ASTM D6696-16(2023)
(Guide)Standard Guide for Understanding Cyanide Species
Standard Guide for Understanding Cyanide Species
SIGNIFICANCE AND USE
4.1 This guide provides standard terminology for use in identifying and describing the different chemical forms of cyanide. The complex nature of cyanide chemistry, existence of numerous distinct chemical forms as well as the various regulatory distinctions that may be made can lead to confusion in technical discussions on cyanide and in the selection of appropriate methods for its analysis. This guide is intended to provide clarification and a common framework of terms and definitions to facilitate discussions and referencing different cyanide chemical species and groups of cyanide compounds.
4.2 The use of such common terminology is particularly important from an environmental perspective because certain forms of cyanide are considered to be toxic. Therefore, their release into the environment is regulated by federal and state agencies. Thus a general understanding of cyanide chemistry and species definitions is needed for proper wastewater management and testing.
SCOPE
1.1 This guide defines guidance based on a consensus of viewpoints for interpretation of test results to identify various chemical forms of cyanide. It is intended to provide a general understanding of the chemical nature of distinct cyanide species as related to chemical analysis and environmental fate and transport.
1.2 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.3 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
General Information
Relations
Standards Content (Sample)
This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: D6696 − 16 (Reapproved 2023)
Standard Guide for
Understanding Cyanide Species
This standard is issued under the fixed designation D6696; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope 4. Significance and Use
1.1 This guide defines guidance based on a consensus of 4.1 This guide provides standard terminology for use in
viewpoints for interpretation of test results to identify various identifying and describing the different chemical forms of
chemical forms of cyanide. It is intended to provide a general cyanide. The complex nature of cyanide chemistry, existence
understanding of the chemical nature of distinct cyanide of numerous distinct chemical forms as well as the various
species as related to chemical analysis and environmental fate regulatory distinctions that may be made can lead to confusion
and transport. in technical discussions on cyanide and in the selection of
appropriate methods for its analysis. This guide is intended to
1.2 The values stated in SI units are to be regarded as
provide clarification and a common framework of terms and
standard. No other units of measurement are included in this
definitions to facilitate discussions and referencing different
standard.
cyanide chemical species and groups of cyanide compounds.
1.3 This international standard was developed in accor-
4.2 The use of such common terminology is particularly
dance with internationally recognized principles on standard-
important from an environmental perspective because certain
ization established in the Decision on Principles for the
forms of cyanide are considered to be toxic. Therefore, their
Development of International Standards, Guides and Recom-
release into the environment is regulated by federal and state
mendations issued by the World Trade Organization Technical
agencies. Thus a general understanding of cyanide chemistry
Barriers to Trade (TBT) Committee.
and species definitions is needed for proper wastewater man-
agement and testing.
2. Referenced Documents
2.1 ASTM Standards:
5. Cyanide Species Terms and Definitions
D1129 Terminology Relating to Water
5.1 Chemistry Related Terms and Definitions:
D1426 Test Methods for Ammonia Nitrogen In Water
5.1.1 Cyanide Ion—The term used to describe a negatively
D3590 Test Methods for Total Kjeldahl Nitrogen in Water
charged ion comprised of one carbon atom and one nitrogen
D6888 Test Method for Available Cyanides with Ligand
-
atom triply bonded to each other (C≡N ). The cyanide ion is
Displacement and Flow Injection Analysis (FIA) Utilizing
reactive and readily forms neutral compounds or anionic
Gas Diffusion Separation and Amperometric Detection
complexes with most metals.
