CYANIDE in WATER and SOIL: Chemistry, Risk, and Management - Chapter 2 pdf

2
Physical and Chemical Forms of
Cyanide
Rajat S. Ghosh, David A. Dzombak, and
George M. Wong-Chong
CONTENTS
2.1 Gaseous Forms of Cyanide 17
2.2 Aqueous Forms of Cyanide 17
2.2.1 Free Cyanide 17
2.2.2 Metal–Cyanide Complexes 19
2.2.2.1 Weak Metal–Cyanide Complexes 19
2.2.2.2 Strong Metal–Cyanide Complexes 19
2.2.3 Cyanate and Thiocyanate 19
2.2.4 Organocyanide Complexes 20
2.3 Solid Forms of Cyanide 20
2.3.1 Simple Metal–Cyanide Solids 21
2.3.2 Metal–Metal Cyanide Solids 21
2.3.2.1 Alkali/Alkaline Earth Metal–Metal Solids 21
2.3.2.2 Other Metal–Metal Cyanide Complex Salts 22
2.4 Summary and Conclusions 22
References 23
Cyanide occurs in many different forms in water and soil systems. The specific form of cyanide
determines the environmental fate and transport of cyanide, as well as its toxicity. Understanding the
specific form(s) of cyanide present in a particular water, soil, or sediment is critical for assessment
of how to manage or treat the cyanide present. This cannot be overemphasized! While “cyanide”
is often discussed as a single entity in the popular press and even in professional publications,
this is a misleading portrayal. The various forms of cyanide are quite different in their reactivity
and their toxicity. Proper professional evaluation, assessment, and design activities pertaining to
cyanide contamination management requires knowledge about and careful consideration of cyanide
speciation.
This chapter provides an introductory overview of the various forms of cyanide that can exist in

water and soil systems. All of the remaining chapters of this book assume a basic knowledge of the
speciation of cyanide as presented here. A detailed examination of the properties and reactivity of the
In water and soil systems, cyanide occurs in various physical forms, including many different
kinds of species dissolved in water, many different solid species, and several gaseous species. The
Chemically, cyanide can be classifiedinto inorganic and organic forms, as indicated in Figure 2.1.
Inorganic forms, which occur in all three physical states, include free cyanide, weak metal–cyanide
complexes, strong metal–cyanide complexes, thiocyanate andmetal–thiocyanate complexes, cyanate
15
© 2006 by Taylor & Francis Group, LLC
most commonly occurring aqueous, gaseous, and solid forms of cyanide is provided in Chapter 5.
cyanide species that occur in the aqueous, solid, and gas phases are indicated in Figure 2.1.
16 Cyanide in Water and Soil
WATER
GAS
SOLID
Free
cyanide
Metal–cyanide
complexes
Cyanate,
thiocyanate
Organocyanides
Free
cyanide
HCN(g)
Cyanogen
halides
CNCl(g),
CNBr(g)
Simple metal

cyanide solids
NaCN(s), KCN(s),
CuCN(s) …
Alkali or alkaline earth
metal-metal
cyanide solids
K
3
Fe(CN)
6
(s), K
4
Fe(CN)
6
(s),
KAg(CN)
2
(s), …
Other
metal-metal
cyanide solids
Fe
4
[Fe(CN)
6

]
3
(s),
Fe

3
[Fe(CN)
6
]
2
(s), …
HCN, CN
–
Weak complexes:
Ag(CN)
2
–
, CdCN
–
, …
Strong complexes:
Fe(CN)
4–
Fe(CN)
3–
…
CNO
–
, SCN
–
Nitriles, cyanohydrins, …
6
,
6
,

