4.3: Procedure, flowchart, and datasheets for separation and confirmation of group II cations
|
Chemical |
Hazard |
|---|---|
|
0.1M ammonium chloride (\(\ce{NH4Cl}\)) |
Toxic and irritant |
|
0.1M bismuth nitrate in 0.3M \(\ce{HNO3}\) |
Toxic, irritant, and oxidant |
|
0.1M cadmium chloride in 0.3M \(\ce{HNO3}\) |
Toxic and suspected carcinogen |
|
0.1M copper(II) nitrate in 0.3M \(\ce{HNO3}\) |
Toxic, irritant, and oxidant |
|
0.1M Tin(IV) chloride in 0.3M \(\ce{HNO3}\) |
Corrosive and irritant |
- *Hazards of 6M ammonia, 6M hydrochloric acid, 6M nitric acid, 3M potassium hydroxide, and 1M thioacetamide are listed in the common reagents table in chapter 2.
- Used heavy metal ion solutions or precipitates are disposed of in a labeled metal waste disposal container, do not drain these solutions down the drain or in the regular trash.
- Take 15 drops of the test solution if the group I cations are not present in the sample or take the supernatant of step 1 of group I analysis . Find its pH using a short-range pH paper. If the pH is 0.5 ±3 there is no need to adjust the pH. If pH is lower, increase it to 0.5 ±3 by adding drops of 0.5M ammonia solution, one drop at a time while stirring. If pH is higher, decrease to 0.5 ±3 by adding drops of 0.5M \(\ce{HCl}\), one drop at a time while stirring. Then add 10 drops of 1M thioacetamide stir and heat for 10 min in a water bath. Add 1 drop of 0.5M \(\ce{NH3}\),stir, centrifuge for 2 min, and add 5 drops of 1M thioacetamide, stir, and heat again for 2 min. Cool in room temperature water bath and add 1 more drop of 0.5M ammonia while stirring and centrifuge for 2 min. Decant and keep the supernatant for group III cations and keep the precipitate for separation and analysis of group II cations. The precipitate may be one or more of the following: \(\ce{SnS2}\) (yellow), \(\ce{CdS}\) (yellow-orange), \(\ce{CuS}\) (black-brown), \(\ce{Bi2S3}\) (black). Record the observation in the datasheet.
- Wash the precipitate from step 1 by re-suspending it in 1 mL (20 drops) of 0.1M \(\ce{NH4Cl}\), centrifuge for 2 min, decant, and discard the supernatant which is just the washing liquid. Re-suspend the precipitate in 1 mL (20 drops) of 3M \(\ce{KOH}\) + 1 drop of 1M thioacetamide, stir, loosely stopper the test tube, and heat in a water bath for 2 min. Centrifuge the hot mixture for 2 min and decant while it is hot. Keep the supernatant for analysis of \(\ce{Sn^{4+}}\) which exists as soluble \(\ce{Sn(OH)_{6}^{2-}}\) ion at this stage and keep the precipitate , if there is any, for analysis of the rest of the group II cations. Record the observation in the datasheet.
- Add 6M \(\ce{HCl}\) drop by drop to the supernatant from step 2 and keep testing with blue litmus paper until the mixture turns acidic. Then add 5 drops of 1M thioacetamide, stir, and heat in a water bath for 2 min. Yellow precipitate at this stage is \(\ce{SnS2}\) which confirms \(\ce{Sn^{4+}}\) is present in the test sample, no yellow precipitate means \(\ce{Sn^{4+}}\) was not present. Record the observation in the datasheet and discard the mixture in a waste container.
- Wash the precipitate from step 2 by re-suspending it in 10 drops of distilled water and then centrifuge for 2 min. Decant and discard the supernatant and wash the precipitate again by re-suspension in 10 drops of distilled water followed by centrifuge for 2 min, decant and discard the supernatant. Re-suspend the precipitate in 10 drops of distilled water + 2 drops of 6M \(\ce{HCl}\) and heat for 2 min. Centrifuge and decant while the mixture is still hot. If the supernatant appears turbid due to some precipitate left in it, use the cotton plug technique to aspire clean supernatant and filter out the residual precipitate. Keep the supernatant for analysis of \(\ce{Cd^{2+}}\) which may exist as dissolved \(\ce{[CdCl4]^{2-}}\) ion at this stage and keep the precipitate , if there is any, for analysis of remaining group II cations. Record the observation in the datasheet.
- Add 6M \(\ce{NH3}\) drop by drop to the clear supernatant from step 4 and keep testing with red litmus paper until the solution turns basic. Add 2 drops of 1M thioacetamide, stir, and heat for 2 min in a water bath. If a yellow precipitate forms at this stage it is \(\ce{CdS}\) that confirms \(\ce{Cd^{2+}}\) was present in the test sample, otherwise \(\ce{Cd^{2+}}\) was not present. Record the observation in the datasheet and discard the mixture in a metal waste container.
- Wash the precipitate from step 4, if there is any, by re-suspending it in 10 drops of distilled water, centrifuge for 2 min, decant and discard the supernatant. Re-suspend the precipitate in 10 drops of 6M HNO 3 and heat in a boiling water bath for 5 min. The precipitate, i.e., \(\ce{CuS}\) and/or \(\ce{Bi2S3}\) will dissolve in the liquid, and \(\ce{Cu^{2+}}\) and/or \(\ce{Bi^{3+}}\) hydrated ions and yellow sulfur particles may form. Remove the sulfur particles by centrifugation and decantation and discard them as there is no ion in them. Keep the supernatant for the analysis of \(\ce{Cu^{2+}}\) and \(\ce{Bi^{3+}}\) and record the observation in the datasheet.
- Add 6M NH 3 drop by drop to the supernatant from step 6 and keep testing with red litmus paper till the solution turns alkaline. Add 10 more drops of 6M \(\ce{NH3}\) solution after the solution turns alkaline to make it strongly alkaline. If the mixture becomes blue color at this stage, it is due to the blue \(\ce{[Cu(NH3)4]^{2+}}\) ion that confirms \(\ce{Cu^{2+}}\) is present in the test solution. If there is a white suspension in the mixture, keep it for testing \(\ce{Bi^{3+}}\). Record the observation in the datasheet.
- Centrifuge the mixture from step 7 for 2 min and decant and discard the supernatant. If there is any white precipitate left after decantation, it is most likely \(\ce{Bi(OH)3}\). Wash the precipitate by re-suspending it in 10 drops of 6M \(\ce{NH3}\), centrifuge for 2 min, and decant. If the white precipitate remains there after the washing, it is \(\ce{Bi(OH)3}\) that confirms \(\ce{Bi^{3+}}\) is present in the test solution, otherwise, \(\ce{Bi^{3+}}\) is absent. Discard the mixture in a metal waste container and record the observation in the datasheet.
- Step number refers to the corresponding step number in the procedure sub-section.
- In “the expected chemical reaction and expected observations column”, write an overall net ionic equation of the reaction that will happen if the ion being processed in the step was present, write the expected color change of the solution, the expected precipitate formed and its expected color, etc.
- In the “the actual observations and conclusion” column write the color change, the precipitate formed and its color, etc. that is actually observed as evidence, and state the specific ion as present or absent.
- In “the overall conclusion” row write one by one symbol of the ions being tested with a statement “present” or “absent” followed by evidence/s to support your conclusion.