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Analysis of metals and acids in Electropolishing Baths for stainless steel

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Q. I operate a tightly controlled phosphoric sulphuric stainless steel electropolishing bath. Analysis by titration for sulphuric and phosphoric acid percentages becomes inaccurate as metals content increases. Is there a procedure to provide the true sulphuric and phosphoric acid percentages?

Al Sutton
- Canada


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A. The presence of hexavalent chromium ion will cause some inaccuracy in titration analysis. In our experience, keeping metal content below 3% via frequent, small decants maintains acid balance, and the inaccuracy of the titration is insignificant. The titration for metal content is usually satisfactory if the bath is not operated as a "full-sludging bath".

Ed Bayha
Metal Coating Process Corporation

Charlotte, North Carolina, USA



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Q. Given task of setting up an electropolish with 40% sulfuric V/V and 48% phosphoric V/V: Anyone have a simple titration to determine percentage of each acid by %v/v? Would save me a trip to the library and some method development.

Thanks.

Mike Howcroft
Environmental Safety Engineer/Chemist - Phoenix, Arizona, USA


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A. After diluting your sample, you can titrate it with NaOH and watch for two end-points: the first, v1 - when both protons of sulfuric acid and the first proton of phosphoric acid have been neutralized (around pH 4.5); and the second, v2 - when the second proton of phosphoric acid has been neutralized (around pH 9.5). You can then calculate the molarity of phosphoric acid from v2 - v1 and the molarity of sulfuric acid from [v1 - (v2 - v1)]/2 = v1 - v2/2 after accounting for proportionality through the known volumes and NaOH concentration.

The end-points are well separated and can best be detected potentiometrically or thermometrically. If you need to rely on indicators (e.g. bromocresol green for the first end-point and thymolphthalein for the second), you may have to titrate two samples, one for each end-point (unless somebody can think of a combination of indicators that works without mutual interference at these particular pH values).

Emanuel Cooper
- Yorktown Heights, New York



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Q. Does anyone know of a simple titration for iron concentration in a sulfuric acid/ Phosphoric acid bath for electro polishing operations? I would like to be able to determine the iron level to establish a tank dump schedule.

mukesh gupta
Chemical engineer - chandigarh, UT, INDIA


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A. The iron can be analyzed by AA spectroscopy. No simple titration exists due to multiple cations: Fe+2, Fe+3, Cr+3, Cr+6, Ni+2.

It is usual to maintain sulfuric and phosphoric acid concentrations, and to estimate total dissolved metals by SG (specific gravity).

Ken Vlach
- Goleta, California  

Ken received a special
"Contributor of the Year" award
from finishing.com for his numerous
helpful and well researched responses


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A. In order to titrate iron in the solution you need to reduce it quantitatively to Fe(2+). A simple way to do that is to pass the sulfuric acid solution through a bismuth column. This is simply a small liquid chromatography column filled with 25-50 (or better, 50-100) mesh high purity Bi particles. I have used a 0.8 or 1.0 cm I.D., 20-25 cm long filled column. My experience has been with dilute (1-2 M) sulfuric acid. I don't think the phosphoric acid would interfere, at least thermodynamically speaking. If you decide to use this method, be sure to keep all the bismuth submerged in acid at all times, to rinse it well with acid before the analysis, to pass the sample solution through it at no more than about 20 ml/min (use a little vacuum in the receiving flask if gravity is not enough), and to rinse the column after the sample into the receiving flask with an amount of dilute sulfuric acid equal to at least 3 times its estimated intergranular empty volume. The solution in the receiving flask is immediately titrated with calibrated KMnO4 solution (watch for stable faint pink).

This technique is similar to the better known Jones (Zn/Hg) and silver reductors, but the column is much easier to prepare (pour Bi particles in; rinse with acid) and to maintain, and selectivity is quite high. I know that Ni, Co, Ti and Sn(4+) do not interfere, and I am sure the same is true of a few additional metals. If Cr is present, a K2Cr2O7 (dichromate) titration may have to be used instead of permanganate because permanganate can oxidize Cr(3+). Unlike permanganate, dichromate requires an end-point indicator or a potentiometric method. For a typical (not necessarily optimal) dichromate procedure see "Volumetric Analysis of Metal Finishing Solutions" [link is to info about the book at Amazon] by Andrew McFadyen, Finishing Publications Ltd.--, p. 154.