D7237 Test Method for Free Cyanide and Aquatic Free
5.1.2 Free Cyanide—The form of cyanide that is bioavail-
Cyanide with Flow Injection Analysis (FIA) Utilizing Gas
able and known for its toxic effect on organisms (1). Free
Diffusion Separation and Amperometric Detection
cyanide refers to the sum of molecular hydrogen cyanide
-
(HCN) and cyanide ion (CN ). Hydrogen cyanide is a colorless,
3. Terminology
poisonous gas having an odor of bitter almonds (mp =
3.1 Definitions—For a definition of terms used in this guide,
-13.4 °C, bp = 25.6 °C). It is readily soluble in water existing
-
refer to Terminology D1129.
as HCN or CN , or both, depending on the pH conditions
(pK = 9.36). At a pH of 7 or less in water, free cyanide is
a
present entirely as HCN; the opposite is true at pH 11 or
greater. Because of its toxicity, free cyanide is regulated in
This guide is under the jurisdiction of ASTM Committee D19 on Water and is
the direct responsibility of Subcommittee D19.06 on Methods for Analysis for
environmental wastewater discharges.
Organic Substances in Water.
5.1.2.1 In Test Method D7237, sum of the free cyanide
Current edition approved Nov. 15, 2023. Published December 2023. Originally
-
(HCN and CN ) and cyanide bound in the metal-cyanide
approved in 2001. Last previous edition approved in 2016 as D6696 – 16. DOI:
10.1520/D6696-16R23.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Standards volume information, refer to the standard’s Document Summary page on The boldface numbers in parentheses refer to a list of references at the end of
the ASTM website. this standard.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D6696 − 16 (2023)
TABLE 1 Selected Weak Acid Dissociable Metal Cyanide
complexes that are easily dissociated into free cyanide under
Compounds and Complexes (2)
the test conditions described in Test Method D7237 at pH 6 and
Stability Constant (log K at
room temperature.
Metal Cyanide Complex
25 °C)
5.1.3 Aquatic Free Cyanide—In Test Method D7237, free
2-A
[Hg(CN) ] 6.22
cyanide measured when the buffer or temperature is adjusted to B
Hg(CN) 32.8
2-
mimic the receiving water environment. [Cd(CN) ] 17.9
2-
[Zn(CN) ] 19.6
5.1.4 Simple Cyanide—A neutral compound comprised of
[Ag(CN) ]- 20.5
3-
an alkali metal, alkaline earth metal or ammonium cation
[Cu(CN) ] 23.1
2-
bound to cyanide. Simple cyanides are so named because of [Ni(CN) ] 30.2
A 2- -
their structural simplicity and their ability to completely Refers to the stepwise dissociation: [Hg(CN) ] ⇔ Hg(CN) + 2CN .
4 2
B
Hg(CN) will be recovered by the available cyanide method (5.2.8) provided that
dissolve and dissociate in water to produce free cyanide and a
ligand-exchange reagents are used.
cation according to the following reaction:
1 2
ACN→A 1CN (1)
where:
(1) Weak Acid Dissociable Metal Cyanide Compounds and
A = alkali metal, alkaline earth metal or ammonium cation.
Complexes—A cyanide compound or complex that dissociates
Examples of simple cyanides include sodium cyanide
under mildly acidic conditions (pH = 3–6) and in dilute
(NaCN) and potassium cyanide (KCN).
solutions, forming free cyanide. Complex cyanides bound with
5.1.5 Metal Cyanide Complex—A negatively charged ionic
cadmium, zinc, silver and copper typically dissociate under
complex consisting of several cyanide ions bound to a single
mildly acidic distillation conditions.
transition metal cation. Also referred to as “metal-complexed
(2) Ligand Exchange Dissociable Metal Cyanide Com-
cyanides,” “metal cyano-complexes” or “transition metal
pounds and Complexes—A cyanide compound or complex that
cyanides,” these species have the general formula:
dissociates under the action of ligand-exchange reagents and
x2
M CN (2)
@ ~ ! # gas diffusion conditions (see Test Method D6888). Complex
b
cyanides bound with nickel or mercury typically require
where:
ligand-exchange reagents for dissociation.
