Unknown identification:

A tube containing roughly 8 mL of an aqueous, green unknown solution identified only as 1246 was obtained, along with a dropper bottle containing 3 M NaCl. 3 drops of the unknown solution was mixed with 3 drops of 3 M NaCl in a test tube, resulting in a goopy, green precipitate being formed. The precipitate was washed with deionized water and centrifuged until only a white precipitate beneath a green supernatant remained. The supernatant was removed, and the white precipitate was exposed to heat, causing it to melt. It was hypothesized that the cation solution contained lead.

To identify the second cation in the unknown solution, three drops of 1246 were put into a test tube, and mixed with 5 drops of 1 M sodium sulfate (Na₂SO₄). The solution into a milky white precipitate, which indicated either lead, silver, barium, and/or aluminum based on the previous experiments. One drop of 6 M HNO₃ was added to the precipitate, which did not seem to cause a reaction. The sample was washed and centrifuged until only a solid white precipitate remained. Again, this did not help to narrow down any potential cations, so something else had to be done.

Within the fume hood, eight test tubes were each filled with three drops of the unknown solution 1246. 2 drops of 6 M HNO₃ were added to all eight test tubes, which were then submerged in a hot bath for several minutes. Upon cooling the test tubes, some precipitates had formed while others had not. The solutions that had formed precipitates were washed and centrifuged, leaving behind a solid mix of white, grey, and black precipitates. Those precipitates were set aside for later.

To the solutions that did not form any precipitates, 3 drops of 15 M aqueous ammonia were added, causing all of the test tubes to form a black precipitate.

To positively identify the cations within each sample of the unknown precipitate, 3 drops of dimethylglyoxime were added to each sample. The sample that turned black after being reacted with 15 M aqueous ammonia became pink and slightly gelatinous, confirming that nickel nitrate was one of the four cations present in the unknown solution 1246.

However, the other three samples that had produced precipitates were harder to identify due to them being mixed together. The assumption was made that the three remaining cations were lead (ii) nitrate, ferric nitrate, and copper nitrate due to the white, grey, and black mixture of precipitates.

5 drops of 6 M HCl were added to one test tube containing the grey, black, and white precipitates, resulting in a white precipitate being formed, while the supernatant turned slightly gold. The test tube was then warmed up, but the precipitate did not dissolve, confirming that it was lead (ii) nitrate within the solution, which was the second cation.

3 drops of 3 M NaOH were added to a second test tube containing the grey, black, and white precipitate, immediately resulting in a thick, rusty precipitate being formed, along with a blue supernatant. It was determined that the third cation was ferric nitrate.

Finally, 3 drops of 3 M ammonium hydroxide were added to a third test tube containing the grey, black, and white precipitate, resulting in a green/brown precipitate being formed, along with a cobalt blue, clear supernatant. This confirmed that copper nitrate was the fourth and final cation within the unknown solution.

In summary, unknown solution 1246 contained lead (ii) nitrate, copper nitrate, nickel nitrate, and ferric nitrate.

Below is the final separation scheme made based on these results.

Figure 2. The final separation scheme based on the results of the unknown solution identification strategy.