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Experiment 43 - Spot Tests for Some Common Ions

Goal: To observe distinguishing chemical tests for some common ions and use these tests to identify the ions in an unknown mixture.

The process of qualitative analysis assumes that each chemical test for an ion is unique to that 2- 3- 2+ ion. For example, if we have an unknown that may contain either SO4 or PO4 , addition of Ba will not be useful, because Ba2+ forms precipitates with both anions (refer to the solubility table).

Again, referring to the solubility table, a better test would be to add Cu2+ ions. I a precipitate 3- 2- forms, then we know PO4 is present. If no precipitate forms, then SO4 is present.

We will be testing for 6 different ions: 2- 2- CO3 carbonate ion CrO4 chromate ion 3- 2- PO4 phosphate ion SO4 sulfate ion Cl- chloride ion SCN- thiocyanate ion

(1) Test for carbonate: Add any acid and bubbling (effervescence) occurs.

+ 2- _____ 2 H (aq) + CO3 (aq) > H2O (l) + CO2 (g)

(2) Test for sulfate: Add barium ions and a white precipitate forms.

2+ 2- _____ Ba (aq) + SO4 (aq) > BaSO4 (s)

(3) Test for phosphate: Add ammonium molybdenate, heat, and a yellow precipitate appears.

2- + 2- + __Δ__ HPO4 (aq) + 3 NH4 (aq) + 12 MoO4 (aq) + 23 H (aq) > (NH4)3PO4·12MoO3 (s) + 12 H2O (l)

(4) Test for chromate: Observation 1 - yellow solution turns orange when acid is added. Observation 2 - rapidly fading blue color appears when 1 drop of peroxide is added. These observations are not apparent from the net ionic equation.

2- + _____ Cr2O7 (aq) + 2 H (aq) + 4 H2O2 (aq) > 2 CrO5 (aq) + 5 H2O (l)

(5) Test for thiocyanate: Solution turns red upon addition of Fe3+.

3+ - _____ 2+ Fe (aq) + SCN (aq) > FeSCN (aq) [this ion is deep red in color]

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(6) Test for chloride: A white precipitate appears when silver ions are added.

+ - _____ Ag (aq) + Cl (aq) > AgCl (s)

SCN- interferes with this test, because it also forms a white precipitate with Ag+.

+ - _____ Ag (aq) + SCN (aq) > AgSCN (s)

we will not have to deal with this problem, as the unknowns will not contain both Cl- and SCN-. You will test for SCN- first. If it is present, the Cl- will definitely not be present. If SCN- is not present, then Cl- may be present.

Pay attention to procedure changes below.

ASA: do all of it (net ionic equations for #3 are above).

Procedure: (1) Omit test for acetate and ammonium (also omit their consideration from the asa). (2) Omit 9:1 dilution. (3) Test known and unknown simultaneously. (4) You will need to heat the phosphate ion test, so get a beaker of boiling water ready first thing. 2- 2- 3- 2- - - (5) Perform tests in the following order: CO3 , SO4 , PO4 , Cr O4 , SCN , Cl . Reminder - if you unknown contains SCN-, it des not contain Cl-. (6) Carbonate ion test: (aq) Watch carefully for bubbling in the unknown. It is more dilute than he known, and will bubble less vigorously and be complete more quickly. (b) Heating is not necessary. (7) Chromate ion test: (a) Omit sentences 2 and 4 (and any mention of heating) from this procedure. (b) Change 1 mL of 3% H2O2 to 1 drop of 3% H2O2. (8) Thiocyanate ion test: Use HNO3 (not HC2H3O2) in this test. (9) Chloride ion test: Will only perform first three sentences of this procedure (omit everything from "If thiocyanate ion...").

Waste disposal: Pour all liquids and rinse all solids into the HEAVY METAL INORGANIC waste container.

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