Qualitative Analysis The Current Status G. Mattney Cole, Jr. and William H. Waggoner University of Georgia, Athens, GA 30602 Qualitative analysis is clearly making a comeback in the undergraduate curriculum. Qua1 is attractive as a pedagogic device because it demands good laboratory technique, and provides a framework within which a variety of chemical nrincinles can he taught. - . includine acid-base and ionic eauilibria, and oxidation-reduction. Perhaps the most important feature of aualitative analvsis is that it ~rovidesa framework for introducing descriptive chemistry into otherwise physicallv-oriented general chemistrv courses. ~ ~ c r e a s i n g l ~ g e n echemistry ral texts are being puhlished in dual editions one of which includes aualitative analvsis. However, the separation schemes presented are definitely not of equal utility. Although all these schemes work, at least in the hands of an experienced person, not all are suitable for large, inexperienced freshman classes. Also, some treatments are completely unsuitable unless the ions are present in equimolar amounts. Most of the more recent treatments present idealized separation schemes, carried out by idealized students, with little attention being given to the realities of the laboratory. In our opinion, a cookbook qualitative analysis scheme completely defeats the purpose of offering qualitative analysis in the first place. This is particularly important in view of the fact that many students have had some exposure to qua1 in high school. This article is the first in what we consider to be a long overdue series. namelv. to reconsider the aual scheme in terms of suitability f i r largenumbers of students and for instructors. We first analyze the accuracy and reliability of several published separation schemes (1-5). However, it should be pointed out that this article is not intended as a review of competing textbooks. Rather, it is to assist the users of these texthooks in designing . .and implementing .a reasonable course in qualitative analysis. Secondly, since teaching qualitative analysis demands more experience than perhaps any other chemistry course, we note those methods where particular difficulties arise. We focus on Groups I1 and 111since the separation and analysis of these Groups are the most troublesome. Finally, we present alternative schemes for the separation and analysis of Groups I1 and 111. These schemes offer an integrated approach, better separation, multiple confirmation tests, and avoid many of the problems associated with the more conventional methods. In order to keep the article to a reasonable length we consider only cation analysis and, in general, do not provide detailed directions. It is also assumed that the procedures are to be carried out by students having no prior experience with analytical procedures. Since the reader is presumed to be reasonably familiar with the qua1 scheme, figures are presented in schematic form in which intermediate steps are often omitted. Group I
All of the various schemes treat this group in an identical fashion. There is, in fact, little room for variation in the separation of this nrouw. . In a later oauer we will ~ r e s e nat variation of the Group I separation; iowever, the ciassical chloride precipitation gives little trouble. It is worth noting that many
texts mention the possibility of a white precipitate prior to separation of Gronp I. The precipitate, which is usually written as bismuth oxychloride, BiOCl and/or antimony oxychloride, ShOCl, is said to be soluble in an excess of HC1. In our experience, this is true only for freshly-prepared samples. If the sample is more than about 48 hours old. the ~recinitate often fails-to dissolve. Group ll
Separation of IIA and IIB. All schemes initially separate the ions of Gronp I1 into two sub-groups. The separation relies on the amphoteric nature of the precipitated sulfides, the base beine" either hvdroxide ion or sulfide ion. The comoounds of lead, bismuth, copper, and cadmium are always included in IIA: com~oundsof arsenic. antimonv. - . and tin are alwavs included in IIB.2 Compounds of mercury(I1) may be in either or hoth dewendina uwon the choice of reaeent. The twdbasic % a t i o n s for separating'I1~ and IIB are illustrated in Figure l. Scheme I utilizes sodium sulfide; Scheme I1 uses ammonium sulfide or dilute potassium hydroxide as reagents. Regardless of the choice. one should expect some mercuric compounds in hoth groups, particularlywhen the procedure is carried out by inexperienced students. Mercury(I1) sulfide is sufficiently acidic that if the separation is not carried out promptly, some will dissolve even in dilute base. In concentrated sodium sulfide or sodium polysulfide solut i ~ n smercury(I1) ,~ sulfide dissolves more slowly than do the SnSz < As& other sulfides, the order being HgS < Sb& ( 6 , 7). Thus, separations using sodium sulfide are liable to leave some mercury(I1) sulfide in Gronp IIA. With ammonium sulfide, copper(I1) sulfide dissolves to some extent. Using either sodium or potassium hydroxide or ammonium or sodium sulfide will sometimes render cadmium sulfide colloidal
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(8).