Emanuel Cooper
- Yorktown Heights, New York



+++++++appended

Q. We currently have an electropolishing bath that is about 5000 gallons in volume. How do we determine the phosphoric and sulfuric acid concentrations and therefore adjust and how critical is the ratio between the two acids relative to productivity?

TY VAN FOSSEN
metal finishing - Lake Orion, Michigan


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A. The ratio of sulphuric acid to phosphoric acid is not particularly critical, but many electropolishing solutions have other additives in them. I therefore suggest you check the details in the process sheet supplied by your solution's supplier. This will tell you all the analyses you will need to do and what the acceptable ranges will be. Successful electropolishing is dependent on how much metal is dissolved in the solution, its viscosity and operating temperature - detail of these parameters will also be in the process sheet.

Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK


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A. An analytical procedure is in the Metal Finishing Guidebook and Directory; hasn't changed in at least 15 years. "Volumetric Analysis of Metal Finishing Solutions" [link is to info about the book at Amazon] by Andrew McFadyen has a very similar procedure under 'Aluminum Electropolishing' (p. 210). Both use colored pH indicators; below is a procedure utilizing a pH meter:
Cool a sample of the well-stirred solution to room temperature.
1. Measure the specific gravity using a narrow-range hydrometer.*
2. Pipette** 2 ml of the solution into a 250 ml beaker and dilute to 100 ml using DI water.
3. Titrate with 1 N NaOH to pH = 4.0 and record as 'A' mls.
4. Continue titrating to pH = 8.5 and record the additional mls as 'B.'

H2SO4, wt. % = (A - B) x 2.452/SG
H3PO4, wt. % = B x 4.900/SG

FYI: 'A' is the titration for sulfuric + 1/3 of the phosphoric, 'B' titrates an additional 1/3 of the phosphoric.

*A 1.600-1.800 hydrometer and 300 ml hydrometer (non-graduated) cylinder are usually suitable for measuring SG.

**Use a TC 'to contain' pipet; it permits flushing out the sticky sample aliquot using DI water.

Making acid additions requires algebra, as the concentrated acids include water and the total solution volume increases. Agitate or stir very, very well before re-sampling. Excess water can be removed by evaporation (heating).

Productivity of an unspecified EP composition at unspecified temperature for electropolishing parts of unspecified geometry and metal using unspecified racking?

Some general comments (opinions):
High phosphoric baths cost more and may have ~infinite life but require closer anode-cathode spacing.
High sulfuric baths cost less and have better throwing power but are perhaps prone to etching at low current densities.
The acid-to-water ratio varies with phosphoric-sulfuric ratio and is perhaps more important. Some of Charles Faust's original phosphoric-sulfuric recipes for electropolishing stainless steels are given in US Patent 2,334,698 (1938).

Search this site for more electropolishing info. Also, several vendors offer electropolishing seminars.
Hope this helps.

Ken Vlach
- Goleta, California  

Ken received a special
"Contributor of the Year" award
from finishing.com for his numerous
helpful and well researched responses



February 27, 2013 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Hello,

We use a hygroscopic electropolishing solution consisting of sulfuric and phosphoric acid. Over the course of electropolishing, metals (Ni, Cr, Fe, Mo) get released into the solution. The Fe content plays a critical role in electropolishing. We have been able to determine the Fe content by Redox titration, but since it is a very tedious method (takes about an hour) we are exploring other techniques...like the XRF.

In order to do the measurements by XRF, we first have to develop a calibration standard. Now since the solution is hygroscopic, how will this effect the readings? (the specific gravity of the solution is sometimes manipulated by the additon of more solution or DI water, or a combination of both). If XRF is not the way to go, is there a better technique to determine metal content?

Thanks,

Aarthi Sundaram
- New York, USA
  ^- Privately contact this inquirer -^

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