Lino1406 said:
After you dissolve the gold in aqua regia, you have to
displace HNO3 with HCl. This is done by adding HCl, vaporizing
3 times, to get a syrup form. Then add the FeSO4
Correct - Hoke also advises:
"As in Chapter V, you must remove the excess nitric acid by evap-
orating the solution to a syrup, adding hydrochloric acid, and boil-
ing down again,
finally diluting with considerable water and getting
rid of all sediment. This procedure is necessary when you use cop-
peras, or sulphur dioxide, or any other precipitant. Be sure that
the resulting solution is perfectly clear and free from sediment or
cloudiness."
Regarding the use of sulfur dioxide gas, which is essentially what SMB introduces, she says:
"The beginner sometimes finds that the gold is very slow to come down.
Usually the trouble is that the liquid contains
too high a concentration of
acid; that is, maybe he failed to evaporate of f all his excess nitric acid;
or maybe he used too much sulphuric or hydrochloric acid in previous
steps.
One remedy is to add water to dilute the liquid and reduce the
concentration of acid; another is to add pinches of sodium carbonate, a
very small quantity at a time, stirring well. When the acid concentra-
tion is right, the dark cloud of precipitated gold will appear promptly."
Those with more experience with different precipitants will need to weigh in,
but remember that lazersteve diluted his AuCl 4:1, forcing dissolved Ag out of solution,
but perhaps also lowering the ph enough to allow easier precipitation?. I just did a small batch
wherein I evaporated off the excess chlorine, but failed to dilute the remaining solution,
I believe thereby
concentrating the acid - it took twice as much SMB to drop & took over a day to settle.
Again, I'm just a beginner, so others with more experience can weigh in with real world info, but thats what I found in Hoke.