Oxidation states of arsenic

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;

NO₂ is a brown gas - you can see it above

• 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:

1. 2HA­sO₂ + 3H₂­SO₄ = As₂(SO₄)₃ + 4H₂O.

2. 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.