QUALITATIVE ANALYSIS without HYDROGEN SULFIDE* A
Comparative Study
L. A. MUNRO Queen's University, Kingston, Ontario
Brockman's scheme for qualitative analysis, avoiding the use of hydrogen sulfide, has been com@red with the accefited methods by student perfomzance in the qualitative analysis laboratory. The procedures have also been tested in a sem~u~ntitative fashion by a research student. The proposed scheme i s much faster but somewhat less accurate than the classical method. Some improvements of Brockman's scheme are suggested.
v
ARIOUS schemes for qualitative analysis have been proposed avoiding the use of HnS. Among the most recent are those outlined by Almquist ( I ) , 1918, Vortman (Z),Mullinix (3), Macchia (a), (5), and Strohal ( 6 ) . In 1930 Brockman embodied such a scheme in a textbook of qualitative analysis (7). In a review of this book in the JOURNAL OF CHEMICAL EDUCATION Browning (8) writes, "While the writer has not had occasion to try out the method so that he can compare it with the conventional one, he finds it of considerableinterest. Whether the suggested system be adopted or not, it certainly merits serious consideration." I could not help agreeing with the comments of the reviewer. Although we have "deodorized the qualitative laboratory by using the individual paraffin-sulfur method for the generation of H2S, it was nevertheless felt that any method that eliminates the time-consuming bubbling of Has without sacrificing accuracy should receive attention. It was, therefore, decided to make a comparative study of the two methods. The study reported in this paper extended over the past two years. Before going into the discussion of the methods of study and the results obtained, i t might be well to glance a t the scheme proposed by Brockman. It will be given in bare outline. The reader should consult the text for details. The metals are divided into six groups. The first group is essentially the same as in conventional methods. Group I1 contains the insoluble sulfates. The precipitation of CaSOh is aided by the use of a little ethyl alcohol-in the solution, of course. Group I11 is the amphoteric group. Group IV contains those metals which form insoluble phosphates in the presence of excess ammonia. The metals which form soluble
-
Presented to the Educational Section at the Canadian Chemi-
cal Convention, Quebec. June 8. 1933.
ammonia complexes are placed in Group V. Group VI comprises the alkalies. They are tested for either a t the start or after Group I. The method of separation and identification is briefly as follows. Group I needs no comments since it is the same as in the classical scheme. Group I1 is metathesized by boiling with Na2COs, filtering, and repeating the procedure. The amounts of sulfates converted to the carbonates can be calculated from the K,*.. Lead and barium are precipitated as chromates. When the mixed precipitate is treated with NaOH the lead dissolves as NaHPbOz. It is reprecipitated by adding acetic acid to the filtrate. Barium is confirmed by the flame. Calcium is precipitated as CaKnFe(CN)s, strontium as sulfate, confumed by the flame. Group I11 is the amphoteric group. The alkaline solution is just neutralized with HC1 and then treated with excess NazCOa. This precipitates the hydroxides or basic salts of Al, Sn, Zn, and Sb. The As and Cr remain in the filtrate. The chromate is detected by the etheriu blue test and the arsenate by arseno-molybdate in acidified solution. Unlike phosphate this precipitate is soluble in alkali. The four remaining metals are separated into two groups by dissolving them in HCl, with subsequent treatment with ammonium hydroxide and ammonium chloride. This precipitates the Al(OH)s and stannic acid, while the zinc and antimony appear in the filtrate. The latter is divided into two parts. The first is acidified and an iron nail is introduced. A black precipitate on the nail is Sb. The procedure recommended by Brockman is to dissolve the precipitate in nitric and tartaric acids, with the addition of hypo and the precipitation of red Sb20S2 on boiling. Potassium ferrocyanide is added to the second portion, giving a precipitate of K2ZnFe(CN)6. The precipitated Al(OH)a and stannic hydroxide are dissolved in HC1 and the solution is divided into two portions. The tin is reduced in the 6rst by an iron nail and the reduced solution is filtered into mercuric chloride. Zinc is added to the second portion to remove the tin and the filtrate is made alkaline with ammonia, confirming with the "aluminon" test. The residue from the treatment of the metals with KOH and NanOnis divided into Groups IV and V by dissolving it in nitric acid and treating with ammonia and di-ammonia acid phosphate. The metals which form soluble complexes are in the filtrate, which is reserved as Group V.