M = transition metal cation,
(3) pH Buffering Dissociable Metal Cyanide Compounds
b = number of cyanide groups, and
and Complexes—A cyanide compound or complex that disso-
x = ionic charge of the transition metal complex.
ciates under the action of pH buffering, forming free cyanide
Metal cyanide complexes are represented by the following
(see Test Method D7237). Simple cyanides bound with sodium
equilibrium in aqueous solution:
and complex cyanides bound with zinc or cadmium are
x2 n1 2
amenable to dissociation using pH 6–8 buffer.
M CN ⇔M 1bCN (3)
@ ~ ! #
b
5.1.5.2 Strong Metal Cyanide Complex—A metal cyanide
where:
complex that requires strongly acidic conditions (pH <2) in
M = transition metal cation,
order to dissociate and form free cyanide. Due to their
n = ionic charge of the transition metal cation,
resistance to dissociation and subsequent low toxicity, the
b = number of cyanide ions, and
strong metal cyanide complexes are distinguished on a regu-
x = ionic charge of the transition metal complex.
latory basis from other forms of cyanide. Although some of the
The degree of dissociation of the metal cyanide complex is
strong metal cyanide complexes are also subject to photo-
dependent of the stability of the complex and the solution pH. chemical dissociation when exposed to UV radiation, the rate
On this basis, metal cyanide complexes are divided into two
of dissociation is generally low in naturally turbid, shaded
categories: (1) “weak acid dissociable metal cyanide com- surface waters. In addition, volatilization and biodegradation of
plexes” and (2) “strong acid dissociable metal cyanide com-
any dissociated free cyanide typically prevents their accumu-
plexes.” lation to toxic levels in the environment thus supporting this
5.1.5.1 Weak and Dissociable Metal Cyanide Compounds regulatory distinction. The term “strongly complexed cyanide”
and Complexes—A cyanide compound or complex that either is also sometimes used to describe a strong metal cyanide
dissociates under conditions of weak acid distillation, pH complex. The most prevalent and well known of such species
buffering, or ligand-exchange reagents. Because of their ability are the iron cyanide complexes namely, ferrocyanide [IUPAC
to dissociate under slightly acidic or slightly basic to nearly nomenclature: hexacyanoferrate(II) ion] and ferricyanide [IU-
neutral, ambient conditions, or through the use of ligand- PAC nomenclature: hexacyanoferrate(III) ion; IUPAC = Inter-
exchange reagents, the weak and dissociable metal cyanide national Union of Pure and Applied Chemistry] as well as gold
compounds and complexes are sometimes regulated along with and cobalt cyanide complexes. Examples of strong metal
free cyanide in wastewater discharges. Several weak and cyanide complexes are presented in Table 2.
dissociable metal cyanide compounds and complexes are 5.1.6 Metal-Metal Cyanide Complex Salts—Neutral com-
presented in Table 1. A weak and dissociable metal cyanide pounds comprised of one or more metal cations and an anionic
compound or complex is also sometimes referred to as a cyanide complex. The metal cations balance the charge of the
“weakly complexed cyanide,” “dissociable cyanide,” “avail- anionic complex thus creating a neutral species. These species
able cyanide,” “directly toxic cyanide,” etc. are divided into two categories: (1) “alkali metal-metal cyanide
D6696 − 16 (2023)
x2
TABLE 2 Selected Strong Metal Cyanide Complexes (2, 3)
T M CN ·yH O⇔tT1c M CN 1yH O (7)
@ ~ ! # @ ~ ! #
t 2 2
b c b
Stability Constant (log K
Cyanide Complex
where:
at 25 °C)
4-
T = transition metal counter cation,
[Fe(CN) ] 35.4
- A
[Au(CN) ] 37
t = number of transition metal counter cations,
3-
[Fe(CN) ] 43.6
b = number of cyanide ions,
3- A
[Co(CN) ] 64
c = number of metal complex anions,
A
This stability constant is considered to be an estimate.
x = ionic charge of the transition metal complex, and
y = number of waters of crystallization.
