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212 Halogen Chemistry

barium chlorite with sulfuric acid followed by removal of of other chlorates and perchlorates. Potassium chlorate,
the precipitated barium sulfate: KClO 3 , is employed as an oxidizer in fireworks, flares,
and safety matches. Heating potassium chlorate in the
Ba(ClO) 2 (aq) + H 2 SO 4 (aq) → BaSO 4 (s) + 2HClO 2 (aq).
presence of a transition-metal catalyst such as MnO 2 is a
(41)
convenient laboratory method for the preparation of small
HClO 2 exists only in aqueous solution. It decomposes
amounts of oxygen:
rapidly in base to give chlorite and chlorate ions and in
acid to give chlorine dioxide and chlorate and chloride 2KClO 3 → 2KCl + 3O 2 . (43)
ions. There is some indirect evidence for the existence of
Bromate and iodate salts are prepared on a much
bromous and iodous acids, but they are even less stable
smaller scale than chlorates. Under appropriate condi-
than HSIO 2 .
tions, these ions undergo oscillating chemical reactions
A number of metal chlorite salts have been isolated.
known as “chemical clocks.” The best known clock
They are normally prepared by reduction of an aqueous
reaction is observed when an acidified solution of sodium
solution of ClO 2 in the presence of the metal hydroxide
sulfite (Na 2 SO 3 ) is mixed with an excess of iodate
or carbonate:
in the presence of starch indicator. After a suitable
2ClO 2 + O 2− → 2ClO + O 2 . (42) induction period allowing for sodium sulfite reduction
−
2 2
of iodate to iodide [Eq. (44)], the blue, starch-iodine
Peroxide is the preferred reducing agent because it adds
color periodically appears and disappears as the iodide
no contaminant to the resulting chlorite solution. Heavy
is oxidized to iodine [Eq. (45)], and the iodine is reduced
metalchloritestendtoexplodewhenheatedorsubjectedto
back to iodide [Eq. (46)].
pressure. Chlorite salts of Groups 1 and 2 are more stable,
2−
−
−
but NaClO 2 is the only one that is of any commercial IO + 3SO 2− → I + 3SO ; (44)
4
3
3
importance. 5I + IO + 6H → 3I 2 + 3H 2 O; (45)
−
−
+
Neutral or basic solutions of NaClO 2 disproportionate 3
so slowly at room temperature that they are considered sta- 3I 2 + 3SO 2− + 3H 2 O → 6I + 6H + 3SO . (46)
+
2−
−
3 4
ble. Heating or exposure to light speeds up the rate of re-
action, and the solutions gradually decompose to chloride
E. Perhalic Acids and Perhalate Salts
and chlorate ions. The main uses of NaClO 2 are in bleach-
ing and removal of pollutants from industrial off-gases. Perchloric and periodic acids and their salts have been
known for some time and are well characterized. Per-
bromic acid was not prepared until 1968 using beta-decay
D. Halic Acids and Halate Salts 83
of radioactive Se.
Chloric and bromic acids exist only in aqueous solution, Perchlorates are the most stable oxy-compounds of
but iodic acid can be isolated in the free state by dissolving chlorine, both as solids and in solution at room tempera-
I 2 O 5 in a minimum amount of hot water. White crystals ture. They are, however, very strong oxidizing agents and
of iodic acid precipitate out on cooling. Alternatively, an undergo violent reactions with readily oxidizable organic
aqueous suspension of iodine may be oxidized electrolyti- or inorganic materials, especially when heated. While
cally, or with concentrated nitric acid, hydrogen peroxide, aqueous solutions at room temperature may be slow to
or ozone. react, the anhydrous form of the acid is particularly dan-
Aqueous solutions of chloric and bromic acids are gerous; great care must be exercised when preparing or
formed by disproportionation of Cl 2 or Br 2 in hot alkaline handling it. Concentrated HClO 4 should never be added
solution [Eq. (15) and (16)] or by treating the appropri- to organic solvents, even if the solution is chilled.
ate barium halate salt with sulfuric acid and removing the Perchlorates are manufactured on a large scale by the
precipitated barium sulfate [similar to Eq. (41)]. Concen- electrolytic oxidation of sodium chlorate. On a smaller
tration of these solutions leads to decomposition of the scale, the acid is prepared by treating anhydrous sodium
acid. or barium perchlorate with concentrated HCl, filtering off
Sodiumchlorate,NaClO 3 ,isthemostcommerciallyim- the precipitated chloride salts, and concentrating the acid
portant halate salt. It is prepared commercially on a large by distillation. HClO 4 is commercially available in con-
scale by electrolyzing hot brine and allowing the chlorine centrations of 60–62% or 70–72% by weight, the compo-
gas liberated at the anode to disproportionate in the ba- sition of the water–acid azeotrope boiling at 203 C. The
◦
sic solution that forms at the cathode. The major uses of anhydrous acid is prepared by distillation of the concen-
sodium chlorate are as a bleaching agent for paper pulp, trated acid under reduced pressure in the presence of a
as a defoliant, and as an intermediate in the production dehydrating agent such as fuming sulfuric acid.