Group IV contains Mn, Fe, Bi, and Mg. The phosphates are redissolved and the manganese oxidized to MnOe by KCIOs. Bismuth and iron are precipitated by ammonium hydroxide in presence of ammonium chloride. This mixed precipitate is then redissolved and specific tests for bismuth and iron .are made on separate portions. The bismuth gives a black precipitate with hypo. Magnesium is confirmed by the usual test. In Group V, mercury and copper are precipitated as sulfides by thiosulfate and separated by dissolving the copper sulfide in nitric acid. The soluble cobalt, cadmium, and nickel ions are tested for in separate portions of the filtrate. Nickel is brought down with dimethylglyoxime. The precipitate is filtered off and the test for cadmium is carried out on the filtrate, using ammonium perchlorate as reagent. The alkali metals are in Group VI. They are detected in portions of the original or after Group I. METHOD Or STUDY
The course in qualitative analysis a t Queens' con.sists of two lectures a week and six hours' laboratory. The text used in the laboratory is by A. A. Noyes (9), supplemented by Treadwell-Hall (10) and various improvements from recent literature. These manuals follow the classic methods, using HB. The writer has always maintained that the prime object of a course in qualitative analysis is to teach the chemistry of the metals and fundamental chemical theory. For that reason the class has always been given alternative methods of analysis and each student has been required to devise a scheme for a limited number of metals. Alternative schemes are studied for two weeks in the laboratory. I t has been found that this serves to emphasize the basic chemistry of qualitative analysis. It also helps to get students away from the "cook-book attitude." For the past two years a class of seventy-five students has examined Brockman's scheme as an alternative method. Each student was given 50 cc. of a known mixture containing 100 mg. of one element in each group. All different combinations of elements were used. The student divided the solution into two equal parts; one he analyzed by Noyes'smethod and theother by Brockman's method. He compared the amount of final precipitate obtained in each individual test, with the size of precipitate from 5 cc. (50 mg. of ion) of test solution. He reported the percentage recovery by both schemes and made any pertinent comments regarding the reactions and time involved. Results were also obtained from two other sources. The second was a half-term study by a group of selected students on the separation and identification of specific individual groups. On completion of their comparative study of the assigned Brockman's group, they were given unknowns on the group, which they examined by Brockman's method. Mr. C. F. Beale, a fourth-year student, was assigned
the task of making a semiquantitative study of the Brockman procedures, and of improving some of the poorer ones. His results served as an excellent cou6rmation of the average results obtained by the first two groups. This will be seen from the following comments on the different separations as given by Mr. Beale and typical student reports. Gnow I1 CFB "A good quantitative precipitate of sulfate. Several treatments with Na&08 necessary. Separation of the P h and Ca not quantitative." Students "Ba better since Bas01 '/LO as soluble as BaC08." "Extremely fine precipitate BaSO,, hard to estimate." "The only one that came down better." CALCrUM
Studrmts "Poor by alternative." "Large loss of calcium." "Group I1 terrible." "Alternative great deal better."
S~ONTIUM StudcntJ
"Very poor precipitate.
Disappointing results."
cHn0mvaa CFB "Some Cr(0H)r is precipitated when boiled with KOH and cannot be redissolved; probably adsorbed or occluded with the other insoluble hvdroxides. Hvdroeen oeroxide was found to he a good oxidizing reagent. preferagle to sodium peroxide." Students "Some chmmic hydroxide does not dissolve." "Sodium peroxide and carbonate do not dissolve the hydroxide completely. Probably some chromic hydroxide does not ao into solution." C ~ B "P 14 seems to give a good separation of the As and Cr from the Al, Sh, Su. and Zn. Test for Su was poor and sometimes negative." "Test for Sn inconclusive." "In the second residue of A1 and Sn hydroxides. HCI does not dissolve all the tin." A~nw~uaa CFB "The Al in P 18 seemed to he all right but rather high."
