Acidified ammonia, also known as blue ammonia, refers to a solution of ammonia (NH₃) dissolved in water and acidified with an acid, typically hydrochloric acid (HCl). The acidification of ammonia solution results in the formation of a blue-colored complex known as "prussian blue," which gives the solution its characteristic blue color.
The reaction between ammonia and the acid can be represented as follows:
NH₃ + HCl → NH₄Cl
The addition of hydrochloric acid (HCl) protonates the ammonia molecules, forming ammonium ions (NH₄⁺) and chloride ions (Cl⁻). The resulting solution contains ammonium chloride (NH₄Cl) along with excess undissociated ammonia molecules.
The blue color in acidified ammonia solution is due to the formation of the prussian blue complex. When a solution containing iron(III) ions (Fe³⁺) is added to the acidified ammonia solution, a chemical reaction occurs, leading to the formation of a blue-colored compound called iron(III) hexacyanoferrate(II), commonly known as prussian blue. The reaction can be represented as:
6NH₄⁺ + Fe³⁺ + 4[Fe(CN)₆]³⁻ → Fe₄[Fe(CN)₆]₃ + 24NH₃ + 6H₂O
The prussian blue complex is insoluble in water and precipitates, giving the solution its blue appearance. This blue coloration is often used as an indicator or visual marker in various chemical reactions and laboratory procedures.
It's worth noting that the term "blue ammonia" is also used in the context of a different process involving the synthesis of ammonia from nitrogen and hydrogen with the capture and utilization of carbon dioxide emissions. This "blue ammonia" refers to ammonia produced from hydrogen derived from fossil fuel sources with carbon capture and storage technology.