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Fe(OH), MnO₂, Ni(OH)₂, CdOH), H₂SO, (7) MnO₂ H₂SO4 H₂O₂ Mn HNO, NaBiO, MnO, purple (9) NH, (10) Fe(OH), brown NatIO NaOH NH₂ Fe³*, Ni, Cd² KSCN (10) FeSCN red NH(NH31 dimethylglyoxime (11) NIC,H, N.O. rose-red Cro,, AKOH), Zn(OH), HCHO. NH, (13) ALOH), HCH O. catechol (14) violet AP complex blue 2+ Cd(NH) * thioacetamide (12) Cds yellow COZNHẠ Bich BaCrO HNO, (16) Cr₂0,- H₂O₂ CrO₂ blue (15) ZHỊNH, K,Fe(CN) (17) K.Zn [Fe(CN) light green NaOH HCI K.Zn [Fe(CN light green THE CATIONIS
lons possibly present Fe, Fe, Al Zn, Mn, Cr, Co, Ni, (Cd) NH,C NH, thinacetamide FeS, ALOHI),, ZnS, MnS, Cr(OH), CoS, Nis, Cds HCI (2) CoS, Nis T HCI HNO, Co, Ni Outline of Procedure for Analysis of Group III Cations (3) NH,SCN (4) Co(SCN), blue dimethylglyoxime (5) NIC,H, NO. rose-red MnO₂ 11,50, H₂O, FelOID, MnO, NOH), Cd011), H₂SO, ( Ma T HNO, NaBiO, MeO, purple (8) (A) ( Fe, AP, Zn, Mn, Cr, Ni, Cd N NaOH NII, plus ions from Group IV Fe, Ni, Cd KIẾN FeSCN red NH, (10) Fe 010), brown (6) NENH dimethylcyosie (11) NICHINO, rose red CO, ALOH, Zn(OH), IC 1,0. (13) NH₂ AKOH), HCHO. catechol (14) violet Al complex blue CANI thinacetamide (12 Cals relline CO, Ze(NH₂ Bộ l BaCrO, HNO, (16) C₁₂0, H₂O, Cro, blue (15) TINH K.FCN (17) K,ZnJFeCN light green NaOH RCI K,Zn/FeCN light greee 82 THE CATIONIS
TORO T. e con- - chlo- with a Group nown ition that 6 M lish hen stir. tes. oid vill lly ter nd he ne if 3- I D ANALYSIS OF GROUP CATIONS 53 III and IV. you cannot make the solution you decanted from the initial sulfide precipitate basic, since that would bring down those groups. Rather, check the pH of the decanted solution. If it is too low (too acidic), add 0.5 M NaC₂H₂O₂, sodium acetate, drop by drop with stirring, until the pH is again 0.5. A precipitate may form during this adjustment; usually it is brown or yellow. Add Lin. 1 M thioacetamide to the mixture, and proceed as in the preceding paragraph, starting with the fourth sentence, the one that reads "Heal the test tube... Step 3. To the precipitate from Step 2 add 2 mL 1 M NaOH, not 6 M. Heat in the water bath, with stirring, for two minutes. Any SnS or Sb₂S should dissolve. The residue will typically be dark and may contain CuS, BigS3, PbS, Cds, and HgS. Centrifuge and decant the yellow liquid into a test tube (Label 3). Wash the precipitate twice with 2 mL. water and 1 ml. 1 M NaOH. Stir, centrifuge, and decant, discarding the wash each time. Step 4. To the precipitate from Step 3, add 2 mL 6 M HNO3. Heat in the boiling-water bath. Most of the reaction will occur within about a minute, as some of the sulfides dissolve and sulfur is formed. There may be a substan- tial amount of residue, which is mainly HgS and free sulfur. Continue heating until no further reaction appears to occur, at least two minutes after the initial changes. Centrifuge and decant the solution, which may contain Cu²+, Bi³*. Cd2+, and Pb2+, into a test tube. Wash the residue with 2 mL water, centri- fuge, and discard the wash. Then add 1 mL. water to the residue and put it aside (Label 4.5 "4° Step 5. To the solution from Step 4, add 6 M NH3 until the solution is basic to litmus. Then add 0.5 mL more and stir. If copper is present, the solution will turn blue. A white precipitate in the solution is indicative of bismuth (or, possibly, lead). Centrifuge and decant the solution, which may contain Cu(NH3)42+ and Cd(NH3)4*, into a test tube. Wash the precipitate with 1 mL water and 0.5 mL 6 M NH₂. Stir, centrifuge, and discard the wash. Step 6. To the precipitate from Step 5, add 0.5 mL 6 M HCl and 0.5 mL water. Stir to dissolve any Bi(OH), that is present. A white insoluble residue may contain lead. Centrifuge and decant the solution into a test tube. Wash the residue with 1 mL water and 0.5 mL 6 M HCl. Centrifuge and discard the wash. Step 7. Confirmation of the presence of bismuth. Add 2 or 3 drops of the decantate from Step 6 to 300 mL water in a beaker. A white cloudiness caused by precipitation of BIOCI appears if the sample contains bismuth. To the rest of the decantate, add 6 M NaOH until it is definitely basic; a white precipitate is Bi(OH)3. To the mixture add 2 drops 0.1 M SnCl₂ and stir; if bismuth is present, it will be reduced to black metallic bismuth. Step 8. Confirmation of the presence of lead. To the white precipitate
THE CATIONS i at iron, a rust-red precipitate of Fe(OH), will form. Centrifuge out the solid transfer the liquid to a test tube. tion from Step 10 may contain Ni(NH36 and Cd(NH₂). To half of that Step 11. Alternative confirmation of the presence of nickel. The sol solution add 0.5 mL. dimethylglyoxime. Formation of a rose-red precipitate proves the presence of nickel. 