Aqua regia Cl3H4NO3 (4HCl + HNO3)
This discussion I cannot cover all of the different reactions that can occur with using aqua regia and its reactions with the metals, as reactions are not simple, and conditions can vary widely, base metals, excess acid or oxidizer, reagents involved and inter-reactions with salts and acids all can make things complicated.
This discussion will just cover some of the basics, and still, try and keep it simple, although the true chemistry involved is actually more complex than can be shown here.
These reactions produce toxic acidic vapors which will corrode the environment around them, form dangerous reactions in your lungs, the toxic solutions need to be treated for hazardous waste properly.
Many dangers can exist from these chemical reactions, anyone working with these acids and metals needs to have a good understanding of the dangers involved, and how to protect himself and others from these dangers, have a good understanding of safe lab practices, and working in the lab, dealing with waste byproducts, an understanding of the chemistry involved, chemistry of metals can have many dangers and can involve explosive reactions…
Without the understanding of the dangers, and the use of safe lab practices, you are not only risking your health but also the health of others, and will destroy things around you, and without this understanding, you will also be lacking in the understanding of how to recover and refine gold.
Aqua regia begins decomposition as soon as the concentrated acids of HCl and HNO3 are mixed, the longer it is mixed before use the less potency the acid is to dissolve metals, storing in a closed vessel can cause the container to burst or explode because of the pressure of the decomposition of these gases.
Decomposition is much more complicated as many reactions can occur, but we can write some of the major reactions in a simple equation (with no metals), 3 moles of Hydrochloric acid mixed with 1 mole of nitric acid combined producing 1 mole of nitrosyl chloride gas and 1 mole of chlorine gas and 1 mole of water.
3HCl + HNO3 --> NOCl + Cl2 + H2O
As this decomposition reaction proceeds the nitric acid is reduced while the hydrochloric acid is oxidized, and as the volatile products of nitrosyl chloride NOCl and chlorine gas Cl2 escapes the aqua regia solution loses its potency to react with metals.
3HCl + HNO3 --> NOCl + Cl2 + H2O
Nitrosyl chloride further decomposes into Nitric oxide gas and chlorine gas.
2NOCl --> 2NO + Cl2
The colorless nitric oxide gas will react with air (oxygen) to form nitrogen dioxide.
2NO + O2 -->2NO2
The red nitrogen dioxide gas reacts with moisture in the air or water (or with the moisture in your lungs) to form nitric acid.
2NO2 + H2O --> HNO2 + HNO3
Nitrous acid only in solution decomposes
2HNO2 --> NO2 + NO + H2O
3NO2 + H2O --> 2HNO3 + NO
Elemental gold has only one electron in its outer shell, does not easily give up its electron or share it with other elements.
Hot concentrated selenic acid is the only pure acid by itself that I know of which can dissolve gold.
2 Au + 6 H2SeO4 → Au2(SeO4)3 + 3 H2SeO3 + 3 H2O
HCl acid or Nitric acid neither acid alone will dissolve gold, but most any combination of these two acids will attack gold.
Chlorine is a gas atom has 7 electrons in its outer shell, which is a good candidate to share the electrons with the golds 1 electron (eight being the magic number for a full outer shell of electrons).
Chlorine gas will attack gold at higher temperatures (150-180 degrees C), but I am getting away from the subject of aqua regia, chlorine gas plays a role in the aqua regia solution, getting back on subject.
2Au + 3Cl2 → 2AuCl3
Gold is not oxidized easily (it will not give up its electron easily), HCl acid alone will not attack gold, HNO3 acid alone will not attack gold, but a combination of these acids will.
Chloroauric acid is an inorganic acid that is widely used in gold refining processes. Gold is one of the least reactive metals, but gold reacts with aqua regia to yield chloroauric acid.
