Oxidation states of arsenic
How many oxidation states does arsenic have?
Arsenic is an element of the 15ᵗʰ group of the 4ᵗʰ period of the Periodic Table. It is a greenish-gray brittle semi-metal. Sometimes it is found in nature in a free state, and among arsenic minerals the most common is arsenopyrite FeAsS.
In compounds, arsenic can display 3 oxidation states - -3, +3 и +5.
Obtaining the pure substance and its chemical properties
At present the main method for obtaining arsenic in a free state is sintering its sulfide ores:
2As₂S₃ + 9O₂ = 6SO₂ + 2As₂O₃;
As₂O₃ + 3C = 2As + 3CO.
Arsenic is often obtained by sintering arsenopyrite FeAsS. In pipes in a muffle furnace, the condensation of free arsenic without access of oxygen takes place:
FeAsS = FeS + As.
At the end of sintering, the remaining arsenic in the furnace is brought to an oxidized form, by letting oxygen into the furnace:
4As + 3O₂ = 2As₂O₃.
Arsenic forms several allotropic modifications – there is gray, black and yellow arsenic.
Chemical properties of arsenic:
4As + 3O₂ = 2As₂O₃;
2As + 3Cl₂ = 2AsCl₃;
As + S = As₂S₃, As₂S₅, As₄S₄ (several products form, the reaction is non-stoichiometric);
As + 3Na = Na₃As;
2As + 3H₂SO₄ = As₂O₃ + 3SO₂ + 3H₂О (hot, concentrated acid is used);
2As + 6NaOH = 2Na₃AsO₃ + 3Н₂ (excess).
With non-oxidizing acids arsenic does not react.
Arsenic in the oxidation state of -3
There are salt-like compounds of arsenic – arsenides. They are crystalline substances with a metallic shine and a grayish or silvery color. Many arsenides are semi-conductors and conductors.
Arsenides can be obtained by smelting pure substances:
3Ca + 2As = Ca₃As₂ (at 1000 ᵒC, or 1832 ᵒF).
Arsenides dissolve in acids:
Ca₃As₂ + 6HCl = 3CaCl₂ + 2AsH₃.
Arsine AsH₃ is a toxic gas which can be obtained in the hydrolysis of arsenides.
Na₃As + 3H₂O = AsH₃ + 3NaOH.
The gas is a strong reducer:
- 2AsH₃ + 3H₂O + 12AgNO₃ = As₂O₃ + 12Ag + 12HNO₃;
2AsH₃ + 3O₂ = As₂O₃ + 3H₂O (this is the combustion of arsine in air);
AsH₃ + 3Cl₂ = AsCl₃ + 3HCl (arsine burns in chlorine);
AsH₃ + 3HCl = AsCl₃ + 3H₂ (concentrated acid is used).
At heating to 500 ᵒC or 932 ᵒF, arsine can decompose into simple substances.
Arsenic at an oxidation state of +3
Arsenic (III) oxide is the raw material for obtaining meta-arsenic acid. It is obtained by sintering arsenic sulfide or the free semi-metal: Some chemical properties of arsenic oxide:
As₂O₃ + 5H₂O + 2Cl₂ = 2H₃AsO₄ + 4HCl (with boiling);
As₂O₃ + 3C = 2As + 3CO (at a temperature of around 700 ᵒC or 1292 ᵒF);
As₂O₃ + 6Zn + 12HCl = 2AsH₃ + 6ZnCl₂ + 3H₂O;
As₂O₃ + 2NaOH = 2NaAsO₂ + H₂O (reaction is carried out in a solution).
HAsO₂ is meta-arsenic acid. There is also ortho-acid – ortho-arsenic acid H₃AsO₃. An acid with arsenic at an oxidation state of +3 can be obtained by diluting oxides or halogenides of arsenic with water.
As₂O₃ + H₂O = 2HAsO₂.
The acid displays amphoteric characteristics (the ability to react with both alkalis and acids) and is a good reducer:
2HAsO₂ + 3H₂SO₄ = As₂(SO₄)₃ + 4H₂O.
HAsO₂ + Cl₂ + 2H₂O = H₃AsO₄ + 2HCl.
Arsenous acid is weak. When heated it decomposes to oxide and water:
2HAsO₂ = As₂O₃ + H₂O.
Salts of arsenous acids are arsenites. Arsenites of alkaline metals hydrolyze.
Arsenic at an oxidation state of +5
Arsenic (V) oxide is a strong oxidizer. It can be obtained by sintering arsenic in ozone or an excess of oxygen:
4As + 5O₂ = 2As₂O₅.
With heating, arsenic (V) oxide can be turned into arsenic (III) oxide:
As₂O₅ = A₂O₃ + O₂ (heating to 700-730 ᵒC or 1292-1346 ᵒF is required);
Other chemical properties of arsenic pentoxide:
As₂O₅ + 6NaOH = 2Na₃AsO₄ + 3H₂O;
2As₂O₅ + 5C = 4As + 5CO₂.
Click here for some experiments with activated carbon C.
Often among compounds in which arsenic displays an oxidation state of +5, arsenates are extracted – salts of arsenous acid (for example, sodium arsenate - Na₃AsO₄). Arsenous acid H₃AsO₄ is a tribasic acid of medium strength. It is an oxidizer, but these properties are primarily displayed in acidic mediums:
2H₃AsO₄ + 4HI = As₂O₃ + 2I₂ + 5H₂O.
The acid may be obtained by the following reactions:
As₂O₃ + 4HNO₃ + H₂O = 2H₃AsO₄ + 4NO₂;
As₂O₅ + 3H₂O = 2H₃AsO₄.
There is a qualitative reaction for the arsenate ion – reaction with dissolved salt of silver with the precipitation of the undissolved sediment of silver arsenate with the color of “coffee with milk”:
Na₃AsO₄ + 3AgNO₃ = Ag₃AsO₄ + 3NaNO₃.
Arsenic is used in many branches of industry – it has found application in dentistry, metallurgy, and the manufacture of semi-conductive materials.