Or NH4OH? I have read contradicting reports about NH4OH stability, long ago it was claimed that ammonia solutions are in fact solutions of NH4OH. Then I have read that such compound in fact doesn't exist.
I read somewhere ( can't remember where) that Arrhenius himself was the one who proposed that ammonia dissolved as NH4OH so that it would fit with his definition of bases.
In practical terms, there is a lab every semester in which the students test the conductivity of several solutions to classify them as non-electrolytes, weak electrolytes, and strong electrolytes. 1M solutions of ammonia always light up the lightbulb with a dim light. Without NH4OH I would find it difficult to explain to the students why it conducts. There are no other ions that I can think of that could be produced to make the solution conducting.
Then again it makes you wonder about the existence of a compound that has no physical properties such as melting point, density, color, etc.
A search through the literature yielded the following
Sing, R.; Rumpf, B.; Maurer, G. Solubility of ammonia in aqueous
solutions of single electrolytes sodium chloride, sodium nitrate, sodium
acetate, and sodium hydroxide. Ind. Eng. Chem. Res. 1999, 38, 2098-
Reactive Gas Solubility in Water: An Empirical Relation
Islam, M. A.; Kalam, M. A.; Khan, M. R.
Ind. Eng. Chem. Res.; (Correlation); 2000; 39(7); 2627-2630.
Influence of a Single Salt (NaCl/Na2SO4) on the Solubility of Ammonia in Liquid Mixtures of (Water + Methanol)
Schäfer, D.; Vogt, M.; Pérez-Salado Kamps, Á.; Maurer, G.
Ind. Eng. Chem. Res.; 2008; ASAP Article;
Influence of NH4Cl, NH4NO3, and NaNO3 on the Simultaneous Solubility of Ammonia and Carbon Dioxide in Water
Perez-Salado Kamps, A.; Sing, R.; Rumpf, B.; Maurer, G.
J. Chem. Eng. Data; (Article); 2000; 45(5); 796-809.
Solubility of Ammonia in Aqueous Solutions of Single Electrolytes Sodium Chloride, Sodium Nitrate, Sodium Acetate, and Sodium Hydroxide
Sing, R.; Rumpf, B.; Maurer, G.
Ind. Eng. Chem. Res.; (Article); 1999; 38(5); 2098-2109
Koneczny, H.; Lango, D.; Lango, M. The investigations on KCl-NH3-
H2O system. Chem. Stos. 1973, 2, 115-125
Trypuc´, M. The solubility investigations of NaCl in aqueous ammonia
solution. Chem. Stos. 1987, 2, 257-265
The first three all point to the formation of ammonium hydroxide to explain some of their results, the other ones I was not able get hard copies of them yet, so I do not know which way they lean if any.