Group IIA
The sulfides of IIA are normally dissolved in concentrated nitric acid: anv residue remaining that is not obviouslv sulfur is probably mmcury(11) sulfide.' Almost all versions precipitate lead as lead sulfate and confirm the presence of PbZt with potassium chromate. Those which state that a white precipitate (which may or may not be lead sulfate proves the presence of Pb2+ ( I ) should be viewed with caution. All versions ~recinitate . . bismuth as bismuth hvdroxide and confirm it with sodium stannite. AU recent versions of the qua1 scheme ( 1 3 )ienore the fact that both Cu2+and He2+interfere with this &. Presented in part at the Southeast/SouthwestAmerican Chemical Society Regional Meeting, New Orleans, December 10-13, 1980. Reference (3) includes only Hg. Pb, Cu, Sb, Sn in Group II. Many older versions of the qua1 scheme call for sodium or ammonium polysulfidecustomarily written M&, which is correctly written as M& (13).This is probably the effectivereagent in a strongly basic Solution containing elemental sulfur. Strictly speaking, Sn(lV)is actually reduced to Sn, but the latter dissolves in acid solution to form Sn(ll).
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There is greater disagreement regarding the identification of copper and cadmium than any other comhination of ions with the possible exception of antimony and tin. In the process of precipitating bismuth hydroxide using concentrated aqueous ammonia, the tetramminecopper(I1) ion and the tetramminecadmium(I1) ion are formed in solution. The [Cu(NH3I4l2+ion is indeed blue; however, the presence or ahsence of a hlue solution does not necessarily prove or disprove the presence of Cu2+ for several reasons. Color alone detects only a few parts per thousand cupric ion (5, p. 163) and then only in excess ammonia. If the solution is nearly neutral, it will he colorless. Also the tetramminenickel(I1) ion is blue. Therefore, a procedure which does not indlude a confirmatory test for Cu2+ ( I ) should he modified. Two basic schemes are used to identify the cadmium(I1) ion in the presence of the copper(11) ion. The first uses potassium cyanide to complex Cn2+ giving the very inert dicyanocu. hy precipitation of the yellow prate(1) c o m a , followed cadmium sul a t ( 2 , 6 ) The second uses sodium dithionite, Na2Sz04, to precipitate copper (4, 5). Both schemes have undesirable features, the most obvious of which is the use of potassium cyanide in large, freshman lahoratories. Additionally, the dithionite ion is unstable in aqueous solution (91, and freshly prepared solutions are not always practical. Group llB
Variations in the analysis of Group IIB occur in two places: confirmatory tests for arsenic and the identification of antimony and tin. The presence of arsenic is confirmed by dissolving arsenic(II1) sulfide in aqueous ammonia, oxidizing to arsenate and precipitating either the white magnesium ammonium arsentate (2, 4) or the yellow ammonium 12-molybdoarsenate, (NH4)3A~Mo12040, (1,5). Both tests are difficult to do consistently, even with good technique. Since magnesium am-
I
PbS
Phs
Phs
I
o
1
r
4
1
Scheme I1
Figure 1. Separation of Group I1 with conventional reagents.
I
PbS
5 Bi& cus CdS HgS
Figure 2. Separation of Group I1 using 1 % lithium hydroxide solution.
136
Journal of Chemical Education
I
HeS
( X = 0 or 8 )
Scheme I
monium arsenate is white, it is indistinguishahle from the other possible precipitates, such as hydroxides, that may occur a t this point, and it often tends Lo supersaturate. The heteropolyarsenate is a distinctive yellow, but the reaction is slow unless the solution is boiled, and it requires a very large excess of acid. The very sensitive and very specific Marsh andlor Gutzeit tests for As(II1) used hy older text (6, p. 301; 7, p. 393) are not recommended for use in large, freshman laboratories since arsine, AsH:q, is very toxic. Antimony and tin can he separated using either aluminum or iron as the reagent ( 2 , 4 , 5 ) .Antimony(II1) is reduced to elemental antimony; Tin(1V) is reduced to tin(III4 which remains in solution. The presence of antimony is then confirmed by taking up antimony in nitric acid and reprecipitating antimony(I1) sulfide. The presence of tin is confirmed by reaction with mercury(I1) chloride. An alternative scheme (3, 6, 7) tests for antimony in the presence of tin(1V). The reagent, sodium thiosulfate precipitates the orange Sh20S2,but does not precipitate the corresponding oxysulfides of either tin(1V) or arsenic(II1). The presence of tin is confirmed by reduction with iron followed by the mercury(I1) chloride test. Finally, one scheme (1) uses the iron reduction described above, but tests for antimony using a piece of tin on a silver strip in an evaporating dish. Antimony and tin can also he separated from a hot (lOO°C) hydrochloric acid solution using hydrogen sulfide (10):antimony(II1) sulfide precipitates, tin(IV) sulfide does not. However, use of hydrogen sulfide directly is not recommended in large laboratory sections, and thioacetamide liberates hydrogen sulfide too slowly for this separation. Either of the first two schemes will work, hut they demand very good technique. The third scheme ( I ) is less useful. We find that it works only some of the time, and besides, silver foil is very expensive. Alternative Scheme for Group N In view of the difficulties encountered in the separation and analysis of the ions of Group 11, we have developed a scheme which avoids the use of potassium cyanide and provides students with alternative routes in cases where problems or uncertainties arise. Alternatives are perhaps the most important feature of this scheme. Wherever possible a qua1 scheme should provide for the nossibilitv of uncertainties in confirmation tests. procedure numbers refer to Figures 1and 2.
