I just guessed that you ended up with about 85% of the 91.5 grams being silver with the remaining 15% being copper and zinc. This isn't rocket science and you'll never be able to estimate the nitric perfectly. However, it's not necessary to do so. What you're trying to do is dissolve ALL of the non-gold metals without using a lot of extra nitric. Everybody has their own way to do this. For a small amount, here's my way. It always works and it is fast. It will take about 20 to 30 minutes to totally dissolve the silver and base metals.
(1) Estimate how much nitric you'll need and measure this out.
(2) Put the 122 grams of inquarted, shotted gold alloy in an adequate sized beaker. In this case, you'll end up with about 300ml of solution, so use at least a one liter beaker (a Pyrex coffee pot will work). Place the beaker in a catch dish that will take the heat. The only ones I have found that won't break are the Pyroceram or Visions dishes made by Corning Ware.
(3) Under the fume hood, cover the metal with an amount of distilled water equal to the estimated nitric. In this case, about 150 ml. Place the CW dish on a hotplate, set to about medium-low, and heat the water until you see a little steam.
(4) Add about 1/6 - 1/4 of the nitric. You should see a reaction almost immediately. When the reaction dies down, give it a gentle stir and add another portion of nitric. Repeat this until a small addition of nitric produces no reaction.
Notes:
(1) Towards the end, raise the temperature a bit to drive the reaction to completion, but never allow the solution to boil.
(2) Towards the end, use smaller additions of nitric, in order to help prevent using an excess.
(3) The estimated nitric you first measured out may be too much or too little. If you have a little nitric left over after a small addition produces no reaction, stop adding. If you use all the nitric, add a little extra and repeat until a small addition produces no reaction.
(4) Especially towards the end, after making a small addition, it may take a little time (often, a minute or two or three) before the reaction occurs. Give it time to react.
(5) When everything is dissolved that's going to dissolve, turn off the heat and add about 50-75 ml of extra distilled water. This compensates for any evaporated water and will prevent any crystallization when cooled. A little extra water won't hurt.
(6) As Oz said, you can do this in 2 stages. In this case, I would only add a total of about 90% of the estimated nitric. After all the nitric has reacted, let it cool a bit, carefully pour off the solution into another container, without pouring off any of the gold particles. Then add a little distilled water, heat, and dissolve the remaining silver and copper with very small increments of nitric. When a small addition produces no reaction, stop adding.
(7) You can do this without applying heat, but it will go much slower. You have to wait much longer before nitric additions. Otherwise, if you have too much unreacted nitric in the solution, it will slowly heat up from the reaction and can then produce a violent reaction and boil over. Also, without heat, it's hard to prevent using too much nitric and it's harder to dissolve that last bit of silver and copper. It's much more iffy without heat.
(8) You can, although I'm not a fan of doing this, pre-mix the estimated nitric and water. Just make sure you have a large enough container so it won't foam over. If I were to do this, I would start with about 90% of the estimated nitric plus water. If you're not using heat, let it work overnight. Then, pour it off, put the residue in a beaker, and dissolve what remains with a little nitric and water, preferably hot.
Chris
