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

A. 1 Ml soln 250 ml flask, add 100 ml D.I. water, 30 ml 50% phosphoric acid,10 ml conc. hydrochloric acid. Add 1.0 g potassium iodide, titrate with 0.1 N sodium thiosulphate to straw colour, add 1ml starch indicator, continue titration colour change from dark blue to colourless, giving titre A.
Sodium dichromate oz/gal= 0.662 X A

Nitric acid= 1 ml sol'n 250 ml beaker, add 50 mls D.I. water,titrate with 1.0 N sodium hydroxide to ph 3.57(calibrate meter ph range 1-4) giving titre B.Nitric acid in %B.V.=6.45 X B. This works for me, Any new analysis procedure it's a good idea to make known standards to verify results. Good luck.

A. Pappu,

Do you mean sodium dichromate? It looks like a passivation formulation to me. The analysis method won't be any different, just the calculation will change.

Nitric acid can be done by a simple acid-base titration using an appropriate indicator, bromocresol green would work well I think.

The chrome can be done by an appropriate REDOX titration. Use ferrous ammonium sulphate with sodium diphenylamine sulphonate indicator, back titrate with potassium dichromate.

A couple of good reference books with lots of analysis techniques common to treatments are The Electroplater's Engineering Handbook =>
and Arthur Vogel's Textbook of Quantitative and Semi-quantitative Analysis =>

A. See Letter 15860

Ed. note: For the readers' convenience we have now incorporating the old Letter 15860 directly into this thread: Letter 15860 (2002) Q. Help! Need formulas for replenishment of passivation tanks. I already have the formulas for replenishment if acid level and dichromate is low but not if the acid and dichromate level is high? I can't seem to transpose the formulas the opposite direction. Can anybody help me out? Thanks, Jason Katorski - Camarillo, California, USA (2002) A. There's a problem here: acid and dichromate should always decrease with use. How did the bath get too high? Are the analyses being performed properly - verify your numbers before you do anything. I say this because to fix the immediate problem, you'll have to decant part the bath - do the math to figure out how much to decant - and add back purified water. You may also need to add back one of the two components if they weren't out of spec to the same degree. Megan Pellenz - Syracuse, New York (2002) A. Here are the two analyses with calculations for a Type II, QQ-P-35 [link is to free spec at Defense Logistics Agency, dla.mil], passivation solution. SCOPE: To determine the concentration of Sodium Dichromate in a solution. EQUIPMENT: 2 mL Pipet 25 mL Buret 250 mL Erlenmeyer Flask Deionized Water 0.1 N Sodium Thiosulfate 100 mL Graduated Cylinder 0.5% Starch Indicator 10 mL Graduated Cylinder Concentrated Sulfuric Acid 10% w/v Potassium Iodide 1.000-1.250 Hydrometer 500 mL Graduated Cylinder PROCEDURE: Using the pipet, transfer 5 mL of the sample into the Erlenmeyer flask. Using the graduated cylinder, add 100 mL deionized water to the flask. Using the graduated cylinder, add 5 mL concentrated sulfuric acid and 10 mL potassium iodide solution to the flask. Titrate with 0.1 N sodium thiosulfate until the solutions turns to a straw color. Add 2 mL starch indicator to the flask and continue to titrate until the blue-black color disappears. Record the mL used as V1. Place 500 mL of sample in the graduated cylinder. Measure the density of the solution with the hydrometer. Record this value as"P". CALCULATION: C1= (V1*F1*N1)/P C1 = Concentration of Sodium Dichromate in %w/w. V1 = mL of sodium thiosulfate used in the titration F1 = 2.483 N1 = Normality of sodium thiosulfate used in the titration. P = Density of the solution REPLENISHMENT: W1 = P*V2 W2 = (C1/100)*W1 W3 = (C2/100)*W1 W4 = W3-W2 V3 = W4/453.6 P = Density of the solution. V2 = Volume of the solution in milliliters. W1 = Weight of the solution in grams. C2 = Optimum concentration of Sodium Dichromate %w/w. C1 = Actual Concentration of Sodium Dichromate in %w/w. W2 = Weight of Sodium Dichromate in solution in grams. W3 = Optimum weight of Sodium Dichromate in grams. W4 = Weight of Sodium Dichromate to add in grams. V3 = Weight of Sodium Dichromate to add in pounds. DILUTION: W1 = P*V2 W2 = (C1/100)*W1 W3 = (C2/100)*W1 W5 = W2-W3 W6 = (W5*100)/C1 V4 = W6 / P V5 = V4 / 3785 P = Density of the solution. V2 = Volume of the solution in milliliters. W1 = Weight of the solution in grams. C2 = Optimum concentration of Sodium Dichromate %w/w. C1 = Actual Concentration of Sodium Dichromate in %w/w. W2 = Weight of Sodium Dichromate in solution in grams. W3 = Optimum weight of Sodium Dichromate in grams. W5 = Weight of Sodium Dichromate to remove in grams. W6 = Weight of solution to be removed in grams. V4 = Volume of solution to be removed in milliliters. V5 = Volume of solution to be removed in gallons. and, SCOPE: To determine the concentration of Nitric Acid in a solution. EQUIPMENT: 5 mL Pipet 25 mL Buret 250 mL Erlenmeyer Flask Deionized Water 1.0 N Sodium Hydroxide 100 mL Graduated Cylinder 0.1% Methyl Orange Indicator Solution PROCEDURE: Using the pipet, transfer 5 mL of the sample into the Erlenmeyer flask. Using the graduated cylinder, add 100 mL deionized water to the flask. Add 5-10 drops of the Methyl Orange indicator to the diluted sample. Titrate with 1.0N sodium hydroxide. The solution will change colors from orange-red to yellow. Record the mL used as V1. CALCULATION: C1= V1*F1*N1 C1 = Concentration of Nitric acid in %v/v. V1 = mL of sodium hydroxide used in the titration F1 = 1.27 N1 = Normality of sodium hydroxide used in the titration. REPLENISHMENT: W1= [(C2-C1)*V2] / 100 W1 = Gallons of Nitric acid to add to the tank C2 = Optimum concentration of Nitric acid in %v/v. C1 = Concentration of Nitric acid in %v/v. V2 = Operating volume of tank in gal. DILUTION: V3 = [(C1-C2)*V2] / C1 V3 = Gallons of solution to remove from the tank C2 = Optimum concentration of Nitric acid in %v/v. C1 = Concentration of Nitric acid in %v/v. V2 = Operating volume of tank in gal. I assume the formatting will be correct as I cut and pasted the required information. Best of Luck, Ira Donovan, M.S.F. Kansas City, Missouri (2002) Q. I incorrectly stated the information. Our tanks lose water from evaporation. When we titrate it shows a higher than spec acid and chromate level. We've been adding water then titrating until we get the levels per spec. Just looking to find how to calculate the correct amount of water to add to our tanks. It would save us a lot of time and guessing. Will try the formulas you attached. Thanks I have the same formulas from my customers specs for replenishment as stated by Mr. Donovan. Problem is they specify only using 50ml of water not 100ml. I tried both procedures for titration and yielded different answers. Which do I follow? Thanks, Jason Katorski - Camarillo, California (2004) A. The correct sample size for the Sodium Dichromate titration procedure should be 2 ml and not 5 ml. Willie Alexander - Colorado Springs, Colorado

A. Hi, Esppi.

Sorry, but I am not understanding in what way the above answers were deficient. Can you please rephrase your question in terms of the answers that were already offered? Thanks!

Regards,

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 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" [link is to info about the book at Amazon] 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.

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 continues to benefit from.

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 sulphuric 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 pipettes, 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.