Help with specific gravity testing of electrolyte and replacement of electrolyte

A. While it is possible to gain some measure of control by using specific gravity, I think you should look at using titration of the solution to accurately determine both acids.

For Sulfuric-Phosphoric mixtures, you can use this procedure:

1) Pipette 20 ml of bath solution into a 100 ml volumetric flask. Fill to the mark with DI water.

2) Pipette 10 ml of this diluted solution into a 125 ml Erlenmeyer flask or beaker, and add 20 ml of water.

3) Add 2 drops of methyl orange indicator solution, and titrate with 1.0 N NaOH to the yellow endpoint. Record the amount of NaOH as volume A.

4) Pipette a second 10 ml of the solution from step 1 into another 125 ml Erlenmeyer flask or beaker, and add 20 ml of water.

5) Add 2 drops of phenophthalein indicator solution, and titrate with 1.0 N NaOH until a persistent pink color appears. Record this volume of NaOH as volume B.

6) Calculate volume C = B - A

%(v/v) Sulfuric Acid (66° Be') = (A - C) x 1.36
%(v/v) Phosphoric Acid (85% or 59.2°Be') = C x 3.4

A. If you take the specific gravity with a hydrometer that reads satisfactory in that range e.g., 1.4 -1.7, after make up,then maintain at that SG with additions of the same mix as make up as loss will be from dragout. A drop in SG will indicate excess water from drag-in or absorption. There will be a slight increase due to some metal dissolving.

A. Hi Jay. If air conditioning or de-humidifying isn't a practical answer, you probably can't do much about the absorption. But as Geoff reminds us, just because absorption is worse in summer and puts you over the edge, doesn't mean that drag-in in playing no part of course. Maybe it's practical to slightly reduce the drag-in, maybe with a warm water rinse?

Regards,

A. Hi BV. If you're just gathering general information and have no personal experience yet, you won't find a quicker or better path to learning about electropolishing than the "Electropolishing" chapter by Dean Ward [dec.] in the "Electroplating Engineering Handbook" [link is to product info at Amazon]. In 20 pages it covers all aspects of Electropolishing.

There are "finite life" and "infinite life" electropolishing baths. For the "infinite life", you must periodically decant or otherwise remove the precipitated salt sludge from the bottom of the tank, and I'm sure there is no 'standard amount' to discard (you discard the 'sludgy' part and retain the 'non-sludgy' part and that is always going to be a matter of balance, judgement, and how much sludge you find there.

For the "finite life" type of solution it is true that in some metal finishing operations it is conventional to retain some portion of the old solution to serve as a "seed" when dumping it, but I don't know if that is commonly done with all electropolishing solutions, none, or some.

Hopefully someone with experience in this will chime in on that ... but as we ceaselessly note, readers tell us they're very drawn to helping people with actual situations where there is back-&-forth from which they can learn, but are mostly uninterested in one-directional technology transfer and abstract questions. Good luck.

Regards,

A. Hi again, BV. Can you give me a couple of synonyms for 'charge'? I previously thought I understood your question but, apologies, now I have no idea what you are asking or talking about :-(

Metal finishing solutions can almost always be replenished and 'juiced up' as time & tide consume their active ingredients. You don't dump because you run out of acid or other solution component, you dump because the contaminants, even if only water, have become problematic. Electropolishing solutions build up in metal if they are 'finite life' and must be dumped due to the metal buildup; they must be decanted or filtered or whatever to remove the metal if they are the 'infinite life' type. You are asking for a 'standard value' for a concept that I don't even understand :-)

Sorry, and Regards,

A. Hi once more. Your question is posted and we'll see if it gets any responses. As explained in the reference that I suggested, you must electropolish at a fixed current density (probably about 150 ASF) because if the current density drops, you get etching instead of polishing. So people will generally set their power supply to deliver that much current density. I suppose it's possible that the current density could drop below that despite their effort, and signal the need to replace the solution, but that is not something I have personally seen, so I doubt that it's 'standard'.

Have you had an opportunity to visit an electropolishing shop or maybe to do some small-scale electropolishing in the lab? It has even been done in shotglass-size processing tanks and smaller. It's difficult to suggest realistic ways to improve an operation based on one single parameter if you don't yet truly understand it :-)

Regards,

A. Hello again, BV:

Let me try again. I have many, many years experience electropolishing, parts ranging from fractions of an ounce to multiple ton weight.

Assuming you are using the common 3:1 phosphoric/sulfuric bath.

After a fairly extended period of use, the metals will begin to form sludge on the bottom of the tank. This sludge causes no problem until it becomes deep enough to interfere with use of the tank.

At that time, the sludge is removed, and the volume made up with fresh acid.

To repeat - there is no testing/measurement of aging. The bath never ages out. It is self regulating through the formation of sludge which is removed periodically.

The above process can be continued eternally. There is never any need to replace the liquid portion of the bath. There is no analytical control beyond measuring specific gravity. If the specific gravity is low, heat the bath to drive off the excess water. The specific gravity will never be too high as dissolved metals are removed by settling to the bottom as sludge.

I repeat: the electropolish solution is never replaced. Not ever. As it is used the excess dissolved metals drop out as sludge. The sludge is removed when it becomes deep enough to interfere with processing, and new acid is added to restore volume.

 Hello Jeffrey,
Thanks for the reply, helped to understand the industrial practice to handle the electrolyte.
Regards,

A. Hi cousin Vignesh. Please read the chapter which I suggested to BV and get back to us with your questions. Good luck.

Regards,

A. One last thing to add to this discussion of maintaining an EP bath in good condition is the importance of keeping the cathodes themselves clean. They will, at least in my experience with copper cathodes, build up a layer of sludge on the surface which needs to be removed regularly with a stiff brush (polypropylene or polyethylene bristles will stand up to the solution just fine) to keep the tank functional and predictable.