1、Designation: D6696 14D6696 16Standard Guide forUnderstanding Cyanide Species1This standard is issued under the fixed designation D6696; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision. A number in parenthe
2、ses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope*1.1 This guide defines guidance based on a consensus of viewpoints for interpretation of test results to identify various chemicalforms of cyanide. It is in
3、tended to provide a general understanding of the chemical nature of distinct cyanide species as relatedto 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.2. Referenced
4、 Documents2.1 ASTM Standards:2D1129 Terminology Relating to WaterD1426 Test Methods for Ammonia Nitrogen In WaterD3590 Test Methods for Total Kjeldahl Nitrogen in WaterD6888 Test Method forAvailable Cyanide with Ligand Displacement and Flow InjectionAnalysis (FIA) Utilizing Gas DiffusionSeparation a
5、nd Amperometric DetectionD7237 Test Method for Free Cyanide and Aquatic Free Cyanide with Flow Injection Analysis (FIA) Utilizing Gas DiffusionSeparation and Amperometric Detection3. Terminology3.1 DefinitionsFor a definition of terms used in this guide, refer to Terminology D1129.4. Significance an
6、d Use4.1 This guide provides standard terminology for use in identifying and describing the different chemical forms of cyanide. Thecomplex nature of cyanide chemistry, existence of numerous distinct chemical forms as well as the various regulatory distinctionsthat may be made can lead to confusion
7、in technical discussions on cyanide and in the selection of appropriate methods for itsanalysis. This guide is intended to provide clarification and a common framework of terms and definitions to facilitate discussionsand referencing different cyanide chemical species and groups of cyanide compounds
8、.4.2 The use of such common terminology is particularly important from an environmental perspective because certain formsof cyanide are considered to be toxic. Therefore, their release into the environment is regulated by federal and state agencies. Thusa general understanding of cyanide chemistry a
9、nd species definitions is needed for proper wastewater management and testing.5. Cyanide Species Terms and Definitions5.1 Chemistry Related Terms and Definitions:5.1.1 Cyanide IonThe term used to describe a negatively charged ion comprised of one carbon atom and one nitrogen atomtriply bonded to eac
10、h other (CN-). The cyanide ion is reactive and readily forms neutral compounds or anionic complexes withmost metals.5.1.2 Free CyanideThe form of cyanide that is bioavailable and known for its toxic effect on organisms (1).3 Free cyaniderefers to the sum of molecular hydrogen cyanide (HCN) and cyani
11、de ion (CN-). Hydrogen cyanide is a colorless, poisonous gashaving an odor of bitter almonds (mp = -13.4C, bp = 25.6C). It is readily soluble in water existing as HCN or CN-, or both,1 This guide is under the jurisdiction of ASTM Committee D19 on Water and is the direct responsibility of Subcommitte
12、e D19.06 on Methods for Analysis for OrganicSubstances in Water.Current edition approved Jan. 1, 2014April 1, 2016. Published January 2014April 2016. Originally approved in 2001. Last previous edition approved in 20102014 asD6696 10.D6696 14. DOI: 10.1520/D6696-14.10.1520/D6696-16.2 For referencedAS
13、TM standards, visit theASTM website, www.astm.org, or contactASTM Customer Service at serviceastm.org. For Annual Book of ASTM Standardsvolume information, refer to the standards Document Summary page on the ASTM website.3 The boldface numbers in parentheses refer to a list of references at the end
14、of this standard.This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Becauseit may not be technically possible to adequately depict all changes accurately, ASTM recommends that users
15、consult prior editions as appropriate. In all cases only the current versionof the standard as published by ASTM is to be considered the official document.*A Summary of Changes section appears at the end of this standardCopyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohoc
16、ken, PA 19428-2959. United States1depending on the pH conditions (pKa = 9.36). At a pH of 7 or less in water, free cyanide is present entirely as HCN; the oppositeis true at pH 11 or greater. Because of its toxicity, free cyanide is regulated in environmental wastewater discharges.5.1.2.1 In Test Me
17、thod D7237, sum of the free cyanide (HCN and CN-) and cyanide bound in the metal-cyanide complexes thatare easily dissociated into free cyanide under the test conditions described in Test Method D7237 at pH 6 and room temperature.5.1.3 Aquatic Free CyanideInTest Method D7237, free cyanide measured w
18、hen the buffer or temperature is adjusted to mimicthe receiving water environment.5.1.4 Simple CyanideA neutral compound comprised of an alkali metal, alkaline earth metal or ammonium cation bound tocyanide. Simple cyanides are so named because of their structural simplicity and their ability to com
