California Association of Chemistry Teachers
George B. Kauffmon, Gary E. FOUS~, and Peggy Tun California State College at Fresno
Fresno, 93726
Pseudohalogens A genera/ chemistry laboratory experiment
O n e of the perplexing problems faced by teachers of today's general chemistry course is how to provide challenging material which is of fundamental importance yet which does not duplicate m a terial already mastered by students with superior high school backgrounds. Indeed, even for college students with no previous training in chemistry, the general chemistry laboratory can often degenerate into a place where the student experimentally verifies the facts and principles that have already been explained in text and lecture. I n our Chemistry 1A course, the student still learns in text and lecture the important trends traditionally illustrated by the halogens-oxidation potential, chemical reactivity, electronegativity, complex formation, covalency, etc.-all with reference to the elements of Periodic Group VIIA. I n the laboratory, however, he applies these principles not to the overly familiar fluorine, chlorine, bromine, and iodine, but rather to a different but analogous class of substances, which to him possess the charm of being new and exotic. An indirect advantage of the experiment described here is that the student's knowledge of descriptive chemistry, a sadly neglected dimension in today's largely theoretical and abstract general chemistry course, is simultaneously expanded with a minimum expenditure of time and effort. The pseudohalogens are of great practical and historical importance, but except for the classic Fe(II1)-SCNreaction used to illustrate equilibrium and mass action they are rarely considered in elementary laboratory and textbooks. On the few occasions when they are mentioned, it is as individual substances out of context, so to speak, and not as representatives of a closely related natural family. The synthesis, properties, and reactions of cyanogen and thiocyanogen, the two most important and easily prepared pseudohalogens, form the subject matter of the following experiment. Because of its lower volatilEDITOR'S NOTE: We reemphasize in this note what the authors repeatedly stress, namely that cyanides and their derivatives are deadly poisons. Poisoning may occur by absorption through the skin, through ingestion, or by inhalation. The experiments described here require special safety precautions and most careful supervision.
ity and consequently lower toxicity, the operations involving thiocyanogen (mp, -3'C) present less of a hazard than those involving cyanogen (mp, -27.92"C; bp, -21.35"C), but with careful supervision and proper safely precautions, both series of experiments have been found to be safe. It should be constantly borne in mind that cyanides and their derivatives are deadly poisons! Poisoning may occur by absorption through the skin, through cuts or abrasions, through ingestion, or by inhalation. If not immediately fatal or if the dose is not lethal, there is constriction of the throat, headache, and vertigo. Cyanide poisoning acts rapidly. Death often occurs before the physician arrives. Under no circumstances should the experiments described in this paper be attempted in the absence of well-ventilated hoods. The experiments have been modified from preparations appearing in the literature. Because of space limitations, details are omitted here. References to the original sources, however, are given. Detailed procedures including amounts, yields, and equations are available on request from the senior author (GBK).
