1、Designation: D6696 10D6696 14Standard 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 standard terminology used for the classification of the guidance based on a consensus of viewpoints forinterpretation of test results
3、 to identify various chemical forms of cyanide. It is intended to provide a general understanding of thechemical 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
4、 measurement are included in this standard.2. Referenced Documents2.1 ASTM Standards:2D1129 Terminology Relating to WaterD1426 Test Methods for Ammonia Nitrogen In WaterD3590 Test Methods for Total Kjeldahl Nitrogen in WaterD7237 Test Method for Free Cyanide with Flow InjectionAnalysis (FIA) Utilizi
5、ng Gas Diffusion Separation andAmperometricDetection3. Terminology3.1 DefinitionsFor a definition of terms used in this guide, refer to Terminology D1129.4. Significance and Use4.1 This guide provides standard terminology for use in identifying and describing the different chemical forms of cyanide.
6、 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 in technical discussions on cyanide and in the selection of appropriate methods for itsanalysis. This guide is intended to provi
7、de clarification and a common framework of terms and definitions to facilitate discussionsand 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 formsof cyan
8、ide are considered to be toxic. Therefore, their release into the environment is regulated by federal and state agencies. Thusa general understanding of cyanide chemistry and species definitions is needed for proper wastewater management and testing.5. Cyanide Species Terms and Definitions5.1 Chemis
9、try 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 each other (CN-). The cyanide ion is reactive and readily forms neutral compounds or anionic complexes withmost metals.5.1.2 Free C
10、yanideThe form of cyanide that is bioavailable and known for its toxic effect on organisms (1). Free cyaniderefers to the sum of molecular hydrogen cyanide (HCN) and cyanide ion (CN-). Hydrogen cyanide is a colorless, poisonous gashaving an odor of bitter almonds (mp = -13.4C, bp = 25.6C). It is rea
11、dily soluble in water existing as HCN or CN-, or both,depending 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 discharge
12、s.1 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 OrganicSubstances in Water.Current edition approved June 1, 2010Jan. 1, 2014. Published July 2010January 2014. Originally approved in 2001. Last
13、 previous edition approved in 20052010 asD6696 05D6696 10.1. DOI: 10.1520/D6696-10.10.1520/D6696-14.2 For referencedASTM 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 Doc
14、ument Summary page on the ASTM website.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
15、recommends that users 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 Bo
16、x C700, West Conshohocken, PA 19428-2959. United States15.1.2.1 In Test Method 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 temperat
17、ure.5.1.3 Aquatic Free CyanideIn Test Method D7237, free cyanide measured when 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 a
18、re so named because of their structural simplicity and their ability to completely 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
19、 sodium cyanide (NaCN) and potassium cyanide (KCN).5.1.5 Metal Cyanide ComplexA 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 s
20、pecies have the general formula:MCN!b#x2 (2)where:M = transition metal cation,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 cati
21、on,n = ionic charge of the transition metal cation,b = number of cyanide ions, 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 divid
22、ed into two categories: 1) “weak acid dissociable metal cyanide complexes” and 2)“strong acid dissociable metal cyanide complexes”.complexes.”5.1.5.1 Weak Acid Dissociable Metal Cyanide ComplexA cyanide complex that dissociates under mildly acidic conditions(pH = 3-6) and in dilute solutions, formin
23、g free cyanide. Because of their ability to dissociate under slightly acidic to nearly neutral,ambient conditions, the weak acid dissociable metal cyanide complexes are sometimes regulated along with free cyanide inwastewater discharges. Several weak acid dissociable metal cyanide complexes are pres
24、ented in Table 1. A weak acid dissociablemetal cyanide complex is also sometimes referred to as a “weakly complexed cyanide”,cyanide,” “dissociable cyanide”,cyanide,”“available cyanide”,cyanide,” “directly toxic cyanide”,cyanide,” etc.5.1.5.2 Strong Metal Cyanide ComplexA metal cyanide complex that
25、requires strongly acidic conditions (pH 2) in order todissociate 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 metal cyanid
26、ecomplexes 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 typicallyTABLE 1 Selected Weak Acid Dissociable
27、Metal CyanideComplexes (2)Metal Cyanide Complex Stability Constant (log K at25C)Hg(CN)42-A 6.22Cd(CN)42- 17.9Cd(CN)42- 17.9Zn(CN)42- 19.6Zn(CN)42- 19.6Ag(CN)2- 20.5Ag(CN)2- 20.5Cu(CN)43- 23.1Cu(CN)43- 23.1Ni(CN)42- 30.2Ni(CN)42- 30.2A Refers to the stepwise dissociation: Hg(CN)42- Hg(CN)2 + 2CN-.D66
28、96 142prevents their accumulation to 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 na
29、mely, ferrocyanide IUPAC nomenclature: 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.
