QUALITATIVE ANALYSIS AND ANALYTICAL CHEMICAL SEPARATIONS WITHOUT THE USE OF SULFIDES' PHILIP W. WEST and MAURICE M. VICK Louisiana State University, Baton Rouge, Louisiana
TEE
teaching of qualitative analysis has suffered an alarming decline in emphasis recently. In some institutions the course has been dropped completely while in others a "watered down" presentation is offered a t the freshman level. This situation is difficult to understand because most chemists agree that qualitative analysis is the best place to learn inorganic chemistry. Unfortunately, part of the difficulty arises from the fact that there are outside pressures to speed up the training in chemistry so that students quickly get to organic chemistry and other more advanced courses, even a t the expense of a sound fundamental background, Undoubtedly, a substantial part of the present difficulty can be attributed to basic defects in the classical qualitative scheme, which has been the basis of the standard courses for over a century and a half. It must be admitted that the classical qualitative analysis system fails miserably from the standpoint of applied analysis. In addition, it is inefficient and troublesome to teach and the use of hydrogen sulfide is both dangerous and extremely unpleasant. Because of these basic defects, the authors have introduced a new nonsulfide scheme of separation and analysis, design to increase pedagogical efficiencywhile eliminating the unpleasantness and dangers of sulfide separations (I). The present discussion mill serve to review some of the history of qualitative analysis and will summarize the principal features and fundamental chemistry of the new scheme. THE NEEDS FOR TEACHING QUALITATIVE ANALYSIS
Before entering into a discussion of pedagogical and technical details, it would seem appropriate to outline some of the aims of a course in qualitative analysis. Quite logically, such aims should be predicated on the needs of t,he chemical curricula and, therefore, must be correlated with the established practices of teaching the various fundamental areas of chemistry. While the authors feel that it would be highly desirable to be able to teach practical qualitative analysis techniques that would find application in the industrial and research laboratory, it is felt that the pedagogical needs must come first. In our own institution, the practical aspects of qualitative analysis are presented at a more advanced level in a course in microchemistry where spot test methods and various microscopical techniques are taught. We feel that the traditional "qualitative analysis" course must be used to teach Presrntedas part of the Symposium on Qualitative A n a l y s i p What, Why, How?-before the Division of Chemical Education at the 130th Meeting of the American Chemical Society, Atlantic City, September, 1956.
VOLUME 34, NO. 8, AUGUST, 1957
chemistry in terms of chemical reactions, including the behavior of a wide variety of acids, bases, and salts and emphasizing the chemistry of the familiar metals. Where else can such chemistry be learned? Who can deny its importance? I n addition to the "wet chemistry" to be taught in the qualitative analysis course, emphasis must be placed on laboratory techniques. The great majority of schools are now teaching semimicro methods with the result that the student is given a particularly valuable background in neat and efficient laboratory procedures. The qualitative analysis course should present as much theory as possible, particularly in regard to equilibrium concepts. This is more than just traditional thinking. It is a considered opinion based on the belief that chemical principles are best learned u hen correlated with experimental studies. The qualitative analysis rourse provides an ideal opportunity for the correlation of equilibrium theory with the practical applications of these concepts. SYSTEMS OF QUALITATIVE ANALYSIS
It is important to note that qualitative analysis is taught by systematic approaches. The traditional course is based on the use of the hydrogen sulfide scheme essentially as elucidated originally by Fresenius almost 120 years ago. Important modifications and even rompletely new qnalitative schemes are now being used. The important thing is that they are all systematic srhemes of separation that provide the student with a logical and understandable means of classifying the chemistry of various ions and, therefore, they all have great pedagogical merit. The classical hydrogen sulfide scheme is outlined in Table 1. TABLE 1 . Classical Hydrogen . . Sulfide Scheme Precipitant
