Sciencemadness Discussion Board - Best solvent for EPNS?

Best solvent for EPNS?

blogfast25 - 31-8-2010 at 06:35

I bought a set of 6 small and rather useless desert spoons from a junk shop because they are made of Electro Plated Nickel Silver (EPNS). I want to turn then into some nickel salts and some relatively pure nickel metal.

What would be a suitable acid solvent? Ye Olde Holleman doesn't recommend HCl or H2SO4, but 'dilute HNO3' instead. I'd rather avoid using too much of the precious HNO3. I was thinking to get 'rid' of the silver plating with some HNO3, then use a HNO3/HCl mix to attack the bulk of the metal.

What say you?

densest - 31-8-2010 at 10:15

You could always put them in a bath (choose your salts for your convenience) and electrolytically dissolve them. If you use a low voltage at the beginning you could selectively strip the silver, replace the bath, and then dissolve the rest. If you have nitrate salts you can use them because the bath doesn't need to be very acid.

DJF90 - 31-8-2010 at 10:40

However you do it, be careful. Nickel (II) is carcinogenic...

blogfast25 - 31-8-2010 at 11:58

Quote: Originally posted by densest

You could always put them in a bath (choose your salts for your convenience) and electrolytically dissolve them. If you use a low voltage at the beginning you could selectively strip the silver, replace the bath, and then dissolve the rest. If you have nitrate salts you can use them because the bath doesn't need to be very acid.

A bit of a slow boat to China, especially when the shape limits the current densities that can be realistically achieved. There's more than I thought too: it's 2 sets of 6 each. Will weigh it up soon.

ScienceSquirrel - 31-8-2010 at 12:37

Quote: Originally posted by DJF90

However you do it, be careful. Nickel (II) is carcinogenic...

I would be far more worried about the toxicity of the nickel salts than their carcinogenic properties.
Chronic exposure eg working with nickel compounds day in day out may raise your likelihood of developing cancer but not by a lot.
http://www.ncbi.nlm.nih.gov/pubmed/7585584
Really it is low down on the risk list compared with cigarettes, bonfire smoke, char grilled steak, etc.
Nickel is quite safe, wear gloves and a mask or do it in a hood, stopper the flasks with cotton wool to minimise any aerosols during boiling etc, cover your work surface with an absorbent covering eg damp newspaper and then bag it up and dump it after use.
Stick to small quantities!

blogfast25 - 31-8-2010 at 12:48

Quote: Originally posted by ScienceSquirrel

Quote: Originally posted by DJF90

However you do it, be careful. Nickel (II) is carcinogenic...

I have to admit I wasn't aware of many toxicity issues associated with nickel compounds, other than with nickel carbonyl...

ScienceSquirrel - 31-8-2010 at 13:06

Simple nickel salts are not toxic at low levels.
They can cause dermatitis in some individuals on skin exposure but others are unaffected.
If you use reasonable containment and do not gratuitously expose yourself to the stuff you will be fine.
Compared with nitric acid, lead acetate, mercury chloride, etc; they are a walk in the park!

blogfast25 - 31-8-2010 at 13:12

I was a bit surprised not to find much mention of toxicity in John Emsley's between-popsci-and-science book 'Nature's Building Blocks' (recommended). I do recall making some nickel salts as an undergraduate from nickel alloys (from electron tubes, IIRW).

ScienceSquirrel - 31-8-2010 at 13:23

Nickel salts are not very toxic but they are shed chemicals as opposed to kitchen chemicals.
Making Rochelle Salt from cream of tartar and sodium carbonate is kitchen work surface stuff.
The materials are not toxic plus sodium, potassium and citrate ions are commonly found in food, beverages and cleaning materials.
It is really just cooking and cleaning!
Iron, copper and nickel chemistry involve elements that are toxic but not madly so and the shed is good enough.

blogfast25 - 2-9-2010 at 07:30

Well, well. It looks like that for nickel, HNO3 really is the best solvent and that it resembles copper in that respect.

Here's what I did.

Six silver plated nickel spoons (about 41 g) were first exposed to 19 % HNO3 and some heating. Reaction was vigorous. It remained hard to see whether the Ag coating had been fully removed or not due to the reactivity of the Ni.

After some minutes, 200 ml of 22 w% HCl and then 80 ml of H2SO4 95 w% were successively and carefully added. On adding the HCl, a white precipitate (AgCl?) first drops out, then redissolves with more HCl (to AgCl2 (-) ?).

After adding the H2SO4, reaction proceeded vigorously, with much NOx evolving, then died down, then restarted upon heating, then died again. The dark green, turbid liquid was decanted off and set aside.

To the spoons (that looked like they hadn't lost much weight at all!) was added 200 ml HCl 22 w%. On heating quite a bit of reaction was noticed and the solution started turning green slowly, slowly being the operative word here. Eventually that reaction died too. But on adding even small amounts of 19 % HNO3 the reaction started up very vigorously again, with NOx formation and the solution turning dark green. Eventually about 50 ml of 19 % HNO3 was used up and my stockpile of HNO3 finished.

One of the surviving spoons was subjected to a 100 ml of boiling, approx. 30 volume % H2SO4, and no reaction was observed at all!

The obtained solutions will now be filtered and converted to NiCl2 hydrate, via Ni(OH)2.

For the remaining nickel and silver I will have to call on The Wide Eyed People for more HNO3 ('pH down!', as they call it)... I would have tried adding some nitrate but I'm clean out of them too! Chlorate I'm not keen on is these conditions.

I think that like Cu, you can leave Ni to react at RT (or its own reaction heat) overnight without external heating.

Update: It appears that by weighing the remaining metal about 70 - 75 w% of the original 41 g went into solution (despite having used only about 50 ml of 38 w% nitric in total - a combination of HNO3 and HCl should thus also work, saving on the more expensive nitric w/o losing much time).

[Edited on 2-9-2010 by blogfast25]

blogfast25 - 9-9-2010 at 12:47

After filtering and neutralising with NaOH: the Ni(OH)2.n H2O on the filter:

After dissolving in 20 % HCl: an emerald green NiCl2 solution, about 0.5 M:

But crystallising the NiCl2 proved difficult. At high concentration the liquid begins to splatter and bang, even with fresh boiling stones. I decided then to convert the lot to NiCO3, by diluting and adding Na2CO3. After filtering, washing and drying, here is the end product:

NiCO3 is stable and a good precursor for most Ni salts, including NiO.

As a 'by product' I've about 10 - 20 g of acid insoluble stuff which I presume is made up mainly of Ni and AgCl, which will be processed when my HNO3 arrives.