Analysis & replenishment of passivation solution for stainless steel: nitric acid, chromate, iron, p.2

A. With 25% nitric acid and 2% sodium dichromate the dissolved iron is already in the +3 oxidation state. Permanganate oxidizes iron +2 (ferrous) to iron +3 (ferric). Your iron is already +3 so this titration has no chance to test for iron +2.
Regards,

A. No it will not work because the iron +2 is already oxidized by the hex chrome in your solution.
Regards,

A. It's much easier to first separate the iron from the nitric acid, nitrates, chromates, etc., via precipitation.

Dilute a 25 ml sample with DI water, raise the pH to 4 using NaOH solution, then collect the precipitated ferric oxyhydroxide (hydrated rust) by filtering through a Gooch crucible.* Rinse sparingly with DI water until the rinseate is colorless (chromate-free).

Treat the precipitate by either 1) complete dehydration to Fe2O3 in a weighed beaker [beakers on eBay or Amazon [affil links] or crucible, or 2) redox titration with permanganate or dichromate following boiling to a clear green Fe+2 solution in a reducing acid.

For dehydration, 1 hour at a minimum of 300 °C in a Pyrex beaker is usually sufficient (250-275 °C may only result in FeOOH). Heating in a crucible over a gas burner will also work. Vogel's Quantitative Inorganic Analysis [on Amazon affil links] mentions heating to 850 °C (bright red) in air.
Weigh after cooling (in a desiccator if available) to room temperature.
Weight of Fe = 0.6994 x wt. of Fe2O3.
For a 25 ml sample, wt% Fe =4 x wt. Fe / SG of passivation solution).
Note: Weighing as Fe2O3 is the classical gravimetric analysis for Fe+3.

To verify the procedure, spike 2 passivation samples with soluble iron (e.g., ferric or ferrous chloride or nitrate) equivalent to 0.1 & 2 wt% Fe, respectively (as Dr. Cook mentioned, the passivation solution will convert any Fe+2 to Fe+3).

*If using filter paper instead of a Gooch crucible, to minimize attack of the paper by chromate, dilute before filtering: Let the precipitate settle, decant off the clear liquid, add DI water to the remaining sample, let settle, decant and filter. Rinse with DI water.

Hi,

I just want to thank everyone who helped me out.
Thanks Ken Vlach, I'm looking forward to giving precipitation a try.

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Ed. note: Hi, Surya. We appended your inquiry to a thread which we think will answer your questions, but please re-post if anything is unclear. Good luck.

Hi Dr. Paul. I'm not a chemist and don't fully understand this dialog, but I do have some experience with chromate waste treatment; the chromium in chromate is Cr+6 and it will not be reduced to Cr+3 without treatment by a strong reducing agent. So I would say it will stay in solution as Cr+6 when you add the hydroxide.

Regards,

A. Hi Aijazullah. Exactly what chemical analysis calculation do you have in mind? Those "factors" are not magic numbers but they are merely the product of all of the conversion involved, and may involve molecular weights, densities, conversions from one measuring system to another, etc.
"9.8" happens to be a useful conversion factor for determining free acid when titrating sulfuric acid with sodium hydroxide, but may have nothing to do with whatever you are trying to determine about hexavalent chromium. What are you trying to determine? Thanks.

Regards,

A. Sorry, Aijazullah, but we are apparently on different wavelengths.

It seems to me that without a specific quotation related to a particular "x" factor, or a description of the specific calculation you want to do, or the reagents involved in a titration, "x" could be anything from zero to infinity because there is no basis for guessing what you are trying to calculate & convert. Maybe another reader has a better insight into what you are asking.

Regards,

A. Hi Mark
I think you misunderstand auditors and their purpose. Most are highly qualified and experienced. In this industry they are certainly not box tickers. Many of them have themselves queried specs which are illogical.
However this misses the point.
You have a contractual obligation to operate within the spec you accepted from the customer and the auditor is tasked to find evidence of this.
Firstly, neither you nor the auditor know precisely why the customer requested this spec., there may have a valid reason for the tight limits.
Secondly, the auditor has found evidence of a failure of process control. Could you show a control graph with evidence of long term compliance?
It matters not that you can satisfy yourself that the parts are OK. The only authority that can accept out of spec parts is the customer.
Why are aerospace customers so particular? Next time you are 35,000 feet up look out and ask the question.

Thanks Geoff!

After a career of specifying stuff as an engineer, then running this site for 20+ years, I'm going to tattoo your
"Neither you nor the auditor know precisely why the customer requested this spec."
on my forehead backwards so I read it when I look in a mirror.

We humans suffer constant temptation to believe the contrary :-)

Regards,

A. Good day David.

I can relate to your situation with auditors = NADCAP = BRUTAL!= shelf life/lot #'s, received dates, open dates, signature, % accuracy TD pipette [pipettes on eBay or Amazon [affil links] , etc.... of ALL chemicals=lab/process. He hunted, but went home hungry!
The small % variance will not impact product, but, as the auditor indicated, out of spec. This is a moot point.
I am surprised the iron concentration was not questioned. As you well know, the max.concentration was increased from 0.5% to 2.0% wt. My auditor dwelled on this, and I have 2 concentrations for iron in my tank matrix analysis.
The spec I am working to for iron analysis involves a quantitative analysis,NaOH precipitation, which precipitates ALL metals as hydroxide = HIGH (Fe) numbers. I also shoot a sample on the AA = 30% value from NaOH analysis.
I walked the auditor through both analysis procedures, and guess what?
He bought it.
Food for thought.

