A Semimicro Scheme of Qualitative Analysis' J. T. DOBBINS and J. A. SOUTHERN2 University of North Carolina, Chapel Hill, North Carolina
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ITHIN the last several years many attempts have thereby enable us to detect smaller quantities of the been made to devise a scheme of inorganic qualita- various metal ions. This would eliminate many of the tive analvsis for thecommonmetal ionswithout the slow filtrations and tedious separations. The second is useof hydrogen sulfide. To be sure, hydrogen sulfidehas to overcome the objections to the use of hydrogen sulits objectionable features, such as repulsive odor, tox- fide. A semimicro scheme of qualitative analysis to icity, and d i c u l t y of administration. Most of these achieve this goal is here described. In the present investigation a number of organic schemes involve separations depending on the insolubilitv of the oxides. chlorides. sulfates. hvdroxides and compounds are used to detect certain of the metal ions. cardonates of the metal ions'. The difference in the So many compounds have been proposed as qualitative solubility products of these various compounds is not reagents that the choice of the most suitable ones has sufficient to give complete and clear-cut separations. been difficult. Those selected were chosen on the Consequently i t is unlikely that as far as logical and following basis: practical chemistry is concerned the classical scheme, 1. That the method of making the test be a simple one. 2. That the reagent be as specific as possible and still meet with the use of hvdrozen sulfide. will be dis~laced. other requirements, thereby making it possible to test Meanwhile, many new reage& for the i2entification for a metal ion in a solution containing several others. of various metal ions have been developed or discovered. 3. That the reagent produce a very striking change in the Strange as it may seem, no extensive work has been appearance of the solution, for example a drastic change done to adapt these new reagents to a complete sysin color, or the formation of a heavy precipitate. 4. That the conditions of the test not be too limited. tematic macro or semimicro qualitative scheme of in5. That the cost of the reagent not be prohibitive. organic analysis. They have been used rather exten6. That the reagent be stable in solution. sively for micro analysis, chiefly because of their sensitivity, and for the so-called "spot tests." Some semiTHE SCHEME micro schemes have been proposed, but these involve, Aside from the fact that magnesium is placed in its for the most part, the same type of reactions and identi- natural group with barium, strontium, and calcium in fication tests as those employed in the classical macro Group IV, the groups are the same as those in the scheme, the only difference being that the operations common scheme of analysis. The precipitating condiare carried out on smaller quantities. tions and reagents are the same as in the usual scheme Realizing that from a scientific and pedagogical except that in Group IV the metal ions are precipitated standpoint the development of a method superior to the as phosphates from an alkaline solution. sulfide separation is quite improbable, there appear to Workmg on a semimicro scale means that the appabe two ways by which this method may be improved ratus must be selected to suit the scale. For all separavery considerably. The first is to make use of the tions of solids and liquids the centrifuge is employed; many specific and sensitive reagents now a t hand, and centrifuge tubes of 3-ml. capacity and 75-mm. test 'Taken from a thesis presented by J. A. Southern to the tubes are used throughout. For transferring liquids, faculty of the University of North Carolina in partial fulfilment droppers of 1.25-ml. capacity are used, and for very of the requirements for the Doctor's Degree. 'At present with the Chemical Warfare Service, New York, small amounts of liquids a semimicro capillary pipet with a capacity of 0.2-ml. is quite serviceable. For all N. Y.
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heating and warming processes the tubes are suspended in a beaker of boiling water, being held in place by a cover of sheet copper with holes large enough to support
the tubes. One-ml. pipets graduated in 0.1 ml., are used for accurate measurement of liquids. The systematic analysis is camed out as follows:
GROUP
Proccdurc 1. Precipitation of Group I . To 5 drops of the unknown, contained in a centrifuge tube, add 1 drop of 3 N HCI. Centrifuge until clear. Test for complete precipitation by adding 1 drop HCl. The precipitate is composed of the chlorides of the Group I metals, and after removal of the centrifugate is
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washed with 2 drops of water and analyzed according to Procedure 2. The catrifugugete containing the metals of Groups I1 t o Vis preservedfor Group I1 (Procedure 3). Procedure 2. Treat the white precipitate Procedure 1with 2 or 3 dropsof water,heat in the water bathoneminuteand centrifuge.