Precipitate
-
Remarks
.4cid-insoluble sulfides
Hydroxides of Altz, Cr+a Carbonates of Cat+, Srt+, z3"
*+
Dt%
Soluble group of Nat,K + , Mgt+
Alkaline-insoluble sulfides and bank salts
Approximately 3,000 technical papers and hooks have appeared dealing with the hydrogen sulfide system since its original inception (Z), and although the scheme was far from perfect, as evidenced by the numerous attempts at its improvement, it remains today essentially unchanged. Most of the earlier work on this system dealt with modifications within the fundamental groups of the scheme. Recently the emphasis has shifted to finding new sources of sulfide ion so as to eliminate the danger and unpleasantness of hydrogen sulfide and at the same time increase efficiency of separations. Almost as soon as the hydrogen sulfide scheme was proposed, Himly (8) advocated that sodium thiosulfate be substituted for the hydrogen sulfide. Sodium thiosulfate will precipitate copper, mercury, bismuth, arsenic, antimony, and tin. The precipitation must be carried out in the absence of nitric acid, and the control of acidity is of a critical nature because of the various equilibria involved. The precipitation is accomplished by boiling the solution, which leads not only to the precipitation of the metal sulfides, but also results in the release of sulfates, sulfites, and free sulfur. A number of investigators utilized this approach in trying to eliminate hydrogen sulfide, but none were able to come up with sufficiently clean separations to justify widespread use.= Other sources of sulfides, such as phosphorus pentasulfide, resin sulfides (Amberlite IR-4 charged with gaseous hydrogen sulfide), insoluble metal sulfides, alkali sulfides, and organic sulfides, have been suggested. The use of resin sulfides, proposed by Gaddis (4), has a number of interesting features. The resin serves mainly as a carrier for the sulfide ion, which being lightly bound is readily available for the precipitation of the various metal sulfides. The resin plays a special role in solutions containing phosphate, because under such circumstances it not only releases sulfide hut serves also to separate the phosphate through ion exchange reactions. The resin absorbs up to 127, of its weight of hydrogen sulfide and the resin sulfide so formed is claimed to be quite stable if kept in tightly stoppered vessels. So far as is known, there has been no general adoption of the use of this approach. This may be accounted for by the newness of the idea but also must be attributed in part to the somewhat objectionable feature of having to distinguish or separate precipitates from the solid resin. Also, there seems to be no clear cut distinction between acid sulfide and alkaline sulfide groups. These same remarks may be applied to the use of solid metal sulfides which have been proposed for use. When solid sulfides are used as precipitants they must be equilibrated with the sample, and none of the schemes proposed based on this principle have been adopted because of the introduction of the extra solid phase and because there is no flexibility in the grouping of the resulting sulfide precipitates. Solutions of alkali or ammonium sulfides serve to precipitate a large number of metal cations from
' Recently, Bdeher and Wilson proposed both an extended as well as a short scheme of nonsulfide separations (BELCHER, R., AND C. L. WILSON, "Inorganic Microanalysis," Longmans, Green & Co., London, 1957, Chaps. VI and VII) employing thiosulfate 8a a major group precipitant. The first edition of this book appeared in 1946, and its revision indicates that satisfactory results are being obtained and that precipitations with thiosulfste are practical m'hen done properly.
alkaline solutions. The scheme of Cornog (5), which has been quite successful and has been used for a number of years, employs ammonium sulfide. The ammonium sulfide was chosen rather than alkali sulfide so as to afford the precipitation of nickel and cobalt which otherwise would not be separated cleanly becauseof their tendencies t o form colloidal suspensions. The major groups utilized by Cornog are shown in Table 2. TABLE 2 The Scheme of Cornos Unknown
I
PPt.
r I
+ HCI soh.
7
Add slight excess ~ H , O H . Add (NH,)B and acidify with HAc. Boll. I
i
Hg, Bi, Fe, Cu Co, Ni, Cd, Pb Zn,As, Sb, Sn
$t,.
1
Ba, Ca.
Mn; Mg Al, Cr
soh.
1 I
Add HaPo, and make slk. wlth NH,OH.
