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topic 44293

Titrimetric procedure for trivalent and hexavalent chromate analysis


Q. I would like to know if there is a titrimetric procedure of analysis in determining hexavalent and trivalent chromates. Can you suggest where I can get them?

L.G. Pinon
Researcher - Philippines


A. Most books on Electroplating Analysis have the method, e.g., Langford's "Analysis of Electroplating and Related Solutions", Electroplating Engineering Handbook, etc.
Hexavalent is by iodometric titration with thiosulphate.
Trivalent is by oxidizing the trivalent with peroxide in alkaline conditions,boiling to destroy excess peroxide and then acidifying and doing the iodometric titration above and
the trivalent is determined by difference.

Geoffrey Whitelaw
Geoffrey Whitelaw
- Port Melbourne, Australia

October 12, 2009 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Is there any simple method that can accurately determine the concentration of Cr6+ and Cr3+ in plating solution? Will Ion-chromatography do the job? Thanks for your help in advance.

Tommy Yuen
Plating engineer - China

September 13, 2011 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Hello, We are doing hard chrome plating (among other platings), well we are having some confusion in the determination of trivalent chromium, we checked some written procedures, and we are confused about how they consider this trivalent chromium, is Cr3+ or Cr2O3)?? In this bath we are controlling ranges of chromic acid,ratio chromic acid sulfate, trivalent chrome, Iron, Temperature.
Well I hope you could help me; thank you very much.

Alan Tacuri
Engineer - Peru

December 30, 2014

Q. My name is Peter Totaro and I am a chemist for a company in Paterson, NJ. I'm also in my last year of my Masters in Chem Engineering. I have a question for you. I would like to know if there is a way to calculate trivalent chromium in a sodium dichromate/nitric acid passivation bath. I need to prove out a density calculation by hand and I'm having trouble determining the trivalent chromium. Is there anyway this is possible?

Thank you,

Peter Totaro
- Paterson, New Jersey

January 7, 2015

A. You can determine total chrome and hex chrome. The difference is tri chrome.

James Totter
James Totter, CEF
- Tallahassee, Florida

January 24, 2015

A. We analyze our hexavalent chromate solutions for Metal, Sulfate, and Fluoride.

Metal analysis has Na2S2O3 as the titrator, Sulfate analysis uses Kocour A & B w/centrifugation, and Fluoride slopes are determined by TISAB III, 20% C2H3NaO2 and 1000 ppm Fluoride standard.

The results determine any additions or dilutions needed.

george arnsberg
OEM decorative chrome plating - milwaukee, Wisconsin usa

Solution Analysis of Immersion bath in Chromate Conversion Coating Process

July 25, 2018 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. My situation:

We are a Aerospace Component manufacturing shop. We have recently introduced a chromate conversion coating process in house. Our customer requirement states that we have to perform solution analysis of the Immersion bath periodically. Is there a standard laboratory process to do that. I have found couple of spectrometers from Hach. Are there any other gages or controls that I can setup to monitor the concentration of the Chromium bath? Any Sampling Procedure?

Thanks in Advance

Sid Achyuta
Engineer - Traverse City, Michigan, USA

July 25, 2018

A. Hello Mr. Achyuta,
Please give the manufacturer and exact chemical name that you are using. Usually, you can find control instructions on their tech data sheet, and with some practice and a couple tricks, get a good result.
The lab analysis for a RoHS-compliant conversion bath is more difficult and requires more equipment, but for traditional hexavalent chromium products, a simple oxidation-reduction titration will give you good data. Let me know, and I'll get back to you with the best information I have!

Rachel Mackintosh
Plating Solutions Control Specialist / Industrial Metals Waste Treatment - Brattleboro, Vermont

July 31, 2018

Q. Thanks for the reply Rachel.

The product that we are intending to use is Bonderite M-CR 600RTU (Henkel Product).

Also some follow-up questions

1.) Since this is an immersion process (not plating just chromate conversion) how can I monitor the contamination in the chromate bath?

2.) Is there a documented procedure for that?

3.) Will a total chrome test on the free chromic acid do the job?

Thanks in advance for considering.

