Page 127 - Academic Press Encyclopedia of Physical Science and Technology 3rd InOrganic Chemistry

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204 Halogen Chemistry

Halogen molecules dissociate into atoms on heating Anhydrous hydrogen fluoride is usually produced com-
(Table IV) and on absorption of light. Halogen atoms mercially by reacting hot, concentrated sulfuric acid with
are sometimes able to initiate chain reactions between fluorspar:
other substances. Specific examples include the chlorine-
sensitized explosion of hydrogen with oxygen, the bro- CaF 2 + H 2 SO 4 → CaSO 4 + 2HF. (18)
mine-sensitized decomposition of ozone, the iodine-
sensitized decomposition of hydrogen peroxide, and The principal use for anhydrous HF is in the preparation of
the halogenation of hydrocarbons. Small quantities of other inorganic fluorides for fluoridation of water, for use
iodine often serve as catalysts in such organic reactions as fluxes and catalysts, and for the manufacture of glass
as halogenation, dehydration, and isomerization. and ceramics.
Hydrogen chloride is produced by a reaction analogous
H. Analysis to Eq. (18) using NaCl as the source of chloride. Alterna-
tively, sodium chloride may be treated with a mixture of
Quantitative determination of the halogens can be accom-
sulfur dioxide, air, and water (the Hargreaves process):
plished by a number of procedures. Accurate analysis of
fluorine is the most difficult, and is generally carried out
4NaCl + 2SO 2 + O 2 + 2H 2 O → 2Na 2 SO 4 + 4HCl.
either by (1) calcium fluoride precipitation, (2) lead chlo-
(19)
rofluoride precipitation, or (3) titration with thorium ni-
When very-high-purity HCl is required, hydrogen and
trate using sodium alizarin sulfonate as an indicator:
chlorine gases are combined directly in a specially de-
Th(NO 3 ) 4 (aq) + 4KF(aq) → ThF 4 (s) + 4KNO 3 (aq). signed burner. A large percentage of commercially avail-
(17) able HCl is obtained as a by-product of the chlorination
More methods are available for chlorine. As free chlorine, of organic materials.
the element may be analyzed by reduction to chloride us- MostcommercialHClisusedfortheproductionofvinyl
ing iodide, arsenite, alkaline hydrogen peroxide, sulfur chloride and chlorinated solvents, for the manufacture of
dioxide, or sodium thiosulfate (Na 2 S 2 O 3 ), or determined a large variety of inorganic chemicals, especially ammo-
colorimetrically by treatment with ortho-toluidine in hy- nium chloride (NH 4 Cl), metal salts and bleaches, and for
drochloric acid. Chloride ion may be precipitated as silver the pickling of metals to remove oxide scale. Food-related
chloride or titrated with silver nitrate in the presence of applicationsincludethehydrolysisofstarchtoglucoseand
potassium chromate (K 2 CrO 4 ). the manufacture of gelatine.
Free bromine and iodine are determined by reduction Hydrogen bromide is prepared by direct combination
with sodium thiosulfate. Both bromide and iodide ions of the elements at elevated temperatures in the presence
are typically analyzed by precipitation or titration using of a catalyst. Hydrogen iodide can also be made in this
silver nitrate or by titration with potassium dichromate way, but is more often prepared as the aqueous acid by
(K 2 Cr 2 O 7 ). reaction of I 2 with hydrazine or hydrogen sulfide:
All six of the naturally occurring halogen isotopes
have been studied by nuclear magnetic resonance (nmr) 2I 2 + N 2 H 4 → 4HI + N 2 , (20)
spectroscopy. Nmr sensitivity and ease of obtaining 19 F
1
spectra are second only to H. All five of the naturally H 2 S + I 2 → 2HI + S. (21)
occurring isotopes of Cl, Br, and I can be investigated
by nuclear quadrupole resonance (nqr) spectroscopy. The A convenient laboratory preparation involves the re-
127 129 duction of bromine or iodine with red phosphorus and
I and I isotopes are the only halogens meeting the
necessary requirements for observation and study by the water:
M¨ossbauer effect.
2P + 6H 2 O + 3X 2 (X = Br or I) → 6HX + 2H 3 PO 3 ,
(22)
II. THE HYDROGEN HALIDES H 3 PO 3 + H 2 O + X 2 → 2HX + H 3 PO 4 . (23)
(HYDROHALIC ACIDS)
A reaction analogous to Eq. (18) may be employed if the
A. Preparation and Use
acid is nonoxidizing (e.g., H 3 PO 4 ), and sodium bromide
The hydrogen halides (HX) make up one of the most im- or sodium iodide serves as the halogen source.
portant classes of inorganic compounds, containing the Hydrogen bromide is used in the manufacture of inor-
halogens in the −1 oxidation state. They are prepared in ganic and alkyl bromides. There is no large-scale use for
a number of different ways. HI outside of the laboratory.