# Rhodium solution, dark plating



## goldandsilver123 (Apr 18, 2017)

Hello,

I'm trying to make a rhodium plating bath, but the layer deposited is darker than the solution that I bought.

The rhodium I started with was already dissolved, a person gave to me because he tried to make this bath several years ago and didn't succeeded. The solution was analysed with AA and by the analysis it was 99,9% Rhodium in relation to metal cations.

The solution was reddish pink, what I assume it was from a pyrossulfate + chloride fusion. I neutralized it to pH 7,5 with potassium hydroxide and additions of hydrogen peroxide, this was boiled for 1 hour, then it was filtered.

To the hydroxide I added enough distilled water and sulfuric acid to make a 2 g/L Rhodium and 9,2 g/L H2SO4.

Do anyone have a clue?

The current draw was the same in both baths and no there was no development of bubbles in the piece. One time I over added the sulfuric and got an darker deposit with bubble development in the cathode. 

Here's some pictures:

Left sample is the one I made, and the right one is the one that I bought.


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## anachronism (Apr 18, 2017)

I really like that clean working environment you've got going on there. 8) 

Jon


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## Lino1406 (Apr 18, 2017)

No bright silver/nickel below Rh?


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## goldandsilver123 (Apr 18, 2017)

Lino1406 said:


> No bright silver/nickel below Rh?



The substrate was treated properly and equally, and yet the one that received the commercial bath looked whiter.

These "keys" are ABS plastic with nickel coating.

The procedure for the plating bath was

Degrease
sulfuric 10%
Basic copper (CuCN)
Acid copper
bright silver (AgCN)
rhodium


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## nickvc (Apr 19, 2017)

If anyone can help it's probably Lou.


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## Lou (Apr 19, 2017)

Looks like chloride is issue to me.

Chloride usually darkens Rh plating baths but destresses coating so that it is less likely to flake. 


People also add stress relievers (i.e. sulfamic, tartaric acid) and brightening agents (i.e. pyridine and other heterocyclic and/or aromatic amines).

Lou


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## goldandsilver123 (Apr 19, 2017)

I made a quick test with silver nitrate dissolved in concentrated sulfuric acid (to not precipitate the Ag2SO4), and the solution turns a bit more cloudy.

I made 3 times the rhodium hydroxide with reagent grade potassium hydroxide and reagent grade hydrogen peroxide. The last time I added the sulfuric acid and filtered in quantitative "slow" filter paper (the pore size is not written, but I believe it's 1-2 um), I tried to use a sintered glass funnel but too much of the hydroxide passed trough, the result was the solution of the picture from the last post. Even with the filtration the solution is cloudy.

Thanks for the info about the brighteners and stress relieve. Right now I want to do a decorative rhodium plating solution, and for what I know this type of plating bath it's just rhodium sulfate and sulfuric acid. Anyway I want to do the best solution possible, but it's difficult to find clear information.

Here's the pictures from the rhodium solution and the silver nitrate test:




I took around 1 mL of the rhodium solution diluted with around 10 mL sulfuric acid 98% and silver nitrate solution. This solution is much more diluted and ir more turbid, that's a conclusive test for AgCl?


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## Lou (Apr 19, 2017)

That's silver chloride.

Yes.


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## Lino1406 (Apr 19, 2017)

Interesting to check use of the Cl-less solution


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## goldandsilver123 (Apr 20, 2017)

Thank you all.


I will boil this solution, then make a filtration, make rhodium hydroxide once again then wash several times with hot distilled water.

After I'm done, I will post the results here.


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## Lou (Apr 20, 2017)

If there is sufficient sulfuric acid, simply boiling will remove Cl- as HCl gas.
I would not so quickly jump to re-make the hydroxide. That is a lot of work. You might consider adding some pyridine first however, a very small amount of that may rectify your situation. Take perhaps a 10 mL aliquot and try.

The color of your initial baths is the telltale indicator for me. If you were to compare the bath to the left (commercial) with the one on the right (yours) on a UV-Vis spectrophotometer, you would see that you have a distinctly different color profile--why?

Chloride!

Your chloride contamination is not insignificant--it is chloride that gives Rh aqua complexes their color, with increasing color as more chloride is tacked on--for example, Na3RhCl6 is very strongly colored red, even dilute solutions (100 ppm level, 2 g/L makes red wine look weak!) in distilled water, which are mostly (H2O) substituted for the chloride, those are very pink. Most of those complexes have very large extinction coefficients and so are easily measured with even a spec20 off ebay (save yourself some AA time!). 

I compliment you on your work though, very good! I see someone here who is one of the most observant of our members!!


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## goldandsilver123 (Jun 7, 2017)

Hello All,

Thank you Lou!

Sorry for the delay, I was making several tests and had to drive 2600 km to get a WDX 

The solution from the last post I boiled to almost syrup without adding anymore sulfuric, and kept that for one hour or more, the colour of the bath did take a slighty less "red" but not as clear as the commercial one and the plating was still darker.

I had to wait to buy the piridine because of stock issues at supplier. I tried it but I think I over added it in the first try, the deposit was yellow in colour, so checked the pH and it was 3 , knowing that I stoichiometrically added sulfuric acid, the rhodium plated "grey" again, the same darker tone as before.


So I decided to try from the start. I bought 999 rhodium sponge, "England Origin" (I think it's JM).

I used the recipe Freechemist gave long time ago, Potassium Pyrosulfate and sodium chloride, the amout dissolved was 80% as he declined.




From this I dissolved everything in distilled water, make the rhodium hydroxide and filtered.





To the filtered rhodium hydroxide I added water and a large amount of sulfuric acid to favor the elimination of Cl- as HCl, around 50 mL for 2 g of rhodium, this solution was heated and almost made a syrup and kept in this situation for 2 hours or more.




After the heating period the solution was neutralized with KOH, as the content of sulfuric was high it took a high amount of KOH to neutralize.

When the stirring stoped there was a layer of "rhodium hydroxide" thar was paller than the other on top of it. The next day there was also crystals on the surface, which I believe is potassium sulfate, so I decanted the liqud, added hot distilled water, agitated and decanted the clear solution, I repeated this 2 more times, then the rhodium hydroxide was filtered and washed several times with hot distilled water on the filter.





The Rh(OH)3 was added to a beaker, then distilled water and a calculated amount of sulfuric acid.




This solid doesn't dissolve for nothing. The pH is 0, it was heated with agitation for hours, kept hot with sonication, and the solution only slightly turned yellow. Rhodium chemistry is new to me but I already dissolve rhodium hydroxide about 6 times already and it's the first time this happens.


I tried add more water, more sulfuric, peroxide, more time with heating and agitation and nothing makes any difference. What happened? Does the Rhodium precipitate as rhodium potassium sulfate? Does the rhodium oxidezed even more (more than 3+)? Does the rhodium reduced from 3+?

I will think of something else, but if I can't solve this, I will do a reduction with magnesium/zinc and start all over again.


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## Lou (Jun 9, 2017)

Perhaps wise to have stopped here.

My advise would be to have use no halide salt at first in the fusion, just pyrosulfate.


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## Lino1406 (Jun 10, 2017)

I think too fusion is the right action, not dissolution, and probably adding some H2SO4 drops without regard to the material's history.


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