First the test needles.
I have never encountered any made for the platinum group----and I'm not convinced you'd find any that worked to your satisfaction. It's easy enough to distinguish palladium from platinum, which are the two metals you're most likely to encounter. A drop of nitric on palladium will provide enough solution to react with stannous chloride or dimethylglyoxime.
Test needles and dropping bottles, plus a touch stone, used to be available from jewelry supply houses----marketed by Vigor. I doubt you'd find them at a jewelry store. Rio Grande in New Mexico could be a source, but I don't have their catalog to verify that they're still available.
I would highly recommend you not try to assemble your own samples if you're interested in testing karat gold. Between dishonest benchmen that under karat, and marking laws that permitted a variance of a full K, depending on how an item was manufactured, is sure to lead to error in your samples, and the test is difficult, at best, with known samples.
Truth is, once you get past the learning curve, it would be rare to have need for them. Mine sat unused once I got established. That might not be true if you ran a pawn shop, or got involved in buying frequently.
The most important thing you can do is learn to distinguish karat gold from gold plate or gold filled. You can do that with nothing more than a small three square file and a dropping bottle of nitric. Bottom line is you have to go through the learning curve and train your eye to recognize the reactions.
SO2:
I started precipitating with ferrous sulfate, and it works very well. If you have a clue about the amount of gold you intend to recover, very little is left behind, and it readily washes from the gold if you use a proper washing technique. My washes were always done by boiling the gold powder in a fume hood, washing with HCL and tap water, followed by a boiling rinse, followed by a wash with ammonium hydroxide, taken to a boil, decanted, followed by a water rinse, also taken to a boil, decanted, then followed by a second wash with boiling HCL and water, with a final tap water rinse, and finally drying, all in the same beaker, all done in a fume hood, with heat control. You have to be very patient in drying the gold, otherwise you experience steam explosions with gold being lost to spattering. Done slowly, it's not a problem.
You can see contamination being removed by the above process. The final wash is generally void of any color, so it's a good indicator that you've removed contamination. Further, the color of the gold powder is a good indicator of purity. The lighter the color, the higher in purity you'll find the gold to be.
The problem with using ferrous sulfate as opposed to SO2 is that it markedly increases the volumes of solutions, so it requires larger and larger vessel with which you conduct your refining. I used nothing but lab grade containers (Kimax or Pyrex glassware), so I was limited to size, which was a problem as greater amounts of gold were processed. Changing to SO2 to precipitate permitted handling larger volumes because there was no increase in solution volume by its use. You can precipitate up to 18 ounces of gold from a 4 liter beaker, assuming you start out with an iced, concentrated solution. Precipitation is an exothermic reaction, so a beaker full of ice ends up a beaker full of warm or hot solution after precipitation. Cooling is important if you expect to precipitate all the gold.
I found that SO2 would drag down a trace of palladium, but only from concentrated solutions. A second refining, which was my routine, would always eliminate the traces, even if they were not washed from the gold powder on the first refining cycle.
You may encounter problems buying SO2. I think it all depends on industry in your area, although, with the changes due to EPA, that may not be true. Remember, I've been out of the game since '94, and have no clue what it's like to buy chemicals currently.
SO2 is commonly used by many, including the wine making industry, but it can be difficult to obtain in small bottles. You'll come to realize, if you try using it, that it borders on the impossible unless you have a fume hood. It's dreadful stuff to work with otherwise. Solutions retain a large amount and are suffocating to work with after the fact. It is discharged readily by boiling a solution, but, again, that requires a fume hood.
The platinum group will usually hitchhike with the gold solution, but in small amounts, too small to precipitate by conventional means. I recovered the traces on scrap iron, which, of course, eventually requires the use of a furnace for recovery. Sort of a vicious cycle, which, in the end, dictates that you have quite a bit of equipment if you intend to recover all values. It took years for me to build everything I needed.
Harold
