# Platinum (II) nitrate - solubility/stability/reactivity



## orvi (Dec 31, 2022)

Hi all skilled PGM masters 
I am planing one process for later tests, and I want to make sure I have collected all possible information.
In one JMatthey article, I found rather interesting figure about solubility of Pt(II) nitrate - only 0,014g/L at pH=1 (pdf attached below). This is in strike contrast with platinum (IV) nitrate, of which commercially available solutions of 15 or more wt.% are commonly sold. I will come to this later in the text.

I intended to explore process like this (based on that information is true):
1. Take solution of PGMs - namely PtPdRh - as chloroacids, prepared by dissolving PPGM cement with HCL/H2O2 to obviate tedious de-noxing.

2. Basify this solution slowly with solution of NaOH or Na2CO3 to the point all Pd and Rh will drop (in theory pH = 6,3, but we will see how far we need to go). My point here isn´t the true separation of PGMs, but rather formation of their hydrated oxides. Platinum will likely somewhat stay in solution, but I am prepared that some significant part would be dragged down to the precipitate. I do not plan to oxidize the solution as with eg bromate hydrolysis.

3. Coagulate the hydroxide cake by conventional means (boiling, addition of some filtering-aid medium like microcrystalline cellulose etc.) and filtration. Washing till there would be no Cl- in the washes.

4. Obtained hydroxide cake would then be redissolved in nitric acid - to switch from chloride to nitrate media. Rh and Pd nitrates are very nicely soluble in water, and Pt (IV) nitrate also. I will see if and how it will go + how much free acid would be necessary to use to redissolve the hydrated oxohydroxides - and also, if they do redissolve.

5. If the solubility data about Pt(II) nitrate given in the JM article are true, I would then proceed by addition of reducing agent (one Chinese patent use oxalic acid, maybe hydroxylamine or hydrazine careful addition will do similarly well, or ascorbic acid will do (ASA))... Nevertheless, by doing this, in theory, platinum should be reduced to Pt(II) and precipitate out.

6. According to one article about use of ascorbic acid for palladium precipitation from nitrate media (pdf attached below), reduction of Pd Rh nitrate solutions with ascorbic acid at low concentrations of ASA in 0,5M HNO3 has marked selectivity towards Pd precipitation - again, if true, with very slow addition of ascorbic acid solution, I should theoretically be able to reduce palladium, with Rh staying in solution. Redox potentials are +0,55V for Pd and 0,34V for Rh (ASA reduction) - not too much, but they claim there was marked separation upon reduction. That could lead to separation of Rh mother liquor.

So there are two major things that I want to check upon with this experiment - first the possibility to reductively precipitate platinum out of the solution as Pt(NO)2 and second, if the ascorbic acid has selective reduction ability for paladium in PdRh nitrate solutions. Anyway, with platinum nitrate also in solution, I think platinum would react as quickly if not quicker as palladium with ASA - maybe simultaneously precipitating PtPd out of solution.

What are your thoughts on this ? 
Thanks for your input and Happy New Year


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## orvi (Dec 31, 2022)

Oh yeah, the pdf-s


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## Platz (Dec 31, 2022)

Hi, I like the idea and thought process but doubt it would work. The solubility data for Pt ii nitrate is in pure water, addition of nitric acid and trace chloride will render it far more soluble. Washing a hydroxide cake free from chloride is never normally done for a few reasons it is terrible to filter, full of fines which can become colloidal on washing creating yield losses. Having traces of bromate and chloride in it mean it will dissolve easily in HCl and you dont have to generate tons of washings. If done well with bromate and bicaronate you can effectivley seprate Pt from Ir Rh and Ru, but depending on the concentration of Pd and pH reached some Pd will remain with the Pt about 8 gL at pH 6. Its pracically impossible to use sodium hydroxide as its too alkaline.

Selective reduction would be a trade off between kinetics and thermodynamics I think you would need to boil the solution to preciptate Pt and Rh, Pd would drop at room temp. 
I would personally master the bromate hydrolyis process before trying anything new as it would provide a baseline for future experiments.


