Ammonia has replaced fluorochlorohydrocarbons (Freon) as a refrigerant due to concerns about oxone layer depletion in the Earth's atmosphere. Only $1.95 for 100 strips.
Because ammonia (NH3) is a very small molecule, it can leak through very small gaps in the fittings used to attach gas lines. The image shows test strips infused with phenolphtalein that change color in response to the alkaline pH of ammonia. Note that the strip on the left which shows the response was wetted with a drop of water while the one in the middle shows no change at all.