Ammonium bromide is a salt that dissolves readily in water. When this compound is introduced into water, it separates into its component ions: the positively charged ammonium ion (\(\text{NH}_4^+\)) and the negatively charged bromide ion (\(\text{Br}^-\)). Although it is a salt, the solution it forms produces an acidic solution with a \(\text{pH}\) value below 7. This acidic nature arises because one of its constituent ions reacts with water in a process that releases acidic particles into the solution.
How Salts Affect Water’s pH
Salts are ionic compounds that are formed from the neutralization reaction between an acid and a base. When a salt dissolves in water, the resulting \(\text{pH}\) is determined by the reaction of its component ions with water, a process called hydrolysis. The ions from the salt can be classified based on the strength of their parent acid or base, which dictates whether they will affect the water’s neutral \(\text{pH}\). Salts formed from the reaction of a strong acid and a strong base create neutral solutions because neither ion reacts with water. Conversely, a salt formed from a strong acid and a weak base will yield an acidic solution, while a salt from a weak acid and a strong base will result in a basic solution. Ammonium bromide is a salt formed from hydrobromic acid, which is a strong acid, and ammonia, which is a weak base.
The Acidic Contribution of the Ammonium Ion (\(\text{NH}_4^+\))
The ammonium ion (\(\text{NH}_4^+\)) is the cation in the ammonium bromide compound and is responsible for the solution’s acidity. This ion is the conjugate acid of the weak base ammonia (\(\text{NH}_3\)). When a weak base forms a salt, its conjugate acid retains the ability to react with water, acting as a proton donor. The ammonium ion undergoes hydrolysis, donating a proton (\(\text{H}^+\)) to a water molecule (\(\text{H}_2\text{O}\)). This proton transfer generates a hydronium ion (\(\text{H}_3\text{O}^+\)), which increases the acidity of the solution. The reaction is represented by the equilibrium: \(\text{NH}_4^+ + \text{H}_2\text{O} \rightleftharpoons \text{NH}_3 + \text{H}_3\text{O}^+\). Because the \(\text{NH}_4^+\) ion is a weak acid, only a small fraction of the ions react with water, meaning the reaction is reversible and reaches equilibrium. Despite this partial reaction, the formation of hydronium ions is sufficient to lower the \(\text{pH}\) below 7.
The Neutrality of the Bromide Ion (\(\text{Br}^-\))
The bromide ion (\(\text{Br}^-\)) is the anion component of ammonium bromide. Its parent acid is hydrobromic acid (\(\text{HBr}\)), which is classified as a strong acid, meaning it completely dissociates in water. The conjugate base of a strong acid is an extremely weak base. Because \(\text{Br}^-\) is the conjugate base of the strong acid \(\text{HBr}\), it has a negligible tendency to accept a proton from water. Consequently, the bromide ion does not undergo measurable hydrolysis reaction. The \(\text{Br}^-\) ion is considered chemically inert in terms of \(\text{pH}\) influence because it does not produce hydroxide ions (\(\text{OH}^-\)) or consume hydronium ions. Therefore, the presence of the bromide ion does not contribute to changing the original neutral \(\text{pH}\) of the water. Its function in the solution is simply as a spectator ion, balancing the positive charge of the ammonium ion.
Conclusion: Is \(\text{NH}_4\text{Br}\) Acidic, Basic, or Neutral?
Ammonium bromide is classified as an acidic salt. The \(\text{NH}_4\text{Br}\) compound is formally a salt of a weak base (\(\text{NH}_3\)) and a strong acid (\(\text{HBr}\)). This combination means that only the cation, the ammonium ion, actively reacts with water to produce acidity. The \(\text{NH}_4^+\) ion contributes hydronium ions, while the \(\text{Br}^-\) ion remains neutral, leading to an overall acidic solution. When \(\text{NH}_4\text{Br}\) is dissolved in water, the resulting \(\text{pH}\) is measurably less than 7. A chemist would use a \(\text{pH}\) meter or \(\text{pH}\) paper to confirm this acidity, observing a \(\text{pH}\) value typically around 5 for a common concentration. The final nature of the solution is determined entirely by the hydrolyzing ion, which, in this case, is the proton-donating ammonium ion.