What is Ammonium Hydroxide?

Ammonium hydroxide is a basic or alkaline solution of ammonia which has several applications, such as cleaning windows, treating water, producing alkyl amine, and processing food. It also has some use in the laboratory for qualitative inorganic analysis, and can be used as a chemical indicator.

Ammonium hydroxide is otherwise known as ammonia solution, ammonia water, ammonia liquor, or aqua ammonia. While some refer to it as simply ammonia, this is actually inaccurate because ammonium hydroxide is an aqueous solution, while ammonia takes a gaseous form.

Despite this, the formula for ammonium hydroxide is the same as ammonia because although the molecules separate into hydroxide and ammonium ions, the ammonium hydroxide molecules are impossible to isolate. 

What is the Formula for Ammonium Hydroxide?

Ammonium hydroxide is an alkaline compound with the formula NH4OH. This is because the solution contains the following ions: [NH4+][OH]. Its standard formula is simply expressed as an aqueous solution of ammonia: NH3(aq).

The main reason for this is that samples of ammonium hydroxide are impossible to isolate from the solution, and because the ions aren’t significantly abundant in the solution. This means that molecular ammonia is still dominant except for in extremely dilute solutions.

In terms of molecular structure, this substance has a trigonal pyramidal geometric shape, as shown below:

Trigonal pyramidal diagram

The alkalinity of an aqueous solution of ammonia is due to the deprotonation (the transfer of a proton) of a small fraction of the water molecules. This results in the formation of ammonium and hydroxide ions, as shown in the balanced equation below:

NH3 + H2O ⇌ NH4+ + OH

Only a very small amount of ammonia in an aqueous solution is converted into ions. For example, in a one mole of ammonia solution, approximately 0.43% of ammonia molecules are converted into ammonium. The equivalent pH of this one mole of solution is 11.3. This is based on the computation below, given the base ionisation constant:

  • [NH4+] = 0.0042 M
  • [OH−] = 0.0042 M
  • NH3] = 0.9958 M
  • pH = 14 + log10[OH−] = 11.62

The base ionisation constant can be computed using the following expression:

Since the constant is just a ratio of concentration, it doesn’t have a unit, and we can substitute the given values. With this in mind, it can be written as:

Kb = [NH4+][OH] / [NH3] = 1.77×10−5

Below is a tabulated list of some weak bases and their respective base ionisation constants. These constants are useful when determining the unknown concentration of ions in a solution. These constants are valid at 25°C.

Is Ammonium Hydroxide Toxic?

The concentration of ammonia in ammonium hydroxide solution, which can be up to 30%, makes it toxic. If you accidentally inhale ammonia vapour, it can lead to the irritation of the air passageway, especially the nasal cavity, where the first initial reflex reactions are sneezing and coughing. If you’re exposed to high concentrations of the substance, it can lead to pulmonary edema or swelling of the lungs. Cyanosis may follow as blood oxygen levels drop, causing shock and convulsions.

Ammonia vapour can also irritate the eyes, skin, and mucous membrane of the gastric walls. It may even perforate the gastrointestinal tract. Other risks associated with long term exposure to ammonium hydroxide include central nervous system depression, ulceration of the cornea, and conjunctiva of the eyes, which can cause temporary blindness. Not only this, it can seriously irritate the skin and even lead to chemical burns. When ingested, ammonium hydroxide may cause the person to become nauseated, and vomit.

When working in the lab using ammonium hydroxide, or when using it to clean windows, for example, you should wear gloves, goggles, and protective clothing to avoid contact with the skin and the mucous membranes. You should also wear a face mask or a gas mask with a respirator when handling large amounts of the chemical.

Brown glass bottle of ammonium hydroxide in lab setting

Is Ammonium Hydroxide an Acid?

Ammonium hydroxide has a very high pH, making it an alkaline or basic substance. By definition, this means ammonium hydroxide isn’t an acid, as acids have pH levels lower than 7, which is the neutral level. By comparison, one mole of this solution has a pH of 11.3.

According to the Bronsted-Lowry theory of acids and bases, an acid donates protons and a base is a proton acceptor. The proton here is a hydrogen ion, which is really just a proton. 

A substance doesn’t necessarily have to have a hydrogen ion or a hydroxide ion to be considered either an acid or a base. As long as the condition of either acting as proton donor or acceptor is fulfilled, a substance can be classified as either an acid or a base. Consider the example below:

Balanced equation showing the reaction between an aqueous solution of hydrochloric acid and dissolved ammonium hydroxide

This equation shows that in an aqueous solution mixture of hydrochloric acid and ammonium hydroxide (dissolved ammonia), the hydrochloric acid donates a proton to ammonia. The reaction produces an ammonium ion, which is positively charged, and a chloride ion, which is negatively charged.

Therefore, the HCl is classified as a Bronsted-Lowry acid and the ammonia is classified as a Bronsted-Lowry base. Furthermore, the chloride ion is known as the conjugate base of the acid, while the ammonium ion is the conjugate of the base ammonia.

Is Ammonium Hydroxide Soluble?

Ammonium hydroxide itself is an aqueous solution of ammonia. It’s not only soluble in water, but also miscible. This means that it can form a completely homogeneous mixture with water. However, it’s not soluble in nonpolar solvents.

Aside from water, ammonia is also soluble in chloroform, ethanol, and methanol. Similar to other types of gases, the solubility of ammonia in water and in other solvents is inversely proportional to the temperature of the solvent and the surrounding. As the temperature increases, the solubility of ammonia decreases.

Optimal solubility, which is the saturation point of ammonia in water, is achieved at 15.6°C. At this temperature, the solution contains 35.6% ammonia by mass.

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