Nitric and hydrochloric acid. Nitric acid is a powerful oxidizing agent, capable of converting small amounts of gold to its ionic form, Au3+. Once this ionic form is present in the solution, the hydrochloric acid provides a source of chlorine anions which react with the gold cations to form tetrachloroaurate(III) anions. As the reaction with hydrochloric acid is an equilibrium reaction favoring the formation of chloroaurate anions (AuCl4-), the gold ions are removed from solution making room for more oxidation to occur. And as the solution is so acidic, the chloroaurate anions are swiftly protonated to form chloroauric acid.
Au + 3NO3 + 6H+ --> Au3+ + 3NO2 + 3H2O
or
Au + 4H+ + NO3- <--> Au+3 + NO + 2H2O (reaction soon moves back to the left)
Then
Au3+ + 3NOCl + 3NO3 --> AuCl3 + 6NO2
or excess nitric
Au + 6H+ + 3NO3- <--> Au+3 + 3NO2 + 3H2O
Then Au3+ + 4Cl- --> AuCl4
Reaction with gold alone
Au + HNO3 + 4 HCl → HAuCl4 + NO + 2 H2O
with excess nitric:
Au + 3 HNO3 + 4 HCl --> AuCl4− + 3 NO2- + H3O+ + 2 H2O
more excess acids...
2Au + 9HCl + 3HNO3 --> 2AuCL3 + 3NOCl + 6H2O
Chloroauric acid shows acidic behavior by releasing proton in solution. It is a strong monoprotic conjugate acid.
Gold is oxidized by halogens; so, a solution of HAuCl4 can be obtained by the action of chlorine or chlorine water on metallic gold in hydrochloric acid.
2Au + 3Cl2 →2 AuCl3
2 Au + 3 Cl2 + 2 HCl → 2 HAuCl4
The gold-chlorine bond is a coordinate covalent bond, where the electron density is shared but the electrons that are shared both come from the same atom. In this case, the chlorine, which donates its electrons through the mostly filled p orbitals.
So far, we have not discussed other metals besides gold.
Silver in the gold alloy can form a passivated coating in aqua regia of silver chloride keeping the gold from dissolving in the acid, we in-quarter the gold (25 parts Au/75 parts Ag), and part the gold and silver in nitric acid before processing the washed gold in aqua regia…
Very little silver will dissolve in aqua regia, it will form insoluble silver chloride AgCl very small amounts of silver will go into a concentrated chloride solution with a higher oxidation state AgCl2 which precipitates from diluted solutions as AgCl, the passivated coating can keep gold from dissolving in aqua regia...
It is hard to explain all of the different roles base metals can play in these reactions, with almost all of these reactions being undesirable, they not only complicate the chemistry but interfere with the total process…
Base metals with gold also go through many reactions, several of these metals will make salts of the acids, which when concentrated and re-acidified can reform acids of their salts, complicating the removal of excess acids and of deNOxing of the solutions, the purity of the gold or the ability to recover the gold easily from solution...
Aqua regia is a good refining process, where base metals are very limited, more base metals = more problems.
Aqua regia makes a bad choice as a recovery method…
Mixing HCl and HNO3 to dissolve gold, for example, using 3 parts HCl and 1-part HNO3 is a waste of acids, and can give problems with using excess acids, and trouble recovering gold or reducing gold with excess oxidizing agents…
Using Excess HCl causes no harm, adding very small additions of nitric acid and letting reaction settle down then adding heat, using only small controlled increments of nitric additions we can dissolve the gold consuming the excess nitric as we work.
For calculating how much acids, I normally figure around 3.8ml HCl and 0.95ml HNO3 per gram of gold (the nitric acid added in increments, with heat as reaction slows).
Or around 118ml HCl and 29.5ml HNO3 (small additions as needed) per ounce of gold.
Adding HCl in excess and using nitric as needed, we may not need to use all of the nitric that we have calculated.
Excess oxidizer in this case nitric acid, chlorine, nitrosyl chloride, NO2, (NO3)2 …will keep gold in solution, they interfere with reduction of the gold and can even keep the stannous chloride test from working.
