ANALYTICAL LABORATORY MANUAL I QUALITATIVE ANALYSIS OF ANIONS CATIONS

Transcription

1 ANALYTICAL LABORATORY MANUAL I QUALITATIVE ANALYSIS OF ANIONS CATIONS ANKARA UNIVERSITY, FACULTY OF PHARMACY DEPARTMENT OF ANALYTICAL CHEMISTRY

2 TABLE OF CONTENTS 1) QUALITATIVE ANALYSIS OF ANIONS Identification of chloride ion (Cl ) Identification of sulfate ion (SO42 ) Identification of nitrate ion (NO3 ) Identification of carbonate ion (CO32 ) By adding acid Identification of ortophosphate ion (HPO42 ) ) GROUP V CATIONS (Mg 2+, Na +, K +, NH4 + ) Ammonium (NH4 + ) Magnesium (Mg 2+ ) Sodium (Na + ) Potassium (K + ) ) SEPARATION OF GROUP IV AND GROUP V CATIONS ) GROUP IV CATIONS (Ba 2+, Ca 2+, Sr 2+ ) ) GROUP III CATIONS (Fe 3+, Al 3+, Cr 3+, Ni 2+, Co 2+, Mn 2+, Zn 2+ ) ) GROUP II CATIONS (As 3+, Sb 3+, Sn 2+, Hg 2+, Cu 2+, Cd 2+, Pb 2+, Bi 3+ ) ) GROUP I CATIONS (Ag +, Hg22 + ve Pb 2+ ) Ag Pb Hg

3 1 drop of liquid is supposed to be 0.05 ml so that 20 drops of liquid is 1 ml. 1) QUALITATIVE ANALYSIS OF ANIONS Cl, SO 2 4, NO 2 3 3, CO 3 and PO 4 each have specific identification reactions so they can be identified in the mixtures containing these anions. That is why there is no need for a separation step for their identification tests. In qualitative analysis of anions, the reactants should be added drop by drop and directly into the solution in the test tube (not on the wall of the tube) Identification of chloride ion (Cl ) With silver nitrate (AgNO3) Preliminary info: In neutral solutions, Cl, SO , CO 3 and PO 4 all reacts with AgNO3 to precipitate as silver salts. When HNO3 is added to this solution, all these silver salts except for silver chloride (AgCl) dissolves. So for identification test of Cl, HNO3 should be added to the solution to prevent Ag2CO3, Ag3PO4, Ag2SO4 salts from precipitation. Procedure: To 0.5 ml of sample in a test tube, add 7 drops of HNO3 and 1 drop of AgNO3. The white precipitate is AgCl and the formation of this precipitate indicates there is Cl ion in the sample. Cl + AgNO 3 AgCl (s) + NO 3 In order to confirm that this precipitate is AgCl, 20 drops of NH4OH is added to the same test tube. The white precipitate dissolves because AgCl salt forms a complex with NH3. AgCl (s) + 2NH 3 [Ag(NH 3 ) 2 ] + + Cl Add 15 drops of HNO3 into the solvated complex and a white precipitate is re-constituted. Because with the addition of HNO3, the silver diammine complex ( [Ag(NH 3 ) 2 ] + ) decomposes and re-precipitates as AgCl. [Ag(NH 3 ) 2 ] + + Cl + 2HNO 3 2NH NO 3 + AgCl (s) 1.2. Identification of sulfate ion (SO 4 2 ) With barium chloride (BaCl2) Preliminary info: In neutral solutions, SO , CO 3 and PO 4 reacts with BaCl2 to precipitate as barium salts. When a mineral acid is added to this solution, these barium salts except for barium sulfate (BaSO4) dissolve. So for identification test of SO 2 4, a mineral acid should be added to the solution to prevent BaCO3, Ba3(PO4)2 salts from precipitation. 2