Residue may contain AgCl and HgKh. Treat with 2 drops of 3 N N&OH and centrifuge. Remove the centrifugate. c a n r v i f w u t e m a y contain ~ b C b . Divide into two portions. 1. To the first, add an equal portion of 2.5 N NaAc and heat in water bath. Whenhot, add 10 drops of bromine water and continue to heat for 1t o 2 minutes. Centrifugeand pouroff theliquid. Wash theresidue with 10 drops of water. Wash again with 5 drops of water to which 5 drops of 3 N NH'OH have been added. Discard the wash water. Then add 5 drops of benzidine solution. The formation of a pnrplish blue solution shows the presence of Lsno. 2. T o t h e s e e o o d p o r t i o n a d d ~ C r O ~ .Ayellowprecipitateshowr the presence of CBAD
. . may contain AgCI. Make acid with HNOa. The formation of whifeprffipiLate s h o w s the presence of SILVBR.
Residua m a y contain HE. wash with 10 drops of water. Discard the wash water. Dissolve in 1 drop of cane. HNOa. Dilute t o 1 ml. with water and divide info two poxtion.. 1. Make first portion neutral with 2 N KOH. add 10 drops of 2.5 N NaAc and 2 drops of diphenylcarbazid solution. The formation of a deep purple color shows the pre3enee of xeacunr. 2. To second portion add SnCI.. A white precipitate showr the ureeenee of ~ X C V R Y .
GROUP I1
Procedure 3. Precipitation of Group 11. To the centrifugate from Group I (Procedure 1) add one drop of 3 NHNOa. Heat in the water bath three minutes. Add 3 N NHaOH until the solution is barely alkaline, then add 3 N HC1 dropwise until the solution is barely acid, concentrate by boiling to 1 ml. if necessary, then add 1 drop of concentrated HC1. Heat t o bailing, place in water bath and pass in H a slowly for three minutes. Cool, dilute t o 2 ml. and again pass in H B for thirty seconds. A white precipitate is sulfur, not Group 11. Centrifuge and test for complete precipitation. When precipitation is complete, centrifuge and remove the centrifugate. Centrijugale contains Groups 111t o V. Heat in water bath for several minutes, or until all H2S has been expelled and prererve far Group I11 (Procedure 7). Residue consists of sulfides of metal3 of Group I1 and is treated according t o Procedure 4.
Procedure 4. Separation of Groups IIA and IIB. The sulfide precipitate from Procedure 3 is washed with 10 drops of water containing s drop of saturated NH,CI. The wash water is discarded. Treat the residue with 4 drops of 2 N KOH and heat in water bath for about two minutes and centrifuge. Remove centrifugate. Wash the residue with 4 drops of KOH and centrifuge. Remove wash solution and add t o centrifugate. c,,trifugate contains this and oxythio salts of G~~~~IIB (as Sb and Sn) and is analyzed according to Procedure 6, Residue contains the sulfides of Group IIA (Hg, Pb, Eli, Cu, and and is according to Procedure 5. Procedure 5. Analysis of Group IIA. Wash the residue from Procedure 4 with 10 drops of water, discard the washings and then add 5 drops of 3 N HNOJ; warm for two minutes and centrifuge.
C r n l r i f w o l c contains nitrates of Cu, Cd, Bi, and Pb.
Add 5 drops of concentrated NH40Handcentrifuge. Wash with 5 drops of water and add washings t o the centrifvpate.
Residue may be black HgS or white ZHgS.Hg(NOs)$. Dissolve in 1 or 2 drops of aqua regia by heating. Divide into two portions.
I. To
portion add diphenylcarhseide solution. Now KOH run down the side of the inclined tube. A bl". irhgurplecolorshowsthepreaenee of MPRCUPIY. of
2. To second portion add SnClr. A white precipitate shows the presence of xnacrrav.
Cu and Cd as ions. Divide into two portions. 1. To first portion add 2 drops of benzoin orime solution. The formation of a greenish precipitate shows the prenence of COPPER. 2. To second portion add a drop of KCN. Divide into two portions. (a) To fir* portion add 6 drops of KOH and 2 drops of "Cadion" and then 6 t o 10 drops of formaldehyde. The separation of a reddish colored precipitate shows the p e e Cenlr;fgzgafa
ence OF C*DNIVX. (6) To second portion add HsS.
A greenish yellow precipitate shows the presence of CADxrUx.
Procedure 6 . Analysis of Group IIB. To the centrifugate (Procedure 4) containing the metals of Group IIB, add dilute
Residue consists of Pb(OH), and Bi(OH),. Add 2 drops of KOH, warm and centrifuge. Removethecentrifugate. Rcridvc may be Bi(OH).. Wash with water. Discard washings. Dirrolve in l d r o p o3NHC1. f
C e n t r i f " g l r m a y contain Pb as PbOs-. Make barely
acid with 3 N HAc. Divide into two portions.
