Chemistry - What is the group reagent for the zero group (ammonium cation) in inorganic salt analysis?
Solution 1:
Ammonium salts are identified by the release of ammonia upon warming with base.
$$\ce{NH4+ + OH- ->[\Delta] NH3 ^ + H2O}$$
Vogel's Qualitative Inorganic Analysis 5th ed., p 400 writes
6. Test for ammonium ions Boil a little (c. 0.1 g) of the substance with sodium hydroxide solution. The evolution of ammonia, detected by its odour and its action upon red litmus paper and upon filter paper soaked in mercury(I) nitrate solution,* indicates the presence of an ammonium salt.
* Drop-reaction paper, treated with tannic acid and silver nitrate solution, may also be used.
Solution 2:
The "Group reagent" is a cocktail of reagents that serves as a prima facie indication of a particular cation, characterized by the formation of a precipitate during a positive test/result and...well... no precipitation in the event of a negative test/result.
You often come across use of "Group reagents" in a school chemistry lab where, as I mentioned earlier, it's used as a preliminary "probe" (if you will) to narrow down possible cations present in a salt.
As far as I know, most common ammonium salts (sulfates. chlorides, etc) do not precipitate when treated with aqueous NaOH, in fact, they dissolve fairly well, resulting in a clear (sometimes slightly turbid) solution. So that's not exactly a "Group reagent" now, is it?
However, treating ammonium salts with NaOH produces vapors of ammonia, in addition to water and the corresponding sodium salt. Vapors of ammonia react with vapors of hydrochloric acid to form dense, white fumes (which you can actually condense over a chilled surface and collect) of ammonium chloride. This reaction is often used as a preliminary test for the ammonium cation.
So essentially what you do is:
Dissolve your salt (let's assume you already know that it's an ammonium salt) in a little (aqueous) NaOH, and heat the test tube. Once the solution begins to boil, it gives off (faintly white) ammonia vapors. Apart from catching a whiff of the characteristic smell of ammonia, you can also introduce a glass rod (that's been dipped in a concentrated solution of HCl) to the mouth of the test tube. You will then observe dense, white fumes of ammonium chloride rising from the mouth of the test tube.
Solution 3:
My theory about group reagent is different. I think that group reagent is that reagent that when reacted with all ions present in a particular group will give the same reaction. Ions present in other group will not give that reaction and that's how ions are differentiated and that is the basis of salt analysis.
I am stressing on the word group and all because all the groups except group zero and six* have more that one ions that will give the same reaction when reacted with the group reagent. Since, group zero has only one cation i.e. ammonium ion, $\ce{NH4+}$, there is no need of calling them group reagent because the group has only one cation. When group has more than one ion, then the reagent should be called group reagent. For instance, the reagent that detect presence of ammonium ion i.e Nessler's reagent is not necessarily called group reagent neither is sodium hydroxide.
Group six is no exception. Though, there are three ions present in group six ($\ce{Na+, K+, Mg^2+}$) mentioned in a detail salt analysis book(Vogel), there is no group reagent for this group as three ions are detected separately and have separate test for their detection. But for the time being, you only need $\ce{Mg^2+}$. :)
*You might think that group 1 has only one ion and still has group reagent. Actually there are three ions $\ce{Pb^2+, Ag+, Hg2^2+}$, but again for the time being, you only need $\ce{Pb^2+}$. :)