FIGURE 2.1 Forms and species of cyanide in water and soil.
and metal–cyanate complexes, and cyanogen halides. Aqueous free cyanide is the sum of hydrogen
cyanide, HCN, and its deprotonated form, the cyanide anion, CN
−
. HCN is volatile under environ-
mental conditions and occurs as both aqueous and gaseous species. Many metals can bond with the
cyanide anion to form dissolved metal–cyanide complexes, as well as metal–cyanide solids. Cyanate,
CNO
−
, requires the presence of strong oxidizing agents for its formation and thus is rarely found
in the environment. Thiocyanate, SCN
−
, can be formed in the environment and is also present in a
variety of industrial wastewater discharges. The cyanogen halides of interest, CNCl and CNBr, form
upon chlorination or bromination of water containing free cyanide. These species are volatile under
environmental conditions, and thus occur as both aqueous and gaseous species. Organic cyanides
contain carbon–carbon covalent bonding between hydrocarbon and cyanide moieties, and are usually
present as dissolved species.
Natural as well as anthropogenic sources discharge a wide range of cyanide species to the envir-
onment. Over 2650 species of plants (130 families) produce cyanogenic glycosides as part of natural
coexisting plant enzyme and release HCN. In addition, almost all fruit-bearing plants release HCN
during ethylene synthesis, which aids in the fruit ripening process (Chapter 3).
Cyanide (as free, organic and metal-complexed cyanide compounds) is used as a raw mater-
ial during the production of chemicals (nylon and plastics), pesticides, rodenticides, gold, wine,
anticaking agents for road salt, fire retardants, cosmetics, pharmaceuticals, painting inks, and other
and hydrometallurgical gold extraction (Chapter 4). One of the earliest uses of cyanide dates back to
1704, when the solid phase iron–cyanide compound ferric ferrocyanide (FFC), Fe
4
[Fe(CN)
6

]
3
(s),
also referred to as Prussian Blue, was first used as a pigment for artist colors [1,2]. In addition, free
cyanide, weak and strong metal–cyanide complexes, and thiocyanates also occur as by-products of
many current and former industrial processes (Chapter 4). Current industries that produce cyanide
as a by-product include chemical manufacturing, iron and steel making, petroleum refining, and
aluminum smelting. An example of a past industry that generated cyanide-bearing wastewaters and
solid wastes in substantial quantities is gas manufacture by coal gasification. There are thousands of
former manufactured gas plant (MGP) sites throughout the eastern and midwestern United States and
© 2006 by Taylor & Francis Group, LLC
defense mechanisms (Chapter 3). Upon stress or injury, cyanogenic glycosides are hydrolyzed by a
materials (Chapter 4). Cyanide is also used directly in a variety of processes, including electroplating
Physical and Chemical Forms of Cyanide 17
Europe with soil containing FFC, which was generated as a process by-product and often managed
onsite as fill [3]. Cyanide contamination exists at many other former industrial sites. It is one of the
most common contaminants identified at Superfund sites in the United States [4].
The aim of this chapter is to provide an overview of the common physical and chemical forms of
cyanide that occur in water and soil systems. In the following sections, the cyanide species of primary
interest in gaseous form, dissolved in water, and in solid form are listed and briefly described.
2.1 GASEOUS FORMS OF CYANIDE
Three gaseous forms of cyanide are of interest in water and soil systems: hydrogen cyanide (HCN),
cyanogen chloride (CNCl), and cyanogen bromide (CNBr). Cyanogen chloride and cyanogen brom-
ide are disinfection by-products formed in water and wastewater treatment [5,6]. HCN is present in
wastewater discharges and leachates from certain industrial waste sites, and can be formed in nature
as well.
Hydrogen cyanide gas is colorless with an odor of bitter almonds. It is highly toxic to humans
HCN has a high vapor pressure (630 mm Hg at 20
◦
C; Ref [7]) and is readily volatilized from water

at pH values less than 9, where HCN remains fully protonated.
The cyanogen halides CNCl and CNBr are also colorless gases with high vapor pressures
(1230 mm Hg and 121 mm Hg at 25
◦
C for CNCl and CNBr, respectively [8,9]). Like hydrogen
cyanide gas, CNCl and CNBr are highly toxic to humans if inhaled or absorbed. These are soluble
in water, but degrade by hydrolysis, very rapidly at high pH [5]. Degradation is rapid at any pH if
there is free chlorine or sulfite present [5]. At pH 10, degradation of CNCl and CNBr by hydrolysis
occurs with half-lives in the range of 20 to 40 min [5]. The hydrolysis degradation product is cyanate
ion (CNO
−
), which can subsequently hydrolyze to CO
2
and NH
3
at alkaline pH conditions (see
2.2 AQUEOUS FORMS OF CYANIDE
free cyanide, metal–cyanide complexes, cyanate and thiocyanate species, and organocyanide com-
pounds. Free cyanide comprises molecular HCN and cyanide anion. Metal–cyanide complexes range
from weakmetal–cyanide complexes (e.g., complexes ofcopper, zinc, and nickelwith CN
−
) tostrong
metal–cyanide complexes (e.g., complexes of cobalt and iron with CN
−
). Cyanate and thiocyanate
form by oxidation of free cyanide, in the presence of sulfide compounds in the case of thiocyanate.
Both of these species are anionic for the environmental pH range, and form complexes with metals.
Finally, there are organocyanide complexes, where the cyanide anion is covalently bonded to a
hydrocarbon group.
2.2.1 FREE CYANIDE