StudmUs "Contains much foreign material as the precipitate was greater than 1000/,." ANTIMONY CFB "The test for Sb in P 19 was poor. There was no feathery deposit. The nail became black but the Sb was hard to scrape off." ~fudentJ
"A little mare completely precipitated by B." "I last some Sb through not boiling with sodium peroxide long enough." "ShO.C% forms a red precipitate but rapidly dissolves." "Antimony spoils the procedure for this group." "Sb and Cr are no good." ZINC CFB "Good positive test for Zn hut low. composes on heating."
The precipitate de-
studntts
in the results of the first two sources made me rather dubious of the value of such a study. However, on averaging the results of the classwork for two years (Table 1). and comparing them with the conclusions ARSENIC amved at by Mr. Beale, it is found that they are in [Arsenic gave better results than with HIS.] agreement in every detail. This will be seen from a CFB "The procedure gives a good separation of the Al, Sh, Sn, comparison of the foregoing with Table 1. The figures and Zn from the As and Cr." represent the average recovery of the diierent metals Students in percentages for twenty to fifty student reports. "Alternative much better for arsenic." Admittedly the figures can hardly represent absolute "Slightly larger precipitate than with Noyes." "Much better. No loss by evaporation as in Noyes, p. 44." values. An accurate statistical analysis should be [A general conclusion is that Group 111 is the most unsatismade when a larger number of results are obtained. factory. We have made an improvement in the separation They do, however, point to very definite conclusions. of antimony but the group requires further work.] I t will be noted that the first two metals which are In the procedure for antimony, described above, separated by the same procedures in both schemes give Mr. Beale found that the red precipitate started to form very close agreement. Although lead is precipitated and then redissolved. Our revised method prevents in both as a sulfate, in the B scheme it is separated this. I t is as follows. Dissolve the antimony in from barium by treatment with sodium hydroxide, nitric and tartaric acids. Boil the solution to drive off forming plumbite. Hence, we would expect that this as much acid as possible. Add water to form SbOC1, recovery would be slightly lower than in the N proa whitish precipitate. Add solid sodium potassium cedure. One hesitates to consider a difference of four tartrate. Heat and add NazSzOa. Boil for 2 or 3 per cent. as being significant, but the results for barium lead one to think that this figure can be considered -minutes. A stable red precipitate is formed at once. within the maximum significant figure. I t is to be expected that a better recovery of barium would be GROUPIV obtained in the B scheme since barium sulfate is CFB "The alternative gave a good positive test for Mn. The much less soluble than barium carbonate. It will MnOl adheres somewhat t o the beaker so that amounts be noted that the difference in the quantity reported is cannot be estimated very satisfactorily." [Procedure gave a good positive test for bismuth. Free sulfur is present in in the right direction. Taking *4 as the significant figure the alternative scheme is more accurate for the precipitate.] four ions, of equal accuraey for eight, and of much BIS?.~TA lower accuracy for nine ions. Probably one should Sludenls "Bismuth completely precipitated. Precipitate too large also add aluminum, since the apparent high separation compared with the test solution precipitate of BiPO.. On is due to undissolved tin. washing i t with ammonium hydrokde i got a test for cobalt The summarized reports on the length of time inbut could not remove i t all by washing it." volved in the two schemes are given below (Table 1). "Very complete." "Could find no Zu in alternative procedure." [Zinc is the flaw in this procedure. Several students got negative results. Others found zinc with Cd.]