19、pletely dissolve and dissociatein water to produce free cyanide and a cation according to the following reaction:ACNA11CN2 (1)where:A = alkali metal, alkaline earth metal or ammonium cation.Examples of simple cyanides include sodium cyanide (NaCN) and potassium cyanide (KCN).5.1.5 Metal Cyanide Comp
20、lexA negatively charged ionic complex consisting of several cyanide ions bound to a singletransition metal cation. Also referred to as “metal-complexed cyanides,” “metal cyano-complexes” or “transition metal cyanides,”these species have the general formula:MCN!b#x2 (2)where:M = transition metal cati
21、on,b = number of cyanide groups, andx = ionic charge of the transition metal complex.Metal cyanide complexes are represented by the following equilibrium in aqueous solution:MCN!b#x2Mn11bCN2 (3)where:M = transition metal cation,n = ionic charge of the transition metal cation,b = number of cyanide io
22、ns, andx = ionic charge of the transition metal complex.The degree of dissociation of the metal cyanide complex is dependent of the stability of the complex and the solution pH. Onthis basis, metal cyanide complexes are divided into two categories: (1) “weak acid dissociable metal cyanide complexes”
23、 and (2)“strong acid dissociable metal cyanide complexes.”5.1.5.1 Weak Acidand Dissociable Metal Cyanide ComplexCompounds and ComplexesA cyanide compound or complexthat either dissociates under mildly acidic conditions (pH = 3-6) and in dilute solutions, forming free cyanide. conditions of weakacid
24、distillation, pH buffering, or ligand-exchange reagents. Because of their ability to dissociate under slightly acidic or slightlybasic to nearly neutral, ambient conditions, the weak acidor through the use of ligand-exchange reagents, the weak and dissociablemetal cyanide compounds and complexes are
25、 sometimes regulated along with free cyanide in wastewater discharges. Several weakacidand dissociable metal cyanide compounds and complexes are presented in Table 1. A weak acidand dissociable metal cyanidecompound or complex is also sometimes referred to as a “weakly complexed cyanide,” “dissociab
26、le cyanide,” “available cyanide,”“directly toxic cyanide,” etc.(1) Weak Acid Dissociable Metal Cyanide Compounds and ComplexesA cyanide compound or complex that dissociatesunder mildly acidic conditions (pH = 36) and in dilute solutions, forming free cyanide. Complex cyanides bound with cadmium,zinc
27、, silver and copper typically dissociate under mildly acidic distillation conditions.TABLE 1 Selected Weak Acid Dissociable Metal CyanideCompounds and Complexes (2)Metal Cyanide Complex Stability Constant (log K at25C)Hg(CN)42-A 6.22Hg(CN)2B 32.8Cd(CN)42- 17.9Zn(CN)42- 19.6Ag(CN)2- 20.5Cu(CN)43- 23.
28、1Ni(CN)42- 30.2A Refers to the stepwise dissociation: Hg(CN)42- Hg(CN)2 + 2CN-.B Hg(CN)2 will be recovered by the available cyanide method (5.2.8) provided thatligand-exchange reagents are used.D6696 162(2) Ligand Exchange Dissociable Metal Cyanide Compounds and ComplexesA cyanide compound or comple
29、x thatdissociates under the action of ligand-exchange reagents and gas diffusion conditions (see Test Method D6888). Complex cyanidesbound with nickel or mercury typically require ligand-exchange reagents for dissociation.(3) pH Buffering Dissociable Metal Cyanide Compounds and ComplexesA cyanide co
30、mpound or complex that dissociatesunder the action of pH buffering, forming free cyanide (see Test Method D7237). Simple cyanides bound with sodium and complexcyanides bound with zinc or cadmium are amenable to dissociation using pH 68 buffer.5.1.5.2 Strong Metal Cyanide ComplexAmetal cyanide comple
31、x that requires strongly acidic conditions (pH 2) 2) in orderto dissociate and form free cyanide. Due to their resistance to dissociation and subsequent low toxicity, the strong metal cyanidecomplexes are distinguished on a regulatory basis from other forms of cyanide. Although some of the strong me
32、tal cyanidecomplexes are also subject to photochemical dissociation when exposed to UV radiation, the rate of dissociation is generally lowin naturally turbid, shaded surface waters. In addition, volatilization and biodegradation of any dissociated free cyanide typicallyprevents their accumulation t
33、o toxic levels in the environment thus supporting this regulatory distinction. The term “stronglycomplexed cyanide” is also sometimes used to describe a strong metal cyanide complex. The most prevalent and well known ofsuch species are the iron cyanide complexes namely, ferrocyanide IUPAC nomenclatu
34、re: hexacyanoferrate(II) ion andferricyanide IUPAC nomenclature: hexacyanoferrate(III) ion; IUPAC = International Union of Pure and Applied Chemistry aswell as gold and cobalt cyanide complexes. Examples of strong metal cyanide complexes are presented in Table 2.5.1.6 Metal-Metal Cyanide Complex Sal
