Friday, July 25, 2014

Assaying galena for silver

In which I create large quantities of a small molecule with unforgettable properties while simultaneously inventing a new Olympic Sport.

I was recently exchanging email with Fergus Milton (a most excellent Brit who teaches a class on copper smelting at Butser Ancient Farm) on the possibility of adding a unit on cupellation to the class. This jogged a memory and I recalled that I had purchased a quantity of galena some time ago. I hadn't had any particular drive to smelt lead, but now that I might get lead and silver out of the effort, I was moved to retrieve it.

The galena was in my Ore Storage Facility, known to the less appreciative as "that big dusty pile of rocks and bags of yet more rocks you keep behind the bar". After a time I had moved enough of the contents of the OSF to identify the correct unlabelled tan canvas sack which contained my galena. I towed it over to the scale and learned that I had almost exactly 7.5 kilos of the stuff.

Now we all know that some galena is argentiferous and some is not. Having been (to date) unsuccessful in coming up with a decent excuse to blow the cash for an X-Ray Fluorescence analyzer, I was seemingly left with two alternatives: Send the sample off to a lab to have it analyzed, or do a smelt & cupel, trusting that I'd get it all right the first time, and find the silver if there was any. Neither appealed.

Then I had a thought. (This happens more often than I like.)

Like any aspiring mineralogist, I have some hydrochloric acid for identifying metals in rocks. Lead chloride is soluble. Silver chloride is not. It should be a simple matter of dissolving the galena in hydrochloric acid and watching for an insoluble precipitate. Silver chloride has some well-defined characteristics, so it should be easy enough to confirm the silver and estimate the silver content of the original lead.

Being a Man Of Action, I grabbed the jug of acid and a handful of galena and headed to the kitchen. I tossed the galena in a glass pie pan and promptly poured some acid over it.

I can hear the chemists in the crowd wincing.

Now the chemistry I had concerned myself with was fairly straightforward:

   PbS + Ag2S + 4HCl = PbCl2(aq) + 2AgCl(s)

But this is not a balanced equation, since it doesn't account for the hydrogen or sulfur on the left hand side. A correct and balanced equation looks like this, with aqueous, solid, and gaseous products:

   PbS + Ag2S + 4HCl = PbCl2(aq) + 2AgCl(s) + 2H2S(g)

Anyone remember hydrogen sulfide? The more enterprising among you may have produced it in stink bombs when you were kids. It has an eye-watering odor of rotting meat crossed with severe flatulence. It's also flammable and quite toxic, but I can personally assure you it's the odor that will speed your pace when clearing the area.

Flash forward in time five minutes. Every window in the house, as well as the front and back doors, are wedged fully open. Every fan in the kitchen, lavatories, and HVAC system is on "high". The pie pan has been transferred to the backyard.

Ever try to run through your living room while squinting and holding your breath? While carrying a pie pan full of corrosive liquid which is actively producing toxic gas? Getting the sliding glass door to the backyard open under these conditions yields a difficulty rating of 6.0. I believe I have a new Olympic Sport in the offing.

Happily hydrogen sulfide is a small and (mostly) hydrophobic molecule, so once it's cleared, it's cleared. It doesn't linger. At that point it's only a problem for the people downwind of you.

The adventure over, I was able to discern that my galena does in fact contain silver. Stay tuned for an account of smelting lead and cupellation of silver.

DANGER: Usually in metallurgy the worst thing that could happen to you is a disfiguring burn. This is different. Every one of the reactants and byproducts of this reaction are toxic to humans. Don't even start this unless you know how to handle and dispose of them safely.
For those qualified to do this properly, the stochiochemistry follows: Galena (lead sulfide) has a molecular weight of 239. Lead chloride has a molecular mass of 278 and a solubility of 10g/L in water. The acid I used is 31.45%, 20° Baume, or 8.5molar hydrochloric acid.
Scaling everything for 0.036 moles, we get the following values: 8.6g of lead sulfide + 4.3cc of 8.5M hydrochloric acid + 995.7cc of distilled water results in 10g of lead chloride dissolved in 1L of water. Excess acid and/or water will assure only the silver precipitates.