4 Procedure: To 0.5 ml of sample in a test tube, add 7 drops of HNO3 and 1 drop of BaCl2, respectively. The 2 white precipitate is BaSO4 and the formation of this precipitate indicates the presence of SO 4 ion in the sample. SO BaCl 2 BaSO 4 (s) + 2 Cl 1.3. Identification of nitrate ion (NO 3 ) Preliminary info: Diphenylamine, which is a colorless reactant, oxidizes to blue-violet colored diphenyl benzidine violet in presence of an oxidant such as NO 3. This reaction occurs in acidic medium. Procedure: To 0.5 ml of sample in watch glass, add 1 drop of diphenylamine solution. In the contact surface of two liquids, blue color appears. The blue color may disaappear after a while. (The diphenylamine solution given in the laboratory has been prepared with H2 SO4 by the laboratory personnel. So there is no need to add acid into the watch glass.) 1.4. Identification of carbonate ion (CO 3 2 ) By adding acid Procedure: To 2 ml of sample in a test tube, add 1 ml of acid (for example CH3COOH, HCl, HNO3) all at 2 once. The formation of gas bubbles indicate the presence of CO 3 in sample and the releasing gas is CO2. The formation of gas bubbles can be accelerated by shaking the test tube. CO CH 3 COOH 2CH 3 COO + H 2 O + CO 2 (g) With BaCl2 This test does not prove the existence of CO 2 3. It will help you to understand why you need 2 to acidify the medium when you perform SO 4 identification test with BaCl2. Procedure: To 1 ml of sample in a test tube, add 1 drop of BaCl2. The white precipitate might be barium carbonate (BaCO3). In order to confirm that it is BaCO3, add 7 drops of HNO3 and observe the dissolving precipitate which releases CO2. CO BaCl 2 BaCO 3 (s) + 2 Cl BaCO 3 (s) + 2H+ Ba 2+ + H 2 O + CO 2 (g) 3

5 With AgNO3 This test does not prove the existence of CO 2 3. It will help you to understand why you need to acidify the medium when you perform Cl identification test with AgNO3. Procedure: To 1 ml of sample in a test tube, add 1 drop of AgNO3. The white precipitate might be silver carbonate (Ag2CO3). In order to confirm that it is Ag2CO3, add 7 drops of HNO3 and observe the dissolving precipitate which releases CO2. CO AgNO 3 Ag 2 CO 3 (s) + 2NO 3 Ag 2 CO 3 (s) + 2H+ 2Ag + + H 2 O + CO 2 (g) 1.5. Identification of ortophosphate ion (HPO 4 2 ) With ammonium molybdate ((NH4)2MoO4) Procedure: To 0.5 ml of sample in a test tube, add 5 drops of HNO3 and 5 drops of (NH4)2MoO4. Heat to 40 C in water bath for 5 minutes. The yellow precipitate is ammonium phosphomolybdate 2 ((NH 4 ) 3 (MoO 3 ) 12 PO 4 ) and the formation of this precipitate indicates the presence of HPO 4 ion in the sample. HPO (NH 4 ) 2 MoO HNO 3 (NH 4 ) 3 (MoO 3 ) 12 PO 4(s) + 21NH NO H 2 O With BaCl2 This test does not prove the existence of HPO 2 4. It will help you to understand why you need 2 to acidify the medium when you perform SO 4 identification test with BaCl2. Procedure: To 1 ml of sample in a test tube, add 1 drop of BaCl2. The white precipitate may be barium hydrogen phosphate (BaHPO4) and the formation of this precipitate may indicate the presence 2 of HPO 4. When 7 drops of HNO3 is added into this tube, the white precipitate disappears because BaHPO4 is dissolved in acidic medium. HPO BaCl 2 BaHPO 4 (s) + 2 Cl BaHPO 4 (s) + 2H+ Ba 2+ + H 3 PO 4 4

6 With AgNO3 This test does not prove the existence of HPO 2 4. It will help you to understand why you need to acidify the medium when you perform Cl identification test with AgNO3. Procedure: To 0.5 ml of sample in a test tube, add 1 drop of AgNO3. The yellow precipitate is silver phosphate (Ag3PO4) and the formation of this precipitate indicates the presence of HPO 2 4. When 7 drops of HNO3 is added into this tube, the yellow precipitate disappears because Ag3PO4 is dissolved in acidic medium. HPO AgNO 3 Ag 3 PO 4 (s) + 2NO 3 + HNO 3 Ag 3 PO 4 (s) + 3H+ 3Ag + + H 3 PO 4 5