Procedure 1 . Separation of Groups IIA and IIB. Holness and Trewick have studied the efferts of a number of reagents on separation of the ions of Groun I1 into the IIA and IIB sub-erouos . (.8.) . Their results are shown in Table 1.They found that in no instance do the conventional reagents give a clean separation. Sodium and ammonium sulfide dissolves the sulfides of copper(I1) and mercury(I1) while rendering cadmium sulfide colloidal. These reagents also produce large amounts of sulfur when treated with acid. Sodium or potassium hydroxide also dissolve mercury(I1) sulfide in amounts proportional to the reagent concentration. Mixtures of the two types of reagents proved to be a worse choice in the hands of inexperienced students. From Table 1 it is clear that the reagent of choice for separating Group I1 is a 1%solution of lithium hydroxide. Even hetter results were obtained when a solution containing 1% lithium hydroxide and 5% potassium nitrate is added t i t h e precipitated sulfides of Group I1 ions and heated just to its boiling point (Fig. 2). Procedure 2: Separation and identification of mercury(II).
Table 1.
Separation of Group IIW i t h Various Reagents
Hgl'
Reagent
Copper group Pb Bi Cu
Table 2.
Arsenic group Sb Snl' SnlV
Cd
As
COI col col col col
S S S S S
S S S S S
S S S S S
S S S S S
S S S S
S S
NS S * *
S S
Warmed
.. .. .. ..
(NH&S LiiS Na2S KzS CaS
..
ss
NS NS NS NS NS
NS NS NS NS
NS NS NS NS
ss 5s
ss 5s
NS NS NS NS NS
PS
ps ps ps ps
Heated Just to Boiling I%NaOH I%LiOH Ba(OHI2 Ca(OHI2
.. . .. ..
NS NS NS NS
NS NS NS NS
NS NS NS NS
*
S
NS = not soluble; S = soluble; ps = panly soluble; ss = slightly solubie; col = colloidal IOIUtiOn: = lnSOlYble hydroxides
.
-
Pb' Bi" CU*' Cd2*
Reactions of Hg2+, CuZt, and Bi3+ w i t h Sodium Iodide a n d Excess Iodide Ion.
ion
Nal
H$+ CuZt Bi3+
Hgl2 Ired) Cui (white) BiI3 (black)
excess I-
Hg142( c o i o r l e ~ ~ ) NO reaction B i l l (yellow)
The tin(l1) chloride test presents no problems. 1f t h k is any doubt regarding the presence of the Hg2+ion, reserve aportion of the test solution. Excess alkali-metal hydroxides precipitate the distinctive and characteristic yellow mercurv(I1) oxide. This test is preferable to the non-distinctive, white mercuric precipitated from aqueous amamidochloride, HRNH~CI, monia
(1)Evaporate (2) NHJNHat
PbS BLSs
3M H,S04
Pbs BiaS3
cus CdS
3M HNO:
A
Figure 3. Separation and identification of Group i i A Procedures 1 through 4
* pbS04
[P-6 I
PbCr04
(1)6M HNOp ( 2 ) 1 8 M H,SO,
PbS BLS,
cus
HOAc
:
( 3 ) H%O IP-51
Figure 4. Separation and identification of Group HA. Procedures 5 through 10.
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L Figure 5 Separation and ldentlflcatlonof Group 118
Fe(0H). Fe(OH). MnO%
HNO, [Co(NH3),12*
Al(OH),CrOs2 Zn(OH)42-
[Zn(OH),li Scheme I1 (4)
Scheme I (1,2 ) Figure 6. Separation of Group Ill using hydrochloricacid or nitric acid and potassium perchlorate.
