sulphate Analysis of Chrome Plating Solution

A. Hi ASHOKKUMAR. While awaiting a personal response, 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 is "Solution Control"; and Chapter 5 of Dubpernell is "Empirical Tests for Catalyst Concentration and Bath Balance".

Luck & Regards,

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.

Ah, for life in simpler times ... Toxic, carcinogenic, hexavalent chrome whirling around atop the desk in fragile glass test tubes. With a "Be Careful" label the sum of the vendor's safety responsibilities :-)

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. 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 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.

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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
February 2014

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 [on eBay or Amazon], 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.

Thanks for the responses. Incidentally, we don't nickel plate, so sulphate from drag-in isn't a problem. We initiated a testing method where once a week we run 5 tests on each chrome tank (there are 4). We look at the results, and reason which ones are most likely to be correct. If no two are alike, of course we resample and retest. We use this weekly result for any adjustments needed during the week. Usually we have to cut the solution weekly due to formation of trivalent chrome (we use platinum-clad anodes, and of course the anode to cathode ratio is skewed towards rapid trivalent generation, and the platinum doesn't allow conversion back to hex. We use the weekly sulphate analysis result to rebuild the solution, and so far it's working great.

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. I am currently thinking of contacting Kocour to see if they would like to distribute my calculator that I wrote with their test kits. With it you tell the program where you want your solution in ration, put in your baumé and your sulphate reading and it tells you the exact amounts of acids you need.

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.

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 [affil link on Amazon] doesn't either agree or disagree, but stresses that what matters is consistency and proving the correlation between your centrifuge results and the actual sulphate content:

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.

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 whole thread is about the standard techniques to check for the sulphate content -- can you study it and phrase your question in terms of the responses already provided please? Thanks!

Regards,

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Ed. note: We don't recommend or comment on brand names on these pages (why?), but if anyone has recommendations on generic issues like tube volume, etc., they are 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.

A. Hi Shahid. The method is explained and discussed on our thread 32838, 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,