Sodium sulfide (Na2S) is highly soluble in water. It typically appears as a colorless solid, though industrial grades may be yellow to brick-red due to polysulfides. It is supplied in various forms, such as crystalline masses, flakes, or fused solids. Its strong interaction with water is a defining characteristic, leading to its widespread use in many industrial processes.
How Ionic Compounds Dissolve
Water’s polarity allows it to dissolve many substances, especially ionic compounds like sodium sulfide. Water molecules are polar, with a slightly positive end (hydrogen atoms) and a slightly negative end (oxygen atom). This polarity enables water to interact strongly with the charged ions of an ionic compound. When an ionic compound is placed in water, the positive ends of water molecules are attracted to the compound’s negative ions, while the negative ends are attracted to its positive ions.
This attraction helps pull ions away from the solid crystal lattice, a process known as dissociation. The separated ions become surrounded by water molecules, a phenomenon called hydration or solvation. For an ionic compound to dissolve, the energy released during hydration must be sufficient to overcome the lattice energy that holds the ions together in the solid state. In highly soluble compounds, the favorable energy of hydration outweighs the energy required to break the ionic bonds, allowing the compound to disperse throughout the water.
What Happens When Sodium Sulfide Dissolves
When sodium sulfide dissolves, it undergoes a two-step process. First, solid sodium sulfide dissociates into sodium ions (Na+) and sulfide ions (S2-). This process is represented by the chemical equation: Na2S(s) → 2Na+(aq) + S2-(aq). Sodium ions remain solvated by water molecules in the solution.
However, the sulfide ion (S2-) is a strong base and reacts with water in a process called hydrolysis. This reaction involves the sulfide ion accepting a proton from a water molecule. Hydrolysis of the sulfide ion produces bisulfide ions (HS-) and hydroxide ions (OH-), which are responsible for the solution’s high alkalinity.
The chemical equation for this hydrolysis is: S2-(aq) + H2O(l) ⇌ HS-(aq) + OH-(aq). The sulfide ion is a strong conjugate base of the weak acid hydrogen sulfide (H2S). When sulfide ions are present in water, they react to form hydrogen sulfide and hydroxide, increasing the solution’s pH. The formation of these hydroxide ions makes sodium sulfide solutions strongly alkaline.
Properties and Safe Handling of Sodium Sulfide Solutions
Sodium sulfide solutions are strongly alkaline, typically with a pH of approximately 12.5. This high basicity can cause severe chemical burns upon contact with skin or eyes, similar to sodium hydroxide. The solutions also have a characteristic “rotten egg” odor, due to small amounts of highly toxic hydrogen sulfide gas (H2S) forming, especially when exposed to moist air or acidic conditions.
Careful handling of sodium sulfide solutions is necessary. Personal protective equipment, including chemical safety goggles, protective clothing, and gloves, should be worn to prevent skin and eye contact. Adequate ventilation is important to minimize exposure to hydrogen sulfide gas. Contact with acids must be avoided, as this rapidly releases hazardous quantities of hydrogen sulfide gas. Sodium sulfide’s solubility and strong basicity are utilized in various industrial applications, such as in the pulp and paper industry for wood pulping, in mining for ore flotation, and in the leather industry for dehairing hides.