soluble hydrogen cyanide, HCN(aq), or soluble cyanide anion (CN
−
). HCN(aq) is a weak acid
with a pK
a
of 9.24 at 25
◦
(Chapter 5). It can dissociate into cyanide ion according to the following
dissociation reaction:
HCN(aq) = H
+
+CN
−
,pK
a
= 9.24 at 25
◦
C (2.1)
where the “=” sign denotes a two-way, equilibrium reaction. Thus, at pH values less than 9.24, HCN
is the dominant free cyanide species, while at greater pH values cyanide ion dominates free cyanide.
© 2006 by Taylor & Francis Group, LLC
(see Chapter 13). HCN(g) is very soluble in water, forming a weak acid, HCN(aq), upon dissolution.
Chapter 5).
Free cyanide represents the most toxic cyanide forms (see Chapters 13 and 14). It refers to either
Common aqueous forms of cyanide, listed in Table2.1, can be broadly divided intofour majorclasses:
18 Cyanide in Water and Soil
TABLE 2.1
Common Aqueous Cyanide Species
Classification Cyanide species
Free cyanide HCN, CN

−
Weak metal–cyanide AgCN(OH)
−
, Ag(CN)
−
2
, Ag(CN)
2−
3
, Ag(OCN)
−
2
complexes CdCN
−
, Cd(CN)
0
2
, Cd(CN)
−
3
, Cd(CN)
2−
4
Cu(CN)
−
2
, Cu(CN)
2−
3
, Cu(CN)

3−
4
Ni(CN)
0
2
, Ni(CN)
−
3
, Ni(CN)
2−
4
, NiH(CN)
−
4
, NiH
2
(CN)
0
4
, NiH
3
(CN)
+
4
Zn(CN)
0
2
, Zn(CN)
−
3

, Zn(CN)
2−
4
HgCN
+
, Hg(CN)
0
2
, Hg(CN)
−
3
, Hg(CN)
2−
4
, Hg(CN)
2
Cl
−
, Hg(CN)
3
Cl
2−
, Hg(CN)
3
Br
2−
Strong metal–cyanide BaFe(CN)
2−
6
, BaFe(CN)

−
6
complexes CaFe(CN)
2−
6
, CaFe(CN)
−
6
,Ca
2
Fe(CN)
0
6
, CaHFe(CN)
2−
6
Fe(CN)
4−
6
, HFe(CN)
3−
6
,H
2
Fe(CN)
2−
6
,Fe
2
(CN)

0
6
K
2
H
2
Fe(CN)
0
6
,K
3
HFe(CN)
0
6
, KHFe(CN)
2−
6
K
2
Fe(CN)
2−
6
, KFe(CN)
3−
6
LiFe(CN)
3−
6
,Li
2

Fe(CN)
2−
6
, LiHFe(CN)
2−
6
Fe(CN)
3−
6
MgFe(CN)
−
6
, MgFe(CN)
2−
6
NH
4
Fe(CN)
3−
6
, (NH
4
)
2
Fe(CN)
2−
6
,NH
5
Fe(CN)

2−
6
NaFe(CN)
3−
6
,Na
2
Fe(CN)
2−
6
, NaHFe(CN)
2−
6
SrFe(CN)
−
6
TlFe(CN)
3−
6
Au(CN)
−
2
Co(CN)
3−
6
Pt(CN)
2−
4
Cyanate HOCN, OCN
−

Metal–cyanate complexes Ag(OCN)
−
2
, and others
Thiocyanate HSCN, SCN
−
Metal–thiocyanate MgSCN
+
complexes MnSCN
+
FeSCN
+
FeSCN
2+
, Fe(SCN)
+
2
, Fe(SCN)
0
3
, Fe(SCN)
−
4
, FeOHSCN
+
CoSCN
+
, Co(SCN)
0
2

CuSCN
+
, Cu(SCN)
0
2
NiSCN
+
, Ni(SCN)
0
2
CrSCN
2+
, Cr(SCN)
+
2
CdSCN
+
, Cd(SCN)
0
2
, Cd(SCN)
−
3
, Cd(SCN)
2−
4
ZnSCN
+
, Zn(SCN)
0

