The Cyanocobaltate Scheme of
Ronald Rich Bethel College North Newton, Kansas
Qualitative Analysis
This paper presents a novel scheme of qualitative analysis for cations. One unique advantage is that the groups and subgroups are quite orderly, according to the periodic chart. Table 1 outlines this organization. Dotted lines separate subgroups. Table 1. The Organization of Elements into Groups
analytical procedure: No element appears in more than one analytical group. Any organic hydroxy acids or other chelating agents which may be present in unknown samples and which always prevent some precipitations are easily destroyed by permanganate after precipitation of the unaffected cyanocobaltate group. Similarly, phosphate, borate, oxalate, or flucride ions (which cause premature precipitation of alkaline earths in other analytical schemes) either do not interfere or are easily counteracted. Group Separotions
~ g l ~ dSn Sb
-Gr. Gr. E D
Group B
Hg
Group A
Pb Bi
-
Group C
The chemical behavior of the cations, which is nsed to establish the separations, is different from that usually emphasized. Hence both students and teachers may be attracted by its educational value. The first precipitant, used to effect separation of the largest group of transition element cations, is the nonbasic cyanocobaltate ion, CO(CN)~-~.Sodium, potassium and ammonium salts of this anion are available at moderate cost.' The concurrent presence of strong acid eliminates most sources of interference. Since only the uni- and dipositive cations of the transitional elements form precipitates with this anion and since halide complexes of Hg(I1) are not precipitated, solutions of unknowns must be treated with a reducing agent. Hydroxylammoninm ion, NH30H+, is effective with iron(II1) as is formic acid with Hg(I1). Ascorbic acid is found more effective with Hg(l1) and just as effective with Fe(II1). Subsequent group precipitations are accomplished by using HzPOI- first with acid and then with base to form insoluble metal phosphates. In addition to the emphasis on somewhat different fundamental inorganic chemistry and on the correlation of groups and subgroups with positions in the periodic table, the scheme offers advantages as an This work was done largely with support from the Research Corporation. ' City Chemical Corp., 132 W. 22nd St., New York 11, N. Y. Solutions of theae sdts are stable for many months in brown bottles and are not appreciably decomposed by hrief boiling with dilute aqueous strong acids. A simple preparation of KaCo(CNh and a quick test for purity are given by BIRELOW, J. H., "Inorganic Syntheses," Val. 2, McGraw-Hill Book Co., Ino., 1946, pp. 225-6.
Table 2 ontlines the procedure. The unknown solution is assumed to have a volume of 2 to 3 ml. The cations should have concentrations in the range 0.001 M to 0.02 M. Some reagents are mentioned only in the ionic form or without exact specification of quantity. This is to direct attention to the essential chemistry involved and to discourage the cookbook approach. Some additional details, including confirmatory tests and possible further improvements, can be obtained from the author. The acid hydrolysis of acetoxime provides the effective reducing agent, hydroxylamine. Hydroxylammonium chloride can be nsed, but with some risk of the partial precipitation of lead chloride on cooling. The greater the acidity the faster the hydrolysis of acetoxime but the slower the reduction of iron(II1). A pH of 0 to 1 is satisfactory. Trichloroacetic acid can be used to provide a buffer a t this pH. Methyl violet is a good indicator, turning green or blue. If a solid unknown is soluble only in more acidic solutions and if it reprecipitates on neutralizing some of the acid, the precipitate is simply retained until after the permanganate treatment. It is then recombined with the solution from which group A has been removed. The solution is heated several minutes with formic acid before adding cyanocobaltate because Hg(II), if not first reduced, may catalyze a slow decomposition of the cyanocobaltate ion. The precipitation of iron(I1) cyanocobaltate could be performed separately, by delaying addition of acetoxime, but with some violence to the simple relationship between the rest of group A and the periodic table. The cyanocobaltate ion appears to be non-poisonous. We gave a mouse 22 mg of sodium salt, equivalent to a human dose of one-tenth of a pound, without serious effects. The cyanocobaltate ion coordinates to other ions through the nitrogen atoms, reminding students that group A elements are those that form the most stable amine and cyanide complexes. Most insoluble cyanocobaltates tend to be colloidal in solutions of low ionic Volume 39, Number 8, August 1962 / 403
Table 2.