Regards,

Geoff,
Point well made, however, I've seen the other side of this too. A subcommittee in charge of a standard may come back at the five year review point, and with fresh eyes on the document say "Why the heck did we write it like this"?

I have a few recent experiences with some line in a standard needing to be rewritten because one or more company's auditors got hung up on something that was really beside the point, and had not been foreseen as an issue when the line was originally written. I witnessed an hour long discussion that amounted to "We know what we mean, but how do we write it in such a way that will inform the processor what they need to do without leaving it open for auditors to nitpick something that doesn't need to be scrutinized?"

Standards are a best effort from people who are experts trying to communicate the important things to people who are not experts, and there's a certain traditional language to standards that doesn't always exactly reflect the situation. Oftentimes the focus is more on giving a recipe than an explanation of overly technical details.

However, yes, the larger point is that regardless of how vital the concentration range is or isn't, falling outside the set bounds shows that your process controls didn't do what they were supposed to. Which only raises the question, what other details are not being followed?

A. Mic,
Commercially available "concentrated" nitric acid is typically a solution of 61.0 to 68.2 weight percent of HNO3. (Source: Mil spec O-N-350) You may also see this referred to as 42 °Baumé.

The density values vary depending on what source you use, but for now let's say 1.4 for a 67 wt% solution. Therefore a liter weighs 1.4 kg. If you add 1.4 kg of water, that cuts the nitric concentration in half, to 33.5 wt%. So that's 1.4 liters of water (density 1) with 1 liter of nitric concentrate. The volume percent of nitric concentrate is 1/2.4 = 41.7%

So let's see how the 25 vol% number plays out. That's 3 kg of water and 1.4 kg of nitric concentrate. That 1.4 kg represents 1.4*0.67 = 0.938 kg of nitric acid in the final 4.4 kg of solution, or 21.3 wt% nitric acid.

So the "20-25 vol% nitric concentrate" does not match the "16-34 wt% nitric acid", but it falls inside the range.

As for your second question, making an equivalence between 14.0-31.5 vol% pure nitric acid and 20-45 vol% nitric concentrate is silly, because you don't want to go anywhere near pure (white fuming) nitric acid. Golly, I hope the "20-25 vol% nitric acid" reference in your type A wasn't meant to refer to white fuming nitric acid. I suppose you could look up the density of that and run the numbers.

Anyway, yes, this shows how formulas can be problematic if you aren't specific about what's being mixed, what the concentration within the stock solution is, and if you miss the distinction between weight percent and volume percent.

A. Timothy,
The industry standards for passivation that I'm familiar with say the bath concentration must be maintained within the assigned range, but offer no advice or directive on how to go about doing so.

Hi Tony,
Here's my titration for the nitric in that tank:

Place a 5mL sample of Bath in a clean beaker with about 30mL DI. Add 10 drops BCG. (bromocresol green ⇦ this on eBay & Amazon [affil links] )
Titrate with NaOH 1.0 N [1N NaOH on Amazon [affil links] to green-blue endpoint. Record mL as "A".
1.333 A = % v/v Nitric Acid 42° Be (67% stock material) = % w/v HNO3

That being said, when I started here, my predecessor had been buying those Kokour pre-fab solutions and was testing with 'N71' which is basically just sodium hydroxide diluted so as to make an equivalence so that the mL used in titration is equivalent to the concentration of Nitric in the tank, and I back-titrated the N71 against a known concentration of monoprotic acid and just recalculated everything stoichiometrically.
I had a conversation with a Nadcap auditor last year about this exact subject- relating to the Citric test, but same deal- and was also frustrated that it has to come from an 'expert', even though you can set up a chemical equation and calculate it yourself. And his comment that 'online or journals do not count as a legitimate source' just annoyed me to no end.
Also considering that shops will be buying Nitric from a chemical distributor who doesn't necessarily provide support specific to our industry, getting the 'official' scoop is like looking for a needle in a haystack!
I'm not sure if you've ever checked out the Carpenter Steel website, which is a rabbit hole of technical goodies, including a recipe for a hard-to-passivate-steels solution that seems to have a cult following (I'm a believer lol). And, arguably, this can be considered a 'manufacturer' method coming from a specialty supplier of stainless steels.
Here's a link to their titration instructions. You can skip the pH meter step; even in other tanks WITHOUT the dichromate, switch the phenolphthalein out for bromocresol green ⇦ this on eBay & Amazon [affil links] and use the blue-green endpoint. Also if you've got a ton of iron buildup the results skew high.
https://www.cartech.com/en/alloy-techzone/technical-information/manufacturing-guides/how-to-passivate-stainless-steel-parts
The nice thing is that if you want to start with different sample volumes (like, if you don't have an accurate 1mL pipet) or reagent concentrations, you can simply recalculate, while still maintaining reference to this work instruction. I'd print it out, do your calcs right on the sheet, squirrel it away in your library, and be ready to defend your math :)
Happy titrating!

Copper (II) Nitrate

A. Hi Clinton. Most salts of divalent copper, including copper nitrate ⇦ this on eBay or Amazon [affil links] , are a strong bright blue. Any reasonable way for a substantial amount of copper to have become dissolved in your nitric acid?

Regards,