experiment16

CHM 1046L Exp. 16 Virtual Unknown Form K

Your unknown gave a yellow color on the first drop tested with methyl violet paper. Next, 15 drops of thioacetamide are added and the sample is heated for 15 minutes in the heating block. Your sample shows an black precipitate.

The black precipitate was mixed with 2 ml of 6M nitric acid and heated for five minutes. The solution was clear except for a solid suspended in the liquid. The solid was discarded. One ml of DI water and 2 grams of solid ammonium sulfate were added to the clear liquid and the result was a cloudy solution. The mixture was centrifuged and the colorless liquid was decanted into a clean test tube to test for bismuth, copper, and cadmium. The white precipitate was washed and dissolved in ammonium acetate with warming. After the addition of acetic acid and ammonium chromate, a yellow precipitate formed. This yellow precipitate was observed to completely dissolve in 6M sodium hydroxide. Two ml of concentrated ammonium hydroxide were added to the clear colorless solution resulting in no change whatsoever. The solution remained clear and colorless.

The same clear colorless solution was treated with 1 gram of sodium dithionite and heated. After centrifugation and discarding of a precipitate, the clear liquid was treated with 10 drops of thioacetamide and heated. No precipitate was obtained.

The liquid saved from group II was made basic, treated with thioacetamide, and heated.

Your unknown sample shows a black precipitate.

The procedure B in Group III calls for 0.5M HCl and you have 6M and 12 M as stock solutions available. Be very specific about steps, reagents, and amounts.

After mixing the black precipitate well with 0.5 M HCl, a black precipitate and a clear solution (save for step F) were obtained. The black precipitate from step B was dissolved with hydrochloric and nitric acid by heating. The resulting solution was diluted to 3 milliliters and the sulfur was removed. After the addition of 6M ammonium hydroxide, the basic solution was divided into two separate test tubes. Ten drops of dimethylglyoxime were added to one of the test tubes and no precipitate was observed. The second test tube was made acidic with 6M HCl and 0.4 grams of KSCN was added. After the addition of 25 drops of isoamyl alcohol-ether solution, the upper alcohol-ether layer appeared dark red. Upon addition of solid ammonium bifluoride, the ether layer showed a deep blue color.

The solution saved for step F was filtered and evaporated down to three milliliters. The solution was made basic with sodium hydroxide and it was allowed to react with sodium peroxide. The result was a clear yellow solution.

One third of the clear yellow solution was acidified with 6M acetic acid and treated with four drops of plumbous acetae, resulting in the formation of a yellow precipitate. Another third of the clear yellow solution was acidified with 6M hydrochloric acid, 4M ethylenediamine, and potassium ferrocyanate. This action produced a white precipitate.

The liquid you saved for group IV was evaporated to 3 ml and filtered. It was made alkaline with excess 6M ammonium hydroxide. 1M ammonium carbonate was added to complete precipitation. This action produced a white precipitate. The white precipitate was washed with DI water. After discarding the washing, the precipitate was treated with 12 drops of 6M acetic acid and 24 drops of 3M ammonium carbonate. The drop-by-drop addition of 1M potassium chromate gave a clear yellow solution. According to step C in group IV, the liquid from step B was treated with 6M ammonium hydroxide until the color changed. After the addition of potassium chromate and ethyl alcohol, the solution was cooled in an ice bath and the formation of a yellow precipitate was observed. The mixture was separated in the centrifuge and the liquid was heated for 10 minutes. The addition of ammonium oxalate and cooling in an ice bath gave no precipitate at all.

The unknown was tested for group V with the following results: To the naked eye, the flame appears a light blue color.