Summary for iron in passivation solution Discussion
iron in passivation solution is the core issue here, and the replies caution that direct titration is not practical in a high-chromium nitric/dichromate bath. Randall Fowler says iron can be measured only with a labor-intensive separation: precipitate the iron with sodium hydroxide, filter and wash the hydroxide, redissolve it, then titrate the iron with potassium permanganate. He warns the result may be inaccurate because iron can reduce chromium, changing the chromium valence during the process. Ken Vlach offers a simpler gravimetric approach: precipitate at about pH 4 so iron comes down as hydroxide while minimizing chromium co-precipitation, let it settle, decant, collect on a pre-weighed ceramic or glass crucible, dry to Fe2O3, and weigh. He notes that higher pH steps can be used to measure combined Fe/Cr or Ni fractions, but not pure iron alone.
Q. I am looking for a titrimetric analysis for determining the concentration of Iron in a passivation solution of 25% Nitric acid/2% Dichromate.
Carol L. Hustis
metal finishing shop – Oxford, Connecticut
2005
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 orprinted circuit grade hydrochloric. Finally, the iron would be titrated against potassium permanganate
⇦ on
eBay or
Amazon [affil link] . 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.
Randall Fowler – Fowler Industrial Plating, LLC
Cleveland, Tennessee, USA
2005
A. Oops! Carol, don’t use nitric acid to resuspend iron hydroxide. Use hydrochloric.
Randall Fowler – Fowler Industrial Plating, LLC
Cleveland, Tennessee, USA
2005
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.
Ken Vlach [deceased]
– Goleta, California
Finishing.com honored Ken for his countless carefully researched responses. He passed away May 14, 2015.
Rest in peace, Ken. Thank you for your hard work which the finishing world, and we at finishing.com, continue to benefit from.
2005
Analysis for iron content in a 50% nitric passivate solution
I’m looking for the method of performing analysis for iron content in a 50% nitric acid passivate solution.
Richard Wright
Manufacturer – Sylmar, California
2005