A pseudohalogen or halogenoid is a substance composed of two or more electronegative atoms which resembles the halogens in physical and chemical properties. The most important pseudohalogens and their corresponding uninegative ions include: cyanogen (CN)2, cyanide CN-; thiocyanogen (SCN)2, thiocyanate SCN- and isothiocyanate NCS-; oxycyanogen (OCN)2, cyanate OCN-, isocyanate NCO-, and fulminate ONC-; selenocyanogen (SeCN)%,selenocyanate SeCN-; ~tellurocyanogen (TeCN)J, tellurocyanide TeCN-; azidocarhondisuliide (SCSN&, azidodithiocarbonate SCNs-; [nitrine (N&], azide Na-; and chloride dioxide CIOz, chlorite C102-. (The species in brackets have not been isolated). History Because of their extreme reactivity, their tendency to polymerize, and their ability to enter into a wide variety of reactions with both inorganic and organic substances, pseudohalogens and their compounds have Volume 45, Number 2, February 1968
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Reactions ofIonic Cyanides Hydvolysis. As salts of an extremely weak acid (Kmcrr = 4 X 10FOat 25'C), soluble cyanides are extensively hydrolyzed in aqueous solution, as the student can confirm by means of suitable indicators. Ozidation. Alkali and alkaline earth cvanides mav he oxidized K,CrlOl. Formation of Cyanoqm Iodide. To a solution of 4 in K I solution, to which starch suspension may be added to intensify the color. 1 AT KCN is added dro~wiseuntil the solution is decolorized. F m a t i m of Thioeyanale. One milliliter of dark (red) (NH& S, (polysulfide) solution is added to 1 ml of 1 M KCN. One millimeter of 6 M HC1 is added, and the solution is hailed to remove excess sulfide (Hood!). Addition of a. drop of FeCh solution confirms the presence of SCN- (see reactions of ionic thiocyanates in section on Thiocyanates and Thiocyanogen below). Reaction with Mermry(1). Excess 1 M KCN is added d r o p wise to a Hg2(NOa)*solution. The resulting gray color is due to metallic mercury formed by disproportionation. If H B or NalS is added to the supernatant solution, no HgS is precipitated a t 25- C), since despite its great insolubility (K,, = 3 X the soluble Hg(CN)* is largely covalent and furnishes little Hg2+. Prussian Blue Reaction. Cyanide ion forms stable complex ions with a, wide variety of transition metals. Not only the salts hut even the parent acids of some of these complexes can be prepared easily in the general chemistry laboratory (5). Its complex with iron(II), the ferrocyanide ion (G), may he prepared by boiling 1M XCN with FeS01.7Hn0and adding HC1. I t forms a. characteristic deep hlue precipitate with iron(III), the formation of which constitutes s. sensitive test whereby one part of Fe(II1) per million can he detected (7, p. 161). Nitroplusside Reaction. NaNOl is added to 1 M KCN, followed bv addition of FeCI. solution. Concentrated Hi301 is added until the solution changes from brown to yellowish green (Hood!). The solution is boiled and cooled. Excess Fe(II1) is precipitated hy addition of aq. NH8, and Fe(OHh is removed by centrifugation. Addition of S2- to the centrifugate yields a red-violet color, the formation of which constitutes s. sensitive test whereby one part of S per 50,000 can be detected (7, p. 303). Relative Stability of Cyanide Complezes. To individual solutions containing Co(I1) and Cd(II), 1 M KCN is added until the precipitates initially formed redissolve (Hood!), and excess KCN is then added. When H.S is bubbled through the resulting solutions, CdS (yellow), hot not CnS (black), is precipitated, a property sometimes employed in separating Cu(I1) and Cd(I1) in qualitative analysk (Kicu(cN).* = 5 X 10P1 at 25°C; K ~ c ~ ( c N )= , ~7.8 - X 10-18 at 25°C). The relative stability uf the NH8 and CN- complexes of Cu(I1) can he demonstrated by adding excess conc. NHI to s Co(I1) solution. When H,S is huhbled through the reunlting deep purple solution, a black precipitate of C11S ~.esult,s( K ~ C ~ ( N N= ~ )8.5 . ~ *X I 0 F 3 &t 2.5T). Relatiue Stability of A m i n e vesus Cyanide Complezes. One milliliter of a Ni(I1) salt solution is placed in each of three test tubes. To the first is added several drops of an ethanolic solution of dimethylglyoxime which produces a, bright scarlet precipitate of an insohthle inner complex, the formation of which permits the detection of one part of Ni in 2 million parts of HZO. Ten drops of conc. a% NHz is added to the second tube, producing the hlue complex ion Ni(NI~Ia),Pt. Addition of a few drops of dimethyglyoxime solrttion produces the scarlet precipitate, showing that s~fficientfree Ni2+ is present in equilibrium with the ammine complex to react with the dimethylglyoxime. If 1 M KCN is added to the third tuhe until the precipitate of Ni(CN). initially formed just redissolves, addition of dimethylglyoxime solution to the resulting yellowish brown solution produces no visible reaction, showing that the [NiP+lin equilibrium with N i ( C N ) F i a too small to cause precipitation with dimethylelvoxime. For NilIII. the CN- com~lex(Ki = 1 X 10F1 a t 5.5"~)is clearly m&e ;table than the NH, cbmplex (Ki= 5.7 X lo-' a t 25°C).
