Unknown 5

Topics: Solubility, Potassium, Solid Pages: 2 (477 words) Published: April 6, 2013
Tiffany Ward
Scheme For Unknown 2A/B

Possible Ions:
Cations: Na+, K+, NH4+
Anions: OH-, Cl-, NO3-, SO42-, HSO4-

1.Describe Sample
Phase, color, odor, crystal shape, amorphous, gel-like, needle-like, powdery, etc.

2.Tests on original sample
(i) Flame test
bright orange Na+
purple K+
no color NH4+

(ii) NH4+ test
Place a small portion of original unknown sample in a test tube or spot plate, then add a few drops of 1M NaOH solution. At the same time suspend a piece of red litmus paper over unknown solution. If red litmus paper turns blue, it indicates the existence of NH4+ ion.

3.Sample solubility and solution tests
(i) Dissolve ~1/2-2/3 of sample in ~10-15mL of DIW.
Always retain a sample of original sample!
This sample/DIW ratio will give ion concentrations of ~0.1-0.2M for soluble salts of unknowns.

(ii) Observe pH to one SF
Use small portion of sample to make a solution with neutral water to test pH with indicators.

≈2 HSO4- (no OH-)
≈5-6 NH4+
≈7 Cl-, NO3-, SO42-, Na+, K+
≥13 OH-

(iii) Perform necessary separations
If Na+ and NH4+ are present, testing for K+ requires separation of NH4+. Place small portion of original sample in crucible and heat strongly until “smoking” ceases.

(iv)Perform specific ion tests
Proceed with K+ precipitation test and specific tests for other ions which are yet to be determined. * Test for K+:
After burning off all possible NH4+, in a small spot plate, add a drop of 6M HAc and 2-3 drops of sodium cobaltinitrite reagent to 10-15 drops of unknown solution. If precipitate forms, then K+ ion is present. * Test for Cl-:

Place about 10-20 drops of unknown solution into individual small test tubes or vials. Add 1M HNO3 dropwise until each test solution is acidic before adding one drop of 0.02M AgNO3. Determine acidity by touching tiny quantities of the test solution to blue litmus paper until the litmus turns red. If precipitate forms then Cl- is present. * Test...
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