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topic 11047

Help with specific gravity testing of electrolyte and replacement of electrolyte


Q. Our firm has been electropolishing stainless steel wire for quite a few years. The process is rather archaic and I am trying to improve the process somewhat.

One such problem that I am trying to solve is that the electrolyte is never tested, it just "loses effectiveness" and the whole lot is thrown out. I have heard mention that testing of specific gravity and metal ion contamination should occur and the electrolyte should be topped up and parts replaced (but not the whole lot).

Can someone please explain specific gravity (in relation to how an electrolyte works) and also some tests that should indicate how effective the electrolyte is and when it needs to be replaced.

We currently use a Phosphoric (81%) and Sulphuric acid bath with the following concentrations: 2.5L phos, 880 mL sulphuric plus 620 mL of water.

The parts that we electropolish are fine wire to small rods between 0.3mm and 5 mm diameter.


Kayte Worrall
- Melbourne, Victoria, Australia


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

David Sugg
- Thorndale, Pennsylvania


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.

Geoffrey Whitelaw
Geoffrey Whitelaw
- Port Melbourne, Australia

Electropolish specific gravity problems

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

Q. Our electropolish tanks fall out of specific gravity range during the summer months probably due to humidity. We try to raise the tank temperatures to burn off the excess water but once we put the tank back in production it falls out in a couple of days. We can't remove much solution and replace it because we have designated iron content levels. Does anyone have any ideas?

Jay Bells
- Denver, Colorado

July 2018

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?


pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
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