The Separation of the Groups
Adjust pH of unknown solution to between 0 and 1.. Add 1 drop 1 M aeetoxime (CHd2NOH, and 1 drop 6 M HCHO,. Addminimum0.5MCo(CN)s-5foreompletep~ec~p~taton. HeatZ-3mmutes.
Heat 5 min.
Precipitate: Solution: Group A, low-valence Add 10 drops sat. KMnO, and 7 drops 0.5 M CO(CN)~-~.Heat 5 minutes, maintaining acidity (e.g., with transition elements HN03) and slight excesses of ~errnanganateand cyanoeobaltltate. Add 1 drop 1 M HIOz and d~scardppt. of Mns(Co(CN)& and MnOp. If fluoride ma, be resent add 1 ml sat. H,BOs. In any case, add 10 drops 1 M as M,(Co(CN)& H2P04-or HPO*-2. Add chloroacetate, ~ H . c ~ oor ~ a- ,base plus CH&lCO.H until ,pH = 3 (Congo Red and Hg&. paper is purple). Heat 5 minutes. Centrifuge hot. Without decantmg, cool and centrrfuge agam. MtP = Mnf2 Fet" Co+* Ni+z &+a Solution: zn+z: ~ A ~ Lcdl: , Pren'pitatr: AlPOI plus group B, high-valence transition elements as Groups D and E. Add excess base and, if necessary, and H ~ S + ~ . TiOHPO,, CrPO,, UO2HPO4,plus group C, post-tran- more phosphate. See Table 5. sition metallic elements aa Pbl(P0J2, BiPO* H2Sn (OH)&and (e.g.1 W(OH)Z. Add 2-3 drops 6 M OH- Group D, alkaline-earth Solution: Discard unless previousreagents, +@ and 5draps 1 M &02, ~~~~~~d stir 3 containing non-metalhe catelements as MdP0.h. 1M H~P,-J- until p~ = 7 or 8 (naphtholphthale,n M+l = Mgt2, Ca+l, ions, permit analysis of group yellow or green). Add 5 drops 1M HaOe. E, the alkali metal ions S a t Sr+', Ba+l. and X+, here. Solution: Precipitate: GroupB as oxyAdd 1 ml CHBCOCH~COCHZ and peroxy(HAcac) and 2 drops 1M CHaCOn-. Heat and stir anions. See Table 8. 5 minutes. Residve plus aqueous HAcae solution: Evaporate on solution: Group C as shove. watch glass over water See Table 3. bath. Residue: A1Acac8. Add 1-5 drops 6 M HCL or HNOa. Evaporate. Solution: Alts.
strength unless only a small excess of the precipitant is used. A spot test to detect the excess would be useful but is not known. Solutions can be treated with solid sodium nitrate to facilitate coagulation. The precipitates are white except for iron (yellow), cobalt (pink), and nickel and copper (pale blue). Fe(CN)s-3 precipitates the same ions as C O ( C N ) ~ - ~ but it is very easily reduced to Fe(CN)s-&, which then precipitates most other cations also. Early precipitation of group A removes the elements whose phosphates have solubilities which would blur the later distinction between groups B and C and group D. Arsenic, a non-metallic element, does not appear in group C hut needs to be sought only as an anion. Aluminum could be considered as a member of group C although it is somewhat different. Beryllium, if included, would he found with aluminum, illustrating the diagonal relationship in the periodic table. The inclusion of vanadium, molybdenum, and tungsten would require some changes in the group separations. Heating is necessary for the precipitation of groups B and C because of the inertness of chromium(II1). The phosphates of group D, as well as the others, are less soluble when hot. This calls for a lower pH than otherwise, with chloroacetic acid providing the appropriate buffer system. Under these conditions, traces of aluminum phosphate will redissolve when the Table 3.
