The little bit of added sulfuric acid is normally used to help remove lead from solution as the insoluble lead sulfate.
Actually if you had nitrates in solution adding an acid like HCl or H2SO4 would convert the nitrate salts in solution to nitric acid, Hydrogen from the acid, and the nitrate salt would make nitric acid.
H + NO3 ---> HNO3
nitric acid which can decompose as NOx gas (normally with added heat, or a reducing agent like a metal copper for example), or even excess SMB...
The sulfuric acid would not decompose excess nitric acid, both would be in solution.
You may have just added enough reducing agent to overcome a small excess of nitric acid, which may have happened if you had just added a bit more SMB or potassium metabisulfite.
The sulfite salt and ferrous sulfate both can reduce gold from solution, but the chemistry and how they do so is completely different, sulfites produce SO2 gas with the acid, it is actually the SO2 gas that is the reducing agent that gives gold ions the missing electrons to become atoms of gold which join together to form a heavy enough powder to precipitate from solution.
A few chemical formula’s using reagents to precipitate gold
Sodium bisulfite and gold chloride solution
2 HAuCl4 + 2 NaHSO3 --> 2 Au + 8 HCl + Na2SO4 + SO2
or
2AuCl3 + 3NaHSO3 + 3H2O --> 2Au + 3NaHSO4 + 6HCl
sodium metabisulfite and HCl
Na2S2O5 + 2 HCl → 2 NaCl + H2O + 2 SO2
gold chloride and sulfur dioxide gas
2AuCl3 + 3SO2(g) + 3H2O --> 2Au + 3SO3 + 6HCl
Sodium metabisulfite in gold solution:
Na2S2O5 + H2O --> 2 NaHSO3
Notice we actually make sodium bisulfite
Then
2 AuCl3 + 3 NaHSO3 + 3 H2O --> 2 AU + 6HCL + 3 NaHSO4
Ferrous sulfate (iron sulfate) (copperas) (FeSO4) and gold in chloride solution:
2 AuCl3 +6 FeSO4 --> 2 AU +2 Fe2(SO4)3 + Fe2Cl4
(Note we do not make sulfuric acid above).
potassium metabisulfite and HCl acid
K2S2O5 +2HCl --> 2KCl + H2O + SO2(g)
Adding SMB (or K2S2O5), and ferrous sulfate in the same reaction to precipitate gold, there is no benefit, and would complicate matters for the most Part, and would be recommended.
just because it happened to work for your particular circumstance or one certain reaction (which may be because you finally decomposed the excess nitric acid redissolving your gold with excess reagents, does not mean it will not complicate the next batch in another circumstance, there is no benefit, and it is not recommended...
There are ways to deal with excess nitric acid in your gold chloride solutions.
Do not use more than needed.
Sulfamic acid can be used to destroy the excess HNO3 in solution.
Dissolving a metal like (an added button of gold) to consume the nitric in solution (with heat), (Harold's method).
Or the 3 evaporation process ( discussed in Hoke's book) where a little HCl acid is added towards the end of each of the three evaporation's in which you concentrate the gold chloride solution to the point of driving off the excess nitric acid as NOx gas, (where HNO3 decomposes to gas before most of the HCL or gold chloride decompose.
A few drops of sulfuric also have the benefit here. if you happen to get carried away by mistake or inexperience, with the evaporation process, and go to far with the evaporation, where you can form salts of the gold chloride or cook those salts, (driving off too much of the HCl or chloride salts as gas by mistake) the sulfuric aids in helping to keep the gold salts in solution, sulfuric acid is less volatile and will not decompose even after the HCl has or the chloride salts decompose to gas...
Actually if you had nitrates in solution adding an acid like HCl or H2SO4 would convert the nitrate salts in solution to nitric acid, Hydrogen from the acid, and the nitrate salt would make nitric acid.
H + NO3 ---> HNO3
nitric acid which can decompose as NOx gas (normally with added heat, or a reducing agent like a metal copper for example), or even excess SMB...
The sulfuric acid would not decompose excess nitric acid, both would be in solution.
You may have just added enough reducing agent to overcome a small excess of nitric acid, which may have happened if you had just added a bit more SMB or potassium metabisulfite.
The sulfite salt and ferrous sulfate both can reduce gold from solution, but the chemistry and how they do so is completely different, sulfites produce SO2 gas with the acid, it is actually the SO2 gas that is the reducing agent that gives gold ions the missing electrons to become atoms of gold which join together to form a heavy enough powder to precipitate from solution.
A few chemical formula’s using reagents to precipitate gold
Sodium bisulfite and gold chloride solution
2 HAuCl4 + 2 NaHSO3 --> 2 Au + 8 HCl + Na2SO4 + SO2
or
2AuCl3 + 3NaHSO3 + 3H2O --> 2Au + 3NaHSO4 + 6HCl
sodium metabisulfite and HCl
Na2S2O5 + 2 HCl → 2 NaCl + H2O + 2 SO2
gold chloride and sulfur dioxide gas
2AuCl3 + 3SO2(g) + 3H2O --> 2Au + 3SO3 + 6HCl
Sodium metabisulfite in gold solution:
Na2S2O5 + H2O --> 2 NaHSO3
Notice we actually make sodium bisulfite
Then
2 AuCl3 + 3 NaHSO3 + 3 H2O --> 2 AU + 6HCL + 3 NaHSO4
Ferrous sulfate (iron sulfate) (copperas) (FeSO4) and gold in chloride solution:
2 AuCl3 +6 FeSO4 --> 2 AU +2 Fe2(SO4)3 + Fe2Cl4
(Note we do not make sulfuric acid above).
potassium metabisulfite and HCl acid
K2S2O5 +2HCl --> 2KCl + H2O + SO2(g)
Adding SMB (or K2S2O5), and ferrous sulfate in the same reaction to precipitate gold, there is no benefit, and would complicate matters for the most Part, and would be recommended.
just because it happened to work for your particular circumstance or one certain reaction (which may be because you finally decomposed the excess nitric acid redissolving your gold with excess reagents, does not mean it will not complicate the next batch in another circumstance, there is no benefit, and it is not recommended...
There are ways to deal with excess nitric acid in your gold chloride solutions.
Do not use more than needed.
Sulfamic acid can be used to destroy the excess HNO3 in solution.
Dissolving a metal like (an added button of gold) to consume the nitric in solution (with heat), (Harold's method).
Or the 3 evaporation process ( discussed in Hoke's book) where a little HCl acid is added towards the end of each of the three evaporation's in which you concentrate the gold chloride solution to the point of driving off the excess nitric acid as NOx gas, (where HNO3 decomposes to gas before most of the HCL or gold chloride decompose.
A few drops of sulfuric also have the benefit here. if you happen to get carried away by mistake or inexperience, with the evaporation process, and go to far with the evaporation, where you can form salts of the gold chloride or cook those salts, (driving off too much of the HCl or chloride salts as gas by mistake) the sulfuric aids in helping to keep the gold salts in solution, sulfuric acid is less volatile and will not decompose even after the HCl has or the chloride salts decompose to gas...