Analyze Chrome Plating Bath; Determine sulphates

A. Hi Marissa,

Angela's posting is very old. I doubt that she monitors the site or that we can reach her -- although we'll try.

Dave Wichern says you neither need to ignite the precipitate, not let it boil over.

Concentrated acetic acid offers hazards, and ethanol is flammable, so I think you'll need to establish a simple safety protocol. Unfortunately I'm not a lab tech, let alone a chemist, and I don't have the D. Gardner Foulke book that Angela relied on; still, I'd hazard a guess that the procedure requires the use of reagent-grade chemicals not full strength industrial supplies.

Luck & Regards,


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

Centrifuges

on eBay or

Amazon

(affil links)

A. Kenneth, your best bet for a centrifugal unit is Kocour Co. in Chicago, IL. They have several types of units available.

A. Kocour used to sell an open hand crank unit --somewhat dangerous-- and an enclosed unit with the chemicals and centrifuge tubes.

The only stand alone unit that I am aware of is ion chromatography. This is a bit expensive.

Ahh, simpler times ... fragile glass tubes full of toxic, carcinogenic, hexavalent chrome whirling at hundreds of RPM atop the desk with probably a "Be Careful" label  🙂

Regards,

A. I think the RPM for the centrifuge should be between 600 and 900 for 3 minutes if using the manually operated one. Kocour has a nice electric one that sure saves your arm if you have more than one or two samples to run.

A. The only reference I found was in an old British lab book, "Electroplating Laboratory Manual", by Armet, 1965.

They say 1000 RPM for 2 minutes.

"Chromium Plating"
by Weiner & Walmsley

on AbeBooks
(rarely)

or eBay
(sometimes)

or Amazon
(sometimes)

(affil links)

A. Thanks Chris. Doing some scouting myself, I see that Weiner & Walmsley ⇨
agrees with that 1000 RPM for 2 minutes .

Dubpernell's Electrodeposition of Chromium from Chromic Acid Solutions ⇦[this on Amazon affil links] doesn't agree or disagree, but stresses that what actually matters is consistency and proving the correlation between your centrifuge results and the actual sulphate content as James Watts, Jeffrey Holmes, Jon Barrows and others keep trying to tell us:

Regards,

A. In the dark ages, when I worked in the Udylite lab in L.A., we used a hand-cranked centrifuge made by Kocour, I believe. We kept a bottle of a standard chrome bath with a known amount of H2SO4 to test our techniques.

Ed. note: That's sounds like an excellent tip, Chris!

A. If you use the Kocour method and want very accurate results, you will want to calibrate the tubes yourself against a prepared 100:1 chromic/sulphate standard. You will find that you will need to apply your own correction factor to most tubes. Repeat the calibration every three months on each tube. This will account for interior scratches that tend to occur with use and cleaning.

A. George,

I hope you have a excellent supplier for your hard chrome solution. The people here at this forum can't not give you the right answer for your hard chrome supplier:). But I like to get a analysis of this, trivalent chrome, sulfuric acid ratio to chrome, iron, copper, and if you use a SRHS Chrome the content's of fluoric silicate.

Regards,

A. Dear GEORGE ABRAHAM,

You must weekly not monthly make a chemical analysis for your tank and add the chemical compounds if the tank needs that . In chemical analysis you must check chromic acid content and sulfuric acid content and the ratio between chromic acid and sulfuric acid . About the method of chemical analysis you can ask your chemical suppler to give you the data sheet . Also you can daily check the (Be) baumé (it is hygrometer scale for density) for the solution in the tank and compare it to the standard point.

A. Dear George,

You must check chromic and sulfuric acid content, baumé, trivalent chrome and most common impurities (iron, copper, zinc,etc.).

Regards

A. Contact your local sewer authority. Usually there is an exemption which allows a few gallons per day of laboratory waste to be disposed directly to sewer without treatment or reporting.

A. It is safe to combine all the waste from the chromic acid titration, sulphate test and trivalent test, just chuck them in a old, properly labeled 25L container.