35、tsNeutral compounds comprised of one or more metal cations and an anionic cyanidecomplex. The metal cations balance the charge of the anionic complex thus creating a neutral species. These species are dividedinto two categories: (1) “alkali metal-metal cyanide complex salts” or “alkaline earth metal
36、-metal cyanide complex salts” and (2)“transition metal-metal cyanide complex salts”.5.1.6.1 Alkali Metal-Metal Cyanide Complex SaltsCompounds comprised of one or more alkali metal cations and an anioniccyanide complex having the general formula:AaMCN!b#yH2O (4)where:A = alkali metal counter cation,a
37、 = number of alkali metal counter cations,M = transition metal cation,b = number of cyanide ions, andy = number of waters of crystallization.Alkali metal-metal cyanide complex salts readily dissolve in water to form a free alkali metal cation and an anionic metalcyanide complex as follows:AaMCN!b#yH
38、2OaA1MCN!b#x21yH2O (5)where:A = alkali metal counter cation,a = number of alkali metal counter cations,M = transition metal cation,b = number of cyanide ions,x = ionic charge of the transition metal complex, andy = number of waters of crystallization.5.1.6.2 Alkaline Earth Metal-Metal Cyanide Comple
39、x SaltsStructurally and chemically similar to alkali metal-metal cyanidecomplex salts, these compounds contain an alkaline earth metal cation in place of an alkali metal cation (see 5.1.6.1).TABLE 2 Selected Strong Metal Cyanide Complexes (2, 3)Cyanide Complex Stability Constant (log Kat 25C)Hg(CN)2
40、A,B 32.8Fe(CN)64- 35.4Au(CN)2- 37CAu(CN)2- 37AFe(CN)63- 43.6Co(CN)63- 64CCo(CN)63- 64AA Hg(CN)2 is actually a neutral species and therefore more correctly identified asa metal cyanide compound rather than a metal cyanide complex.B Hg(CN)2 will be recovered by the available cyanide method (5.2.8) pro
41、vided thatligand-exchange reagents are used.A This stability constant is considered to be an estimate.D6696 1635.1.6.3 Transition Metal-Metal Cyanide Complex SaltsCompounds consisting of one or more transition metal cations and ananionic metal cyanide complex having the general formula:TtMCN!b#cyH2O
42、 (6)where:T = transition metal counter cation,t = number of transition metal counter cations,M = transition metal cation,b = number of cyanide ions,c = number of metal complex anions, andy = number of waters of crystallization.Transition metal-metal cyanide complex salts, also referred to as “double
43、 metal cyanide complex salts” when the counter ionand the metal cation bonded to the cyanide ligands are the same, are salts are extremely stable and generally insoluble under acidicand neutral pH conditions (4-67). They are, however,All transition metal-metal cyanide complex salts, however, are sol
44、uble underalkaline conditions. Dissolution into aqueous solution is represented by the following equilibrium:TtMCN!b#cyH2OtT1cMCN!b#x21yH2O (7)where:T = transition metal counter cation,t = number of transition metal counter cations,b = number of cyanide ions,c = number of metal complex anions,x = io
45、nic charge of the transition metal complex, andy = number of waters of crystallization.An example of a transition metal-metal cyanide complex salt is the ferric ferrocyanide species IUPAC nomenclature: iron(III)hexacyanoferrate(II) known as prussian blue: Fe4Fe(CN)6 3.NOTE 1Metal cyanide complexes t
46、hat contain other ligands besides cyanide may also exist in aqueous solution. Examples of such complexes include:Hg(OH)CN and Fe(CN)5H2O3-(7).NOTE 2When both the transition metal counter cation and metal cation bonded to the cyanide ligands are the same metal, the species is referred toas a double m
47、etal cyanide complex salt.An example of a double metal cyanide complex salt is the ferric ferrocyanide species IUPAC nomenclature: iron(III) hexacyanoferrate (II) known as prussian blue: Fe4Fe(CN)63.5.2 Operationally Defined Definitions:5.2.1 Inorganic CyanideThis category includes all inorganic com
48、pounds or ionic complexes containing one or more cyanideligands bonded directly to either a metal or an ammonium ion.5.2.2 Organic CyanideOrganic compounds containing a cyanide functional group. Examples of naturally occurring organiccyanides are the cyanogenic glycosides. These species are comprise
49、d of a cyanide group bound to a carbon atom that is in turnbound by a glycosidic linkage to one or more sugars as depicted in Fig. 1. Specific examples of naturally occurring organiccyanides include linamarin, dhurrin, and amygdalin (Fig. 2). Organic cyanides also include nitriles, which are commerciallyprepared, substituted hydrocarbons such as acetonitrile (CH3CN) or cyanobenzene (C6H5CN). Because the chemical bond to thecyanide functional group in organic cyanides is very stable, fr