Reactions of Covalent Cyanides Pseudohalides are ambidentate, i.e., they can form
covalent bonds through more than one atom. For example, among SCN- complexes, metals of the first transition series usually bond through the X atom, whereas heavier transition metals usually bond through the S atom. After long speculation as to whether a given metal might bond either way, linkage isomers containing S-bonded and N-bonded SCN- have been recently prepared (8). Among simple compounds, the ambidentate nature of pseudohalides may also lead to z c polymerization," i.e., the formation of giant molecules. For example, inasmuch as CN- can bond through either C or N (O), AgCN consists of infinite chains of alternate Ag atoms and CN groups fulfillimg the coordination number of 2 for Ag: . . . Ag-C=N Ag-C=N A=. . . Ni(CN)2, on the other hand, in order to fulfill the coordination number of 4 for Ni, has a square planar configuration. The compound Ni(CN)2.iVH3.CeHsis a typical and easily prepared example of a clathrate or inclusion compound, i.e., a single-phased solid consisting of two distinct components, the host and the guest (10-13). The Ni atoms are of two kinds-tetracoordinate, bonded to C, and hexacoordinate, bonded to N. The benzene molecules occupy the interstices in the structure:
-
-
6 Ill
:I
NH1 N
111
-
NH,' N
Ill
C
Ill
Silwr Cyanide. This compound may he obtained in quanbitative yield by mixing stoiehiometric amounts of AgNOa and KCN solutions. An alternative preparation involves the aeration of a warm solution containing Ag(NH&+ and CN- ions (14, p. 151). may he used to synthesize (CN),. (See preparation . AaCN of cyanogen below). Clathrate Compound. Ni(CN)..NII,. CaH6may he prepared in 84% yield as a pale violet crystalline precipitate (14, p. 561; 15). The product gives no odor of CsHe,hut on application of a lighted match the CeHaignites, and a deposit of NiO is left. The clathrate compound is soluble in warm aq. NHa; when the resulting violet mlntion is cooled, droplets of CsHe appear. Cyanogen ( 1 6 )
Caution. Because of its great toxicity (maximum allowable concentration in air, 10 ppm) cyanogen will not be collected but will be used immediately. All operations should be carried out in a well-ventilated hood, and ali fittings of the gas generator and collecting flasks should be inspected by the instructor. In order to conserve l h i t e d hood space and reduce the instmctor's supelvision problems, students should work in groups. Preparation Cyanogen is most conveniently prepared by adding an aq. KCN solution to a CuSOn.5H20solution (17; 18,p. 660), a reaction analogous to that employed in the widely used iodometric determination of copper. Inasmuch as the reaction is exothermic, the KCN solution should be added slowly with shaking of the genevator. Volume 45, Number 2, February 1968
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Reactions of Ionic Cyanides Hydrolysis. As salts of an extremely weak acid (Koxm = 4 at 25'C), ~olublecyanides are extensively hydrolyzed in aqueous solution, as the student can confirm by means of suitable indieatom. Ozirlalion. Alkali and alkaline earth cvanides mav be oxidized X
KzCraOi. Formatia of Cyanogen Iodide. To a solution of 1%in K I solution, to which starch suspension may he added to intensify the color. 1 M KCN is added dro~wiseuntil the solution is deeoloriaed. Formation o j Thiocyanale. One milliliter of dark (red) ( N H d r S, (polysulfide) solution is added to 1 ml of 1 M KCN. One millimeter of 6 M HCI is added, and the solution is boiled to remove excess sulfide (Hood!). Addition of a drop of FeCls solution confirms the presence of SCN- (see reactions of ionic thiocyanates in section on Thiocyanates and Thiocyanogen below). Reaction with Mermry(1). Excess 1 M KCN is added dropwise to a Hga(NOa)*solution. The resulting gray color is due to metallic mercury formed by disproportionation. If HaS or NalS is added to the supernatant solution, no HgS is precipitated at 25- C), since despite it8 greet insohtbility (K., = 3 X the soluble H ~ ( C N ) Z is largely covalent and furnishes little
.