1
Cornog's scheme has the advantage of requiring few reagents. The separations are generally quite satisfactory, and the analytical results obtained are about as good as those obtained with most hydrogen sulfide schemes and in some cases may even be better. The scheme suffersfrom the production of great quantities of fumes which must be eliminated by means of an efficient hood system. The fumes are generally quite harmless but even with semimicro procedures may become quite disagreeable unless removed from the Iahoratory. A considerable amount of complication results from the large number of metals included in the one sulfide group. Cornog has made effective use of a number of subgroup separations but many users prefer to have a more even distribution of elements between the various major groups. A number of other schemes have been proposed utilizing ammonium or alkali metal sulfides but none has demonstrated any great advantage over the hydrogen sulfide procedures. Various organic compounds have been proposed as potential sources for sulfide including thioacetic acid, thiourea, sodium diethyldithiocarbamate, and ammonium dithiocarbonate. These have all failed for various reasons, but thioacetamide, which apparently was first proposed by Iwanof (6),has been established to be widely acceptable and a number of excellent schemes have been proposed utilizing it. The studies of Barber (7, 8),Flaschka (9),and Swift (10) have presented convincing evidence of the validity of using this reagent. I t seems safe to say that for anyone who has a reverence for sulfide precipitation, this should become the recommended approach. The precipitations are essentially homogeneous, the separations are JOURNAL OF CHEMICAL EDUCATION
reasonably sharp, and the reagent is relatively pleasant and safe to use. I n concluding the brief summary of sulfide precipitations, mention must he made of various alternative schemes devised for introducing hydrogen sulfide itself. A relatively concentrated solution of hydrogen sulfide can he prepared in acetone, and the use of such solutions has been proposed by Perronnet and Remey (11). They claim that the sulfide solutions give results comparable with those obtained through use of the gaseous reagent. The acetone solutions are somewhat more pleasant to use and are considered safer, although it should he pointed out that such concentrated solutions spilled on the skin would be far from safe. Switching to semimicro procedures has been brought about in part as a means of reducing the amount of sulfides used and so reducing the air pollution hazards in the laboratory. The use of sulfide generating solid mixtures is well known. Prothiere proposed as early as 1903 the generation of hydrogen sulfide by heating sulfur and vaseline mixtures (I$). Other sulfur mixtures have been proposed from time to time since and individual generating units have been used in many laboratories. At one time a considerable amount of interest was directed toward the use of thionalid (thioglycolic acid P-aminonaphthalide). Berg and his co-workers have made extensive studies of this reagent (15) and have demonstrated its usefulness as a substitute for sulfides. Although thionalid is a mercaptan and might be thought to be a source of sulfide ion, as such similar t o thioacetamide, it actually remains intact and precipitates metals as innercomplex salts. It is remarkable in precipitating essentially the same group of metals as hydrogen sulfide. lt has a great advantage over sulfide in having a much greater precipitation sensitivity. The precipitates in some cases may he extracted into chloroform, which is useful for further separat,ions, and the various precipitates ohtained are almost all of definite composition which is in contradistinction to the metal sulfides. As might be expected since it is a mercaptan, thionalid is readily air oxidized and so must he utilized in the form of freshly prepared solutions. This together with its expense and the fact that it is an organic reagent, which detracts from its pedagogical usefulness a t the freshman and sophomore level, has det,racted from its ultimate development for use in qualitative analysis courses. NONSULFIDE SEPARATIONS
The idea of avoiding sulfides completely is not new, and numerous attempts have been made t o use such reagents as hydroxides, phosphates, sulfates, carhonates, chlorides, oxalates, and chromates as group precipitants (14). I n addition, a number of systems have been proposed based on the use of organic reagents, ion exchange and chromatographic separations, spot tests and microscopical procedures. These latter approaches have limited pedagogical value at the elementary level, however, and are not to he considered here. One of the nonsulfide schemes that gained a considerable amount of popularity was that of Brockman (15). His procedure, which is summarized in Table 3, provides a good distribution of the metals. VOLUME 34, NO. 8, AUGUST, 1957
TABLE 3 The Scheme of Brockman Unknown
1
PPt.
r
+ HCI soln.
I
~ a , ' ~ r Ba, Ph
Add KOH and NanOl. , Boil
Sn,Al, Sb Zn, Cr, As
~ n Fe, : Bi, Mg and Cu, Hg, Co, Ni, Cd
According to Munro (16), Brockman's scheme provides equal or better accuracy for 12 cations and is less satisfactory for nine when compared with classical hydrogen sulfide procedures. It is worthy to add that Brockman's scheme is more rapid than conventional schemes and the precipitates are generally easier to work with. The scheme is pleasant and safe t o use. The principal difficulties with the scheme are associated with the large hydroxide group which requires a subgroup separation. A second nonsulfide scheme that is typical of the attempts made t o avoid sulfide is t.hat of Rane and Kondaiak (17). This is essentially a phosphate procedure which gives group separations as indicated in Table 4. TABLE 4 The Scheme of Rana and Kondaiak Unknown
+ HCI
Evap. with HNO*.
I
ppt.
r
i
Ba, Sr, Pb PPtL
r
soh.
i+
Add NH,OH (NH,),HPO,. j soln.
1
Al, ~ r Fe, Mn,Bi, Ca, Mg ppt.
r
Cd, Cu, Ni
I
.4dd NaOH.
/
80;".
