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topic 11047

Help with specific gravity testing of electrolyte and replacement of electrolyte


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


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


If you take the specific gravity with a hydrometer that reads satisfactorally 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 dragin or absorption. There will be a slight increase due to some metal dissolving.

Geoffrey Whitelaw
Geoffrey Whitelaw
- Port Melbourne, Australia

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