I have an unknown white powder, which is an ionic solid, and I need to identify what it is by using qualitative analysis. I've run quite a few tests, but I am having a lot of trouble identifying the anion. The data I've collected is below.

General Properties:

  • It is a white powder, and I know that it is an ionic solid.
  • It's density is around $\pu{2g/mL}$ (this is not an exact number)
  • It appears to be a hydrate (as it loses mass when heated over a bunsen burner)
  • It is fairly soluble in water. It definitely isn't very insoluble, but I have to add a significant amount of water in order to get it to dissolve.
  • The powder has a very strong smell (although it's hard to describe).
  • A solution of the solid is colorless.

Cation Test

  • A flame test produced a deep orange / orange-red flame

  • Adding $\ce{NaCO3}$ to a solution of the solid results in the formation of a white precipitate

  • Adding a base, such as $\ce{NaOH}$ to a solution of the solid results in the formation of a white precipitate

Based on this data, I am fairly confident that the cation is calcium, but please correct me if there are any other metals that fit the pattern described above or if additional tests are necessary.

Anion Test

This is what has really been messing with me, because I can't seem to get a conclusive test for any anion.

  • Adding $\ce{AgNO3}$, $\ce{BaCl}$, or $\ce{Pb(NO3)2}$ to a solution of the solid did not result in the formation of any precipitate, regardless of how much I added.
  • Adding $\ce{H2SO4}$ results in the formation of a white precipitate (and no bubbles or anything). However, I'm fairly sure that this precipitate is not related to the anion, but rather producing $\ce{CaSO4}$ (from the cation).
  • Adding $\ce{HCl}$ or $\ce{HNO3}$ to a solution of the solid did not result in any bubbles (neither did adding $\ce{H2O}$ directly to the pure solid)
  • Adding Ferric Nitrate to the solution just retained the orange color of the ferric nitrate. A dark red color or precipitate was not observed.
  • Adding $\ce{HCl}$ did not result in the smell of vinegar.
  • Adding $\ce{AgNO3}$ followed immediately by $\ce{HNO3}$ also did not result in a precipitate.
  • Adding ammonium molybdate and nitric acid to the solution and heating it caused the solution to turn yellow, but did not yield a precipitate (even after centrifuging). This test was done twice with different ratios of ammonium molybdate to nitric acid, and the exact same result was observed both times.
  • The brown ring test was conducted, and came back with a negative result. It is possible that this test was done wrong, because the acidified ferrous sulphate was added drop by drop on top of the solution, rather than by rolling it down the side of the test tubes, which may have interfered with test results.

Are there any tests for ions that I have missed? I've done almost every test I know trying to identify the anion, but no dice. The results for the ammonium molybdate test are also confusing to me, because I expected either a yellow precipitate (to confirm phosphate) or no reaction, but the test seems to have gone halfway by just turning yellow. Any guidance on what I should do next would be very much appreciated.

  • $\begingroup$ If it were the only reasonably soluble phosphate of calcium, monocalcium phosphate, then the phosphate would have dropped out with the addition of Pb(NO3)2. Regarding solubility, do you know about how much water you had to add to how much solid? Did you ever notice a change in smell or the appearance of an oily layer upon adding acids to the solution? Did you observe any boiling, popping, melting when heated with bunsen burner? Maybe most importantly, did you quantitatively determine the weight loss on heating (masses before and after heating)? $\endgroup$ – airhuff May 24 '17 at 1:25
  • $\begingroup$ For the solubility, I do not have any quantitative measurements, but the volume of water I have to add is easily 10x the volume of the solid present. I can give more details in a few days. When adding acids to solution there is no change in appearance of the oily layer - it just seems to sink. I haven't smelled the solution after adding acid. When heating the solid, there is a white fume/vapor emitted that rises up, but there was no boiling, popping, or melting. The mass of the solid before heating was 0.26g, and after heating it was reduced to 0.21g (so a 0.05g difference). $\endgroup$ – L. R. J. May 24 '17 at 3:08
  • $\begingroup$ 1) Is it hydroscopic (i.e. adsorb water from air to form solution) ? 2) Could it loose oxygen on heating and not water? Or maybe some other gas? $\endgroup$ – permeakra May 24 '17 at 5:30
  • $\begingroup$ @permeakra 1) I'm not sure if it is hydroscopic, how would I test for that? One thing I did notice is that after a few days, the powder in the container started to slightly clump together. Would this indicate anything? 2) I don't believe it would lose any gas, because it didn't bubble or anything. Most likely, everything that was lost was water. $\endgroup$ – L. R. J. May 24 '17 at 5:57
  • $\begingroup$ @L.R.J. 1) leave a small quantity on open air overnight and see what happens 2)incorrect. thermal decomposition of perchlorates produces oxygen, but they won't produce gases when mixed with (diluted) acid. $\endgroup$ – permeakra May 24 '17 at 10:14

What about a possible F- anion? While the nature of the cation might be guessed based on the flame test, since the AgNO3 test gave no precipitate, it might have produced AgF (soluble in water).

This said, by looking at the densities of M-F systems with a cation with orange flame and density around 2 g/ml, I'd say it's probably NaF.

You might test for fluoride with a selective electrode, if you have one.

Otherwise, here are listed some other analytical methods for the indentification of fluoride anions: https://pubchem.ncbi.nlm.nih.gov/compound/sodium_fluoride#section=Analytic-Laboratory-Methods

  • 2
    $\begingroup$ Maybe, but only if the OP is wrong (as you suggest may be true) about $\ce{Ca^2+}$ being the cation. What bothers me most is that $\ce{PbF2}$ is only soluble to about 600 ppm, which you'd think would have formed a precipitate. $\endgroup$ – airhuff May 24 '17 at 1:34
  • $\begingroup$ You are right, actually I'm not familiar with the lead test. Anyway, I excluded calcium simply because the density is quite different from the one provided (3.18 g/ml). $\endgroup$ – The_Vinz May 24 '17 at 3:07
  • $\begingroup$ What is the best way to conclusively determine if it is Calcium or Sodium? Would adding Sodium Oxalate do the trick? $\endgroup$ – L. R. J. May 24 '17 at 3:10
  • $\begingroup$ My idea is quite nasty since it produces HF (maybe someone has a better idea): by reaction with H2SO4, NaF would yield N2aSO4, and CaF would give CaSO4. Other than having different appearance, Na2SO4 is highly soluble in water 28 g/100 ml, while CaSO4 is poorly soluble: 0,2 g/100ml. You could use this property to differentiate them. $\endgroup$ – The_Vinz May 24 '17 at 5:01
  • 1
    $\begingroup$ @L.R.J. Atomic Emission Spectroscopy. $\endgroup$ – CoffeeIsLife May 24 '17 at 5:07

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