1
Zn, i s
The procedure provides a good distribution of the metals into groups but suffers seriously from the use of phosphate which interferes with subsequent separations within the group and tends t o inhibit many of the confirmatory tests. The precipitation of carbonates or basic carbonates has been employed for the key separation in a number of schemes. One of the simplest of such systems is that of Gerstenzang (IS), outlined in Table 5. A general defect of such schemes is the size of the carbonate group which often requires the division of 12 or more metals into subgroups. Some investigators have 395
will be designated as the LSU scheme, is outlined below (Table 6). Appropriate remarks pertaining to critical steps are included. The common cations are divided into six groups. These are: (1) the soluble group, (2) the chloride group, (3) the basic benzoate group, (4) the fluoride group, ( 5 ) a nonamphoteric group, and (6) an amphoteric group.
TABLE 5 The Nonsulfide Scheme of Gerstenzang
Unknown,
+ HCI
I
PPt.
soln.
7
Hg, ~ i Mn, , Fe Pb, Cu, Cd, Ni Co, Ca, Sr, Ba, Mg
Add NH,OH And NrtCl
TABLE 6 The M U Scheme
I sol".
I
7
~issolvkin HCI and H?03. Add NH,OH and HzO..
Sd, Sn, Al
Solution of metallic ions
Zn, ~ r , ' A l
Add HCl to Jot solution. Centrifuge while hot.
I
PPt.
r-
I
I
soln.
A ~ C I ,Hg2Clr ' (Chloride group) Ca, ~ r Ba, , Mg
Add
Adjust pH to 4'; add ammonium benzoate and sodium benzoate. Heat at boiling point 10 minutes. Centrifuge.
~dI and KOR.
I
I
Cu, Cd Ni, Co
attempted to simplify the group by prior separations of such metals as tin, antimony, and bismuth, but none have been able to evolve simple dependable procedures. I n spite of the discouraging history of attempts at devising substitutes for the hydrogen sulfide scheme, certain helpful observations resulted from the study of various of these schemes. It became apparent that any new system of separation and analysis should meet two principal requirements. First, the various groups should each have proportionate shares of the cations involved, a t least to the extent that no one group would contain so many metals that complicated subgroup separations x~ouldbe required. Second, certain "trouble makers," such as tin, antimony, and bismuth, should be separated early so as to avoid unexpected or unwanted precipitations commonly encountered with these metals during pH changes. Quite naturally, rare or exotic reagents should be avoided and the gronp precipitants should not form complexes or otherwise interfere with identification reactions. Separations should be sharp or even quantitative and the precipitates should be dense and readily filterable. THE LSU SCHEME
Based on the above aims and observations a new nonsulfide system of analytical separation and qualitative analysis has been evolved. The group separations are quantitative and the procedure is a t least 50% more rapid than the classical procedure. There are no toxir fumes released, and in fact, there are so few fumes of any kind that there is no need for any kind of hood or special ventilation system. The scheme gives analytical results that are a t least as good as those obtained with sulfide separations, and the theory involved is definitely easier t o understand. The scheme, which
P ~ F ?IllgF*, : BaF2 S r h , CaFn (Fluoride group)
Make alkaline dith excess NaOH. Expel NHs by evaporation. Centrifuge. I
The analysis of the soluble group is similar to that which has been used in the classical scheme with the added use of spot tests to accompany the usual flame tests for Na+ and K + and the NH, tests for NH4+. The appreciable solubility of PbC1, has long placed Pb+2 in two different groups. By precipitation of the group in hot water and separation of the precipitate and solution while still hot, Ph++ is eliminated from the chloride group and appears in a subsequent one. The separation and identification of Ag+ and Hg+ follow the standard procedure. Since the basic compounds of the metals are difficultly soluble in aqueous solutions, with the exception of the alkali metals, most analytical schemes for separation of the metallic ions involve a hydroxide group at some point in the procedure. However, due to the nature of such precipitates, especially those of the hydrous oxide types, inclusion early in the analysis results in excessive entrainment of ions normally remaining in solution. Also, the size of the group is too large to be subdivided easily into individual ions. An inspection of solubility product constants will serve to point out that the basic compounds of the metals in the higher oxidation states are less soluble, or precipitate a t lower pH than those of the lower oxidation JOURNAL OF CHEMICAL EDUCATION
TABLE l Schematic Separation of the Basie Benzoate Group
--
Centrifugste from the chloride group. Adjust pH to 4, add NHIBz, heat to boiling add NsBz and heat in a boiling water bath for 10 minutes. Centrifuge and wash precipitate with 1 M NH.NOa to remove C1-. Add 6 M HNO8, warm and oentrifugo. Re eat HNOI NH4NOI. I treatment and join c~ntrifngrttcs.Wash precipitate with I $ I
I
H.SnOa SbOCl ~issolv;in HCI. Divide into 2 portions.
