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Sodium Dichromate passivation question
My name is Rob. I am running a Sodium Dichromate passivation tank and have some questions about specific gravity. By the ASTM A967 [link is to spec at TechStreet] the tank should be 20-25% acid by volume. We use 42-degree baumé and 3 oz per gallon of Dichromate to mix the tank. Every month I send out a sample of the tank to a local lab. The lab results indicate that I am within the spec however my company has an SOP that requires us to stay within a specific gravity of 1.06 to 1.10. I recently started logging the specific gravity before I send it out to the lab. The last sample I sent out had a specific gravity of 1.116 and the lab said it was 20% acid. If you look at the density table out of Chemical Engineers Handbook [link is to info about book at Amazon] at 20% Nitric acid and 50 degrees Celsius the specific gravity should be 1.0966. I understand the specific gravity should change due to adding Dichromate. My question is are the tables in the Perry's book for % acid concentration or % by volume. I need to prove that the tank is in spec on the 20-25% acid by volume and the specific gravity parameter of 1.06-1.10 is wrong or that I am wrong and have been going by the wrong numbers I am getting back from the lab. I hope this makes sense and someone can help.
Rob SmithSr. electropolish/passivation tech - South Bend, Indiana
^- Privately contact this inquirer -^
First of two simultaneous responses -- January 26, 2009
After the first parts go through and/or after the first day the specific gravity changes without regard to the analysis of the acid and the dichromate.
Iron dissolves.
Dichromate which is hex, breaks down to tri, you add more hex to satisfy the analysis, but the specific gravity keeps going up.
Oh, be sure to dump or bleed/feed when the iron reaches 2%/weight.
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Robert H Probert Robert H Probert Technical Services  Garner, North Carolina
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Second of two simultaneous responses -- January 26, 2009
Sorry, you do not run a chromate tank by specific gravity as SG includes the amount of water, acid, dissolved aluminum and any other material like chlorides and sulfates. In short, it is grossly unreliable as the tank ages.
It does not take much of a lab to be able to do your own testing for both chromate and acid. You just need to train a couple of reliable employees.
- Navarre, Florida
January 27, 2009
Hello Rob,
Don't forget that specific gravity can also go up due to iron concentration.
At 2-3% is when I will change my tank out.
- Rincon, Georgia
January 29, 2009
My next question is the ASTM standard just says 20 to 25% by volume. Is that volume by weight or how is that measured? I am obviously no chemist, however I am very inquisitive. I will not stop until I figure this out. If I am using 42 degree Nitric in a 15 gallon keg how do I get a tank that is 350 gallons to be a 20% mixture? Knowing that the 42 baumé nitric is 67.2% weight/volume I am not sure how to dilute it to the target mixture without using the same weight/volume. But is the ASTM spec calling for weight/volume or concentration/volume?
Rob Smith- South Bend, Indiana
January 29, 2009
It means percent by volume of the commercial strength of the acid. Easy. Non chemist needed until you get over 2%/weight of dissolved iron, then you will need outside instrumental analysis.
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Robert H Probert Robert H Probert Technical Services Garner, North Carolina
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February 10, 2009
Mr. Probert,
Just curious, but what kind of bath loading is required to exceed
2%/weight Fe in a passivation bath? We run relatively small quantities of steel, usually 15-5PH, through our bath for months and barely exceeds 20ppm Fe. As a matter of fact, we only monitor Fe because AMS 2700 requires it.
We are not a job shop and only perform captive processing, so that may explain our low concentrations.
Aerospace - Dayton, Ohio
May 4, 2009
If you mean only HNO3 %volume,it is better to analyse HNO3 gms/litre by a simple titration with standard NaOH solution and calculating %Vol/Vol HNO3 by dividing the gms/litre HNO3 by the sp. gravity of the fresh solution of 25% by volume HNO3 solution and the dividing the product by 10 for percentage.Dissolved Fe may interfere with the observation of the end point.Color change may be seen only in supernatant clear solution.
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Subramanian Ramajayam consultant Bangalore, Karnataka, India |
December 15, 2010
Dear Mr. Rob:
Sir, I believe Perry's Handbook is using the "standard" method of expressing percent composition in weight solute/weight solution (w/w) though frequently, we also find it being expressed in volume (v/v) or weight over volume (w/v) such as prescription medication which render the standard (if I may say so) expression ambiguous. Nevertheless, determine the solute's weight (i.e. nitric acid) from the expression, calculate its corresponding volume by dividing it with its density, and divide it with the volume of the solution to get the percent volume expression. See if it tallies with the ASTM standard similar to what Sir Ramajayam is saying.
Moreso, following Sir Watts, you may want to use a total iron test first that is not being affected by chromium levels such as TPTZ (2,4,6-tri(2-pyridyl)-1,3,5-triazine) iron method of Hach company including a portable equipment - even neophytes can do the test with confidence. You have to determine the amount of iron contamination because it could interfere with your hexavalent or total chromium test.
Hope this helps in any way.
Kind regards.
| Barlow Campano galvanizing chemist - Jeddah, KSA |
August 8, 2011
My name is Doug. I'm setting up a Sodium Dichromate passivation tank and have some questions about correct amounts of nitric acid and sodium dichromate. The tank is 400 gal. To make a 20-25% acid solution, I add 100 gal. 42-degree baume nitric acid, but to get to 2.5% sodium dichromate how much do I add? Also any information on where to get titration procedures would be greatly appreciated.
Douglas Laukaitisindustrial finishing - Deer Park New York USA
First of two simultaneous responses -- August 10, 2011
Douglas
Please see letter 39558.
- Colorado Springs, Colorado
Second of two simultaneous responses -- August 10, 2011
2.5 %/wt = 3.28 av oz/gal = 24.6 grams per liter
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Robert H Probert Robert H Probert Technical Services Garner, North Carolina |
August 11, 2011
Doug-
1% sodium dichromate by weight is 1 gram sodium dichromate per 100 grams of solution (or 1 pound per 100 pounds of solution). So, you have to determine the density of the solution if you want to figure out how many grams to add to a liter or how many pounds to add to a gallon. Density of the solution varies depending on the nitric concentration, sodium dichromate concentration, and iron concentration, so you have to check it every time you analyze. That may not have been what the authors of the Mil spec intended for you to have to do, but that is the way it is written.
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Jon Barrows, MSF, EHSSC Chemical Process Engineer Polyfab Plastics & Supply Springfield, Missouri |
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