DRAFT: This module has unpublished changes.

Introduction:

The goal of this experiment is to properly identify the anion (or anions) present within a solution or mixture of solutions. This is done via qualitative analysis, in a manner not unlike that used for cations, albeit simpler, due to the lesser need for separation.

 

I hypothesize that by subjecting the unknown solutions to the 3 procedures given, that this qualitative analysis should enable the deduction of the anion (or anions) contained within the unknown.

 

Equipment:

  • Test Tubes (13), 10-mL
  • Test Tube Rack
  • Pipets (13), 1-mL, disposable
  • Stirring Rods
  • Masking Tape
  • Pen
  • Centrifuge
  • 0.2 M Sodium Sulfate, 10 mL
  • 0.2 M Monopotassium Phosphate, 5 mL
  • 0.2 M Sodium Nitrate, 5 mL
  • 0.2 M Sodium Chloride, 5 mL
  • Unknown anion solution #1, 20 mL
  • Unknown anion solution #2, 20 mL
  • 0.2 M Barium Nitrate, 5 mL
  • Saturated Iron(II) Sulfate, 2 mL
  • 0.1 M Silver Nitrate, 5 mL
  • 6 M Nitric Acid, 5 mL
  • 5 M Ammonia, 5 mL
  • 3 M Sulfuric Acid, 5 mL
  • Concentrated Sulfuric Acid, 2 mL
  • Deionized Water

 

Method:

  • Prepared a set of 5 test tubes. Cleaned them and set them in the test tube rack. Labeled 4 of them 1-4, and the 5th "Unknown #1"
  • Labeled 6 disposable pipes, identically to the test tubes.
  • Repeated, preparing a similar set of test tubes for "Unknown #2".
  • Added 5 drops of the following substances to the associated test tube as follows:

  • (Note: Unknown solutions each received 15 drops.)
  • Added 5 M ammonia to each, until basic.

 

Procedure 1:

  • Added 3 drops of Barium Nitrate to test tubes 1-4.
  • Added Barium Nitrate drop by drop to the unknown solutions.
  • Centrifuged the mixtures from the unknown solutions, to separate them into precipitate and supernatant. Separated them into different test tubes, labeled "Supernatant 1", and "Precipitate 1", and labeled them according to their respective unknown solution.
  • Added 6 M Nitric Acid, drop by drop, to each solution from test tubes 1-4 which developed a precipitate.
  • Added 6 M Nitric Acid, drop by drop, to Precipitate 1.
  • Recorded partial or complete dissolution.
  • Separated the unknown solutions into Precipitate 2 (if any), and Supernatant 2, and labeled according to unknown solution.

 

Procedure 2:

  • Prepared another set of test tubes as outlined, (1-4) excepting unknown solutions.
  • Diluted each with 1 mL of deionized water.
  • Added 2 drops of Silver nitrate to each of the test tubes (1-4).
  • Added ammonia to the solutions containing chloride, until the precipitate dissolved.
  • Added 2 drops of Silver nitrate to each of the Supernatant 1 solutions.
  • Added 2 drops of Silver nitrate to each of the Supernatant 2 solutions.

 

Procedure 3:

  • Added 10 drops of 0.2 M Sodium nitrate to a clean test tube.
  • Added 3 M Sulfuric acid, drop by drop while stirring, until the solution was acidic.
  • Added 5 drops of saturated Iron(II) sulfate.
  • Tilted the test tube to a 45º angle, and allowed 5 drops of Sulfuric acid to flow down the side of the tube, and float atop the solution, taking care not to mix the two.
  • Recorded that the presence of nitrate ions was indicated by the formation of a brown ring between the layers of the two liquids.
  • Repeated test, substituting 0.2 M Sodium nitrate with Unknown Solution #1, and then repeating again with Unknown Solution #2.

 

Data Analysis:

Procedures:

 

Unknown solutions:

 

We can conclude that the anion present in Unknown Solution #1 was Chloride.

We cannot draw an accurate conclusion for Unknown Solution #2, given the anomalous results for procedure 2, and the premature disposal of the supernatant needed. We can however conclude that it contains either Phosphate and/or Sulfate.

 

Conclusion:

The goal of this experiment was to determine the anions present in a solution or mixture of solutions via qualitative analysis (a means for determining the chemical composition of an unknown substance by systematically reacting the unknown substance with a number of different reagents).

 

We can conclude that the anion present in Unknown Solution #1 was Chloride.

We cannot draw an accurate conclusion for Unknown Solution #2, given the anomalous results for procedure 2, and the premature disposal of the supernatant needed. We can however conclude that it contains either Phosphate and/or Sulfate.

 

Possible sources of error include:

  • Contamination of test tubes, due to improper cleaning prior to the experiment
  • Failure to keep the test tube at a 45º angle during Procedure 3.
  • Contamination of the solutions due to improperly labeled pipets, or pipet use for multiple solutions.
  • Inaccuracies in pH measurement, due to lack of litmus paper
  • Failure to follow directions
  • Human error is always in effect, given that the laboratory does not function under ideal conditions. As such, there is always the possibility of inaccuracies with measurement, perception of measurement, inaccuracies of equipment, and other such errors. (However, this is not likely to be the sole cause of the inaccuracies within this experiment, though it may contribute to it.)

Possible improvements with subsequent experiments include:

Using a larger sample size, to lessen the possibility that an anomalous result, or procedural error would disrupt the end result. Keep pipets separated. Use litmus paper. Ensure that lab partners have a functional understanding of the experiment and the reactions involved, prior to beginning the experiment.

 

During this test, our results contained significant error, due to anomalous results during procedure 2, and the premature disposal of the supernatant from Unknown Solution #2. Human error was a significant factor, though the cause of the failure within procedure 2 is unclear. Possibilities might include, contamination of the test tubes from prior experimentation, failure to keep pipets separated, which could in turn contaminate the solutions used for the test, or even the main containers from which they were taken.

Ideally, given the opportunity and time, we could repeat the test to derive a more accurate result.

 

DRAFT: This module has unpublished changes.