1. To first portion add 10
drops of 2.5 N NaAc. Test f ~ ~ ~ , " , " for ~ L~E A D~as $in Group ~ & I.~
An intensely colored precipitate shows the presence of R l S M V T A .
2. To second portion add K?CrO.. A yellow precipitate ahow3 the presence of L E A D .
HCI until neutral, centrifuge and discard the centrifugate, then add 6 drops of concentrated HCI, stir, and centrifuge.
c e n r r i f i ~ n r rcontains s b and s n as chlorides. ~ i v i d into e two portions. 1. T o the first portion add 0.05 g. NaNOa slowly, then 2 drops of 3 N HCI. Finally, to 2 drops of solution on a spot plate add 2 drops of Rhodamine B ~olution. A purplish color shows the presence of m r r ~ o a u . 2. T o second portion add an equal volume of 3 N HCI. Add a piece of magnesium ribbon about 2 cm. longand allowto stand until the magnesium is diiiiIved. Add 5 drops of water and 1 drop of phosphomolyhdie aeid. f he formation of a bluecolor shows the presence of r r N .
R E ~ ~ ~ may U Econtain AS.
~ i s s o l v cin a drop of concentrated HNOa by heating and add 6 drops of (NH.)IM~O~.Heat 1 to 2 minutes, dilute to 3 ml. and cool. ~ d d 2 drops of SnClx. The formation of a deep hlue color shows the presence of * a s e ~ r c .
GROUP 111
Procedure 7. Precipitation of Group 111. To the centrifugate from Group I1 (Procedure 3) add 2 drops of N H E l and concentrated NH,OH until distinctly alkaline (0.1 ml. should be sufficient). Pass in H B until precipitation is complete and centrifnge. Preci@ats is to be analyzed according to Procedure 8. Cmtrifugatc contains Groups IV and V. Make acid with acetic acid and heat in the water bath until analysis of Group 111 is completed. Centrifuge and discard any precipitate which may form while the solution is being heated. Save the clear, colorless centrifugate for analysis of Groups IV and V (Procedure 11). Procedure 8. Analysis of Group 111. Wash the precipitate from Procedure 7, consisting of the hydroxides of C r and A1 and 1st Portion Test for Cr by heatingfor one minute and adding an equal volume of water, and making barely acid with HISO.. Then add 2 drops of diphenylcarbazide solution. T h e formation of I reddish violet color shows t h e prcsence of C x n o M l u M .
sulfides of Zn, Fe, Co, Ni, and Mn, with 10 drops of water to which 2 drops of 4 N NHX1 have been added, and dissolve by adding first 1 or 2 drops of concentrated HCl and heating in water bath for one minute, then add a small drop of concentrated HNOa. Continue heating until the solution is clear. Make the solution alkaline with KOH and slowly add 0.1 to 0.2 gm. Nas09,heat for one minute in water bath and centrifuge. cenlrifugate contains cr,Zn, and and is analyzed according to Procedure 9. P7ecifitate contains Fe, Co, Ni, and Mn as Or hydrated oxides and is analyzed according to Procedure 10. Procedure 9. Centrifugate from Procedure 8 contains AIO1-, CrOa-and ZnOz-. Divide into three portions.
2nd Portion Test for A1 asfollowr: Acidify with 3 N HAe. Add 2 dropsof "Aluminon." Make barely alkaline with NHdOH and centrifuge. The separation of a red precipitate shows t h e presence of ALUMINUM.
Procedure 10. The precipitate from Procedure 8 consists of the hydroxides and hydrated oxides of Fe, Co, Ni, and Mn.
3rd Portion Test for Zn as follows: Add an equal volume of water and 10 drops of dithizone solution. Shokr aigorously. A change in the color of the aqueouslayer to pink, not hrown,showr the presence of ZINC.
Wash with 10 drops of water. Dissolve in concentrated H( Dilute and divide into four portions.
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1st portion Test for M o as follows: Add 2 drop5 of saturated NaKCaH 4 0 . Make alkaline with 2 N KOH and shake for one minute. Add 1 drop of benddine. A blue color shows the presence of M A N G A N E S E
. ..... ...... T e ~ tfor F e by adding 1 drop of 3 N N H E N S . The formation of a deep wine red color show. t h e nrcnence of m o w .
3rd Portion Test for Ca as follows: Add d i d NalFl until t h e solution is colorless. Now add 2 drops of NHaCNS. The formation of a blue eolor shows t h e presence of COBALT.
4th Portion Test for Ni as follow.: Add solid NasFs until t h e solution in colorlers. Make alkaline with 3 N NHaOH, then add 2 drops of dimethylglyoxime. The orecioitate form*:ion of a red . show3 t h e presence of NICKEL.