7 2) GROUP V CATIONS (Mg 2+, Na +, K +, NH 4+ ) There is not a common precipitating agent for the group V cations. Sodium and potassium are alkali metals. NH4 + is also classified in the group V because the compounds containing NH4 + have similar properties with alkali metals. Magnesium is an alkaline earth metal, but it does not precipitate with the common precipitating agent of group IV. Therefore, it is also classified in the group V. The salts of colourless anions and group V cations are colourless and have ionic bonds. Therefore, most of them are soluble in water and this is why group V cations don t have a common precipitating agent. Important points: If acidity or alkalinity of a solution is stated, it must be controlled by litmus paper. Litmus paper must not be contacted with the tube. Care should be taken that the solution which is splashed from the tube can also change the color of the litmus paper. We use HCl to clean the platinum wire while changing from one sample to another. If the platinum wire has a colored flame, this coloring is eliminated by immersing the wire in the HCl solution within the tube and the wire is reignited. This process is continued until the color is not visible in the flame. If you have a mixture of K + and Na + salts, the violet color of K + is masked by the yellow color of Na +. In this case, a cobalt glass must be used to prove the existance of K +. If NH4 + is present in the sample, the analysis of Na + and K + ions becomes more difficult. Because NH4 + is yellow in the flame test like Na +. For this reason, it should first be checked whether there is NH4 + in the sample. If NH4 + is not present in the sample, Na + and K + are searched for, if any, NH4 + is removed first. Removing NH4 + from sample solution: 50 drops of sample are taken to a porcelain crucible and the sample solution is evaporated to dryness on wire gauze. When the solution is reduced to half, 10 drops of concentrated HCl is added and the evaporation process is continued. The wire gauze is removed after a while and the crucible is taken to the triangle for the application of naked flame heating. This process is continued until the all white ammonium vapors is 6

8 exhaustad. After the crucible is cooled, add 50 drops of distilled water onto the residue to completely dissolve precipitates. The solution obtained is used for the analysis of Mg 2+, Na + and K + ions Ammonium (NH4 + ) Add 20 drops of NH4 + sample in a test tube and add 10 drops of distilled water. The solution is made alkaline with 10 drops of NaOH. Put the solution is in a water bath and hold a red litmus paper, which previously wetted with distilled water, on the top of the test tube to interact with the evaporated ammonium. Otherwise, the presence of NH4 + ion can not be monitored since NH3 gas is moving away from the medium. Red litmus paper turns blue in the presence of NH4 + ion Magnesium (Mg 2+ ) With Diphenylcarbazide reagent Add 20 drops of Mg 2+ sample in a test tube. Add 3 drops of NaOH to observe white precipitate which is Mg(OH)2 (magnesium hydroxide). Then, add 3 drops of diphenylcarbazide reagent to the test tube. Diphenylcarbazide change the color of the precipitate of Mg(OH)2 to a redpurple color because Mg(OH)2 adsorbs the diphenylcarbazide Sodium (Na + ) Add 10 drops of Na + sample, 10 drops of distilled water and 10 drops of concentrated HCI in a test tube. Immerse clean platinum wire in it and hold the platinum wire on hottest region of the flame. The presence of Na + ions is observed with yellow color in the flame Potassium (K + ) Add 10 drops of K + sample, 10 drops of distilled water and 10 drops of concentrated HCI in a test tube. Immerse clean platinum wire in the tube and hold the platinum wire on hottest region of the flame. The presence of K + ions is observed with violet color in the flame. 7

9 3) SEPARATION OF GROUP IV AND GROUP V CATIONS Before separating group IV and group V cations, NH4 + should be tested. Add 1 ml (~20 drops) of sample that includes group IV and group V cations in a test tube. Add 1 pinch of spatula or 20 drops (if it is liquid) 1M ammonium chloride (NH4Cl) into the tube and mix. Add NH4OH until the sample solution is alkaline. Check the alkalinity of the medium litmus paper and then add (NH4)2CO3 is until the precipitate formation is completed. Heat tube in a water bath and centrifuge it. The supernatanat contains group V cations while the precipitate contains group IV cations. Separate precipitate and supernatant. Add 20 drops 5 M acetic acid (CH3COOH) onto the precipitate and begin group IV experiments. Important points: The order of reagent addition is important for the separation of group IV cations from the group V. To buffer the solution with NH4OH - NH4Cl buffer, first NH4Cl, and then NH4OH must be added. If NH4OH is added first, Mg 2+ which is a member of the group V will precipitate as Mg(OH)2 and it will remain at the precipitate with the Group IV cations. When NH4Cl, NH4OH and (NH4)2CO3 are added subsequently, all group IV cations will precipitate as their carbonates and Mg 2+ will remain at the supernatant with the other group V cations. 8

