Dissolving Silver away from Gold using Parting Solutions
Parting is the process of dissolving the Ag (Silver) in the bead from the cupellation to leave the Au (Gold) behind for weighing or to be read on the AAS.
If Ag is to be determined gravimetrically as well, the bead is weighed before parting. The very small beads are weighed using sensitive micro-balances.
Parting solutions: Nitric Acid (HNO3) – Dissolves Ag and Pt group metals & leaves the gold behind.
Sulphuric Acid (H2SO4) – dissolves Ag ; leaves the Pt group metals & gold behind.
Hydrochloric acid (HCl) – In the AA finish, HCl is added after the Ag is dissolved to form aqua regia (3:1 HCl to HNO3) . Aqua regia dissolves the gold
Ammonia (NH4OH) – used to dissolve any possible silver chloride which may have precipitated out.
Nitric acid is almost used all the time but sulphuric acid can be used as well. Nitric can not be used unless there at least three times the amount of silver than gold. The idea of parting is to prevent the gold from breaking up into small particles. The gold that remains in one piece is much easier to handle.
6:1 HNO3 for gravimetric finish
3:1 HNO3 for gravimetric finish
1:1 HNO3 for gravimetric finish
1:5.5 H2SO4 for gravimetric determination Of Pt Group metals.
0.5 ml HNO3 of concentration for AA finish
1.5 ml of conc. HCl for AA Finish