H +.- d +
Prussian Blue Reaction. Cyanide ion forms stable complex ions with a wide variety of transition metals. Not only the salts but even the parent acids of some of these complexes can be prepared easily in t,he general chemistry laboratory (6). Its complex with iron(II), the ferracyanide ion (b'), may be prepared by boiling 1 M KCN with FeSOn.7H20and adding HC1. I t farms s characteristic deep blue precipitate with iron(III), the formation of which constitutes s. sensitive test whereby one part of Fe(II1) per million can be detected (7, p. 161). Nitroplusside Reaction. NaNO. is added to 1 M KCN, followed bv addition of FeCI. solution. Concentrated HzSOI is added uhtil the solution changes from brown to yellowish green (Hood!). The solution is boiled and cooled. Excess Fe(II1) is precipitated by addition of aq. NIT*, and Fe(OH), is removed by centrifugation. Addition of Sa- to the centrifugate yields a red-violet color, the formation of which constitutes a sensitive test whereby one part of S per 50,000 can be detected (7, p. 303). Relative Stability of Cyanide Complezes. To individual solutions containing Cu(I1) and Cd(II), 1 M KCN is added until the precipitates initially formed redissolve (Hood!), and excess KCN is then added. When H2S is bubbled through the resulting solut,ions, CdS (yellow), but not CuS (black), is precipitated, a property somet,imesemployed in separating Cu(I1) and Cd(I1) s t 25-C; in qualitative analysis (Kicu(c~),a-= 5 X = 7.8 X 10-la&t25°C). The relative stability of the IZicd(c~),>NHa and CN- complexes of Cu(I1) can be demonstrated by adding excess cone. NHa to a. Cu(I1) solution. When ITS is bubbled through the resulting deep purple solution, a black precipitate of = 8.5 X 10-l3 a t 25°C). CuS remlts (K~C,,(NE~).~+ Relatiue Stability of Ammine versus Cyanide Complezes. One milliliter of a Ni(1I) salt solution is placed in each of three test tubes. To the first ia added several drops of an ethanolic sohtion of dimethylglyoxime which produces a, bright scarlet prreipitat,e of an insoluble inner complex, the formation of which permits the detection of one part of Ni in 2 million parts of H20. Ten drops of conc. aq. NHa is added to the second tube, producing the blue complex ion Ni(NIT#+. Addition of a few drops of dimethyglyoxime solution produces the scarlet precipitate, showing that s~fficientfree Ni2+ is present in equilibrium with the ammine complex to react with the dimethylglyoxime. If 1 M KCN is added to the third tube until the precipitate of Ni(CN). initially formed just redissolves, addition of dimethylglyoxime solut,ion to the resulting yellowish brown solution produces no visible reaction, showing that the [Niz+1in equilibrium with Ni(CN)rZ-is t w small to cause precipitation with dimethylglyoxime. For Ni(II), the CN- complex (Ki= 1 X 10-** a t 25°C) is clearly more stable than the NH, complex (Ki = 5.7 X lo-' a t 2.5-C).