10 4) GROUP IV CATIONS (Ba 2+, Ca 2+, Sr 2+ ) For separation of precipitate and supernatant, use centrifugation. Only use filtration if the solution is hot. (SrSO4 precipitates only in the hot medium). Acidity or alkalinity must be checked with litmus paper. After centrifugation, several millilitres of distilled water are added to the precipitate for washing and if necessary the precipitate is centrifuged again. Procedure: Add 20 drops of sample, 8 drops of 5 M CH3COOH and 12 drops of 0.25 M potassium chromate (K2CrO4) in a test tube are add. If there is Ba 2+ in the sample, yellow colored barium chromate (BaCrO4) precipitates. Precipitate and supernatant are separated with centrifugation (weak acid is used because BaCrO4 does not precipitate in strong acidic medium). Wash the precipitate and dissolve it with concentrated HCl. In flame test on Pt wire, a green flame is observed. Add 5 M Ammonium hydroxide (NH4OH) to the supernatant until it becomes alkaline. Then, add 12 drops of 1.5 M ammonium carbonate ((NH4)2CO3) and centrifuge it. Discard supernatant and wash the precipitate with distilled water. After the precipitate is washed, add 25 drops of 5 M CH3COOH. Heat until all CO2 is removed. After heating, add 20 drops of 1 M ammonium sulfate ((NH4)2SO4) and 1-2 crystal sodium thiosulfate (Na2S2O3). Heat it until boiling and filter when it is hot and keep fitrate and precipitate for further experiments. Wash the precipitate and dissolve it in concentrated HCl. Carmine red color in flame test indicates the existence of Sr 2+ Add 20 drops of 5 M NH4OH and a pinch of spatula of ammonium oxalate ((NH4)2C2O4) on the filtrate. Centrifuge it and wash precipitate with distilled water. After the precipitate is washed, dissolve it in concentrated HCl. Tile red color in flame test proves the existence of Ca 2+. 9

11 5) GROUP III CATIONS (Fe 3+, Al 3+, Cr 3+, Ni 2+, Co 2+, Mn 2+, Zn 2+ ) These cations precipitate as sulfides and hydroxides when reacted with hydrogen sulfide (H2S) or thioacetamide solution in basic medium buffered with NH4OH NH4Cl. These cations do not precipitate with the reagents used for precipitation of the group I and II cations. Group III cations are divided into two: 1) Iron Group (Group 3A): Fe 3+, Al 3+, Cr 3+ (precipitates in the form of hydroxides). 2) Zinc Group (Group 3B): Ni 2+, Co 2+, Mn 2+, Zn 2+ (precipitates in the form of sulfides). Systematic analysis of group III cations: Add 20 drops of sample containing Fe 3+, Al 3+, Cr 3+, Ni 2+, Co 2+, Mn 2+, Zn 2+ in a tube from the sample. Add 10 drops of 4 M ammonium chloride (NH4Cl) solution and 10 drops of 2 M ammonium hydroxide (NH4OH, %33) solution dropwise to the sample and check the alkalinity of the solution with a litmus paper. Then add 20 drops of NH4OH solution and Fe 3+, Al 3+ and Cr 3+ precipitate as their hydroxides. Heat the solution for 5 min at water bath and add 20 drops of thioacetamide solution (11%, w/v) and observe precipitates of sulfides of Ni 2+, Co 2+, Zn 2+ and Mn 2+ (COMMON PRECIPITATE) This precipitate contains Al(OH)3, Cr(OH)3, Fe(OH)3, NiS, CoS, MnS, and ZnS and it is washed several times with 0.1 M NH4Cl. Add 20 drops of 6 M HCl onto the washed precipitate. Centrifuge and separate the supernatant A and the precipitate A. Precipitate A (CoS ve NiS): Precipitate A is taken into cruicible, heated on bunsen burber till the solute part evaporates if exist, and add 20 drops of aqua regia (15 drops of 6 M HCl + 5 drops of 6 M HNO3) to dissolve the precipitate. Add from NH4OH (%33) solution until it is basic (check with red litmus paper). Divide the solution into two parts. To the first part, add 10 drops of 0.1 M potassium ferricyanide (K4[Fe(CN)6]) solution and the dark green color is followed, which is the precipitate of cobalt ferricyanide (Co2[Fe(CN)6]). This shows the presence of Co +2. To the second part, add 2 drops of 6 M NaOH and 2 drops of dimethyl glyoxime (%1 in 96% alcohol), the rose color shows the presence of Ni