2
, Zn(SCN)
−
3
, Zn(SCN)
2−
4
, and others
Organocyanides Nitriles (e.g., acetonitrile)
Cyanohydrins
Cyanocobalamin and others
© 2006 by Taylor & Francis Group, LLC
Physical and Chemical Forms of Cyanide 19
2.2.2 METAL–CYANIDE COMPLEXES
The cyanide anion is a versatile ligand that reacts with many metal cations to form metal–cyanide
complexes. These species, which aretypically anionic, have ageneral formulaof M(CN)
n−
x
, where M
is a metal cation, x is the number of cyanide groups, and n is the ionic charge of the metal–cyanide
complex.
The stability of metal–cyanide complexes is variable and requires moderate to highly acidic pH
conditions in order to dissociate. Metal–cyanide complex dissociation yields free cyanide:
M(CN)
n−
x
= M
+
+xCN
−

(2.2)
Metal–cyanide complexes are classified into two broad categories, namely, weak metal–cyanide
complexes and strong metal–cyanide complexes, based on the strength of the bonding between
the metal and the cyanide ion. Complexes with greater strength of the metal–cyanide bond are more
stable in aqueous solution, that is, they dissociate only to a limited extent, and the dissolution process
may be very slow.
2.2.2.1 Weak Metal–Cyanide Complexes
Weak metal–cyanide complexes are those in which the cyanide ions are weakly bonded to the metal
cation, such that they can dissociate under mildly acidic conditions (pH = 4 to 6) to produce free
cyanide. Because of their dissociative nature, they are often regulated along with free cyanide in
water. Common examples of weak metal–cyanide complexes include copper cyanide (Cu(CN)
2−
3
),
zinc cyanide (Zn(CN)
2−
4
), nickel cyanide (Ni(CN)
2−
4
), cadmium cyanide (Cd(CN)
2−
4
), mercury
cyanide (Hg(CN)
2
), and silver cyanide (Ag(CN)
−
2
).

2.2.2.2 Strong Metal–Cyanide Complexes
Strong metal–cyanide complexes include cyanide complexes with transition heavy metals such as,
iron, cobalt, platinum, and gold that require strong acidic conditions (pH < 2) in order to dissociate
and form free cyanide. Strong metal–cyanide complexes are much more stable in aqueous solution
than the weak ones and are relatively less toxic. Common examples of strong metal–cyanide com-
plexes include ferrocyanide (Fe(CN)
4−
6
), ferricyanide (Fe(CN)
3−
6
), gold cyanide (Au(CN)
−
2
), cobalt
cyanide (Co(CN)
3−
6
), and platinum cyanide (Pt(CN)
2−
4
).
2.2.3 CYANATE AND THIOCYANATE
Free cyanide can be oxidized to form cyanate, CNO
−
, or, depending on the pH, its protonated
form HOCN (pK
a
= 3.45 at 25
◦

C). Cyanate is substantially less toxic than free cyanide. It is rarely
encountered in aqueoussystems, as a strongoxidizing agent and a catalystare required for conversion
of free cyanide to CNO
−
or HOCN [10]. When cyanate does form it can react with metals to form
Free cyanide can reactwith various formsof sulfur toform thiocyanate, SCN
−
, which is relatively
nontoxic. The two forms of sulfur in the environment most reactive with free CN
−
are polysulfides,
S
x
S
2−
, and thiosulfate, S
2
O
2−
3
(Chapter 5). Thiocyanate can protonate to form HCNS
0
, but this
rarely occurs in natural systems as the pK
a
for this reaction is 1.1. Thiocyanate can form complexes
with many metals (Chapter 5).
© 2006 by Taylor & Francis Group, LLC
metal–cyanate complexes, though these reactions have not been studied extensively (Chapter 5).
20 Cyanide in Water and Soil

(sugar O)
n
C
C
ϵ
N
H,R
R
FIGURE 2.2 General structure of cyanogenic glycosides (R represents CH
3
group).
O
OCH
CN
O
CH
2
OH
CH
2
OH
O
HO
OH
OH
HO
O
HO
HO
OCH