78 mium failed to precipitate in Group II, it should be in the other half of the Step 12. Alternative confirmation of the presence of cadmium. If cad solution from Step 10. To that solution add 6 M HC₂H₂O₂, acetic acid, until after mixing, the solution is just acidic to litmus. Then add 1 mL thioacets mide and put the test tube in the water bath. If cadmium is present, a yellow precipitate of CdS will form over a period of several minutes. Step 13. Returning to the solution from Step 6, add 6 M acetic acid slowly until, after mixing, the solution is definitely acidic to litmus. If neces sary, transfer the solution to a 50-mL beaker and boil it to reduce its volume to about 3 mL. Pour the solution into a test tube. Add 6 M NH₂, drop by drop, until the solution is basic to litmus, and then add 0.5 mL in excess. Stir the mixture for a minute or so to bring the system to equilibrium. If alumi num is present, a light, translucent, gelatinous precipitate of Al(OH), should be floating in the clear (possibly yellow) solution. Centrifuge out the solid and decant the solution, which may contain CrO2 and Zn(NH3)4*, into a test tube. Step 14. Confirmation of the presence of aluminum. Wash the precipi tate from Step 13 with 3 mL water once or twice, while warming the test tube in the water bath and stirring well. Centrifuge and discard the wash. Dissolve the precipitate in 2 drops 6 M HC₂H₂O₂, acetic acid, no more, no less. Add 3 mL water and 2 drops catechol violet reagent and stir. If Al³+ is present, the solution will turn blue. Step 15. If the solution from Step 13 is yellow, chromium is probably present; if it is colorless, chromium is absent. If you suspect that chromium is there, add 0.5 mL 1 M BaCl₂. In the presence of chromium, you obtain a very finely divided yellow precipitate of BaCrO4, which may be mixed with a white precipitate of BaSO4. Put the test tube in the boiling-water bath for a few minutes; then centrifuge out the solid and decant the solution into a test tube. Wash the precipitate with 2 mL water; centrifuge and discard the wash Step 16. Confirmation of the presence of chromium. To the precipitate from Step 15, add 0.5 mL 6 M HNO, and stir to dissolve the BaCrO4. Add 1 mL water, stir the orange solution and add two drops of 3 percent H₂O₂. A blue solution, which may fade quite rapidly, is confirmatory evidence for the presence of chromium. presence of chromium, use the solution from Step 15. If you were sure Step 17. Confirmation of the presence of zinc. If you tested for the chro COMMENTS mium was al acidic to lit- excess and t cyanide, ane K₂Zn, Fe(C amination. add 5 drop possibly sc centrifuge 0.5 ml. wa drops). Re DISCARD COMMENT ANALYSIS Step ble and Group I sulfide buffer is the con 10 M Group insolub more s In and sc (greer fides, COS reduc be th itate liqui out a the real sep no obt bo
& Bei lout H₂S # PROCEDURE FOR ANALYSIS OF GROUP II CATIONS 52 WEAR YOUR SAFETY GLASSES WHILE PERFORMING THIS EXPERIMENT. Unless directed otherwise, you may assume that a 10-mL. sample co tains the equivalent of about 1 mL of 0.1 M solutions of the nitrate or chlo ride salts of one or more of the Group II cations. If you are general unknown, your sample is the HCl solution decanted from the C I precipitate and may contain ions from Groups II, III, and IV. working with Group Step 1. Pour 5 mL of your Group II sample or your general unknow into a 50-ml beaker. Add 0.5 mL 3% H₂O2, and carefully boil the solution down to a volume of about 2 mL. Step 2. Swirl the liquid around in the beaker to dissolve any salts that may have crystallized, and then pour the mixture into a test tube. Add 6 M NaOH a little at a time, until the pH becomes 0.5. (The way to accomplish this is discussed in the comments on procedure following this section.) When the pH has been properly established, add 1 mL. 1 M thioacetamide and stir. Heat the test tube in the boiling-water bath for at least five minutes. CAUTION: Small amounts of H₂S will be liberated. This gas is toxic, so avoid inhaling it unnecessarily. If any Group II ions are present, a precipitate will form; typically, its color will be initially light, gradually darkening, and finally becoming black. Continue to heat the test tube for at least two minutes after the color has stopped changing. Cool the test tube under the water tap and let it stand for a minute or so. Centrifuge out the precipitate and decant the solution into a test tube. Add 1 mL 1 M NH4Cl and 1 mL water to the precipitate and put it aside. Under the conditions described here, cadmium ion will not precipitate if the concentration of Cl ion is too high. If CdS does not come down, cad- mium will carry over into Group III. If your unknown contains only Group II cations, you should force the precipitation of CdS, and any other sulfides that were reluctant to precipitate, by adding 6 M NH3, drop by drop, to the decanted solution until the solution is basic to litmus. Then add 1 mL 1 M thioacetamide and heat the test tube in the water bath, stirring occasionally. for three minutes. Centrifuge out any precipitate, and decant the liquid. which should be saved only if it contains jons from groups to be studied later. Add 1 mL 1 M NH4Cl and 1 mL water to the precipitate, stir, and pour t slurry into the test tube containing the first portion of Group II precipitate. Stir well, centrifuge, and decant the liquid, which may be discarded. Wash liquid. the precipitate once again with 3 mL water. Centrifuge and discard the wash the If you are working with a general unknown, containing ions from Groups