Table 3.
Procedures 3 and 4: Separation and identification of cadmium(I1). Cadmium sulfide is the only sulfide of Group IIA that is insoluble in warm 3M sulfuric acid. Lead sulfide may dissolve to some extent hut is reprecipitated as lead sulfate. This separation has the decided advantage of avoiding the use of potassium cyanide to separate copper(I1) and cadmium(I1). Double confirmation of cadmium is desirable since the yellow cadmium sulfide may be confused with sulfur or obscured by darker impurities. Only cadmium sulfide dissolves in dilute hydrochloric acid and the yellowish cadmium ferrocyanide, Cdz[Fe(CN)&is distinctive. Procedures 5 and 6: Separation and identification of lead(I1). These are standard procedures and present no difficulties. T h e presence of lead(I1) should be confirmed since a white precipitate from sulfuric acid only implies the presence of lead(I1). Procedures 7 and 8: The separation and identification bismuth(III). These procedures are standard and present difficulties only when considerable copper(11) or mercury(I1) are present. If there is any doubt, reserve a portion of the test solution and add sodium iodide. The pertinent reactions are indicated in Table 2. Procedure 9: Identification of copper. This procedure is 138
Journal of Chemical Education
Colors of cornnounds used in confirmatory tests in Figures 3, 4 and 5.
Compound
Color
Procedure 2 HgO Procedures 3 and 4 Cd2[Fe(CNle1 Procedures 5 and 6 PbCr01 Procedures 7 and 8 Bi BiI3 Biln-
Yellow Pale Yellow Yellow Black Black Yellow
Procedure 9 Cu21Fe(CNle1 Procedures 10 and 11 MgNH4AsOn AgdsOl C U ~ A S O S C~(C2Hs0212 I~ Procedures 12 and 13 Sb& Procedure 14
White Red-brown Green
Hg HSd%
Black White
.
Red
Orange
several separatory routes for Group 111, hut virtually standardized confirmation tests. The onlv feature common to most Group I11 schemes relies on the amphoteric character of the hydroxides of Al", Cr3+,and Zn2+.Unfortunately, this does not lead to the best separations. I n schematic outline, Figures 5 and 6 show popular variations for separating the ions of Group 111. All of these variations work fairly well for the separation and identification of cohalt(II), iron(III), (or iron(IIl), manganese(I1) and nickel(11). Since the confirmatory tests are so distinctive and there is so little interference among these ions, Scheme I is preferred on the grounds that the fewer separations, the better. That is, in any scheme that separates the amphoteric hydroxides from manganese(II), iron(III1 cohalt(II), there is no reason to isolate maneanese(II1.
i\z(oH)3 ~
"::'
C o Fe3*
Mn(OH),
Ni2* NaOH
Ni(OH),
coz+
Scheme 111 (3,5,61
~~
Figure 7. Separation of Group Ill using excess sodium hyroxide
Figure 8. Separation of Group ill using aqueous ammonia or aqueous ammonia and potassium perchiorate
standard; however, the presence of copper(I1) must be confirmed since the blue color in aqueous ammonia is only indicative of copper(I1). We prefer the reagent potassium ferrocyanide as giving amore distinctive test for copper(II), ( 5 , w. 163).