12 Supernatant A (Fe 3+, Al 3+, Cr 3+, Mn 2+, Zn 2+ ): Add 20 drops of 6 M NaOH and 20 drops of 3% H2O2 (v/v) are added to the supernatant. Centrifuge and separate supernatant B and the precipitate B. Transfer precipitate B to a clean test tube, add 10 drops of HNO3 (37.5%) and pinches of spatula sodium bismuthate (NaBiO3) from the side of the tube. The color of violet proves the presence of Mn 2+. Add 0.1 M K4[Fe(CN)6] to the precipitate B, the prussian blue precipitate proves the presence of Fe 3+. Supernatant B (Zn 2+, Al 3+, Cr 3+ ): Add 20 drops of HNO3 and NH4OH to the solution until it is basic. Centrifuge and separate the supernatant C and the precipitate C Precipitate C is white aluminum hydroxide (Al(OH)3). The yellow color of the supernatant C, which contains Cr 3+ and Zn 2+, indicates the presence of Cr 3+. Add 20 drops of 2 M CH3COOH to the supernatant C to make the medium acidic. Add pinches of spatula of sodium acetate (NaCH3COO). Heat the solution in a water bath for 20 min and add 2 drops of 0.5 M barium chloride (BaCl2) solution to the hot solution. The formed yellow precipitate (precipitate D) is barium chromate (BaCrO4). Add 2 drops of thioacetamide solution (11% w/v) to the supernatant D and heat it up in the water bath with stirring for 5 minutes until the white precipitate is visible. The formation of dirty white zinc sulfide (ZnS) shows the presence of Zn 2+. When excess NH3 is added to the starting solution, red brown color indicates the presence of the Fe 3+, and/or colorless and gelatinous precipitate indicates the presence of Al

13 6) GROUP II CATIONS (As 3+, Sb 3+, Sn 2+, Hg 2+, Cu 2+, Cd 2+, Pb 2+, Bi 3+ ) In systematic analysis of cations, 13 cations are precipitated as their sulfides. Among these cations, 8 of them have lower solubilities in water and they are called as Group II cations while the remaning 5 cations with higher solubility in water are called as Group III cations: Ksp values of Group II and Group III cations Group II Group III Sulfides Ksp Sulfides Ksp HgS 3.0x10 53 ZnS 1.2x10 23 CuS 1.0x10 44 CoS 7.0x10 23 Bi2S3 1.6x10 72 NiS 1.4x10 24 Sb2S3 1x10 30 FeS 3.7x10 19 CdS 3.6x10 29 MnS 1.4x10 15 PbS 3.4x10 28 SnS 1.8x10 28 As2S3 4.4x10 27 Add 40 drops of the sample and 4 drops of concentrated HCl in a tube. Then, add 14 drops of 11% (w/v) thioacetamide and heat the test tube in a water bath for 5 minutes. Centrifuge it (3000 rpm, 4 min) and discard the supernatant. In another test tube, prepare a washing solution containing 2 drops of saturated NH4Cl, 2 drops of thioacetamide and 16 drops of distilled water. Wash the precipitates obtained from the previous step using the washing solution. The obtained precipitates are sulfides of II. group cations. Then, the group is seperated to insoluble sulfides (group II A) and soluble sulfide complexes (group II B) by adding 20 drops of 3 M KOH solution and heating while 12