2
HO
O
HO
OH
OCH
CN
Amygdalin (Cherry, Apricot) Dhurrin (Cassava)
O
C
CH
3
CN
CH
3
FIGURE 2.3 Common plant cyanogenic glycosides.
2.2.4 ORGANOCYANIDE COMPLEXES
Organic cyanide compounds contain a cyanide functional group that is attached to a carbon atom of
the organic molecule via covalent bonding. Common examples include nitriles, such as acetonitrile
(CH
3
CN) or cyanobenzene (C
6
H
5
CN), which are used as industrial solvents and as raw materials for
making nylon products and pesticides. Nitriles can also exist in the natural environment in shale oils
[11], in plants [12], or as a plant-growth hormone [13]. Several classes of nitriles can be produced
naturally or synthesized chemically, the most common of which are the cyanogenic glycosides and
cyanohydrins. Cyanohydrins, also known as α-hydroxynitriles, are organic cyanides with the general

structure R
1
R
2
C(OH)(CN), where the hydroxide group and the cyanide group are attached to the
same carbon atom.
Cyanogenic glycosides are produced by the plants under natural environmental conditions to aid
bonded to a carbon atom, which in turn is bound by a glycosidic linkage to one or more sugars
depicted in Figure 2.2. Some common cyanogenic glycosides produced by plants are shown in
Figure 2.3. Certain groups of nitriles such as, cyanogenic glycosides, exhibit high stability in water
as far as dissociation to free cyanide is concerned.
Other organocyanide compoundsof interest includecyanocobalamin, also known as Vitamin B
12
.
It consists of single cyanide group bonded to a central trivalent cobalt cation. Vitamin B
12
is syn-
thesized by microorganisms, not by plants, and is found in animal tissues as a result of intestinal
synthesis [14]. It is essential for human life, serving numerous functions and being an especially
important vitamin for maintaining healthy nerve cells and aiding the production of genetic building
blocks DNA and RNA [15]. There are cyanide and noncyanide forms of Vitamin B
12
. The noncyan-
ide forms include methylcobalamin, adenosylcobalamin, chlorocobalamin, and hydroxycobalamin.
These compounds, also produced by microorganisms, are less stable than cyanocobalamin but also
essential to human life.
2.3 SOLID FORMS OF CYANIDE
In systems with metals and cyanide present in sufficient quantities, metals can react with cyan-
ide to form a wide range of solids. The solid forms of cyanide may be divided into two general
© 2006 by Taylor & Francis Group, LLC

in their defense mechanism (Chapter 3). These species comprise a cyanide anion that is covalently
Physical and Chemical Forms of Cyanide 21
TABLE 2.2
Common Solid Phase Cyanide Species
Classification Cyanide species
Simple metal–cyanide solids KCN(s)
NaCN(s)
AgCN(s)
CuCN(s)
Hg(CN)
2
(s)
Alkali or alkaline earth metal–metal K
4
Fe(CN)
6
(s)
cyanide solids K
3
Fe(CN)
6
(s)
K
4
Ni
4
(Fe(CN)
6
)
3

(s)
K
2
CdFe(CN)
6
(s)
K
2
Cu
2
Fe(CN)
6
(s)
KZn
1.5
Fe(CN)
6
(s)
Other metal–metal cyanide solids Fe
4
[Fe(CN)
6
]
3
(s)
Fe
3
[Fe(CN)
6
]

2
(s)
Fe[Fe(CN)
6
](s)
Fe
2
[Fe(CN)
6
](s)
Ag
4
Fe(CN)
6
(s)
Cd
2
Fe(CN)
6
(s)
Cu
2
Fe(CN)
6
(s)
Zn
2
Fe(CN)
6
(s)

categories: simple metal–cyanide solids, which are relatively soluble, and metal–metal cyanide com-
plex solids with varying degree of solubility. Some common metal–cyanide and metal–metal cyanide
solids are listed in Table 2.2.
2.3.1 SIMPLE METAL–CYANIDE SOLIDS
This class of cyanide solids consist of structurally simple, metal cyanides of the form M(CN)
x
,
where M is an alkali, alkaline earth metal or a heavy metal. Common examples include sodium
cyanide (NaCN(s)), potassium cyanide (KCN(s)), calcium cyanide, (Ca(CN)
2
(s)), zinc cyanide
(Zn(CN)
2
(s)), and others (see Table 2.2). Most of these solids are highly soluble in water and readily
dissociate, releasing the cyanide ion, and therefore are potentially toxic.
2.3.2 METAL–METAL CYANIDE SOLIDS
This class of cyanide solids consists of one or more alkali, alkaline earth, or transition metal
cations combined with an anionic metal–cyanide complex. Based on whether the metal cation is
alkali/alkaline earth or transition metal, this class of compounds is again subdivided into two cat-
egories: alkali/alkaline earth metal–metal cyanide solids and other metal–metal cyanide solids. In the
latter, the metals involved are B-type or transition metals [16].
2.3.2.1 Alkali/Alkaline Earth Metal–Metal Solids
This class of structurally complex solids comprises one or more alkali or alkaline earth metal
cations ionically bonded to an anionic metal–cyanide complex with the general formula of
A
x
[M(CN)
y
]·nH
2