I can't help but wonder what the caustic test is for on a ACIDIC chrome plating solution? it must be to test your cleaners. this one should be disposed of in a separate container or you will probably end up with a drum full of nasty sludge.

A. I can easily see where the ratio could cause problems with plating control of a gun bore, especially at the top.

My personal feeling is that the Kocour method is very repeatable and accurate if and only if your spin cycle is exactly the same RPM and length of time.

The sulphate ion is a catalyst and is not consumed during the process and the only loss is dragout. Therefore, if you are having a problem in a 10 hour cycle it has to be due to the change in the chrome concentration, which would tell me that you do not have enough replenishment of the plating solution in the gun bore.

The sulphate can be analyzed by a gravimetric analysis method, but it is long and complicated. So bad, that virtually no one will do it. Look at any college level chemistry book for the method. Also, the Metal Finishing Guidebook has the identical method in the analysis section.

I would make up a lab solution of 100:1 plating solution from Reagent grade material, both the chromic acid and the sulfuric acid, and analyze it several times by the Kocour method and then apply a "correction factor", if required, to your analyses of the tank solution. Assuming that you have highly repeatable results, which you should have.

That was a great response, James. Thanks!

Unfortunately the MFG doesn't include that info anymore, but it can be found in olde editions --

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

A. Someone may suggest an alternative method for sulphate analysis, however my experience with the Kocour method is that it is quite reliable. Make sure you are doing it by an exactly repeatable method -- same centrifuge time, etc.

Since sulphate is not consumed during plating, and since you can easily control chromic acid within 10%, why does the ratio get away from your range? If you maintain constant chromic acid concentration, the ratio should change only very slowly as solution is lost to fume scrubber or drag out.

Is something else changing during plating? It is easy to imagine temperature and chemistry changes within the bore unless provision is made for constant circulation of the solution in the bore.

A. I cannot quote the method from memory. It involves mixing the bath sample, acetic acid ⇦ this on eBay or Amazon [affil links] , ethanol, and water, boiling, then adding a good excess of barium chloride solution to drop the sulphate as BaSO4. The resulting mixture is then filtered through a Gooch crucible of known weight, the precipitate rinsed, dried, then weighed.

I once attempted to develop a method for doing this analysis via turbidimetry, a method used for sulphate in water. I had NO LUCK AT ALL. I messed with it for over a year, off and on.

An important source of sulphate in some shops is drag in from the nickel bath that precedes the chrome bath. Good rinsing here is essential.

A. Sir:
In Hot Dip Galvanizing the flux solution becomes contaminated with sulphate (in plants using sulfuric acid) and a volumetric titration using standard barium chloride is used to determine sulphate. The test takes 5 minutes. It determines sulphate as 0.1%, 0.2%, 0.3% etc. These percents are changed to grams/liter by multiplying by 11.4. This method can be modified by changing the sample size and/or the standard barium chloride solution to attain the required accuracy. A centrifuge is required, which is low cost on eBay .
Regards,

A. Working for Boeing can be difficult at times, especially with some inspectors.
Step 1 is to ask Boeing for a copy of their procedures, including analysis for chrome plating.
If it were me, I would use their procedure and the Kocour method on split samples and track their agreement. Eventually, you will have enough data to ask for a variance in the procedure.
The gravimetric method is a time consuming pain in the rear method that is only minorly better than the Kocour method after you have run it on a bunch of reagent grade standards to come up with any fudge factors required.

A. Just to add to James' post, we carry out the sulphate analysis by the gravimetric method and it is a right royal pain in the posterior. Add to that that it is a multiple stage process, with each stage open to inaccuracies ... by the time you get to weigh your sample it is no more accurate than the Kocour method, in fact if you are carrying out analysis on several baths then I bet you will end up trying to find shortcuts and introduce new inaccuracies.

Follow James' advice, run concurrent samples by both methods, build up a database of results and offer these as objective evidence of satisfactory analysis.