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## orvi (Dec 31, 2022)

Platz said:


> Hi, I like the idea and thought process but doubt it would work. The solubility data for Pt ii nitrate is in pure water, addition of nitric acid and trace chloride will render it far more soluble. Washing a hydroxide cake free from chloride is never normally done for a few reasons it is terrible to filter, full of fines which can become colloidal on washing creating yield losses. Having traces of bromate and chloride in it mean it will dissolve easily in HCl and you dont have to generate tons of washings. If done well with bromate and bicaronate you can effectivley seprate Pt from Ir Rh and Ru, but depending on the concentration of Pd and pH reached some Pd will remain with the Pt about 8 gL at pH 6. Its pracically impossible to use sodium hydroxide as its too alkaline.
> 
> Selective reduction would be a trade off between kinetics and thermodynamics I think you would need to boil the solution to preciptate Pt and Rh, Pd would drop at room temp.
> I would personally master the bromate hydrolyis process before trying anything new as it would provide a baseline for future experiments.


I have experience with Pd oxohydroxide precipitate from basifying Pd nitrate solutions with NaOH. With care, good filtering area and additive like microcrystalline cellulose or Celite, filtration time can be cut down 20x the original duration (tested). Altough, I do not know how the precipitates of combined PtPdRh would behave. You are right about retention of chlorides in the cake, as the precipitates do not need to be exactly fully hydrolysed to precipitate. That is one thing I didn´t considered.

_To explain further, I do not refine PGMs to their purest forms. If it somewhat happens, good for me. But my objective is to enrich Rh into the fraction with content above 15% to make it easily sellable - and avoiding to loss of Rh into outcoming fractions of Pd and Pt. I am usually starting at 1-2% Rh. _

Other possibility would be to avoid the HCL/H2O2 dissolution completely, and dissolve what can be dissolved of the cement with HNO3. I had troubles with PtPdRh powders to dissolve in the nitric in the past (not full dissolution of Pd), but PdRh cement powder (from Zn cementation) with about 2% Rh content dissolved without a trace in regular nitric. That would be a nice bypass to obtain nitrate solution to start with.

I used ascorbic acid several times to precipitate gold and palladium, but I never implemented this procedure on more complicated systems. First to happen would probably be reduction of Pt(IV) to Pt(II), probably with Pd being reduced simultaneously in this manner. This is probably the thing I would experiment with firstly - to see what will happen with Rh in this system, if it will also reduce alongside Pd in significant/insignificant rate.

Regarding of model solution, Pd would be easy to make into the nitrate. Do you think it would be possible to prepare small quantities of Pt(NO3)4 and Rh(NO3)3 by addition of AgNO3 into the corresponding neutral solutions of Na2PtCl6 and Na3RhCl6 respectively ? In theory, AgCl should drop as solid, leaving corresponding PGM nitrates, alongside with some NaNO3 in solution.

Also, do you have experience with precipitation of Pd from chloride solutions ? Does it work, or the potential is just too shifted due to complexation, that ascorbic is rendered unreactive ?

I really appreciate your input and carefully implement your advice in my future experiments  Thank you.


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## Platz (Tuesday at 4:27 PM)

Yes I can see the trouble with trying to upgrade the Rh and not spreading it thinly in the platinum and palladium.

Regarding adding AgNO3 into chloride solutions of platinum silver-chloroplatinate insoluble salt may form which will look like orange crystals, silver nitrate will lead to a the cream precipitate of silver chloride as you mentioned, since the solubility of silver chloride is linked to the chloride concentration, it may take a lot of silver nitrate to remove the chloride but theoretically would work.

Personally I would burn the material at as high temperature you can to oxidise as much of the Rh, then try to dissolve the palladium in parting nitric acid ~33-49% and repciriate using you usual method. Rh should not dissolve, fingers crossed some of the palladium will oxidize so wont dissolve and remain with the Rh and Pt. Playing with the chloride salts of palladium and platinum is not the best if there is an alternative.


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