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Verifying the Validity of Sodium Dichromate Test for Passivation

adv.   

August 17, 2009

Hello,

We have a passivation line using 20-25% Nitric with 3-4.6 oz/gallon sodium dichromate dihydrate.

When we initially mix the tank, we put 118 oz. (weight) of sodium dichromate dihydrate crystals for our 37 gallon batch.

After the tank is mixed we test using the below procedure and the results indicate only 2 oz. per gallon.

Can someone please review the test procedure and comment on what exactly it is testing for and whether it appears to be valid? I am obviously not a chemist and am only working with SOP's that were inherited.

Thank You

Test procedure

1 ml sample of passivation solution
Add 100 ml water
add 20 ml Ammonium Bifluoride (10% solution)
add 5 ml Potassium Iodide (20% solution)
add 15 ml Hydrochloric acid 50%

Titrate using 0.1N sodium thiosulphate [on eBay or Amazon] until straw colored

add 3 ml 1.0% starch indicator

Titrate until blue turns clear.

Calculation

Titration X .4444 = oz per gallon sodium dichromate.

My question is:

1. whether the test is valid?
2. what is it actually testing for (i.e., chromium level etc.)?
3. why when we add 3 oz/gallon does the test only indicate 2 oz. per gallon?
Any other comments would be appreciated.

Dave Gallagher,Manufacturing Engineer
aerospace parts mfg. - Linwood, Pennsylvania

I do not love several of the quantities that you are using, but it may work. I think that your ).44 fudge factor is giving you the amount of chrome in the solution and not the chromate di hydrate. Check out a couple of other references.

James Watts
- Navarre, Florida
August 22, 2009

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August 25, 2009

These titrations with starch complexes are subject to oscillating between forward and reverse reactions. The reaction is sometimes referred to as the "iodine clock".

If you titrate it too quickly, the solution will turn clear prematurely.

Once the clear endpoint is achieved, let the mixture sit for a minute or two, to ensure it has gone to completion. If you've titrated too quickly, the dark blue color will return.

Robert H Kinner
- Toledo, Ohio

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You can always run a test parallel with a Koslow Passivation Tester to validate you procedure. Very fast and easy.

Lauren Keane
Koslow Scientific Company

Englewood, New Jersey

September 18, 2009

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The difference in the wts of the dichromate used for preparing the solution and the wt by analysis may be due to the following reasons 1.dichromate purity may be less and 2.calculation from the titre value gives Sodium dichromate in gpl and not sodium dichromate dihydrate 3.Besides analytical errors, if any may be looked into.
Ramajayam

Subramanian Ramajayam
consultant - Bangalore, India
September 25, 2009

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The multiplier should be .662 (not .444). This analysis method is detailed at www.anoplex.com/Iodine.html [a finishing.com supporting advertiser], where the multiplier is traced back to the periodic table --

- the final sentence of the explanation is "Since the sample is 10 mL, there will be 24.833 grams / 10 mL = 2.4833 g/L of Na2Cr2O7-2H2O per mL of titrant". We have to convert from a sample size of 10mL to the 1mL (from your reader's instructions), and we have to convert the normality of the sodium thiosulphate titrant from the 0.5N used at anoplex.com to(your reader's) 0.1N. So, the 2.4833 g/L at anoplex becomes 2.4833 g/L x 10mL/1mL x 0.1N/0.5N = 4.9666 g/L. Converting g/L to oz/g gives 4.9666/7.5 = 0.662 oz/gal. So, for a 10mL bath sample titrated with 0.1N sodium thiosulphate, the computation is 0.662 oz/gal of sodium dichromate for each 1 mL of titrant.

Bill Fazakerly
- Pleasanton, California, USA
November 2, 2009

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Bill,

Thanks so much for taking the time for such a detailed reply. It looks as if that was the error. This has been a great help!

Robert,

I am incorporating the suggested stand period into our test procedure as well. Thank You.

Thanks again to all who took the time to respond.

Dave Gallagher
- Linwood, Pennsylvania, USA
January 20, 2010

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Q. I've been requested by the QA Section to validate the passivation process we use for our plant systems, e.g purified water piping. This job is performed by a contracted company which we have asked them about their passivation process if validated or not to proof effectiveness. Their answer is "no, we have been using this procedure for years in several companies with good results". Would be enough and acceptable for QA if I make reference to the ASTM A967 only?

Regards,

Olga Diaz
- Puerto Rico
November 14, 2012