1、Designation: D6696 10Standard 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 parentheses indi
2、cates 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 theclassification of the various chemical forms of cyanide. It isintended to provide a general understanding of
3、 the chemicalnature of distinct cyanide species as related to chemicalanalysis and environmental fate and transport.2. Referenced Documents2.1 ASTM Standards:2D1129 Terminology Relating to WaterD7237 Test Method for Free Cyanide with Flow InjectionAnalysis (FIA) Utilizing Gas Diffusion Separation an
4、dAmperometric Detection3. 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 inidentifying and describing the different chemical forms ofcyanide. The complex nature of cyanide c
5、hemistry, existenceof numerous distinct chemical forms as well as the variousregulatory distinctions that may be made can lead to confusionin technical discussions on cyanide and in the selection ofappropriate methods for its analysis. This guide is intended toprovide clarification and a common fram
6、ework of terms anddefinitions to facilitate discussions and referencing differentcyanide chemical species and groups of cyanide compounds.4.2 The use of such common terminology is particularlyimportant from an environmental perspective because certainforms of cyanide are considered to be toxic. Ther
7、efore, theirrelease into the environment is regulated by federal and stateagencies. Thus a general understanding of cyanide chemistryand species definitions is needed for proper wastewater man-agement and testing.5. Cyanide Species Terms and Definitions5.1 Chemistry Related Terms and Definitions:5.1
8、.1 Cyanide IonThe term used to describe a negativelycharged ion comprised of one carbon atom and one nitrogenatom triply bonded to each other (CN-). The cyanide ion isreactive and readily forms neutral compounds or anioniccomplexes with most metals.5.1.2 Free CyanideThe form of cyanide that is bioav
9、ail-able and known for its toxic effect on organisms (1). Freecyanide refers to the sum of molecular hydrogen cyanide(HCN) and cyanide ion (CN-). Hydrogen cyanide is a colorless,poisonous gas having an odor of bitter almonds (mp = -13.4C,bp = 25.6C). It is readily soluble in water existing as HCN or
10、CN-, or both, depending on the pH conditions (pKa= 9.36). Ata pH of 7 or less in water, free cyanide is present entirely asHCN; the opposite is true at pH 11 or greater. Because of itstoxicity, free cyanide is regulated in environmental wastewaterdischarges.5.1.2.1 In Test Method D7237, sum of the f
11、ree cyanide(HCN and CN-) and cyanide bound in the metal-cyanidecomplexes that are easily dissociated into free cyanide underthe test conditions described in Test Method D7237 at pH 6 androom temperature.5.1.3 Aquatic Free CyanideIn Test Method D7237, freecyanide measured when the buffer or temperatu
12、re is adjusted tomimic the receiving water environment.5.1.4 Simple CyanideA neutral compound comprised ofan alkali metal, alkaline earth metal or ammonium cationbound to cyanide. Simple cyanides are so named because oftheir structural simplicity and their ability to completelydissolve and dissociat
13、e in water to produce free cyanide and acation according to the following reaction:ACN A11 CN2(1)where:A = alkali metal, alkaline earth metal or ammonium cation.1This guide is under the jurisdiction of ASTM Committee D19 on Water and isthe direct responsibility of Subcommittee D19.06 on Methods for
14、Analysis forOrganic Substances in Water.Current edition approved June 1, 2010. Published July 2010. Originally approvedin 2001. Last previous edition approved in 2005 as D6696 05e1. DOI: 10.1520/D6696-10.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Se
15、rvice at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.1*A Summary of Changes section appears at the end of this standard.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19
16、428-2959, United States.Examples of simple cyanides include sodium cyanide(NaCN) and potassium cyanide (KCN).5.1.5 Metal Cyanide ComplexA negatively charged ioniccomplex consisting of several cyanide ions bound to a singletransition metal cation. Also referred to as “metal-complexedcyanides,” “metal
17、 cyano-complexes” or “transition metal cya-nides,” these species 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 followingequilibrium in aqueous solu