mixture is cooled unless first compacted by centrifuging The precipitate containing antimony is not a phosphate and is slightly soluble in the hot mixture. This is counteracted by cooling and a second centrifugation, thus permitting a more complete separation of antimony than is possible in other non-HrS schemes such as those of West and VickZor Br~ckrnan.~ a-Naphtholphthalein is recommended as the indicator for getting the pH below 8 because of its good color change and stability in the presence of chromate ion. The test can be applied on a spot-plate. The separation of aluminum(II1) from group C may be facilitated by adding chloroform to the mixture. The resulting boiling action substitutes for stirring. Also the organic layer, a t first, stays a t the bottom and has better contact with the precipitate. The whole procedure provides a simple answer to the problem of separating aluminum ions from those of tin and antimony without H2S. It can also be used to illustrate solvent extraction, chelation, and resonance. It would be possible to treat group E on a par with the other groups, if this is desired, and thus to avoid 2 WEST, P. W., AND VICE, M. W., "Qualitative Analysis and Analytical Chemical Separations," 2nd ed., The Maemillan Co., New York, 1959, p. 18. a BELOCKMIIN, C. J., "Qualitative Analysis," Ginn and Co., Boston, 1930.p. 63.
Separation of Low-Valence Transition Elements into Subgroups
Heat the group A ppt. 2 min with 2 ml sat. NaCl plus 2 drops 0.5 M CO(CN)~-~.Cool. Add 3 drops 3 M 4- ( I Residue: M8(Ca(CN)&and AgI. Add 1-2 ml 1M or sat. thiourea, CS(NH&. Solution: Residue: M8(Co(CN)s)r. Add 2 drops 1 M H2O%and 10 drops 6 M NHa. Periodic table group Ib, as CU(CS(NH~)~),+Z and Destroy HIOl by heatmg. Ag(CS(NH,)&+. See Table 6. Solution: Residue: Co(II1) and N~(NHS)I+~. MnO* and Fe(OH)8. See Table 6. See Table 7.
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+ I>).
Stir 2 min.
Solution: Periodic table group IIb, as ZnCb-, CdC1,-: HgL-a. See Table 4.
Table 4.
Analvsis of Periodic Table G r a u ~Il-B
Add 5 drops 6 Mnitrite, NO1- and, all a t once, 10 drops 6 M HCI. Remove expelled solid b. Extract remaining L with CHCL. Boil off NO and NO2. Make the solution basic with 6 M NH8. Add 1 ml 1 M or sat. CS(NH2h. Heat 10 seconds (in a &inch test tube) in the water bath t o a temperature around 70°C. Precipitate: HgS
Solution: Add 10 drops sat. K9COa. Heat 4 minutes. Precipitate: CdS
Solution: Add 1 m l 6 M OH- and heat a t least 10 minutes. Pmcipitate: ZnS
starting again with the original sample. Ammonium, tetramethylammonium, and guanidinium salts could be used as the reagents in separating the previous groups. Analysis of Group A
Tahles 3, 4, 5, 6, and 7 outline the essential steps. The cyanoeobaltate ion helps retain copper ion in the residue, while iodide precipitates silver ion from the partly soluble chloride complex. The high concentration of chloride ion is required for the complete dissolut,ion of the zinc precipitate. The concentration of free I- is held down, by complexation with 12,to keep from dissolving AgI as Ag1,-. The tri-iodide is best added to a cold solution because of a slight reaction with the copper precipitate when hot. Mercury(I), if present, is oxidized and dissolved somewhat slowly by I,-. The cobalt(II1) ion may be a mixture of ammoniated species hut it is not CO(NH&+~ and is not precipitated by iodide ion. Table 5.
