Help needed - anion and cation tests

[floppy]

We did these experiments in college to find out what type of ions they contained (anions or cations), but can someone explain to me as to how each of the tests actually work? I really need to know this...

 

Test for chloride ion, Cl-. Known solution is sodium chloride.

 

Test for sulfate ion, SO42-. Known solution is sodium sulfate.

 

Test for hydrogen carbonate ion, HCO3-. Known solution is sodium hydrogen carbonate.

 

Test for phosphate ion, PO4-3-. Known solution is sodium phosphate.

 

there are some more but these will do for now....

 

I'd really appreciate any help

[Theophrastus]

I shall address your ideas in turn. To test for chloride, you must find a way to precipitate it. Knowing that AgCl is insoluble means that all one needs to do is add a silver salt or compound, to achieve the desired effect. In this case, silver nitrate would work finely. Implying the solution is sodium chloride, you would be left with dissolved sodium nitrate, and a white tell- tale silver chloride precipitate.

 

To test for sulfate, in a solution, you can perform a similar procedure with a barium salt, and if the sulfate ion is present, you will be left with a white barium sulfate precipitate.

 

The hydrogen carbonate ion, you can test in a similar fashion to the carbonate; simply by adding dilute 0.1 M hydrochloric acid (or really any other sufficiently strong acid for that matter, it's just, the latter is common procedure in the lab) If this results in the release of a nonflammable gas (carbon dioxide), then you know that there are carbonate, or hydrogen carbonate, present in the solution. Its key that the gas is nonflammable, as hydrogen is commonly released, if there are metallic cations floating about. Other flammable organic compounds can also be produced- which is of course- undesirable.

 

As for phosphate ions, that I'm actually unsure about, as I'm not so familiar with phophate salts, however, you can play off of insoluble compounds in a similar fashion, and as long as it holds no common with another salt bearing the same cation- you're clear. (Or you can simply perform further testing upon attaining the insoluble residue, however, unless it isn't possible, option 1 would be your best bet for simplicity's sake.

 

Anyway, hope this helps answer your question. If you have any others, feel free to ask,

 

,Theo

Edited July 12, 2009 by Theophrastus
modifying paragraph structure

[onequestion]

perhaps using the common ion effect? take the NaCl solution and evaporate it so you have NaCl crystals. you know NaCl's solubility is idk lets say 500g/liter. now judging by the volume of the unknown solution, you know that you should be able to dissolve idk lets say 20g. however if you start getting a precipitate before you dissolve 20g, that means the unknown solution already has either sodium cations or chloride anions.

 

or something similar to that. but that's the only way to make sense of why they would give you a "known solution".

Edited July 13, 2009 by onequestion

[Melvin]

For sulfate and phosphate, just use a soluble calcium salt (like calcium chloride). Calcium sulfate and phosphate are both insoluble in water.

 

For chloride, you could always use electrolysis...any chloride in solution would become chlorine gas at the anode.

 

Hope this helps some!

[UC]

The standard lab test for phosphate is to use ammonium heptamolybdate solution. On addition to either phosphate or arsenate, an intense yellow coloration or precipitate is developed. This is due to the formation of complex phosphomolybdic acids.

 

Barium nitrate or chloride will give a white precipitate for both phosphate and sulfate. Once you know it must be one of the two, use the molybdate to test further.

 

Theo has the others correct. Bicarbonate is very hard to distinguish from carbonate, although, you should be able to add a very small amount of calcium chloride solution into the bicarbonate solution without precipitate. Upon heating or standing, you'd get familiar calcium carbonate. Calcium and magnesium bicarbonate are what make hard water "hard."