Add iron wire and boil to a small voh~me. Remove solid matter and add 1 drop HgCI2. White ppt. turning p a y proves Sn.
Make stronb~yalkaline with NaOH and add H201. Boil. Chtrifnze.
Add saturated H I C ~ J , solution. Add 2 drops 0.1 M NarS. Yellow orango ppt: ~ I . O V P~~ ~h ,
B~(oH)~,~F~(oH), Diasolvem HCI. Divide into 2 ~ortions. (li (2) Make alkaline with Add ffiFe(CN)* NaOH. Add NQor NH,NCR to SnOl. Black n d . nrovo Fe. proves Bi
(1)Aeidify. Add I d. sliza m . Make alkaline with NH40H. Red lake proves AI.
(2) .4dd ethyl acetate. Acidify with HCI-HxOz mixture. Blue oreanir laver proves Cr.
TABLE 8 Schematic Separation of the Fluoride Group
-
Centrifugate from the bask benzoate group. Add N H 9 and allow to stand for 10 minut,rs. Cmt,rifu~cand wash precipitate.
1
PbF*, MgR, BsF2,SrR, CaF* Wash with KOH-K2COamxture Centrifuge. Repeat treatment, and ioin centrifueates. soln
-1 MgF,, B~F:,SrF., CaF* Dissolve in HC1-H8BOs. Saturate with solid NH4CI. Make alkaline with NH40H. Add (NH&COa reagent. Centrifuce. I
I
PbOl-' Acidify with HOAe. Identify lead with S--, C I ~ or' I-. soln.
7 I
Mg+Z Destroy NH,+. Add drop S and O reagent. Make alkaline with NsOH. Bluelake provrr pwsenee of Mpi9 PPt
r I
-
--
BaCrO, Dissolve in HCI. Use flame test for Ba. Add (NH4).S04. White ppt. insoluble in NaOH proves presence of Ba +a.
1
soh.
-1
1
Srt4 Cat¶ ~ a l &alkaline with NH40H Add equal volume of 95Y0 ethyl alcohol. Centrifuge.
1
PPt. SrCrO, Flame test proves Sr+'.
VOLUME 34, NO. 8, AUGUST. 1957
Burn off Alcohol. Add
397
-
...
~
TABLE 9 Systematic Separation of the Hydroxide Group
-----
~-~~~
~
Centrifugate from the fluoride group. Make alkaline with NaOH and evnporate to dryness to destroy NH,+. Repent if nert3asary. .Add wat,er, stir and rcnt,rifuge. I
Rln(OH)*, ~ ~ ( o H IHgO ,, Cu(OH)q Co(OH)., Ni(OHh, Cd(OH)? Diasolve m HCI. Add excess NH,OH. Cwtrifugr. Wash ppt. with NH40H. Join washings and rentrifugatv
Subsequent group.
I
MnO(OH)*, HgNHGI, F e ( 0 q ) ~ . Dissolve m HNOsH102. D m d e into three pelt& (2) (1) Boil. Add NaBiOs. Boil and centrifuge. Pur le solution
Evaporate to a small volume. Add SnCle. Ppt. Hg2C11 Hg.
+
~fnK.rl.
I
(3)
Add &F~(CN)G. Dee blue Fe,&e(~~)dl.
I
Cu?(NCS),. Dissolve in aqun wna, Boil. Make solution strongly alkaline with NHIOH. nppp~ I U RCOIOI. C I I ( N H & + ~ .
pPL
- ... .
. 4 p ?
Cu(NH3),+', Co(NH8)aiZ, Ni(NHa)s+2,C ~ ( N H Z ) , + ~ Add HCI until stronelv acidic. Heat to boiling, add i d i d Na.SOa and boil. Add NH Centrifuge.
CO(NCS],+~, Ni(NHdqtZ, c ~ ( N H . ) . ~ ~ Test one drop of salutlon with 10 drops of acetone. A blue solution indicates CoC'. If Co+%iis present, evaporate the main solution to dryness roast while dry, and dissolve residue in ssturated ~ C I .Make this solution acidic with HOAc, add KNO., and let stand 10 minutes. Centrifuge. (If Cots is absent, prowed directly to Ni . .
..
.- . .
+' /
31.
1
'I
I
