Unknown 9

Topics: PH indicator, Solubility, PH Pages: 5 (1438 words) Published: February 3, 2013
CHM 2046 L
Scheme of Analysis for Unknown 9A/9B

“Fair Game” Unknowns
* Cations: Na+, K+, NH4+, Ca2+, Mg(H2O)62+, Al(H2O)63+, Zn(H2O)62+, Cu(H2O)2+, Ag(H2O)+ * Anions: Cl-, NO3-, SO4-2, HSO4-, OH-, CO3-2, HCO3-, S-2 * Insoluble Salts: Ca(OH)2, CaCO3, CaSO4, Mg(OH)2, MgCO3, Ag2O, AgCl, Ag2S, CuO, CuCO3, CuS, Zn(OH)2, ZnCO3, ZnS * Alums: [Al(OH2)6]2(SO4)3∙6H2O, Na[Al(OH2)6](SO4)2∙6H2O, K[Al(OH2)6](SO4)2∙6H2O, NH4[Al(OH2)6]2(SO4)2∙6H2O * Hydrated Ions: Na2CO3∙10H2O, Na2SO4∙10H2O, Ca(OH2)6Cl2, Mg(OH2)6Cl2, Mg(OH2)6(NO3)2, Mg(OH2)6SO4∙H2O, Al(OH2)6Cl3 * Note: Sample 9A is a guaranteed pure salt, unless it is an alum.

Procedure of Analysis for Sample 9A/9B

* Describe sample, noting color, shape, and odor.

Test on Original Sample

* Flame Test (Remember to clean with 16 M HONO2 in between each test) * Orange flame indicates presence of Na+, with K+, Ca2+, and NH4+ as possibilities. * Purple flame indicates presence of K+, with NH4+ as a possibility. No Na+ is present. * Red flame indicates presence of Ca2+, with K+ as a possibility. No Na+ is present. * Green flame indicates presence of Cu2+, with other ions as possibilities. No Na+ is present. * No color indicates presence of NH4+, Al3+, or Mg2+. No Na+, K+, or Ca2+ is present.

* NH4+ Test
* Method I
* Place portion of sample in test tube and add 1 M NaOH, and then suspend a piece of red litmus paper over the tube. Remember to add a drop of deionized water to the paper before hovering over the tube. If it turns blue, NH4+ is present. * Method II

* Place portion of sample in a crucible and heat. If gray fumes and smoke are emitted, then NH4+ is present.

* K+ Test
* If NH4+ is present, then place small portion of the same in a crucible and heat strongly until fumes and smokes are no longer discharged. Next, dissolve any residue in 1-2 mL of deionized water and place in a spot plate. Add a drop of 6M HAc and 2-3 drops of sodium cobaltnitrite (Na3Co(NO2)6). If a yellow precipitate forms, K+ is present. * If NH4+ is not present, dissolve a small portion of the sample in 1-2 mL of deionized water and place in a spot plate. Add a drop of 6M HAC and 2-3 drops of sodium cobaltnitrite. If a yellow precipitate forms, K+ is present.

* CO32-/HCO3- Test
* Treat a small portion of the solid unknown with 6 M HCl. * If there is rapid formation of bubbles, then either CO32- or HCO3- is present. * Confirm which with pH test.
* If there is a weak formation of bubbles, then it is likely a hydroxide salt (this should be confirmed by the pH test).

* Prepare a Solution for Further Testing
* Dissolve approximately half of the sample in 10 or so mL of deionized water for further tests. * If the solution does not readily dissolve in water, heat while stirring to attempt to dissolve the solute. * If it still does not dissolve, then centrifuge the solid. Save the supernatant solution in a vial, rinse with water, and centrifuge again. * If the solubility test fails, then it is likely not an alum, but confirm with aluminum test on supernatant solution.

Tests on (Supernatant) Solution

* pH Test
* Prepare a solution of the sample in neutral water, and then test the pH with indicators. * Remember, if the salt is insoluble, this test will be on the supernatant solution after centrifuging. * The following pHs correspond to presence/absence of ions: * pH: 1-2 implies HOSO3– is present while HO–, CO32–,and HCO3– are absent (likely calcium is also absent because of precipitation). * pH: 3-3.5 implies Al(OH2)63+ is present while HO–, HOSO3–, CO32–, and HCO3– are absent. * pH: 5-6 implies possibly NH4+, Mg(OH2)2+, SO42–, or no pH-affecting ions are present as well as absence of HO–, HOSO4–, Al(OH2)63+, CO32–, and HCO3–. * pH: ~7 implies possibly Cl–, NO3–, implies absence of HO–, HOSO3–, CO32–, and HCO3–, Al(OH2)63+, NH4+, and Mg(OH2)2+. * pH: 8.5-9.5 implies...
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