18、tion:MCN!b#x2Mn11 bCN2(3)where:M = transition metal cation,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 isdependent of the stability of the complex and the solut
19、ion pH.On this basis, metal cyanide complexes are divided into twocategories: 1) “weak acid dissociable metal cyanide com-plexes” and 2) “strong acid dissociable metal cyanide com-plexes”.5.1.5.1 Weak Acid Dissociable Metal Cyanide ComplexAcyanide complex that dissociates under mildly acidic condi-t
20、ions (pH = 3-6) and in dilute solutions, forming free cyanide.Because of their ability to dissociate under slightly acidic tonearly neutral, ambient conditions, the weak acid dissociablemetal cyanide complexes are sometimes regulated along withfree cyanide in wastewater discharges. Several weak acid
21、dissociable metal cyanide complexes are presented in Table 1.A weak acid dissociable metal cyanide complex is alsosometimes referred to as a “weakly complexed cyanide”,“dissociable cyanide”, “available cyanide”, “directly toxiccyanide”, etc.5.1.5.2 Strong Metal Cyanide ComplexA metal cyanidecomplex
22、that requires strongly acidic conditions (pH 2) inorder to dissociate and form free cyanide. Due to theirresistance to dissociation and subsequent low toxicity, thestrong metal cyanide complexes are distinguished on a regu-latory basis from other forms of cyanide. Although some of thestrong metal cy
23、anide complexes are also subject to photo-chemical dissociation when exposed to UV radiation, the rateof dissociation is generally low in naturally turbid, shadedsurface waters. In addition, volatilization and biodegradation ofany dissociated free cyanide typically prevents their accumu-lation to to
24、xic levels in the environment thus supporting thisregulatory distinction. The term “strongly complexed cyanide”is also sometimes used to describe a strong metal cyanidecomplex. The most prevalent and well known of such speciesare the iron cyanide complexes namely, ferrocyanide IUPACnomenclature: hex
25、acyanoferrate(II) ion and ferricyanide IU-PAC nomenclature: hexacyanoferrate(III) ion; IUPAC = Inter-national Union of Pure and Applied Chemistry as well as goldand cobalt cyanide complexes. Examples of strong metalcyanide complexes are presented in Table 2.5.1.6 Metal-Metal Cyanide Complex SaltsNeu
26、tral com-pounds comprised of one or more metal cations and an anioniccyanide complex. The metal cations balance the charge of theanionic complex thus creating a neutral species. These speciesare divided into two categories: 1) “alkali metal-metal cyanidecomplex salts” or “alkaline earth metal-metal
27、cyanide complexsalts” 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 andan anionic cyanide complex having the general formula:AaMCN!b# yH2O (4)where:A = alkali metal counter cation,a = numb
28、er 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 inwater to form a free alkali metal cation and an anionic metalcyanide complex as follows:AaMCN!b# yH2OaA 1
29、MCN!b#x21 yH2O (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 ComplexSal
30、tsStructurally and chemically similar to alkali metal-metal cyanide complex salts, these compounds contain analkaline earth metal cation in place of an alkali metal cation(See 5.1.6.1).TABLE 1 Selected Weak Acid Dissociable Metal CyanideComplexes (2)Metal Cyanide Complex Stability Constant (log K at
31、 25C)Hg(CN)42-A6.22Cd(CN)42-17.9Zn(CN)42-19.6Ag(CN)2- 20.5Cu(CN)43-23.1Ni(CN)42-30.2ARefers to the stepwise dissociation: Hg(CN)42- Hg(CN)2+ 2CN-.TABLE 2 Selected Strong Metal Cyanide Complexes (2, 3)Cyanide Complex Stability Constant (log K at 25C)Hg(CN)2A,B32.8Fe(CN)64-35.4Au(CN)2-37CFe(CN)63-43.6
32、Co(CN)63- 64C Corrected editorially.AHg(CN)2is actually a neutral species and therefore more correctly identified asa metal cyanide compound rather than a metal cyanide complex.BHg(CN)2will be recovered by the available cyanide method (5.2.8) providedthat ligand-exchange reagents are used.CThis stab
33、ility constant is considered to be an estimate.D6696 1025.1.6.3 Transition Metal-Metal Cyanide Complex SaltsCompounds consisting of one or more transition metal cationsand an anionic metal cyanide complex having the generalformula:TtMCN!b#c yH2O (6)where:T = transition metal counter cation,t = numbe
34、r 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 referredto as “double metal cyanide complex salts” when the counterion and th
35、e metal cation bonded to the cyanide ligands are thesame, are extremely stable and generally insoluble under acidicand neutral pH conditions (4-6). They are, however, solubleunder alkaline conditions. Dissolution into aqueous solution isrepresented by the following equilibrium:TtMCN!b#c yH2OtT 1 cMC
