Qualitative Analysis of Group II Cations Collect: (2020/05/08 - PowerPoint PPT Presentation

Qualitative Analysis of Group II Cations Collect: (2020/05/08 revised)  5 centrifuge tubes Centrifuge tube  Evaporating dish  Test tube holder  Crucible tongs  Two droppers Test tube  Latex gloves  Labels Prepare:  Test tube rack, test tubes, and beaker  Take out centrifuge 1 *Conc. NH 3 (aq) and HCl: in hood

Objective  To learn the techniques of systematic separating and identifying some common cations  To understand the principles of precipitation and equilibrium of complex formation Techniques  Vortex mixer  Precipitation  Centrifuge  Decantation Decantation Vortex Mixer  Litmus and universal indicator paper 2

Introduction: Qualitative Analysis of Group I~V Cations Cationic Solutions 2+ , Ag + , Pb 2+ (I) Insoluble chlorides: Hg 2 Insoluble sulfides in acidic medium: Hg 2+ , Pb 2+ , Cu 2+ , Bi 3+ , (II) Cd 2+ , As 3+ , Sb 3+ , Sn 4+ ( metallic sulfide precipitates with smaller K sp ) (III) Insoluble sulfide or hydroxides in alkaline medium: Al 3+ , Fe 3+ , Co 2+ , Ni 2+ , Cr 3+ , Zn 2+ , Mn 2+ ( metallic sulfide precipitates with greater K sp ) (IV) Insoluble Carbonates: Mg 2+ , Ca 2+ , Sr 2+ , Ba 2+ + , Na + , K + (V) Soluble cations: NH 4 3

Group 2 Cations - Copper and Arsenic Subgroup Group 2 cations – Insoluble sulfides in acidic medium － HgS, PbS, CuS, Bi 2 S 3 , CdS , As 2 S 3 , Sb 2 S 3 , SnS 2  Copper subgroup – Hg 2+ , Pb 2+ , Cu 2+ , Bi 3+ , Cd 2+ The sulfides are insoluble in KOH solution, only soluble in nitric acid  Arsenic subgroup – As 3+ , Sb 3+ , Sn 4+ The sulfides are thioamphoteric that are soluble in KOH(aq) and nitric acid  Mostly are toxic heavy metals thus we only examine Cu 2+ , Bi 3+ , Sb 3+ , Sn 4+ 4

Flow Chart for Separating Copper and Arsenic Subgroups Cationic Solution S △ Cu 2+ , Bi 3+ , Sb 3+ , Sn 4+ CH 3 CNH 2 + H + + 2H 2 O (2, 2, 2, 8 drops) O (pH 0.5) 2 d 13%TA,  + + H 2 S(aq) CH 3 COH + NH 4 Centrifuge and separate (Repeat 13%TA precipitation once) Ppt 2-1 Soln 2-1 Bi 2 S 3 , CuS, Sb 2 S 3 , SnS 2 Wash ppt with 1 d 6 M NH 4 Cl and 20 d of water, cfg. Add 10 d 0.5 M KOH to ppt and mix well Heat in boiling water bath Cfg. and separate the ppt and supernatant (Repeat this extraction with KOH once) Ppt 2-2 (copper subgroup) Soln 2-2 (arsenic subgroup) Bi 2 S 3 , CuS 2- , SnS 2 OH － , (KOH) 3- , SbO 3 3- , SnS 3 SbS 3 5

Separate and Ppt 2-2 (Copper Subgroup) Identify Cu 2+ and Bi 3+ Bi 2 S 3 , CuS  Wash ppt with NH 4 NO 3 /water - → CuS, Bi 2 S 3 + NO 3 Cfg. to get ppt. Cu 2+ , Bi 3+ + S↓ + NO↑ Add 5 d. 6 M HNO 3 / 5 d. H 2 O Heat in boiling water bath Cfg. and obtain soln Soln 2-3 Discard Residue Bi 3+ , Cu 2+ (Contains S) Add and mix with 15 M conc. NH 3 (aq) until basic (confirm with litmus test) Cfg. and separate ppt and supernatant Ppt 2-4 Bi(OH) 3 (s) (white) Soln 2-4 2+ (deep blue) Cu(NH 3 ) 4 Add sodium stannite reagent - (freshly prepared) Sn(OH) 3 2Bi(OH) 3 (s)+ 3Sn(OH) 3 － + 3OH － Bi(s) (black) → 2Bi↓ + 3Sn(OH) 6 2- 6

Separate and Identify Sn 4+ and Sb 3+ Soln 2-2 (arsenic subgroup) 2- , SnS 2 OH － 3- , SbO 3 3- , SnS 3 SbS 3 3- + H + → Sb 2 S 3 ↓ SbS 3 Sb 2 S 3 ↓ + H + + Cl - → 2 SbCl 4 － (aq) Add ca. 20 d of conc. HCl Heat in boiling water bath, till ppt dissolves Cfg, pour supernatant into evaporating dish Soln 2-6 2- + 3H 2 C 2 O 4 → Discard SnCl 6 2- (stable) Sn(C 2 O 4 ) 3 2- , SbCl 4 － Residue SnCl 6 - + H 2 S → Sb 2 S 3 (orange) SbCl 4 Evaporate till approx. 4 d left Add 1 mL water and divide into 2 parts Sn 2+ + 2HgCl 2 → Hg 2 Cl 2 ↓ + Sn 4+ + 2Cl - SbTest Sn test ¼ small spatula H 2 C 2 O 4 (s) Small Al strip /6 M HCl 2 d 13%TA Heat in boiling water bath to dissolve Warm in water bath Cfg. and obtain soln Add 0.1 M HgCl 2 to solution Hg 2 Cl 2 (s) (white) Sb 2 S(s) (orange) Hg (black) 7

