The idea of a planet where ammonia replaces water as the dominant liquid in the environment is quite popular in science fiction. I tried to develop a scenario where these kind of oceans are plausible from an astrophysical point of view. There seem to be several problems with the development of such an environment.
The big issue is water and how it interacts with ammonia. Any planet with ammonia will also have gotten water during its development as these ices tend to hang around together. Ammonia now acts as an antifreeze for water. While water would be solid at the temperatures one would expect to find liquid ammonia at, the antifreeze situation will lead to below freezing point water oceans saturated with ammonia. This is interesting and probably what is happening in the subsurface-oceans of several ice-moons, but it isn't and ammonia ocean.
So getting rid of the water is key. This is, however, harder than one might think. We cant get rid of it by removing hydrogen as we need it for the ammonia. Removing oxygen sounds like a good idea. This is however impractical as it is the third most abundant element in the universe. And to make matters worse most of what we call rock is oxygen.
The mass-abundance of the nine most abundant elements in the Earth's crust is approximately: oxygen 46%, silicon 28%, aluminum 8.2%, iron 5.6%, calcium 4.2%, sodium 2.5%, magnesium 2.4%, potassium 2.0%, and titanium 0.61%. Other elements occur at less than 0.15%.
This leaves me with carbon planets, which could be expected around white dwarfs and pulsars, or iron planets without a mantle. Both are bound to have their own, very interesting biochemistries and will probably be unsuitable for ammonia oceans. Gas-giants are also not an acceptable solution.