36、N!b#x21 yH2O (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 = ionic charge of the transition metal complex, andy = number of waters of crystallization.An example of a transition metal-metal c
37、yanide complex saltis the ferric ferrocyanide species IUPAC nomenclature: iro-n(III) hexacyanoferrate(II) known as prussian blue:Fe4Fe(CN)63.NOTE 1Metal cyanide complexes that contain other ligands besidescyanide may also exist in aqueous solution. Examples of such complexesinclude: Hg(OH)CN and Fe(
38、CN)5H2O3-(7).5.2 Operationally Defined Definitions:5.2.1 Inorganic CyanideThis category includes all inor-ganic compounds or ionic complexes containing one or morecyanide ligands bonded directly to either a metal or anammonium ion.5.2.2 Organic CyanideOrganic compounds containing acyanide functional
39、 group. Examples of naturally occurringorganic cyanides are the cyanogenic glycosides. These speciesare comprised of a cyanide group bound to a carbon atom thatis in turn bound by a glycosidic linkage to one or more sugarsas depicted in Fig. 1. Specific examples of naturally occurringorganic cyanide
40、s include linamarin, dhurrin and amygdalin(Fig. 2). Organic cyanides also include nitriles, which arecommercially prepared, substituted hydrocarbons such as ac-etonitrile (CH3CN) or cyanobenzene (C6H5CN). Because thechemical bond to the cyanide functional group in organiccyanides is very stable, fre
41、e cyanide is generally not releasedfrom organic cyanides in aqueous solution under normalambient conditions.5.2.3 Total CyanideTotal cyanide is an analytically de-fined term that refers to the sum total of all of the inorganicchemical forms of cyanide that dissociate and release freecyanide when ref
42、luxed under strongly acidic conditions. Totalcyanide is determined analytically through strong acid distil-lation or UV irradiation followed by analysis of liberated freecyanide (8-10) on aqueous samples preserved with NaOH (pH 12). In water, total cyanide includes the following dissolvedspecies: fr
43、ee cyanide, weak acid dissociable metal cyanidecomplexes and strong metal cyanide complexes. However, itshould be noted that because of the sample preservation,certain suspended or colloidal forms of cyanide will dissolveprior to the distillation step; the recovery of which during theacid distillati
44、on step is variable and depends on varioussolution parameters, such as cyanide concentration in sus-pended solids, ionic strength of the sample, sample tempera-ture, acid digestion times, and so forth. Also, some of thestrong metal cyanide complexes, such as those of gold, cobaltand platinum, might
45、not be fully recovered during the totalcyanide analytical procedure. Additionally, total cyanide mayalso include some organic forms of cyanide such as nitriles thatmay release free cyanide under the conditions of the analysis.5.2.4 CyanidesThis term as used by the U.S. Environ-mental Protection Agen
46、cy, and appears as a Clean Water Act(CWA) pollutant as Item No. 23 in the list of toxic pollutantspursuant to Section 307(a)(1) of the CWA(11). The operationalmethods employed by the EPAfor this category are listed in 40CFR Part 136 (12).5.2.5 Diffusible CyanideThe form of operationally de-fined cya
47、nide that diffuses as HCN gas at room temperatureand at a pH of 6. Diffusible cyanide is recovered anddetermined using microdiffusion analysis (10). In water, thisform of cyanide generally includes dissolved free cyanide.Because of this, diffusible cyanide may provide a relativelyaccurate estimate o
48、f cyanide toxicity.5.2.6 Cyanides Amenable to Chlorination (CATC)A classof operationally defined cyanide species that undergo dissocia-tion and oxidation when exposed to chlorine under alkalineconditions. These species are determined analytically by com-puting the difference in total cyanide of an u
49、ntreated sampleand a sample treated with chlorine at room temperature (8-10).In water, this group of cyanide species generally includes freecyanide and the weak acid dissociable metal cyanide com-plexes. Cyanides amenable to chlorination provides a conser-vative estimate of toxicity because, in addition to free cyanide,it recovers some weak acid dissociable metal cyanide com-plexes that may or may not actually release free cyanide in theenvironment.FIG. 1 Cyanogenic Glycoside General StructureD6696