Reactions of Coualent Cyanides Pseudohalides are amhidentate, i.e., they can form
covalenl bonds through more than one atom. For example, among SCN- complexes, metals of the first transition series usually bond through the N atom, whereas heavier transition metals usually bond through the S atom. After long speculation as to whether a given metal might bond either may, linkage isomers containing S-bonded and K-bonded SCN- have been recently prepared (8). Among simple compounds, the amhidentate nature of pseudohalides may also lead to "polymerization," i.e., the formation of giant molecules. For example, inasmuch as CN- can bond through either C or N (9),AgCN consists of infinite chains of alternate Ag atoms and CN groups fulfilling the coordination number of 2 for Ag: . .. + Ag-CGN Ag-*N Ag-. . . Ni(CN),, on the other hand, in order to fulfill the coordination number of 4 for Ni, has a square planar configuration. The compound Ni(CN)z.NHa.C8H8 is a typical and easily prepared example of a clathrate or inclusion compound, i.e., a single-phased solid consisting of two distmct components, the host and the guest (10-15). The Ni atoms are of two kinds--tetracoordinate, bonded to C, and hexacoordinate, bonded to N. The benzene molecules occupy the interstices in the structure:
-
-
Silver Cyanide. This compound may be obtained in quantitative yield by mixing staichiometric amounts of AgNOs and KCN solutions. An alternative preparatian involves the aeration of a warm solution containing Ag(NH&+ and CN- ions (14, p. 151). mav he used to synthesize ( C N h (See -preparation . AeCN of cymogen below). Clathrate Cummound. NiiCNL.NHs.CrH. mav , .~ . be .~revaredin 84% yield ss a pale violet crystalline precipitate (14, p. ,461; 15). The product gives no odor of CsHr, but an application of a lighted match the C6H6ignites, and a deposit of NiO is left. The elathrate compound is soluble in warm itq. NH8; when the resulting violet solution is cooled, droplets of CsHa appear. ~
~
~~
Cyonogen ( 7 6)
Caution. Because of its great toxicity (maximum allowable concentration in air, 10 ppm) cyanogen will not be collected but will be used immediately. All operations should be carried out in a well-ventilated hood, and all fittings of the gas generator and collecting flasks should be inspected by the instructor. In order to conserve l k i t e d hood space and reduce the instructor's supervision problems, students should work in groups. Preparation Cyanogen is most conveniently prepared by adding an aq. KCN solution to a CuS04.5H20solution (17; 18, p. 660),a reaction analogous to that employed in the widely used iodometric determination of copper. Inasmuch as the reaction i s exothennic, the K C N solution should be added slowly with shaking of the generator. Volume 45, Number 2, February 1968
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Half of the KCN solution is added, and the evolved (CN), is bubbled through a gas-collectmg bottle containing CzHSOHwhich has been saturated with HIS:
When (CN)*is no longer evolved, the bottle of C2HSOH is removed and replaced with a gas-collecting bottle containing 1 M KOH. The remaining KCN solution is added slowly to the generator as above, and the liberated (CN)%is allowed to bubble through the 1 'If KOH. Before the generator is washed out, FeS04.7 H,O should be added in order to react with all CN-. Alternatively, (CN)2 may be generated by heating the AgCN prepared in the preceding sect,ion on reactions of covalent cyanides (18, p. 661).
Dithiooxamide This sensitive analytical reagent can be obtained in 40% yield from the H,S-saturated ethanol through which (CN)%has been bubbled (19, p. 93). It has not only found use as a metal sequestrant, but it has also been employed in the manufacture of pigments, herbicides, vulcanization accelerators, and organic intermediates. It forms highly stable, intensely colored, insoluble, polymeric complexes with a variety of transition metal ions (SO), as can be shown by adding a few drops of its acetone solution to solutions of AgN08, CuSO1, EiSOn,CoC12,etc. and observing the characteristic colors of the resulting precipitates. To demonstrate the sensitivity of some of these reactions, a penny is pressed on the palm of the hand for 1 minute and then removed. A drop of dithiooxamide solution on the skin clearly outlines the imprint of the copper coin.