Step 1 Separate Sulfide into Copper and Arsenic Subgroups • Cu 2+ , Bi 3+ , Sn 4+ , Sb 3+ soln • Separate ppt and • Centrifuge 1 min. • Add 13%TA supernatant by * Balance diagonally • Heat in warm water decantation • Produce sulfide ppt 1) Add 1 d 6 M NH 4 Cl/20 d H 2 O to wash ppt, then cfg. 2) Add 10 d 0.5 M KOH to ppt 3) Heat in boiling water bath 4) Cfg. and separate Ppt 2-2 Soln 2-2 5) Repeat KOH extraction once Copper subgroup Arsenic subgroup Sulfide ppt 8 Contains Sn 4+ , Sb 3+ CuS, Bi 2 S 3

Step 2-1 Analysis of Copper Subgroup Ppt 2-2 • Heat till reaction starts to • Add 5 d water and 5 d 6 Copper subgroup evolve bubbles* M HNO 3 CuS, Bi 2 S 3 • Ppt is dissolved and • Heat in boiling water bath appears white and milky • 1 d 0.2 M NH 4 NO 3 /20 d H 2 O to wash ppt, • Cfg. to get ppt. - (aq) + 8H + (aq) * 3(CuS, Bi 2 S 3 )(s) + 2NO 3 → 3(Cu 2+ 、 Bi 3+ )(aq) + 2NO(g) ↑ + 3S↓ + 4H 2 O  The water bath should keep boiling  Mix the HNO 3 and ppt thoroughly  Heat to the black sulfides disappear, and the solution appears off-white for elemental sulfur forms 9

Step 2-2 Separate and Identify Cu 2+ and Bi 3+ Add conc. Cfg. and take NH 3 to basic Centrifuge supernatant Test with litmus paper Contains 2+ Cu(NH 3 ) 4 Cu 2+ , Bi 3+ Add sodium stannite reagent* *Prepare sodium stannite freshly: Bi Excess NaOH Sn(OH) - SnCl 2 (aq) + NaOH(aq ) → Sn(OH) 2 (s) 3 (aq) Add no more NaOH while ppt dissolved. 10

Step 3 Separate and Identify Sn 4+ and Sb 3+ 1) Add excess conc. Add conc. HCl HCl (ca. 20 d) drop by drop 2) Heat to ppt. dissolve Insoluble sulfide 3) Transfer to appears* evaporating dish Soln 2-2 Contains 3- + 6H + → Sb 2 S 3 ↓ (orange) + 3H 2 O 3- + SbO 3 Sn 4+ , Sb 3+ * SbS 3 * SnS 2 OH - + H + → SnS 2 ↓ (yellow) + H 2 O 1) ¼ small spatula H 2 C 2 O 4 (s) Sb 2) 2 d 13%TA 3) Heat in water bath 1) Evaporate to 4 d 2) Add 1 mL H 2 O Sn 3) Separate into 2 tubes 1) Small Al strips / 6 M HCl 2) Heat till dissolve Al 3) Cfg. to get soln 11 4) Add HgCl 2 to liquid portion

Expected Results Condensed report • Keep the final resuls for TA to check Cu 2+ Bi 3+ Sb 3+ Sn 4+ • Record all the observations, i.e. malfunction color of soln or ppt, rxn rate etc. 12

Notice  Wear gloves and goggles at all time  Always use test tube holders, test tube racks, and crucible tongs  Add reagents on top of the centrifuge tube to avoid contamination  Do not take excess chemicals to reduce chemical waste  Solutions must be mixed thoroughly after adding reagents  Dip solution on testing paper to determine acidity  All heating should be done in the hood  Conc. NH 3 (aq) and HCl(aq) are in hood  Recycle heavy metal waste 13

T8 - Centrifugation T8 Video on YouTube (Click)  Check the casing inside the machine is intact. If corrosion causes holes in casing or there is an unknown object inside, clean or replace the casing.  Use centrifuge tubes in centrifugation, do not use test tubes.  Centrifuge tubes should be placed in opposite sides to keep balancing.  The lid should be closed during use; the centrifuge should be started from slow rate to check if there is any malfunction, then the speed can be increased.  If there are unusual sounds or shaking in the centrifuge, the power should be turned off first in order to fix up.  There must be at least one person look after the centrifuge when in use.  Centrifuge for approx. 1~2 min , open lid when the centrifuge has completely 14 stopped.

T5 - Decantation T5 Video on YouTube (Click)  Decantation is a simple method in lab to separate solids and solutions. When specific gravity of the solid precipitate is large, the solid settles quickly and tightly. When the solid settles to the bottom of the liquid and is no longer suspended in the solution, the liquid can be carefully poured out and the solid will left in the vessel and therefore separates liquid and solid.  Stand the suspended solution by allowing the solid to settle to the bottom of the mixture  Pour off the particle-free part of the liquid. 15

T15 - Litmus Paper  Litmus paper is filter paper which has been treated with a T15 Video on natural water-soluble dye obtained from lichens. YouTube (Click)  Blue litmus paper will turn red when encountering acidic substances.  Red litmus paper will turn blue when encountering basic substances.  Another widely used universal indicator paper which is a combination of a variety of indicators to obtain various color changes.  Stick solution with a clean glass rod and touch it on a litmus paper or universal indicator paper to observe the color.  Do not dip litmus paper into solution directly to avoid contaminating the solution.  When testing with gas, wet the litmus paper first then place it on the opening of vessel. After the gas goes out and absorbed by litmus paper, the acidity and alkalinity can be judged by color change. 16