Or add a soluble sulfide.
with the heavier elements than with lighter ones of the same valence in the same column of the periodic table. These facts provide a convenient basis for discussing various aspects of the mechanisms of reactions. The successive precipitation of mercury, cadmium, and zinc sulfides from the same solution (as the pH and heating time are raised) is very striking and the separations are nearly quantitative. Kone of the heavier metal ion is left in solution to darken the precipitate of the next lighter one. The removal of iodide ion by nitrous acid is necessary since HgIpc2reacts with thiourea much more slowly than HgCla-2 does. Table 7.
Analysis of Manganese and Iron
Add 1 ml H.0 and 2-5 drops 6 M HNOI.
I
Residue: Mn02
Solution: Fe+a
Analysis of Remaining Groups
Tables 8 and 9 show analyses for groups B and C, respectively.
Analysis of Periodic Table Group I-B
Add 10-20 drops 6 M OH- and heat 10 minutes. Or add 2 drops 1 M S-2. 1)i~card ~olution. Dissol~eprecipitate, CuS and A&, by heating in 10 drops 6 M HNOI. Discard residue, Ss. Add 1 in! H30 and 1 drop 5-6 M C1-. Pvecipitate: AeCl
I
Solution: Cu+z
It does not seem necessary or desirahle to eliminate sulfides completely from qualitative analysis. The advantages of thiourea over thioacetamide for the separations proposed here include its greater selectivity and stability and much loaer cost. Hydrolysis in acid would be much too slow to use. Even in these hot basic solutions hydrolysis in the absence of heavy metals is rather slow. However, students can verify by quick semi-quantitative tests that the rate of reaction increases with pH and with the concentration of the heavy metal. The rate is also much greater Table 6.
Analysis of Cobalt and Nickel
Table 8.
Analysis of the High-Valence Transition Elements
Heat the group B solution 15 minutes to decompose peroxide. Pwcipitate: Solutim: TiOHP04, UOzHP04. Add 1 ml H 2 0 and 2-5 drops CrO.-* 6 d'l CC1,CO.H.
1
Residue: TiOHPO< Table 9.
Analvsis of the Past-Transition Elements
To the group C residue add 1 m l 6 M KOH and 2 ml H.0. Residue: Solution: BiPO. and Add 5 drops 1 M or sat, CS(NH.)* and heat 5 minBi(OH), utes. Or add 2 drops 1 M Ki? or ( N H M . P~ceipitate:Solution: Add 10 drops 1 M &O1. (S,-P SO4-2). Destroy H.0: by herttmg. Add 2 ml 6 M HCI. Heat 5 minutes with Fe wire.
-
IPbS I !
lsb
Precipitate:
Solution:
Add 10-20 drops sat. NaI or sat. NaCI04. Wait 2 minutes. Precipitate: Ni(NH.).L or Ni(NH.)s(ClO&
Solution: Add 1 drop sat. dimethylglyoxime (C&CN0H)s in acetone. Precipitate: Solutim: Ni(Dmg-H), Add 10 drops 6 M OH- plus 10 dro s 1M or sat. CS(NH&, a n l h e a t 5 minutes. Or add 2 drops 1 M S-3 and hettt 1 minute. Discard solution.
KOH is required rather than NaOH because of the low solubility of sodium antimonate and some polymerized sodium stannates. A particularly convenient confirmatory test for bismuth, since thiourea is already used in this scheme, depends on the soluble yellow complex of thiourea and bismuth ion formed in acid solutions. It is good for students to realize the versatility of various reagents as well as the ubiquity of complex ions. Volume 39, Number 8, August 1962
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A white antimony compound may partially precipitate with the lead sulfide during the heating. The analysis of the alkaline-earth phosphates can be camed out as ~ bv d Gilreath.4 Since barium * ro * ~ose G ~ E E AE.~ S. , "Quantitative Analysis," McGraw-H~IIGo., New York, 1954, p. 216.
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phosphate is not highly soluble in acetic acid, however, i t is better to use cbloroacetic acid. Acetate ion can then be introduced to raise the pH for precipitation of BaCr04. The alkali metal ions can be detected bv conventional methods. Suggestions for the improvement of this analytical scheme are ea,gerlysolicited.