Tests for Cyanide and Cyanate Ions
washed with ethanol and ice water, and then air-dried. The product is dissolved in H,O and (SHJPSOIis added. The solution is evaporated to dryness, and the resulting urea is extracted from the residue with boiling CH30H (19, p. 114; St, p. 19; 25; 24). The product (43% yield) is identified as urea by the following tests: Melting Point. The melting point of the product is measured both alone (131-132T) and when mixed with commercial urea (no depression of melting point). Biuret Reaction. This general test for proteins and other compounds which contain multiple amide linkages is described in standard laboratory manuals (22, p. 20; $6). Urea-Formaldehyde Resin. By heating urea with formdin (40% HCHO) and HaBOa,s white insoluble polymer inert to most common laboratory reagents is obtained (26). Miscellaneous Tests. Addition of eonc. HNOl produces crystals of urea. nitrate, one of the few insoluble nit,rates. Urea. can be distinguished from the KCNO from which it was prepared by treatment of it8 solution with HISO, or HCI which yields no visible reaetion. Similar treatment of a CNO- sohtion yields CO. bubbles. Reversion of Urea Synthesis. An aqueous solution of urea is tested with a conductivity apparatus. The lamp does not light, and addition of AgNOa solution produces no visible reaction. The tests are repeated with a n identical urea solution which has been boiled for several minutes. Since the Wohler synthesis is a reversible reaction, the lamp now lights and a precipitate of AgCNO is farmed (23, t5).
Thiocyanates and Thiocyanogen Thiocyonofes
Reactions of Ionic Thiocyanates The thiocyanates are similar to the cyanides (see the section on cyanides above) as well as to the cyanates. For example, just as cyanates are formed from cyanides by addition of oxygen so are thiocyanates formed from cyanides by addition of sulfur (see reactions of ionic cyanides above). Like the cyanides, the thiocyanates form stable complexes with many transition metals. Unlike the cyanides, however, soluble thiocyanates show little tendency to hydrolyze, for their parent acid HSCN, a gaseous substance, is a strong acid. Thiocyanates of the alkali metals are soluble in water and have low melting points and limited thermal stability.
The following tests are performed with the solution prepared by bubbling (CN)%through KOH solution (see preparation of cyanogen above). CN-. A few drops of the test solution are added to a drop of a freshly prepared CuS suspension in a spot plate. Disappearance of the black color indicates the presence of CN-, a test which is sensitive to one part in 20,000 (7, p. 277). Halides and other pseudohalides do not interfere. For an alternative test see the Prussian Blue reaction above. CNO-. To a few drops of the test solution on a spot plate a drop of Co(NO& solution is added. A deep blue color indicates the presence of CNO- (7, p. 147; 81). The color reaction also takes place merely upon mixing the finely powdered solids, CoCI2.6Hz0 and KCNO.
Reaction with Iron(l1I). Because of the intense red color developed by t,he reaction between Fe(II1) and SCN-, Berzelius named (SCN)z "Rhodan," the name by which it is still known in German. By means of this test, one part of iron in 200,000 may he detected (7, p. 164). The color is due to several compounds which are in equilibrium with one another (27, 28). Readion with Cobalt(I1). To .5 drops of a. saturated solution of NI-LSCN in acetone in a. spot plate a drop of a Ca(I1) salt solution is added, whereupon a. blue color develops, by which one part of cobalt in 100,000 may be detected (7, p. 147). The color reaction also takes place merely upon mixing the finely powdered solids, CaCL.BH10 and KSCN (29). The blue complex can be extracted into ether (No o p n j h m e s s h u l d be present!). Reversion of Thiourea Synthesis. I n strict analogy to its oxygen analogne, thiourea is in equilibrium with NH8CN. An aq. thiourea solution is a nonconductor of electricity and gives no reaction with solutions of FeC18or AgN03. If thiourea, however, is maintained in the molten state far 10 min. the aoueaus solution
Wohler's Urea Synthesis
A ~ N O (SO). I
To the CN--CNO- solution prepared above, 30% HeOzis added in order to oxidize all the CN- to CNO-. The solution is evaporated to of its original volume, cooled in an ice bath, and the resulting crystals of KCNO (80% yield) are collected by suction filtration, 144 / Journol of Chemical Education
Reactions of Covalat Thiocyanates A number of thiocyanates, particularly those of Group I I B metals such as Zn, Cd, and Hg, are largely covalent and behave as weak electrolytes.