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GROUP N
Procedure 11. Detection of Mg. The clear, colorlesscentrifugate from Group I11 (Procedure 7) should have a volume of not greater than 1.5 ml. If larger, hest in the water bath until the volume is reduced. Discard any residue which may settle out. Take one-fifth of the solution (save the rest for Procedure 12) and test for Mg as follows: add 2 drops of the Mg reagent (p-nitrohenzeneazo-8-naphthol) and then 4 drops of 2 N KOH. Shake well and centrifuge. The farmstian of a purple precipitate shows the presence of MAGNESIUM. Pmceduie 12. Precipitation of Group IV. To the remainder of the solution, saved from Procedure 11, add NHIOH nntil
alkaline, then 0.6 N (NH&HPO, until precipitation is complete and centrifuge. Precipitate contains phosphates of Ba. Sr. Ca, and Mg and is analvzed accordine - to Procedure 13. Centrifugate contains Group V and is analyzed according to Procedure 14. Procedure 13. Wash the precipitate from Procedure 12 with 10 drops of water containing a small drop of 3 N NH,OH. Dissolve in a drop of 3 N HAc. Add one drop of K2Cr04,allow to stand for one minute and centrifuge. Wash the precipitate with a drop of water, adding the washings to the centrifugate.
Cenlri7ugale contains Sr, Ca, and M g an acetates. Make alkaline with 3 N NH.OH and centrifuge. Discard t h e ceotrifugate. PreriPaolr consists of the phosphates of sr. c n , and ~ g .wash with 3 dropr of water, discarding the washings. Disrolve t h e precipitate in a .mall drop of 3 N HAe and add an equal volume of water. Add a small drop of 2 N (N&),SOI and centrifuge. P ~ e t i P i W e may be BaCrO. and indicate3 t h e presence confirm with of a*a,uY. name test.
Cenlrifugolc contains CaSO, and MgSO..
Add a small drop of
(N&).CtO& and centrituge.
PmiPi1.de may be Srs01 and indicate* the prese n c e of s r a o r r r r u ~ . Confirm with flame test.
Pracipil.de may be CaClO. and indicates t h e prerenee o l c~rcrur.Confirm with flame test.
Crnlrifugolr contains Mg but is fa be discarded, as Mg was detected in Procedure
GROUP V
Procedure 14. Removal of PO4= ions. To the centrifugate from Group IV (Procedure 12) add Mg(NO& nntil precipitation of MgNH4PO& is complete and centrifuge. Precipitate is discarded. Centrifugate contains Na, K, and NHI salts and is analyzed according to Procedure 15. Procedure 15. Removal of ammonium salts. Place the centrifugate from Procedure 14 in a small crucible, add 4 drops of 3 N
HSO, and evaporate to dryness. Continue to heat just below red heat until all fumes cease to be evolved. Treat the residue as directed in Procedure 17. Procedure 16. Test for the ammonium ion. Place a drop of the original unknown in a small test tube and add 2 drops of 2 N KOH. At the mouth of the tube hold a piece of filter paper moistened with Nessler's reagent, or a piece of moistened red litmus paper. Warm the solution. The Nessler's reagent will
turn yellow t o brown, depending on the amount of NHI+ present. The litmus will turn blue, which shows the presence of NHlf. Procedure 17. Test for Na and K. Cool the residue from Test for Sodium Take 1drop of solution from crucible and put in centrifuge tube. Add 2 drops of vrnnyl acetate reagent far sodium, allon t o stand for one to two minutes and centrifuge. A greenish gdlow precipitate shows the presence of soorur.
Procedure 15 and add a drop of water and make flame test for sodium and potassium. To further confirm the presence of these metals add another drop of water and proceed as follows: Test far Potassium T o solution remaining in crvcible add 1 drop of dipicrylamine solution. Allow to stand for several minufen. The formation of orange-red cryhtnls and fading of the color of the sololution from orange-red t o yellow shows the presence of m r ~ s s r a l a .
CONCI.USION
The proposed seheme has been used with approximately 1200 students takmg qualitative analysis. It is being used a t the University of North Carolina and four other colleges in the mimeograph form and the results have been most satisfactory. Students have had fewer "repeats" with unknowns using the scheme than with schemes previously used. The proposed scheme has a good pedagogical value and observations have led us to
feel that it is an improvement over the macro and semimicro schemes to be found in the literature and textbooks in the following respects: (a) ease of mauipulation (b) accuracy of results (c) economy of materials and ( d ) a saving of time due to the reduction of the number of steps required to cany out the procedure. We have completed an analysis of a solution containing all metal ions in less than two hours.