A. I have used the Kocour test myself for many years. The key is making sure you add your B solution slowly and that your math in processing your data is correct.

A. You may want to get the book Colorimetric Analysis of Metal Finishing Solutions ..." ⇦ on Amazon, AbeBooks [affil links] by Aubrey Knowles that will describe various methods. If not, you can try a simple direct method with hex chrome at 370nm and tri chrome at 640 nm. Dissolved iron will likely interfere with the hex chrome measurement, but it may be accurate enough for your purposes.

A. The Kocour test is easy and very accurate. Repeatable if you follow the steps accurately. I personally have used it 16 years off and on as they had some others testing the baths over the years. Our company has used it for 20 something years and our tanks stay optimum.

I put the entire testing process on my site, here is the link. Hope it helps:
rodneystephens316.webs.com ⇩

Ed. update: Sorry readers, that link is broken :-(

We ask folks to please put stuff directly here if possible -- we've got all the room in the world and have been a permanent reference site since 1989, whereas most links break very quickly, making them hardly worth trying to maintain across the decades and our quarter million postings. If you do provide a link, please give title & author or other info so we have a fighting chance at fixing broken links.
We thank Rodney for his input! ... but we heard the librarian whisper that he's on her "naughty" list for ripping out and swallowing a page from this reference work of ours :-)

A. Hi James. George Brackett seems to have implied that if you do nickel plating the sulphate will run high in the pre-dip. Do you do nickel plating before the chrome plating or not?

This entire thread is about the standard techniques to check for the sulphate content -- can you review it and phrase your question in terms of the responses already provided please, so it doesn't keep getting longer & longer running in circles? Thanks!

Regards,

Ed. update: That link is broken. We try to avoid brand names on these pages (why?), but if anyone has recommendations on generic issues like tube volume, etc., they are very welcome of course.

A. The centrifuge you've posted the link to will not work for that analysis: it's a medical centrifuge and only holds vials that hold 15 mL.
When I formerly worked in a lab doing metal finishing analyses, we used a simpler centrifuge that held Kokour tubes that are specifically designed for the sulphate analysis. for instance, check out this link:
www.kocour.net/productDetail.asp_Q_catID_E_91_A_subCatID_E_92_A_productID_E_92 ⇩
These tubes are glass, hold 50 mL, and have a completely different shape that 'normal' centrifuge tubes.
The sulphate analysis requires 20 mL of sample, 5 mL of 50% HCl, 5mL of 30% BaCl2, so the 50 mL is optimal.

Ed. update 2024: That link is also broken

A. Hi Shahid. The method is explained and discussed here , and at
https://www.hettweb.com/sulphate.html ⇩
and
http://www.plating.com/anapro.htm ⇩

If you tell us the model number of the centrifuge maybe someone can find a copy of the manual. Good luck.

Regards,

Ed. update 2024: Yikes, both of these links broke too, making 7 out of 7
With almost all plating books out of print with no replacements forthcoming except in Chines, and nothing but broken links everywhere, is finishing.com the only English language site in the world which still values reference materials?  🙂

A. Hi ASHOKKUMAR. You might want to review both this thread and Geoff Whitelaw's article in our on-line library "Standard Chrome Bath Control".

Chapter 10.1 in Weiner & Walmsley [on eBay, Amazon, or AbeBooks affil links] is "Solution Control"; and Chapter 5 of Dubpernell is "Empirical Tests for Catalyst Concentration and Bath Balance".

Luck & Regards,

A. Hi Anil. We appended your inquiry to this long thread explaining the methods of checking concentrations.
Current density is more difficult to answer but never less that 1 Amp per square inch, usually 50 to 100% more, although often higher.

How to "maintain smooth working of process" is the subject of many large books and thousands of Q&A's on this site, so not something that can be fully dispatched ... but please give the full situation and an example of a single problem and readers will probably be able to tell you how to avoid it :-)

Luck & Regards,