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& 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
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 necessary to dissolve any lead-containing salts. Add 1 mL water and 1 ml. from Step 6, add 1 ml 0.5 M NaC₂H₂O₂, sodium acetate. Heat gently it 3 M H₂SO₂ A white precipitate of PbSO4 establishes the presence of lead Step 9. Confirmation of the presence of copper. If the solution from Ca(NH₂ ion. To further confirm copper(II), add about 0.3 g of solid Step 5 is blue, copper must be present, since the color is characteristic of the NagSO4, sodium hydrosulfite, to the blue solution. It should quickly decol orize as copper(II) is reduced to copper(I). Put the test tube in the boiling. water bath, where further reduction to reddish or black copper metal will occur. After about a minute, centrifuge out the solid and decant the liquid 2- into a test tube. Discard the solid. 54 Step 10. Confirmation of the presence of cadmium. To the decantate from Step 9 add 1 ml. 1 M thioacetamide and heat in the water bath. Precipi tation of yellow Cds will occur within a few minutes if cadmium is present. Step 11. Pour off the water from the precipitate from Step 4 and add 1 mL 6 MICI and 1 mL 6 M HNO₂. Put the test tube in the water bath. The dark precipitate, which is mainly Hgs, should dissolve in a minute or two, leaving some insoluble sulfur residue. Pour the contents of the tube into a 50 ml beaker and boil gently for about a minute. Add 3 mL water to the solution, stir, and pour the liquid into a test tube. Centrifuge out any solid. * Step 12. Confirmation of the presence of mercury. Pour half of the liquid from Step 11 into a test tube. Immerse an 8 cm length of copper wire in the liquid for a minute; a shiny deposit of liquid mercury on the wire confirms the presence of mercury. To the other half of the liquid from Step 11, in a test tube, add 3 mL water and stir. Then add 0.1 M KI, potassium iodide, drop by drop, without stirring. If mercury is present, you will get a red-orange precipitate of Hgl, at the interface between the upper (yellow) Kl layer and the lower colorless solution. Step 13. To the decantate from Step 3 add 6 M HCl drop by drop until the mixture becomes acidic (pH-0.5, green color on methyl violet). The precipitate that forms may contain the sulfides of Sb(III) and Sn(IV). Stir well for half a minute, centrifuge, and discard the liquid. Step 14. To the precipitate from Step 13, add 2 mL 6 M HCl. Stir and transfer the mixture to a 50-mL beaker. Boil the liquid gently for one minute to drive out H₂S and to dissolve the sulfides. A black residue is mainly HgS. Add 1 mL 6 M HCl and then pour the solution into a test tube. Centrifuge out any solid residue and transfer the liquid to a test tube. Discard the solid. Step 15. Confirmation of the presence of tin. Pour half of the solution from Step 14 in a test tube and add a 1-cm length of 24-gauge aluminum wire. Heat the test tube in the water bath to promote reaction of the Al and produc tion of H₂. In this reducing medium, any tin present will be converted to ANALYSI Sn² an for two. decant HgCl₂. lishes t St the sol add al add 1 bath. DISCA COM ANA the diza tin, sho hy S
ANALYSIS OF GROUP II CATIONS Sn² and any antimony to the metal, which will appear as black specks. Heat for two minutes after all the wire has reacted. Centrifuge out any solid and decant the liquid into a test tube. To the liquid add a drop or two of 0.1 M HgCl₂. Awhite or gray cloudiness, produced as HgCl₂ slowly forms, estab- lishes the presence of tin. Step 16. Confirmation of the presence of antimony. To the other half of the solution from Step 14 add NaOH to bring the pH to 0.5. If tin is present, add about 0.5 g of oxalic acid, and stir until no more crystals dissolve. Then add 1 mL of 1 M thioacetamide and put the test tube in the boiling-water bath. Precipitation of red-orange Sb₂S3 confirms the presence of antimony. DISCARD ALL REACTION PRODUCTS IN THE WASTE CROCK COMMENTS ON PROCEDURE FOR ANALYSIS OF GROUP II CATIONS Step 1. Here the solution is concentrated, and all ions are brought to their higher states of oxidation. Tin(II) and antimony(III), if present, are oxi- dized to tin(IV) and antimony(V). This is important, particularly in the case of tin, because unless SnS₂ is the precipitated sulfide, tin will not behave as it should in Step 3. Step 2. At this point all the Group II sulfides are precipitated with hydrogen sulfide. The reaction that occurs is in all cases analogous to that for copper: Cu²+ (aq) + H₂S(aq) → CuS(s) + HgS CuS CdS PbS Since the solubility products of the Group II sulfides are very low, they precipitate in the presence of even extremely low concentrations of S2- ion. The precipitation is carried out at a pH of 0.5, where [S2-] is only about 1x 10-20 M. The values of the solubility products of some common metallic sulfides are as follows: Group II + 2 H+ (aq) 1x 10-2 1x 10-35 1x 10-26 1x 10-27 55 Cos NIS Group III ZnS FeS 1x 10-20 1x 10-19 1x 10-20 6 x 10-18 B Under the conditions of the precipitation, the metallic cations are all about 0.01 M. therefore, [M²+ ][S2]= (1 x 10-21 x 10-20 1x 10-22. This means that those sulfides in the left column, K, <10-22, will precipitate,
lons possibly present: Fe, Fe, Al Zn, Mn, Cr, Co, Nitt, (Cd) NH,C NH, thinacetamide CoS, Nis T HCL HNO, Co, Ni NH,SCN Co(SCN) blue Outline of Procedure for Analysis of Group III Cations (4) (3) FeS, ALOHI),, ZnS, MnS, Cr(OH), CoS, Nis, Cds HCI (2) dimethylglyoxime (5) NIC.IN.O. rose-red MnO₂ FeO,, MnO, NOH,, Cd011), H₂SO, ( 11,50, H₂O₂ Ma T HNO, NaBiO, MeO, purple (1) (8) 3 Fe, AP, Zn, Mn, Cr", Nd, Cd N NaOH NII, plus ions from Group IV Fe, Ni, Cd" KIẾN FeSCN red NH, (10) Fe(OH), brown (6) NENH dimethylglyoxie (11) NICHINO, rose red CO, ALOH, ZOH)," HC,1,0, NH, (13) ALOH, HCHO. catechol (14) violet Al complex blue CHÍNH, thoacetamide (12) Gads rellow CO, BÌNH, Balo BaCrO, HNO, (16) C₂0, H₂O, Cro, blue (15) TNH K.FCN (17) K,ZnJFeCN light given NaOH RCI K.ZnFeCN hebt greee 82 THE CATIONIS