that in aqueous ammonia zinc(II1 forms a tetrammine complex, whereas aluminum(I1I) precipitates as the hydroxide. Zinc is easily lost or confused with aluminum in the absence of sufficient ammonia due to precipitation of zinc hydroxide. Two schemes which avoid these difficulties are shown in Figure 7. Of these, scheme V is the most easily carried out. The distinctively-colored precipitates are always preferahle to difficulties with the ammonia seuaration have been discussed white precipitates, the presence of the arsenate ion is cona t length by Noyes and ~ r a y ( 1 i )Manganese(I1) . is very diffirmed by precipitation as the red-brown silver arsenate. If ficult to sewarate due to rawid oxidation to manpanese(III1. any doubt exists, add 3M nitric acid to a portion of the test Incomplete separation of &her elements occurs-due to absolution and heat gently. Mild oxidizing conditions convert sorption by the prec~pitatedhydroxides. Zinc(I1) and cosome of the thioarsenite, ASS&, to arsenite, ASO:~". The halt(I1) are particular problems; in some cases, as much as 90% wresence of arsenite ion is detected bv ureci~itatinethe disof the cohalt(I1) andlor zinc(I1) will he carried down by the . tinctive green compound, C U ~ ( A ~ O ~ ) ; . ~ ~ ( C ~ H : ~ O ~ ) ~precipitated hydroxides. We have, nevertheless, retained the Procedures 12 and 13: S e ~ a r a t i o nand identification o i ammonia separation because it works well enough for simple antimony. These procedures'are straight-forward. separations. the presence of antimony must he confirmed since it is very The seuaration of Grouu I11 is ereatlv" simnlified hv reeasy to confuse the black antimony with unreacted aluminum moving manganese as manganese(1V) oxide initially, then or iron. Precipitation of antimony(I1) sulfide from an oxalic ~ r o c e e d i-n with e the analvsis of the remainine ions. acid solution is preferable to the alternative use of sodium thiosulfate to precipitate Sb2OS2 only because thiosulfate will The confirmatorv tests for G r o u ~111all work. includine the air oxidize and hence has a short shelf-life. SCN- test for co5+, provided t i e directions are followed Procedure 14: Identification of tin. This is a standard carefully. However, use of PbOs (11 to confirm Mn2+ is not procedure and has been discussed above. recommended since it is dark enough to ohscure the MnOacolor. Also, precipitation of Al(OHI3 from ammonical solution Group Ill ( 1 )is not a good confirmatory test for A13+since Zn(OH)?will In contrast to Group I1 for which there is essentially a single precipitate unless conditions are just right. There is no good separation route hut a variety of confirmatory tests, there are reason to avoid using aluminum reagent.
ow eve;,
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Volume 60
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Groups IV and V
All versions of the qua1 scheme treat these groups in an identical manner, the only exception being those versions that precipitate strontium from concentrated nitric acid as strontium nitrate (5,6,7) and one version that does not use magnesium reagent (1).These groups are undoubtedly the most difficult in terms of lab technique. Flame tests only provide supporting evidence and should not he relied on exclusively. Literature Cited (1) Keenan,C. W., Kleinfelter,D. C..and Wood.J. H.,"GeneralCallepe Chemirtrywith Qx~alitativeAnalysis/ 6th Ed., Harper and Row Puhliahen, New York, 1980. (2) Neberpall. W. H., Huitzclaw, H. F., and Robinson. W. K., "College Chemistry with Qualitative Analysis," 6th Ed., D.C. Heath and Cu., New Ymk. 1860. ( 5 ) Bailer. J. C., Moeller. T., Kleinberg, J., Cuss. C. 0.. Castellion, M. E.. and Metz. C.,
140
Journal of Chemical Education
"Chemistry iwiih qualitative Analysis): 2nd ~ d .~, c a d e m i c~ r o s aI. ~ CNPW . YO& 1980. 14) ~ i n gE. , d., "hnic ti^^^ and separatlonr," ~~~~~~~tLira"e J ~ v ~ ~I ~oc .vN, ~~ W~ ~ . York, 1973. 15) Layde.D. C., snd Bu8ch.D. H.,"Introduction faQuvlitativeAnalysis,"2ndEd..Allyn and Bscon,lnc.,Ner ~ o r k1968. , (6) ~ o d e rT.. , " ~ u a ~ i t a t i v,2ndysia: e M C C ~ ~ ~ T TBOO^ - H ~ ICU.. I I ~ C N. ~ WYWL. ,358, p. 287. (71 ~ o g n e s i T. , R.. ~ o h n s o n ,w c., and ~ ~ ~A. R..~ ~ u at ~ i t ~a t l Analysis v ~r ~ and g . C h e m d Bquilihnum." ~ u l t ~. i n e h a r tand winston, h c . , NEW YO& 1966, p. 370. 181 Hoiness, H., and Trewick. R. F. G.,Anoiy.?i,7i, 276 (1950). (91 M O ~ I I ~ T., I .~ ~ chemistry: o ~ h~h n w~ i k y~and isons, ~ i n c ,N*W YO& 1962, P. 535. (101 Noyes. A. A.. "Qualitative Chemical Analysis," The Macmillan Co., N e w York, 1926, P. 86. (11) Noyes. A. A,, and Bray. W. C., "A System olQuslitative Analysis HgO for the Rare Elementr,"The MacmillanCo.,New Yoik. 1948.v. 153. Ref. (61. p. 865. i l 2 i McAlpine.H.K.,andSoule.B.A., "QualitstiveChemiral Anuiysa,"D. rreNoitrand Co. he., New York, 1956. (13) Cottan, F. A., and Wilkinsan. G., "Advanced Inorganic Chemi6Lry.l. lntericienre Publishers. New Ymk, 1972, p. 431.
