cr(+3) co-precipitation. Let settle overnight, decant off the clear solution, collect and wash (a weak ammonium nitrate solution is preferred to avoid peptization) the precipitate on a pre-weighed ceramic or glass filtering crucible (cannot use filter paper), slowly heat to 350 F to dehydrate, cool, and weigh as Fe2O3. All of the iron is Fe(+3); no need to oxidize.

For further analyses, refer to the pH-solubility curves, www.finishing.com/library/metalpH.html.
Precipitate at pH 7.5, and the above procedure yields total Fe + Cr(+3) as (Fe,Cr)2O3.
Precipitate at pH 9.5, etc., (rinsing with slightly alkaline water to avoid Ni loss) to additionally yield Ni as NiO. Note that Ni co-precipitates with Fe & Cr more readily than alone.
Alternatively, Cr(+3) could be separately precipitated from the supernate decanted from the Fe precipitation, and Ni could be precipitated at pH 10.5 from the Cr supernate.

Passivation solely with hexavalent Cr should give solution values such that moles Cr(+3) = moles Fe(+3) + 2/3 moles Ni(+2). Greater Fe and Ni concentrations result from action of the nitric acid.