Pins in HCl

[Militoy]

I'm trying a limited experiment with HCl and gold-plated brass military pins - treating them kind of like foils (maybe a newbie mistake?). I started a 1 pound batch of mixed pins, varying from .03 to .100 inches in diameter; averaging 1 inch length in a crock pot of 10 molar HCl on Saturday morning. No heat or aeration as of this point - though I'm set up for both. The batch started off bubbling nicely, with lots of fumes condensing as an emerald-green liquid on the inside of the lid, and mostly returning to the brew. I stirred a few times over the weekend, and noticed lots of foils floating in the mix on Sunday, along with some flecks of blue paint that was on some of the pins (from the manufacturer). I also added acid 2 times when I thought the reaction might be slowing a bit. Tonight I checked the mix, and only noticed a few bubbles. Added maybe 100-150 ml more acid, and stirred. During my stirring, I noted that some types of the pins seem totally untouched by the acid. They look unchanged from when I put them in. I'm usually pretty patient with this kind of exercise - but I'm second-guessing myself a bit at this point. I'm guessing that the untouched pins might have a heavier gold plating, and that I may have underestimated the volume of acid I might need for the volume of base metals in the pins. If the reaction hasn't progressed better by tomorrow PM, I'm considering filtering out what I have now, and starting the pins in fresh acid. I will be digging around in my stuff for my PH paper later tonight. I would appreciate any constructive comments or guidance. Thanks!

 

[Shor]

Try adding a fish air bubbler and give it a day or two.
Patience 8)

 

[Barren Realms 007]

Go to Steves web site and read the pdf on copper chloride. And have some patience.

 

[Militoy]

Go to Steves web site and read the pdf on copper chloride. And have some patience.

Thank you for your advice. I briefly read his material on copper chloride last week; and went through some of his fine videos as well. His site is one of the reasons I have been careful to try to keep the acid strength up. The patience isn't a problem - I specifically decided to start out without the bubbler or heat, to slow the process so I wouldn't have to be constantly on top of it. I'm just trying to get a better feel for the volume of copper dissolved vs the volume of acid used up before saturation and precipitation. I will re-visit Steve's site today. Maybe I also need to haul out my chemistry book and calculator.

EDIT 5-3: OK – I looked up the free-machining brass (alloy 360) that most of my pins are made up of. A good average of the alloy will come out around 61% copper; 35.5% zinc; 3.1% lead; and 0.4% iron. I pulled out my calculator – and worked out the relative weight of each material I should have in my soup. Converted to mols, it looks like my total mass started at around 6.92 mols of combined metals (ignoring the weight of the gold). Since each metal combines the same way to form a chloride salt (CuCl2, ZnCl2, PbCl2 and FeCl2) – I should use up 13.84 mols of HCl to put all the metal into solution – or 504.8g of reagent grade HCl. Since I’m using 10.17 molar HCl (32%), it looks like I’ll need 1.36L of acid – plus some extra to make sure the acid doesn’t saturate and start to drop salts before I’m done. This is more than I initially figured – but I believe I have around 1.7L in the mix so far – so I think I should be OK, barring too much loss to fumes or evaporation. I’ll probably top up to around 2L and see what happens.

 

[Militoy]

Try adding a fish air bubbler and give it a day or two.
Patience 8)

Thank you for your suggestion. I'm not concerned with speeding the reaction at this point - just concerned about saturating the acid and precipitating copper chloride.

 

[maxwellclager]

I can remember way back when I was in the military that we could buy gold plated brass at the PX. Since it did not oxidize, it was always in demand. I learned early on that it was not gold but actually so lightly plated in platinum that it retained its brass color.

 

[Militoy]

I can remember way back when I was in the military that we could buy gold plated brass at the PX. Since it did not oxidize, it was always in demand. I learned early on that it was not gold but actually so lightly plated in platinum that it retained its brass color.

I served in the US Army at the end of the VN war, and the only brass I recollect for sale in the PX had to be polished regularly with Brasso. Never heard of either the gold or platinum plated stuff. Of course - we didn't wear brass too often - most of our uniform insignia for daily use were painted black. I did notice recently looking over my old Class A uniform (which I still have) that my marksmanship medals are marked "sterling Davorn". I never even realized that they were silver.

 

[maxwellclager]

I was in from '78 to '84 when I found the white gold brass. Thanks for the info on marksmanship medals. Now I know why they cost me so much to replace. I just thought the PX was ripping me off.