NOTE 1—Metal cyanide complexes that contain other ligands besides
cyanide may also exist in aqueous solution. Examples of such complexes
3-
complex salts” or “alkaline earth metal-metal cyanide complex include: Hg(OH)CN and [Fe(CN) H O] (7).
5 2
NOTE 2—When both the transition metal counter cation and metal
salts” and (2) “transition metal-metal cyanide complex salts”.
cation bonded to the cyanide ligands are the same metal, the species is
5.1.6.1 Alkali Metal-Metal Cyanide Complex Salts—
referred to as a double metal cyanide complex salt. An example of a
Compounds comprised of one or more alkali metal cations and
double metal cyanide complex salt is the ferric ferrocyanide species
an anionic cyanide complex having the general formula:
[IUPAC nomenclature: iron (III) hexacyanoferrate (II)] known as prussian
blue: Fe [Fe(CN) ] .
4 6 3
A @M~CN! #·yH O (4)
a 2
b
5.2 Operationally Defined Definitions:
where:
5.2.1 Inorganic Cyanide—This category includes all inor-
A = alkali metal counter cation,
ganic compounds or ionic complexes containing one or more
a = number of alkali metal counter cations,
cyanide ligands bonded directly to either a metal or an
M = transition metal cation,
ammonium ion.
b = number of cyanide ions, and
5.2.2 Organic Cyanide—Organic compounds containing a
y = number of waters of crystallization.
cyanide functional group. Examples of naturally occurring
Alkali metal-metal cyanide complex salts readily dissolve in
organic cyanides are the cyanogenic glycosides. These species
water to form a free alkali metal cation and an anionic metal
are comprised of a cyanide group bound to a carbon atom that
cyanide complex as follows:
is in turn bound by a glycosidic linkage to one or more sugars
x2
as depicted in Fig. 1. Specific examples of naturally occurring
A @M CN #·yH O⇔aA1@M CN # 1yH O (5)
~ ! ~ !
a 2 2
b b
organic cyanides include linamarin, dhurrin, and amygdalin
where:
(Fig. 2). Organic cyanides also include nitriles, which are
A = alkali metal counter cation,
commercially prepared, substituted hydrocarbons such as ac-
a = number of alkali metal counter cations,
etonitrile (CH CN) or cyanobenzene (C H CN). Because the
3 6 5
M = transition metal cation,
chemical bond to the cyanide functional group in organic
b = number of cyanide ions,
cyanides is very stable, free cyanide is generally not released
x = ionic charge of the transition metal complex, and
from organic cyanides in aqueous solution under normal
y = number of waters of crystallization.
ambient conditions.
5.1.6.2 Alkaline Earth Metal-Metal Cyanide Complex
5.2.3 Total Cyanide—Total cyanide is an analytically de-
Salts—Structurally and chemically similar to alkali metal-
fined term that refers to the sum total of all of the inorganic
metal cyanide complex salts, these compounds contain an
chemical forms of cyanide that dissociate and release free
alkaline earth metal cation in place of an alkali metal cation
cyanide when refluxed under strongly acidic or alkaline con-
(see 5.1.6.1).
ditions. Total cyanide may also include some organic forms of
5.1.6.3 Transition Metal-Metal Cyanide Complex Salts—
cyanide that may release free cyanide under the conditions of
Compounds consisting of one or more transition metal cations
the analysis. Total cyanide is determined analytically through
and an anionic metal cyanide complex having the general
strong acid distillation or UV irradiation followed by analysis
formula:
of liberated free cyanide (8-10) on aqueous samples preserved
with NaOH. In water, total cyanide includes the following
T @M~CN! # ·yH O (6)
t 2
b
...
Questions, Comments and Discussion
Ask us and Technical Secretary will try to provide an answer. You can facilitate discussion about the standard in here.