Oxidizer removal
Base metals involved in the solution can play a very big role in the chemistry, and can even complicate the removal of the oxidizers which keep gold dissolved, or dissolve it back into solution…
The reactions below are mostly just to show examples of some reactions of base metals, (not necessarily what we are dealing with here, but some of which can be involved, and are included as food for thought.
Base metals can form salts of an acid or concentrate upon evaporation then upon addition of an acid these salts can reform an acid of that salt (similar reaction after concentrating the gold chloride solution to syrup and adding HCl to wet, where we see a red cloud of NOx gases, from the decomposition of the nitrates to nitric acid and them NOx formed from the reaction of the HCl on the nitrate salt...
Reactions below included mostly just for added thought…
AgCl + Cl- --> AgCl2
AgCl2 + H2O --> AgCl + Cl-
Ag(NO3)2 + NOCl --> AgCl2 + NO2
AgNO3 + CuCl2 --> 2AgCl + Cu(NO3)2
Pb(NO3)2 + CuCl2 --> PbCl2 + Cu(NO3)2
PbCl2 + H2SO4 --> PbSO4 + 2HCl
Cu(NO3)2 + HCl --> CuCl2 + 2HNO3
CuCl2 + H2SO4 --> CuSO4 + 2HCl
CuCl2 + SO2 + 2H2O --> 2CuCl + H2SO4 + 2HCl
Cu(NO3)2 + H2SO4 --> CuSO4 + 2HNO3
Evaporation
This is used heating solution below its boiling point, a steam bath is useful for controlling the heat…
Boiling would cause loss of gold, evaporation will not if you are careful with the process.
If we just look at aqua regia alone, it begins to decompose as soon as the two acids are mixed, with volatile gases reacting and escaping the acidic water solution, the heat of evaporation would speed up these reactions, and concentrate the solution.
Note here without metals being involved, or excess acids being involved we could theoretically evaporate aqua regia to toxic vapors and steam easily.
More base metals more reactions, more trouble, impure gold…
Excessive nitric acid uses = more trouble.
With gold only in solution (no other base metals), the evaporation process would be simpler than with base metals involved, Gold will not react as much and make the same (nitrate) salts that base metals will upon evaporation, these base metal salts can reform oxidizers when reacidified, leading to more evaporations and reacidifications being needed to decompose these salts to gases and De-NOx the solution…
During evaporation acids concentrate in solution, water, and the very volatile gases come off first, base metals can form salts with the acid, after most of the water has evaporated the concentrated nitric begins decomposition and to evaporate as NOx gases with the water vapors, HCl is less volatile but will also come off as gases, (Excess HCl is no problem and can be beneficial), as mentioned above we can be left with base metal salts in an acidic syrup, adding a little HCl can convert nitrates to nitric which can begin decomposition to NOx gases, water involved in the HCl acid can be evaporated out again , and a little HCl to help remove more nitric or its byproducts, repeating three times is normally sufficient…
By limiting Nitric acid use we can help to reduce the problems of getting rid of it and its products…
By limiting base metals, we also limit some of the troubles…
Sulfuric acid can be beneficial during the evaporation process, the sulfuric acid is not as volatile as the water, nitric and HCl, and will help to break the azeotrope of the more volatile acids (making it more easy to remove the free nitric acid), and keep from evaporating the solution to dryness, and for its benefit in removing lead from the solution…
As The solution gets concentrated and any free nitric acid reaches its azeotrope the acid can begin to decompose fairly violently all at once, I am fairly sure base metal reactions also play a big role in the increase of this feverish rush of decomposed gases and nitric acid in the red cloud to try and escape the vessel.
We must also take precautions to prevent Boil overs of solutions as the reaction goes through this stage.
By watching the concentrated solution --we can see small bubbles forming around the rim of the acid, lowering heat at this point so gases can escape much slower (and we are not driving the chemical reactions so hard, we can get past this point of a (possible boil over), and then regain the heat of evaporation after getting past this critical stage…
After the evaporation process of deNOxing the gold chloride solution is increased in volume with water and normally left overnight to settle out insoluble chlorides like silver chloride, and other insoluble salts like PbSO4…
Urea
Urea and nitric are a dangerous combination.