14 stirring in a water bath. After centrifugation, the precipitates (mercury, cupper, cadmium, lead and bismuth) and supernatant (arsenic, antimony and tin) is seperated. Precipitate A: may contain HgS, CuS, CdS, PbS, and Bi2S3. Supernatant A: may contain soluble complexes of As, Sb, and Sn (AsO3 3-, AsS3 3-, SbO 2-, SbS 2-, SnO2 2- ). Precipitate A: Wash with 10 drops of 3 M KOH for removing As, Sb and Sn if still exist and discard the solvent. Then wash with 0.5 ml distilled water for removing remaining KOH. Then, add 24 drops of 6 M HNO3 and heat it in a water bath for 5 minutes. After centrifugation, Precipitate B is kept for mercury test while Supernatant B is transferred to a cruicible. For Hg test, aqua regia is prepared in a test tube by mixing 12 drops of HCl and 4 drops of HNO3. Precipitate B is dissolved in aqua regia solution. Then it is transferred to a cruicible and the solvent is evaporated. After it is cooled to room temperature, it is dissolved in 2-3 ml distilled water. A piece of cupper wire is put onto a watch glass and the solution is added onto it. Hg test is positive if the cupper wire is covered with gray colored Hg. Add 2 drops of 6 M H2SO4 into the cruicible containing Supernatant B. Heat it until a few drops of liquid remains in the cruicible and cool it to the room temperature. Add 40 drops of distilled water and after stirring, transfer it to a centrifuge tube for centrifugation. Supernatant C is transferred to another tube while Precipitate C (lead (II) sulfate: PbSO4) is kept for Pb test. For Pb test, Precipitate C is firstly washed with 20 drops of distilled water. Then, add 20 drops of ammonium acetate (NH4CH3COO), 6 drops of 6 M CH3COOH ve 4 drops of 1 M potassium chromate (K2CrO4) respectively. Yellow precipitates indicate the presence of Pb 2+. Add NH4OH to Supernatant C until it becomes alkaline (check with litmus paper). Then, add 3 more drops of NH4OH and centrifuge it. Decant the Supernatant D to another tube and apply bismuth test for Precipitate D (bismuth hydroxide: Bi(OH)3). Wash Precipitate D with 20 drops of distilled water and add 10 drops of freshly prepared sodium stannite (Na2SnO2) solution. The change of the color of precipitate to black indicates the presence of Bi 3+. Cu 2+ present in the sample if Supernatant D has a blue color and if it is blue, add drop by drop 1 M potassium cyanide (KCN) until the color disappears. When the solution becomes colorless add a few drops of excess KCN. Then add 6 drops of thioacetamide (11% w/v) and heat it in a water bath for 5 min. Yellow Precipitate D formation indicates the presence of Cd 2+. If Supernatant D is colorless, then without addition of 13

15 KCN, directly add 6 drops of thioacetamide and heat it in a water bath for 5 min (again formation of yellow colored CdS precipitate is tested). Supernatant A: Add drop by drop 3 M HCl until it is mildly acidic (test with litmus paper). Precipitates will be formed if at least one of the As, Sb and Sn present in the sample. Discard the supernatant after centrifugation. Add 20 drops of concentrated HCl and heat it in a water bath for 5 min while stirring. Centrifuge it and separate Precipitate E and Supernatant E. Precipitate E may contains arsenic (III) sulphide (As2S3) and arsenic (V) sulphide (As2S5). Wash Precipitate F with 10 drops of distilled water for three times. Then, add 20 drops of concentrated HNO3 followed by heating in a water bath for 5 minutes while stirring. Add 10 drops of 0.5 M AgNO3 and mix them (If white precipitate (AgCl) is formed, discard the precipitate by centrifugation). Add 10 drops of 2.5 M sodium acetate (NaCH3COO) and red precipitate (silver arsenate: Ag3AsO4) formation indicates the presence of As 3+. On the other hand, Supernatant E may contain Sn +4 ve Sb +3 cations and it is divided into two parts for tin and antimony tests. For tin test, NaOH is added drop by drop onto the sample so that it becomes basic. Then add a few drops of basic sodium bismuthate (NaBiO3) reagent. Tin test is positive if black precipitates (reduction of bismuth) are formed after 1-2 min. For antimony test, add 6 drops of thioacetamide (11% w/v) to the other sample and heat it in a water bath. Orange precipitate (Sb2S3) formation indicates the presence of Sb

16 7) GROUP I CATIONS (Ag +, Hg 2 2+ ve Pb 2+ ) This group contains Ag + 2+, Hg 2 and Pb 2+ ions, and the main prominent feature of this group is that they precipitate as insoluble chloride salts in water in the presence of diluted HCl solution. However, when the solubility products (Ksp) of this group are taken into consideration as presented in Table 1, the lead ion will not be able to precipitate in this group because the solubility of lead (II) chloride (PbCl2) in water (more soluble in hot water) is high. Lead ions are still analyzed in the first group. Table 1. Solubility products of group I cations Substances Ksp AgCl 1 x PbCl x Hg2Cl2 2.0 x The cations in this group can be easily analyzed as their chloride precipitations with 0.3 M HCl. When the acid concentration is increased, the precipitated salts in the medium are re-soluble again. At this point, the concentration of the acid is highly important parameter Ag Chloride ions: Ag + ions (20 drops) precipitate as a white-colored chloride when the 1 M HCl (4-5 drops) solution is added to the medium. Ag + + HCl AgCl + H + (White) The obtained white-colored solution is centrifuged, and the precipitate is taken up from the media. The ammonia (NH3) (6 M) solution is added to the medium until the AgCl is dissolved. The AgCl precipitate is dissolved as the diammine complexes (Ag(NH3)2 + ) with NH3 solution. AgCI + 2NH3 Ag(NH3)2 + + Cl Potassium iodide (KI) solution: Ag + ions (20 drops) precipitate as a yellow creamy silver iodide (AgI) when 1 M KI (4 drops) is added to the solution. Ag + + KI AgI + K + (Yellow) Separate the filtrate from the resulting solution by centrifugation, and if 1 M KCN solution is added onto the remaining precipitate soluble [Ag(CN)2] - complex is obtained. 15