30、1.6 Metal-Metal Cyanide Complex SaltsNeutral 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 comp
31、lex salts” or “alkaline earth metal-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)wher
32、e:A = alkali metal counter cation,a = 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 metalcy
33、anide complex as follows:AaMCN!b#yH2OaA1MCN!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 Alkali
34、ne Earth Metal-Metal Cyanide Complex 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(see 5.1.6.1).5.1.6.3 Transition Metal-Metal Cyanide Complex SaltsCompounds consisti
35、ng of one or more transition metal cations and ananionic metal cyanide complex having the general formula:TtMCN!b#cyH2O (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 anion
36、s, andTABLE 2 Selected Strong Metal Cyanide Complexes (2, 3)Cyanide Complex Stability Constant (log Kat 25C)Hg(CN)2A,B 32.8Fe(CN)64- 35.4Fe(CN)64- 35.4Au(CN)2- 37CAu(CN)2- 37CFe(CN)63- 43.6Fe(CN)63- 43.6Co(CN)63- 64CCo(CN)63- 64C Corrected editorially.A Hg(CN)2 is actually a neutral species and ther
37、efore 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) provided thatligand-exchange reagents are used.C This stability constant is considered to be an estimate.D6696 143y = number of waters of
38、 crystallization.Transition metal-metal cyanide complex salts, also referred to as “double metal cyanide complex salts” when the counter ionand the metal cation bonded to the cyanide ligands are the same, are extremely stable and generally insoluble under acidic andneutral pH conditions (4-6). They
39、are, however, soluble under alkaline conditions. Dissolution into aqueous solution is representedby 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 comp
40、lex anions,x = ionic 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 cy
41、anide complexes that contain other ligands besides cyanide may also exist in aqueous solution. Examples of such complexes include:Hg(OH)CN and Fe(CN)5H2O3-(7).5.2 Operationally Defined Definitions:5.2.1 Inorganic CyanideThis category includes all inorganic compounds or ionic complexes containing one
42、 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 comprised of a cyanide group bound to a carbon a
43、tom 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
44、such as acetonitrile (CH3CN) or cyanobenzene (C6H5CN). Because the chemical bond to thecyanide functional group in organic cyanides is very stable, free cyanide is generally not released from organic cyanides in aqueoussolution under normal ambient conditions.5.2.3 Total CyanideTotal cyanide is an a
45、nalytically defined term that refers to the sum total of all of the inorganic chemicalforms of cyanide that dissociate and release free cyanide when refluxed under strongly acidic conditions. Total cyanide isdetermined analytically through strong acid distillation or UV irradiation followed by analy
46、sis of liberated free cyanide (8-10) onaqueous samples preserved with NaOH (pH 12). In water, total cyanide includes the following dissolved species: free cyanide,weak acid dissociable metal cyanide complexes and strong metal cyanide complexes. However, it should be noted that becauseof the sample p
47、reservation, certain suspended or colloidal forms of cyanide will dissolve prior to the distillation step; the recoveryof which during the acid distillation step is variable and depends on various solution parameters, such as cyanide concentrationin suspended solids, ionic strength of the sample, sa
48、mple temperature, acid digestion times, and so forth. Also, some of the strongmetal cyanide complexes, such as those of gold, cobalt and platinum, might not be fully recovered during the total cyanideanalytical procedure. Additionally, total cyanide may also include some organic forms of cyanide suc
49、h as nitriles that may releasefree cyanide under the conditions of the analysis.5.2.4 CyanidesThis term as used by the U.S. Environmental Protection Agency, and appears as a Clean Water Act (CWA)pollutant as Item No. 23 in the list of toxic pollutants pursuant to Section 307(a)(1) of the CWA (11). The operational methodsemployed by the EPA for this category are listed in 40 CFR Part 136 (12).5.2.5 Diffusible CyanideThe form of operationally defined cyanide that diffuses as H