Urea will not destroy nitric acid,
Although It can remove some of the byproducts or NOx gases from a nitric acid solution.
But considering the dangers and it not really being effective with excess nitric.
I do not even care to discuss it or use it in the processes.
Gold to consume oxidizers and use up acids
Adding gold to consume free acids this with the heat and evaporation is an excellent choice.
This goes along with the lines of adding gold to excess HCl and nitric only in tiny increments, with heating and concentrating along the process and leaving just a little gold undissolved after the DeNOxing process.
Sulfamic acid H3NSO3
Nitrous acid and sulfamic acid --> products of sulfuric acid, water, and nitrogen gas.
HNO2 + H3NSO3 --> H2SO4 + N2 + H2O
Nitric acid and sulfamic acid --> sulfuric acid water and nitrous oxide gas.
HNO3 + H3NSO3 --> H2SO4 + N2O + H2O
I like to use a combination of these methods, added gold in the reaction, limiting excess nitric, controlled evaporation and the sulfamic acid…
Excess reagent (low oxidizer in solution)
If Most all of the nitric is consumed in the reaction of the solution and only a little of the oxidizers are left sometimes an excess reducing agent like SMB can complete the de-NOxing through reaction with the acids and base metals…
Testing for gold in solution
We can test for gold in solution by reducing the gold to metal and observing a color change.
In the stannous chloride test, the gold is reduced to a metal colloid held in solution by static charges, giving the solution a violet color, in the ferrous sulfate test the gold is reduced from solution to the metal on a crystal of the iron salt leaving a brown colored spot in the test.
In both of these reactions, the gold is reduced to metal in the test, excessive nitric or oxidizers can keep the gold from being reduced or re-dissolve the gold again in the reaction, the test needs to have the oxidizer or free nitric removed before this test will work.
SnCl2
2AuCl3 + 3SnCl2 --> 2Au (colloid) + SnCl4 (purple of Cassius)
Au violet
Ag white
Pt orange
Pd deep yellow to blue-green
FeSO4
3FeSO4 + AuCl3 --> Au + FeCl3 + Fe2(SO4)3 (brown-elemental gold)
In a spot plate several drops of solution to be tested and a crystal of copperas, the gold precipitates from solution as a brown ring of gold, the remaining solution can be moved to the next well in the spot plate (or a Q-tip or paper) and tested for other metals with SnCl2
There are several other reagents or tests like:
DMG used to test for Pd yellow.
Iron in solution can have several colors depending on the oxidation state, from green, brown, yellow or reddish rusty, which can resemble gold or platinum in solution, we can also test for iron:
Ammonium thiocyanate gives a blood red reaction with iron in solution in the spot plate exposed to air.
Potassium ferrocyanide gives intense (Prussian) blue with iron solution exposed to air.
We should also have a known gold solution, to test the reduction potential of our stannous chloride reagent.
Aqua regia is not the only thing strong enough to dissolve gold and form a gold chloride solution.
HCl and NaClO sodium hypochlorite (household bleach) will also dissolve gold fine gold or powders and form gold chloride
Au + 4HCl + NaClO --> HAuCl4 + NaCl + H3O
HCl and 30% H2O2 Hydrogen peroxide will also dissolve fine gold (powders) to form gold chloride
2Au + 3H2O2 + 8HCl --> 2HAuCl4 +6H2O
Hydrochloric acid in the presence of oxidants such as nitric acid, H2O2, NaClO, nitrate salts, oxygen, cupric or ferric ions, and manganese dioxide can dissolve gold.
There are other oxidizers that will work.
There are even salts that when fused with gold can make the gold water soluble when the fusion is dissolved into water.
There are many reducing agents that can reduce ionic gold into the metal, such as: SMB (sodium metabisulfite), sodium sulfite, copperas (Iron II Sulfate), Sulfur dioxide gas (SO2), Sulfurous acid, metals like copper and zinc, oxalic acid, sodium nitrite, H2O2, and many more.