MAGNESIDM CFB "The accuracy varies with the amount present. When 50 mg. were used there was little difference in accuracy but when larger quantities were used the alternative seemed better. Considerably more than 10 cc. of ammonium phosphate had to be used." Students
"It took 30 ec. of ammonium phosphate. Magnesium precipitates along with iron." "The MgNHSO, was pale brown, probably due t o some iron." GROWV
Mr. Beale reports that some mercury is not precipitated as HgS but probably as sulfo-salts. There is accordingly no good quantitative separation of the copper and mercury. Good tests were obtained for nickel and cobalt. Sometimes negative tests were obtained for cadmium. One student reports that the precipitate of cadmium sulfide was brown even after the removal of all the cobalt and nickel.
The identification of Group VI ions by the flame tests does not permit any semiquantitative observations. The writer prefers the method of Caley (11) for sodium, and the cobaltinitrite test for potassium. I must confess that the possible experimental error
-
Thirty students reported actual time requkd for analysis. Average (N) 4 hr,., 0 min.; (B)2 hrs.. 60 min. + 5.
Orasa G B N B ~ ACO-BNTS E "Alternative much fsstnbut not nearly ns good" - ~ ~ t ~ . leter t i ~ .but not as '"Alternativefaster and of equal or greater accuracy'' '"Preferelnssic method although slower". . w s . i e method fastmore e ~ e i e n t . r . . .
..... ... .. .
16
........ ....... . . . . .. .... . ....... . . . . . .. .
9
....... . . .. .. . . .
40 10 3
CONCLUSIONS
Perhaps the best way to terminate the paper is to quote some of the conclusions reached by students using this scheme. 1. "The alternative procedure is faster although it takes longer for a precipitate to become evident, thereby en. tailing a possibility of error in estimation. It daes not seem to be quite as e5cient as Noyes' scheme. For rough qualitative work it isbetter than Noyes' because there are not so many evaporations and no long H2S precipitations." 2. "Noyes is the more accurate. The alternative is quicker and for less accurate work would be the better of the two."
Possibly many of us feel like the student who states: "Although the alternative is much quicker I prefer Noyes, possibly because I am more familiar with it and can interpret results better or a t least more easily. I t is rather difficult t o estimate amounts with the alternative procedure but it avoids the long H2S precipitation and has fewer filtrations."
It may be concluded from this study that the proposed scheme is admirably suited for a short course in qualitative analysis. Unfortunately, perhaps, all
of the texts on the theory of qualitative analysis are built around a hydrogen sulfide separation. This makes the substitution of such a scheme in a long course in qualitative analysis rather difficult. We shall still retain the classical methods but shall certainly use Brockman's as a reference text, because i t furnishes excellent supplementary illustrations of the fundamental chemistry involved in qualitative analysis; &., (1) similarities of chemical properties, (2) amphoteric metals, (3) complex ions, (4) solubility-product principles, and (5) the necessity for control of hydrogen-ion con~entration. It is proposed to assign another fourth-year student the task of improving the less satisfactory tests, LITERATURE
CITED
1) ALMQ~ST. Z. amrg. allgem. Ckern., 103. 221 (1918). 2) VORTMM. Chmr. Abstr., 16, 2458 (1922). J. C ~ MEDUC.. . 1,67 (1924). 3) MULLINIX, (4) MACCHI&No&. them.-id.. 2, 191-6 (1927); C h . Abstr., 21, 2625 (1927). (5) MAcCHrA* Noti& chfB.-ind.s 2, 561 (1927); Chm. Abstr.. 22, 559 (1928). (6) STROHAL. Arkiu. kern. farm.2, 77-85 (1928); Chmr. Abstr., 22,2723 (1928). (7) ROCKMAN, "Qualitative analysis," Ginn & Co., Boston, 1930. ROWNING. J. CHEM. EDUC., 7, 1217 (1930). 9) NOYES, A. A,. "Qualitative chemical analysis," Macmillan Co., New York City. (10) TREADWELLAND HALL."Anal~tical chemistry." Vol. I 8th ed.. John Wiley & Sons. Inc.. New York City, 1932. (11) CALEY.J. Am. Chern. SOC.. 51,1965 (1929).
?
