1、Designation: D 6696 05e1Standard Guide forUnderstanding Cyanide Species1This standard is issued under the fixed designation D 6696; 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
2、indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.e1NOTETable 2 was corrected editorially in January 2006.1. Scope1.1 This guide defines standard terminology used for theclassification of the various chemical forms of
3、cyanide. It isintended to provide a general understanding of the chemicalnature of distinct cyanide species as related to chemicalanalysis and environmental fate and transport.2. Referenced Documents2.1 ASTM Standards:2D 1129 Terminology Relating to Water3. Terminology3.1 DefinitionsFor a definition
4、 of terms used in this guide,refer to Terminology D 1129.4. Significance and Use4.1 This guide provides standard terminology for use inidentifying and describing the different chemical forms ofcyanide. The complex nature of cyanide chemistry, existenceof numerous distinct chemical forms as well as t
5、he 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 framework of terms anddefinitions from which to discuss and reference d
6、ifferentcyanide 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. Therefore, theirrelease into the environment is regulated by federal and stat
7、eagencies. 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:5.1.1 Cyanide IonThe term used to describe a negativelycharged ion comprised
8、 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 bioavail-able and known for its toxic effect on organisms (1). Freecyanide ref
9、ers 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 partitioning itself asHCN or CN-, or both, depending on the pH conditions(pKa= 9.36). At a
10、pH of 7 or less in water, free cyanide ispresent entirely as HCN; the opposite is true at pH 11 orgreater. Because of its toxicity, free cyanide is regulated inenvironmental wastewater discharges.5.1.3 Simple CyanideA neutral compound comprised ofan alkali metal, alkaline earth metal or ammonium cat
11、ionbound to cyanide. Simple cyanides are so named because oftheir structural simplicity and their ability to completelydissolve and dissociate 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 catio
12、n.Examples of simple cyanides include sodium cyanide(NaCN) and potassium cyanide (KCN).5.1.4 Metal Cyanide ComplexA negatively charged ioniccomplex consisting of several cyanide ions bound to a singletransition metal cation. Also referred to as “metal-complexedcyanides,” “metal cyano-complexes” or “
13、transition metal cya-nides,” these species have the general formula:1This guide is under the jurisdiction of ASTM Committee D19 on Water and isthe direct responsibility of Subcommittee D19.06 on Organic Constituents in Water.Current edition approved July 1, 2005. Published July 2005. Originally appr
14、ovedin 2001. Last previous edition approved in 2001 as D 6696 01.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM webs
15、ite.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.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 followi
16、ngequilibrium in aqueous solution: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
17、 of the complex and the solution 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”.5.1.4.1 Weak Acid Dissociable Metal Cyanide ComplexAcyanide complex that dissociate
18、s under mildly acidic condi-tions (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
19、discharges. Several weak aciddissociable metal cyanide complexes are presented in Table 1.A weak acid dissociable metal cyanide complex is alsosometimes referred to as a “weakly complexed cyanide” or“dissociable cyanide”.5.1.4.2 Strong Metal Cyanide ComplexA metal cyanidecomplex that requires strong
20、ly 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 cyanide complexes are
21、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 toxic levels in the en
22、vironment 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: hexacyanoferrate(II) io
23、n 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 2.5.1.5 Metal-Metal Cyanide Complex SaltsNeutral com-pounds comp
24、rised 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 cyanide complexsalts
25、” and 2) “transition metal-metal cyanide complex salts”.5.1.5.1 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 = number of alkali metal c
26、ounter cations,M = transition metal cation,b = number of cyanide ions, andy = number of waters of cyrstallization.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 MCN!b#x21 yH2O (5)wh
27、ere: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.5.1.5.2 Alkaline Earth Metal-Metal Cyanide ComplexSaltsStructurally and c
28、hemically very similar to alkali metal-metal cyanide complex salts, these compounds contain analkaline earth metal cation in place of an alkali metal cation(See 5.1.5.1).5.1.5.3 Transition Metal-Metal Cyanide Complex SaltsCompounds consisting of one or more transition metal cationsand an anionic met
29、al cyanide complex having the generalformula:TtMCN!b#c yH2O (6)TABLE 1 Selected Weak Acid Dissociable Metal CyanideComplexes (2)Metal Cyanide Complex Stability Constant (log K at 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:
30、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)63-43.6Co(CN)63- 64C Corrected editorially.AHg(CN)2is actually a neutral species and therefore more correctly identified asa meta
31、l 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 stability constant is considered to be an estimate.D669605e12where:T = transition metal counter cation,t = number of transitio
32、n 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 the metal cation
33、 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 cMCN!b#x21 yH2O (
34、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 cyanide complex
35、 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(CN)5H2O3-7.5.2
36、 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 group. Examples
37、 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 cyanides include linama
38、rin, 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, free cyanide is gen
39、erally 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 refluxed under stro
40、ngly 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: free cyanide, weak
41、 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 distillation step is varia
42、ble 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 not be fully rec
43、overed 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 Agency, and appears
44、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 cyanide that diffus
45、es 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 of cyanide toxici
46、ty.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 untreated samplea
47、nd 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 cya
48、nide,it recovers some weak acid dissociable metal cyanide com-plexes that may or may not actually release free cyanide in theenvironment.5.2.7 Weak Acid Dissociable (WAD) CyanideAn opera-tionally defined group of cyanide species that undergo disso-ciation and liberate free cyanide when refluxed unde
49、r weaklyacidic conditions (pH 4.5-6). Weak acid dissociable cyanide isdetermined analytically through weak acid distillation andanalysis of liberated free cyanide (8, 10). Similar to cyanidesamenable to chlorination, weak acid dissociable cyanide alsoFIG. 1 Cyanogenic Glycoside General StructureD669605e13provides a conservative estimate of toxicity as it recovers bothfree cyanide and weak acid dissociable metal cyanide com-plexes. This term should not be confused with the term “weakand dissociable