Sulfur dioxide gas
AuCl4 + SO2 + H2O --> AuCl2 + H2SO4 + 2HCl
Then AuCl2 + SO2 + 2H2O --> Au + H2SO4 + 2HCl
2AuCl4 + 3SO2 + 6H2O --> 2Au + 3H2SO4 + 6HCl
Sodium sulfite
3Na2SO3 + 6HCl --> 6NaCl + 3H2O + SO2
3Na2SO3 + 2AuCl3 +H2O --> 2Au + 2H2SO4 + 6HCl
SMB Na2S2O5 sodium metabisulfite
Na2S2O5 + H2O --> 2NaHSO3
2HAuCl4 + 3NaHSO3 + H2O --> 3NaHSO4 + 8HCl + 2Au
Sodium bisulfite
NaHSO3 + HCl --> NaCl + H2O + SO2
Sulfurous acid H2SO3
2AuCl3 + 3H2SO3 + 3H2O --> 2Au + 3H2SO4 + 6HCl
Copperas FeSO4 ferrous sulfate
3FeSO4 + AuCl3 --> Au + FeCl3 + Fe2(SO4)3
HAuCl4 + 3FeSO4 --> Au + Fe2(SO4)3 + FeCl3 + HCl
2AuCl3 + 6FeSO4 --> 2Au + 2Fe2(SO4)3 + Fe2Cl6
Gold precipitation with Hydrogen Peroxide
(2)AuCl4[-] + (3)H2O2 + (6)OH[-] --> (2)Au + (8)Cl[-] + (6)H2O + (3)O2
Hydrazine
4AuCl3 + 3N2H4 --> 4Au + 12HCl + 3N2
Oxalic acid
2AuCl3 + 3H2C2O4 --> 2Au + 6HCl + 6CO2
2 HNO3 + 8 HCl + 4 SnCl2 → 5 H2O + 4 SnCl4 + N2O
2AuCl3 + 3SnCl2 --> 2Au (colloid) + SnCl4
SnCl2 (aq) + 2 FeCl3 (aq) → SnCl4 (aq) + 2 FeCl2 (aq)
It also reduces copper(II) to copper(I).
SnCl2 + 2CuCl2 --> 2CuCl + SnCl4
Tin(II) chloride also finds wide use as a reducing agent. This is seen in its use for silvering mirrors, where silver metal is deposited on the glass:
Sn2+ (aq) + 2 Ag+ → Sn4+ (aq) + 2 Ag (s)
In HCl solution, either metallic Fe or metallic Al will reduce Sn(IV) to Sn(II)
Fe(s)+[SnCl6]2−(aq) ⟶Fe2+(aq)+[SnCl4]2−(aq)+2Cl−(aq)
Reactions for thought:
HNO3 + 3 HCl → Cl2 + 2 H2O + NOCl
HNO2 + HCl → H2O + NOCl
Cl2 + 2 NO → 2NOCl
NO2Cl + NO --> NOCl + HCl
HCl + NOHSO4 → H2SO4 + NOCl
HNO2 + H2SO4 → NOHSO4 + H2O
Cl2 + H2O ⇌ HOCl + H+ + Cl−
The presence of light or oxides of metals like copper can accelerate the decomposition of chlorine and water to HCl
2 Cl2 + 2 H2O → 4 HCl + O2
HClO + HCl → H2O + Cl2
2 NO + O2 → 2 NO2
2 HNO3 → N2O5 + H2O
2 N2O5 → 4 NO2 + O2
2 NO2 <--> N2O4
2 NO2 -->2NO + O2
2 NO2 (N2O4) + H2O --> HNO2 + HNO3
4 HNO3 --> 4 NO2 + 2 H2O + O2
2NO --> N2O2
2NO + Cl2 --> 2NOCl
N2O2 + Cl2 --> 2NOCl
NOCl + NO2 --> NO2Cl + NO
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