17 Potassium chromate (K2CrO4) solution: Ag + ions (20 drops) precipitate as a tile red colored silver chromate (Ag2CrO4) when K2CrO4 (4 drops) is added to the solution. 2Ag + + CrO4 2- Ag2CrO4 (Tile red) Sodium hydroxide (NaOH) solution: Ag + ions (20 drops) precipitate as a darkish-brown colored silver hydroxide (AgOH) when 1 M NaOH (4-5 drops) is added to the solution. However, the compound immediately decomposes into silver oxide (Ag2O) due to it s unstable. Ag + + OH - AgOH (Darkish-brown) 2AgOH Ag2O + H2O (Black) 7.2. Pb Chloride ions: Pb 2+ ions (20 drops) precipitate as a white-colored chloride when the 1 M HCl (4 drops) solution is added to the medium. Pb HCl PbCl2 + 2H + (White) The obtained white precipitate is separated from the solution by centrifugation, 2 ml of distilled water is added, and the mixture is heated in the water bath for 5 minutes. If the precipitate dissolves, there are Pb 2+ ions in the medium KI solution: 16

18 The distilled water (20 drops) is added to the Pb 2+ sample solution (20 drops), and then 1 M KI (3-4 drops) is added to the resulting mixture. The yellow colored lead (II) iodide (PbI2) precipitate is obtained after the experimental process. Pb 2+ + KI PbI2 + K Thioacetamide (CH3CSNH2) solution: (Yellow) The distilled water (20 drops) is added to the Pb 2+ sample solution (20 drops), and thioacetamide solution (3-4 drops) is added to the mixture. Then, the solution is heated in the water bath for 5 minutes. Pb 2+ ions in the sample precipitate as black colored lead (II) sulfur (PbS). Pb 2+ + H2S PbS (Black) The obtained precipitate should dissolve in the 3 M HNO3 solution K2CrO4 solution: Pb 2+ ions (20 drops) precipitate as a yellow colored lead (II) chromate (PbCrO4) when K2CrO4 (3-4 drops) is added to the solution Hg Chloride ions: Pb 2+ + CrO4 2- PbCrO4 (Yellow) Hg2 2+ ions (20 drops) precipitate as their white chloride salts when 3 M HCl (3-4 drops) is added to the solution. The obtained precipitate is insoluble in cold and hot water or diluted HNO3 solution, however, is soluble in NH3, concentrated HNO3 or aqua regia solutions. Hg Cl - Hg2Cl2 (White) 17

19 K2CrO4 solution: Hg2 2+ ions (20 drops) precipitate as a reddish-brown mercury (I) chromate (Hg2CrO4) salt when K2CrO4 (3-4 drops) is added to solution KI solution: Hg CrO4 2- Hg2CrO4 (Reddish-brown) Distilled water (20 drops) is added to the Hg2 2+ sample solution (20 drops), and 1 M KI solution (4-5 drops) is added to the mixture. Yellow-green colored precipitate of mercury (I) iodide (Hg2I2) is obtained after this process. This precipitate is soluble when the excess amount of KI is added to the precipitate, however, insoluble in dilute acids. Hg KI Hg2I2 + 2K + (Green-Yellow) Copper strip (Amalgam test): Firstly, the copper wire (strip) piece is cleaned with 8-10 drops of concentrated HNO3 solution in a watch glass, then it is dried by filter paper and Hg2 2+ sample solution is added to the copper strip. Hg2 2+ ions coat the copper strip (bright gray colored). 18