Sid Achyuta [returning]
- Traverse City, Michigan

July 31, 2018

Hi there, I see that the Tech Sheet DOES have a solution analysis section, but it appears to require ready-mixed reagents that are not identified as to composition or concentration:
That being said, you should be able to set up a standard calibration curve and run it through Excel and get your multipliers. It will take extra precision and care setting it up, but give you valid results, and you only need to generate the curve once. You'll want to document the heck outta the process of generating your curve.
This is how I would approach this, if I didn't want to buy their pre-mixed stuff:
-Use only pure deionized water when making your standards and dilutions
-Use a calibrated analytical balance that weighs to 3 digits
-Use class A volumetric flasks for your dilutions
-Test your pipets using DI water and your balance... and write down the results!
-DOCUMENT EVERY STEP OF THIS PROCESS because an auditor will ask where you came up with your multipliers!
Set up your standards as a blank (just the DI water you are using), a standard that is about half of your operating concentration, a standard that is at or near your operating concentration, and one that is twice your concentration. Don't be tempted to do serial dilutions (preparing a standard from a more concentrated standard), as that will just pass any imprecisions along! Also, it's a great idea to make each standard in duplicates so that if you run two of the same, and they don't agree, you know that you had an error.
Okay. This is a pretty basic oxidation-reduction titration. I've seen it done with permanganate, but I hate that stuff, and prefer the iodometric method.
You need:
Sodium thiosulfate 0.1N
Sulfuric acid 50% (this does not have to be exact, it just acts as a conditioning agent for the sample, but it DOES have to be pure)
Starch solution in water. 0.5% - 1% is fine. Again, does not have to be exact. It will often be preserved with salicylic acid, don't worry about that, as it does not affect the outcome
Potassium iodide solid
a 5mL pipet that you have proven to be accurate

Load up a 50mL buret with Sodium thiosulfate and zero it.
Set up a stir plate. You really need one; the iodine goes in as a solid and takes a little while to dissolve, and also the endpoint is too twitchy to hit while swirling and dripping and watching all at once.
Using the pipet, put 5mL of Sulfuric acid and 10mL of your standard or sample in a 250mL beaker. Add about 50mL DI water. Doesn't have to be exact, but the stir bar needs to spin freely and you want enough liquid to dissolve the KI.
Add about 2g Potassium iodide solid. Does not need to be exact. The solution will turn very dark brown and temporarily cloudy. When the cloudiness is dissipated and the solid chunks are gone, you can start titrating.
Gradually add Sodium thiosulfate out of your buret. The solution will start to lighten. Don't take TOO long to do this, as the reaction is sensitive to atmospheric oxygen.
The solution will eventually reach a pale straw color, possibly with a greenish hue. Stop titrating and quickly add about 2mL starch indicator. The solution will turn inky blue-black. NOTE: IF THE SOLUTION DOES NOT TURN BLACK, YOU WENT TOO FAR. START OVER, AND ADD THE INDICATOR A LITTLE EARLIER. Start titrating again but this time more slowly, a drop at a time. Not TOO slow, but you now need to watch like a hawk because the endpoint comes up fast. There will be a point where the addition of a single drop will turn the solution very suddenly clear, with a very pale color that can range from straw to blue. But CLEAR.
Write down your mL titrated. If you needed more than 50mL, stop titrating at exactly 50.0 and re-zero, and just add the two values.
Repeat with all standards and a blank, and use this to set up your curve.
If you have multiple technicians, test them all to ensure that their technique gives the same results- this titration does have a bit of an art to it. Adding the starch too early and also titration speed can somewhat skew the results. Also, you may notice that after the solution clears, if you leave it on the stir plate it will cloud up and blacken again. That is the atmospheric interference. Don't second-guess your initial result!

Okay so for the rest of your questions:
-Just testing for Chromium isn't going to give you as accurate a picture of bath health as the titration described above.
-As you noted, contamination DOES play into it, and things you might consider testing for are Sulfate, dissolved Aluminum, and Silicon. Now, that being said, the formulation as-bought will have baseline levels that you should establish by sending your as-mixed-in-tank level standard out for testing at a lab and then periodically (depending on throughput) sending out bath samples to make sure nothing has spiked. Conversely, substances that you may NOT be testing for, added to the material as activators, can be depleted, and a bath that appears to be 'healthy' may not behave as expected. This is a conversation to have with your sales rep who can tell you what other users test for. I use Iridite 14-2, and I test for Sulfate buildup. Your mileage may vary ;)
Sulfate is typically run by EPA 300 (ion chromatography) and metals are run by EPA 200.7 (plasma spectrophotometry). Save yourself a hundred thousand bucks and a Volkswagen sized chunk of floor space, and outsource that mess!
Once you have your analysis method down, you also will want to set a schedule for testing. You can set your own schedule, but you have to stick to it, and may be asked to justify your interval to an auditor. If you find your tank goes out of spec between tests, you have to test more often. Auditors will torch you on that!
Good luck- if you have any issues, come on back :)

Rachel Mackintosh
Plating Solutions Control Specialist / Industrial Metals Waste Treatment - Brattleboro, Vermont

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