Mermry(I1) Thioeyanale. This interesting compound may be obtained in almost quantitative yield by metathetical precipitation (99, p. 15; 51). Inasmuch as it is soluble in an excess of both reagents from which it is prepared because of complex formation, a. Fe(II1) salt is used as an indicator, and the Hg(NO& solution is titrated with a KSCN solution until a permanent red color is obtained. The reaction is andogous to the classical Volhard titration and may be used far the quantitative determination of mercury. To a. portion of the dry product KNOI and dextrin or gum arahic is added, and the resultant thick "dough is molded into small conical "eggs" (Pharaoh's serpents). When dry, the eggs are ignited under the hood and give a voluminous serpentine ash (Poismousfumes!) (58). MercurviZl) TetrathiocvanotoeobaUate(ZI~.A broad dichotomv
eleotronegativity, whereas B-type cations form much stronger complexes with mare polarizable ligrtnd atoms of lower electranegativity. The ambidentate nature of SCN- has been meutioned above (under reactions of covalent cyanides); SCN- n n stitutes the best known case of a ligmd with a. N end appealing to A-type cations and a S end appealing to B-type cations (54). Both these functions are simultaneoosly satisfied in Hg[CoNCS),] which contains the chromophores Co(II)N, and Hg(II)& (56). The metathetical precipitation of this insoluble, deep blue compound can be used as a. sensitive test for the detection of as little as 0.01 mg of mercury (19, p. 147), and the compound itself is the most widely used calibrant for magnetic suscep tibility measurements. Lead(I1) Thiocyanate. Although (SCNj* can he prepared by the oxidation of an ether suspension of Hg(SCN)2, the reection goes much more smoothly when Pb(SCN). is used. The latter may be prepared in virtually quantitative yield by metathetical precipitation (58,57). 'Thiocyanogen
The oxidation potential in acid solution of the SCN-(SCN)2couple falls between those of the I-& and Br-Bra couples (58) : Couple 21e I. 2e s 1.- 2e 312SCN- a (SCN)% 2e 2Br~3Brp(l) 2e s C1, 2e 2Cls F l 2e 2F-
++ ++
++
E" (volts) -0.5355 -0.536 -0.77 -1.0652 -1.3595 -2.65
Accordingly, Brz should liberate free (SCN)%from a SCN-, while (SCN)2in turn should liberate free Izfrom an I-. Since (SCN)e is readily hydrolyzed, all reagents, solvenls, and apparatus must be absolutely dry. Preparation To a suspension of Pb(SCN)2in dry ether is added in portions with vigorous shaking a solution of Br, in dry ether (56, 37). (There should be no open flames in the laboratory when ether is being used!) The velocity of the reaction is markedly influenced by the solvent used. Some students may substitute CS, (flammable) for &her. Inasmuch as the yellow solution of (SCN)* in ether is unstable, the following tests should be performed without undue delay. Oxidation ojlodide Ion A few drops of conc. aq. KI are shaken with a portion of (SCN)z solution. The presence of free I2 may be made more obvious by addition of a starch suspension. This redox reaction is the basis for a number of "thiocyanometric titrationn."