O, where A is an alkali or alkaline earth metal cation (or ammonium ion), M
is a transition metal atom, x is the number of alkali metal atoms, y is the number of cyanide groups,
© 2006 by Taylor & Francis Group, LLC
22 Cyanide in Water and Soil
and n is the number of water molecules incorporated in the solid structure. A common example
of this class of compound is potassium ferrocyanide (K
4
Fe(CN)
6
(s)). Alkali/alkaline earth metal–
metal cyanide complex salts can readily dissociate in aqueous solutions, releasing the alkali metal
cation and the anionic metal cyanide complex according to the following equation:
A
x
[M(CN)
y
]·nH
2
O = xA
+
+[M(CN)
y
]
m−
(2.3)
where m is the ionic charge of the metal–cyanide complex released to solution.
2.3.2.2 Other Metal–Metal Cyanide Complex Salts
This class of structurally complex compound comprises one or more transition metal cations
ionically bonded to an anionic transition metal cyanide complex with the general formula of
M

x
[M(CN)
y
]
z
·nH
2
O where M is a B-type or transition metal cation, x number of transition metal
cations, y is the number of cyanide groups, z is the number of metal–cyanide complexes, and n
is the number of water molecules in the structure. Due to the versatility of the cyanide anion as a
ligand, there are many different kinds of metal–metal cyanide compounds that exhibit a wide range
of structural properties [17].
Metal–metal cyanide solids involving all B-type and transition metals are very stable and relat-
these compoundsare relatively soluble, releasingmetal cations andanionic metal–cyanide complexes
to solution according to the following general reaction:
M
x
[M(CN)
y
]
z
·nH
2
O = xM
+
+z[M(CN)
y
]
m−
(2.4)

where m is the ionic charge of the metal–cyanide complex released to aqueous solution.
A well-known example of a transition metal–metal cyanide is ferric ferrocyanide
4
6
3
2.4 SUMMARY AND CONCLUSIONS
• Cyanide is present in gas, liquid, and solid forms in water and soil systems.
• Many different species of cyanide occur in water and soil systems. The specific form
of cyanide determines the environmental fate and transport of cyanide, as well as its
toxicity. Understanding the specific form(s) of cyanide present in a particular water, soil,
or sediment is critical for assessment of how to manage or treat the cyanide present.
• Cyanide mostly occurs in inorganic forms. The dissolved forms of primary interest are
free cyanide (HCN and CN
−
) and metal–cyanide complexes. Solid forms of cyanide
include simple metal–cyanide solids (e.g., NaCN(s), KCN(s)), which are relatively sol-
uble, and more complex, less soluble metal–metal cyanide solids (e.g., Fe
4
(Fe(CN)
6
)
3
(s),
or Prussian Blue). The gaseous form of cyanide of primary interest is HCN(g).
• Free cyanide, either in dissolved (HCN and CN
−
) or gaseous form (HCN(g)), are the
species of primary interest with respect to human health and aquatic toxicity.
• Dissolved inorganic metal–cyanide complexes can be categorized as weak metal–cyanide
complexes and strong metal–cyanide complexes, based on the strength of the bonding

between the metal and the cyanide ion.
• Cyanate (CNO
−
) is formed from oxidation of free cyanide. It can react with metals and
form metal–cyanate complexes.
• Thiocyanate (SCN
−
) is formed from reaction of free cyanide with various forms of sulfur.
It can react with metals to form metal-thiocyanate complexes.
© 2006 by Taylor & Francis Group, LLC
ively insoluble under acidic and neutral conditions (Chapter 5). However, under alkaline conditions,
Fe (Fe(CN) ) (s), or Prussian Blue, which has various commercial and medicinal uses (Chapter 4).
Physical and Chemical Forms of Cyanide 23
• Organic compounds containing cyanide are produced by both natural and anthropogenic
activities. They consist of molecules with carbon–carbon covalent bonding with the
–CN group. Common organocyanide compounds include the nitriles, such as acetonitrile
(CH
3
CN).
REFERENCES
1. ACC, The Chemistry of the Ferrocyanides, American Cyanamid Co., New York, NY, 1953.
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