Okay, it would be a worthless exercise for a reloader to determine exactly how much lead, tin, antimony, arsenic, copper, bismuth, and zinc is in his casting alloy.

Granted.

For the amount of alloy I'll ever use in one batch, the assayer's charge for assaying a batch of metal would greatly exceed the value of my hundred or so pounds of metal.

I know.

I'm just curious how it's done. I've done a little searching, and I don't know what the assayer's analytical technique is. Just curious.

Anyone know how it's done?

H. C.

Well, I did some more digging. Unfortunately for anyone who thought they might do this in their garage, you probably need to buy yourself a flame, graphite furnace, or ICP atomic absorption spectrometer. You can check eBay for prices on those.

The old chemical literature has some methods that usually involve dissolving a sample in sulfuric acid, filtering off and weighing the lead sulfate (gives you the percentage of lead). The filtrate is titrated with a solution of potassium permanganate to determine the amount of antimony present. The tin (now tetravalent) is reduced with iron powder, an excess of iodine is added, and the portion of iodine that didn't react with your reduced tin is titrated with sodium thiosulfate. Separately, copper is determined by precipitation with potassium thiocyanate and then determined by potassium permanganate or iodimetric titration. Arsenic is distilled out of the alloy and determined by a separate method.

There are numerous variations on this "wet chemical" method.

The good news is that all these chemicals are cheap, and any most chemical suppliers will ship them to your house. Measuring weights on your powder scale is easy enough. The problems are the nasty fumes you will generate and the nastier solutions of soluble lead, antimony, and arsenic. Yu could probably swallow a bullet and crap it out without much ill effect. Let me know how that goes if you try it. Don't get any of these soluble lead salt solutions on you. Or spill them. Or drink them. They are nasty poisons, and it will be very bad for you.

Oh, and if you want to obey the law and not send toxic waste to a residential sewage treatment facility (which they can trace back to you and for which you will pay tens of thousands of dollars in fines), you need to send your waste solutions to a real waste disposal facility.

Don't ever dump lead solutions down the drain or on the ground. By dissolving lead in acid, you have converted it from a fairly benign and unreactive metal into a salt that is absorbed by living things readily and will cause all sorts of problems. Ask anyone who knows someone who got lead poisoning.

H. C.

(corrected for a typo)

Well done, Henry! ... felix

Great post!

I wonder if one might look into having a local high school or college lab doing the assay? They'd already be set up with the equipment, and for disposing of the toxic salts.

Well, I did a little more digging, and I found a method that may even be suitable for garage chemists. It doesn't avoid the use of strong acids to dissolve the lead based alloy, but it potentially avoids any problems storing or disposing of highly toxic heavy metal salts.

Controlled potential electrolysis is a technique where you electroplate a metal out of solution at a controlled, fixed voltage. Different metals electroplate from solution at different voltages, so you can plate out all the lead, and then all the antimony, and then all the tin in stages by stepping up the voltage appropriately.

You can weigh the thing you're electroplating before electroplating and again after the lead, after the antimony, and after the tin have been deposited. Divide the mass increases at each stage of electroplating by the mass of metal you started with to get the percentages of lead, antimony, and tin.

In principle, since you have exhaustively electroplated all the lead, antimony, and tin from the solution, you now do not have a highly toxic heavy metal salt solution to dispose of.

That's the short version. The devil is found upon close inspection of the details.

Refinements are needed to avoid copper artificially increasing the lead number (generally a small correction on account of the low solubility of copper in lead) and to get accurate percentages for arsenic and zinc.

I will probably read a little more over the next week and in all likelihood abandon this project. My hobbies are shooting and thinking about stuff like this.

H. C.

And if you've followed all that...

It is tempting to propose a reverse of the controlled potential electrolysis method.

In electroplating, the piece getting plated is called the cathode, and the piece that's providing the metal is the anode.

Why not make an anode out of the alloy to be assayed? A simple experiment would be to hang a bullet in a beaker full of salt water and plate the lead, then the antimony, and then the tin onto something else. The same jumps in voltage and weighings before and after each plating step would apply. The wonderful advantage of this approach is that you would never have to dissolve the lead in anything. Just plate it away.

It doesn't work in practice. Electrochemists call the thing that thwarts you "anode mud", If you use a mixed alloy as an anode and one of the metals plates away selectively, the remaining metal gets pores and fissures in it, and pieces of it slough off and fall to the bottom of the container. In our lead/antimony/tin example, as the lead (the predominant portion of the anode) is removed, the anode loses structural integrity. By the time the lead and most of the antimony are gone, there is almost no way to avoid chunks of the remaining material falling off, and when you get to the stage of trying to analyze tin, the tin-containing portion of the anode will have fallen to pieces. The pieces are no longer in contact with the rest of the anode, so they don't move electrochemically. They sit in a pile of mud underneath what's left of the anode.

Maybe, just maybe a conductive platform could be built for the anode. As the bullet falls apart, the anode mud would sit there, and as long as electrical contact between the metal powder mud and the platform was maintained, maybe the metal would electroplate.

The rub here is that the platform would have to be a very unreactive metal. I have no doubt platinum would serve admirably. Just about anything gold plated would probably do the job, but you'd want to be sure there were no pinholes in the gold plating. Oh, and you probably need 24 K plating so the copper doesn't etch out of the plating. I don't know, but I think that rules out common gold plated spoons you might find cheap (maybe an odd spoon not in a set) in a pawn shop or thrift store. In America, gold tends to be 14 K. I guess a Canadain Maple Leaf coin could be used, but that's stupid. Maybe 24 K gold leaf or gold foil. I don't know. If I think of something feasible-sounding, I'll come back and write about it.
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H. C.

"If I think of something feasible-sounding, I'll come back and write about it."

Graphite! A graphite cup anode. Drop about a 100 grain piece of what's to be analyzed in there. Start electolyzing. The electrolyte composition and pH problems have to be tackled. If the pH is wrong, the lead deposits on the anode as PbO2. Maybe that's exactly what you want to happen (weigh it as such). I came up with 100 grains, because if you have a 1% constituent such as tin and a powder scale that weighs to +/- 0.1 grain, that will let you weight the amount of tin (about 1 grain in a 100 grain sample) to +/- 10% relative to the amount there.

I have a potentiostat and am reviewing my college electrochemistry a little. My old textbook shows how to do this experiment using essentially a 6 volt lantern battery and a variable resistor. I still think this is a stupid endeavor and I don't imagine I'll ever have it set up in my garage. Knowing that I could. That's all I'm after.

H. C.