Portions of the freshly prepared (SCN), solutioli are shaken with finely powdered Fe and with Hg, and the reactions are compared with those of halogens. Reaction with Unsaturated Organic Compounds
If (SCN), solution is added with shaking to an uusaturated oil (cottonseed oil or oleic acid), the color disappears, and if concentrated aq. ICI is added, little if any I2is liberated (36). The reaction may he used to evaluate the degree of unsaturation of fats, oils, and resins (thiocyanogen number). Hydrolysis A portion of (SCN)2solution is shalceu with an equal volume of H20, and the ether layer is decanted. ilfter acidification with 6 M HN03 (Hood!), additiou of a drop of 1 M BaCI2 produces a white precipitate of Ba-
son.
Decomposition and Polymerization One portion of (SCN)2solution is allowed to stand in a closed vessel, and another is placed in a desiccator over H2S04. The samples are examined after a week. Qualitative Analysis
A valuable learning experience for the student is to integrate the pseudohalide ions into the standard qualitative analysis schemes for the common anions. For example, he may be asked to devise a scheme for pseudohalides in thc presence of halides (CN-, CNO-, SCS-, Cl-, Br-, and I-). 4 typical procedure might be to test the original solution for CN- by the CuS suspension test (see tests for cyanide and cyanate ions above). CNO- can be detected in a separate portion of the unknown solution by addition of CoCl2.6H20. If CNO- is absent (no blue color develops), SCN- may be tested for by merely adding acetone to the pink solution (see reactions of ionic thiocyanates above). If CNO- is present, SCN- may be tested for by acidifying the original solution with 6 dl HCI, removing interfering I- by prccipitation with Pb(NO& solution, and adding a drop of FeC13 solution. A deep red color indicates SCN- (59, p. 547; see reactions of ionic thiocyanates above). A separate portion of the original unknown solution may be tested for halides as follows. All six ions (CK-, CNO-, SCN-, C1-, Br-, and I-) are precipitated with 0.2 d4 AgNOa, and the silver halides and pseudohalides are collected by centrifugation and washed with H20. The interfering pseudohalide ions are removed by heating the dry silver salts strongly for 10 min (Hood!). The residue is acidified with 6 M HCzH302,a piece of mossy Zn is added, the solution is heated in a water bath for 15 min, 2 ml more of HCzHaOzis added, and the mixture is centrifuged. The centrifugate is tested for CI-, Br-, and I- by standard methods (SO, pp. 523546). Acknowledgment
The authors gratefully acknowledge the experimental assistance of Ronald Majors and the financial assistance of the National Science Foundation (UnderVolume 45, Number 2, February 1968
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graduate Rcsenrch I'articipatior~ Program GY 372 and G T 2R07), and the CSCF Research Committee. They are also indebted to t,he entire Spring 1967 Chenlistry 1rZ class at the California Stat,e College a t Ipresno who served as "guinea pigs." Work-shdy funds were furnished by the Unitcd States Government as provided by the Economic Opportunity Act of 1964. Literature Cited ( 1 ) WALDEY,P., A N D ACDKIETH, L. F., C h m . Revs., 5 , 339 (lW28). (2) XOELLER, T., "11mt.ganic Chemistry," John Wiley B Sora, Ine.. New York. 1952., o. 463. BRISTED,It. C., "Comprehensive Inorganic Chemistry," D. Van Nostrand Co., Inc., Princeton, New Jersey, 1954, I ' d . 3, Chap. 9. E M E I . ~11. ~ J., ~ , m u ANDERSOY, J . S., "Modern Aspects of Inorganic Chemistry," (3rd Ed.), I). Van Xostrsnd Co., Iue., P~.incetan,New Jersey, 1960, p. 457.
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K A ~ F F M AG. X .B..
AND
I~~OCTGHTEN. R. A,. J . CHEII. EDUC..
44, 408 (1967). '
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Ninth Annual Summer Conference, California Association of Chemistry Teachers Asilomar, California, August 13-19, 1967
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