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Analysis & replenishment of passivation solution for stainless steel



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Current postings:

Q. I am looking for the source (book, ASTM, etc.) of the nitric acid replenishment equation posted by Ira_Donovan I would like to understand how the replenishment equation was derived since it does not appear to follow a normal dilution equation. Thank you in advance for your help.

Jill Miller
- Williamsport, Pennsylvania
April 13, 2022



A. Hi Jill,

Looking for the source book sounds like you might be trying to find a "not found on the internet" derivation, as auditors seem to require, no matter how trustworthy the source? It's definitely a dated requirement, and I seriously wish they'd just publish the details in an ASTM spec, so we can all use it!

I can give the titration for both nitric and dichromate from memory ... but the calculations? Ehh ... I can give a nadcap accepted full correction once I'm back at my desk.

There is a source online that is related to an approved software supplier:
https://www.anoplex.com/Titration_Passivate_T2.html
It's not quite what I use (even though I use that software... I heavily modified mine in the nuts n bolts ), or had approved. But if you need to prove a source, it's an option. In conversations with auditors, they accept vendors and consultants as approved sources.

In a pinch, I've actually run enough tests to develop an internal titration technique and calibration curve, shot the whole thing off to my chemical supplier for formal verification, and filed the approval letter away for posterity. You can do this, it is totally legit. The process is a pain in the toosh, but no one can argue with a documented repeatable self-derived titration. Scientific method and all that...

Opinion here: it's a bit funny that auditors will accept a whole mess of internal lab technique of dubious documented origin - as long as it's documented, and they approve of it- over internet community confirmed SOPs. I guess any of us with a decent set of glassware and a .001 scale can develop our own methods, if we document them properly. And I don't disagree with this at all!

One thing that has caught folks (myself included) up is that it's not a straight g/L for dichromate in nitric. The spec asks for g/kg.

What process specification are you adhering to? I'll keep an eye on this thread and if I can get you a good equation and a corroborating source, I'll be back. Even if only to drop an excel doc with some self-calibration prompts ;)

rachel_mackintosh
Rachel Mackintosh
Lab Rat and some other things - Greenfield, Vermont



Q. Rachel, we passivate and titrate using basically the same formulation that you state in your post. My question to you or anyone willing to respond, is deionized water required? Can you do this test w/distilled water?
Thanks
Doug

DOUGLAS MAYER
Special Processes - DoD - NICKTOWN, PA
June 24, 2022





Closely related Q&A's, oldest first:

Q. HOW TO ANALYZE THE BATH CONTENT IN NITRIC ACID AND SODIUM CHROMATE SOLUTION? WE ARE USING NITRIC ACID 21-24% AND SODIUM CHROMATE 2.5% -3.0%. CAN ANYONE HELP OUT IF YOU HAVE THE TEST PROCEDURE FOR THE SAME?

PAPPU G
PLATING SHOP EMPLOYEE - INDIA
May 10, 2011



"Electroplating Engineering Handbook"
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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 [affil. link to info/product on Amazon] 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.

Eric Bogner
- Toronto, Ontario, Canada



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 .

Brian Terry
Aerospace - Yeovil, Somerset, UK



A. See thread 15860

Willie Alexander
- Colorado Springs, Colorado


Ed. note: For the readers' convenience we have now incorporated that old thread 15860 directly into this thread:

Thread 15860

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
Megan Pellenz
- Syracuse, New York



A. Here are the two analyses with calculations for a Type II, QQ-P-35C [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



thumbs up sign "Equipment 2 ml pipette ..."
"Using the pipette transfer 5 ml ..."


I think any auditor would raise an eyebrow.

geoff smith
Geoff Smith
Hampshire, England
August 13, 2022

----
Ed. note: Indeed if that isn't a typo it certainly conjures a humorous procedure :-)


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
2002



A. The correct sample size for the Sodium Dichromate titration procedure should be 2 ml and not 5 ml.

Willie Alexander
- Colorado Springs, Colorado



Q. Hello all,
Looking for a titration procedure for Nitric acid in a Sodium Dichromate bath

Esppi Martinez
employee - Fullerton, California
May 24, 2011



A. Hi, Esppi.

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

Regards,

Ted Mooney, finishing.com
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey



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