Transcript
Unknown Scheme 2
Fair game Cations: Na+, K+, NH4+
Fair game Anions: OH-, Cl-, NO3-, SO42-, HSO4-
Description of sample:
Dissolving the unknown sample:
When dissolving the unknown, make sure to use deionized water. Add about ½ of the unknown to the deionized water and mix thoroughly. Do not use a lot of solvent as this may dilute the unknown too much.
Determining the Cation(s):
Flame Test: Differentiates between Na+, K+ and NH4+.
Do this test before dissolving unknown sample. If the flame test is orange, then Na+ is present in the unknown sample. If not, then the sample must contain either K+ or NH4+.
After flame tests, check the pH of the unknown. In order to do this, first start with a counted number of drops of deionized water. Add some Bromthymol Blue. The color should be yellow. Next, add some tap water using a dropper and count the number of drops it takes for the solution to turn green. At this point the pH of the solution should be neutral and you now have the ratio of drops in order to make pH neutral water.
After making some pH neutral water, add a small amount of the unknown solution to the solution. Add the Bromthymol Blue. If the color is green, then K+ is present and NH4+ is absent. However, if the indicator turns yellow it means NH4+ maybe present because
NH4+ + H2O ? NH3 + H3O+
Verify the pH by using Methyl Red or Bromcresol Green.
If the solution is green, then K+ can be verified by performing a cobalt nitrite test. If solution is yellow, then to verify that it has NH4+, add 1M NaOH to the solution and perform a red litmus test. F the litmus paper changes color, then NH4+ is present. Also, if there is a distinct odor, then NH4+ is present since:
NH4+ + OH- ? NH3 + H2O
However, since K+ does not affect the pH of a solution, it is necessary to test for it. To do so, first ignite the NH4+ until it is gone. Then perform a cobalt nitrite test. If there is a yellow precipitate, then K+ is present.
Be sure to acidify the solution with 6M HAc before performing the cobalt nitrite test.
Flame Test results:
pH test Results:
If pH test was neutral:
Then verify K+
If pH test was acidic:
Test for NH4+
Test for K+
Determining the Anion(s):
Once again, it would be wise to perform a pH test for the possible anions as well. If the pH is acidic, check the acidity of it by using pH paper. If it is around 1-3, then HSO4- is present. If the pH according to the indicator is basic, add HIC in a separate test to verify it. If it is around pH 13, OH- is present. Since Cl-, SO42-, and NO3- do not affect pH, it is necessary to test for them afterwards.
In order to test for SO42-, one must acidify the solution with excess 1M HNO3 drop wise and then perform a Ba(NO3)2 test. If a white precipitate forms, then SO42- is present. (Note that this is also a useful way to verify the presence of HSO4- after the pH test as it forms the same precipitate.)
For Cl-, acidify the solution with 1M HNO3 drop wise and then add AgNO3. If a precipitate forms, then Cl- is present.
Finally, for NO3-, first place 1 crystal of Fe(OH2)6(NH4)2(SO4)2 into a spot plate. Have some 18M H2SO4 ready. Cover the crystal with a few drops of the solution. Immediately add two drops of 18M H2SO4. If a brown solution forms, then NO3- is present.
pH Test results:
Test for SO42-:
Test for Cl-:
Test for NO3-:
