Barium sulfate (\(\text{BaSO}_4\)) is a white, crystalline inorganic compound, a salt of the metal barium and the sulfate ion. This compound is insoluble in water, meaning only a minute amount dissolves when mixed with the solvent. Its natural form is the mineral barite, which is the primary commercial source for barium materials.
Defining Chemical Solubility
Solubility refers to the maximum amount of a substance (solute) that can dissolve in a given amount of liquid (solvent) at a specific temperature. When an ionic solid like \(\text{BaSO}_4\) is placed in water, it reaches equilibrium between the undissolved solid and the separated ions in the solution. This equilibrium is quantified using the Solubility Product Constant (\(K_{sp}\)).
A very large \(K_{sp}\) indicates a highly soluble compound, while a very small value indicates extremely low solubility. Barium sulfate has an exceptionally low \(K_{sp}\) of approximately \(1.1 \times 10^{-10}\) at 25°C. This minuscule value confirms that only a minute fraction of the compound dissolves, classifying it as essentially insoluble.
Why Barium Sulfate Does Not Dissolve
The insolubility of \(\text{BaSO}_4\) is rooted in the energetic competition between two fundamental forces: lattice energy and hydration energy. Lattice energy is the energy required to break the strong electrostatic attractions holding the positive barium ions (\(\text{Ba}^{2+}\)) and the negative sulfate ions (\(\text{SO}_4^{2-}\)) together in the rigid crystal lattice. Since both the barium cation and the sulfate anion are relatively large, the strong forces between them result in a high lattice energy for the solid.
For dissolution to occur, the energy released when water molecules stabilize the separated ions (hydration energy) must be greater than the lattice energy. In \(\text{BaSO}_4\), the energy needed to break the strong crystalline structure far outweighs the hydration energy. This unfavorable net energy change means the system prefers to remain in the solid, undissolved state, preventing water molecules from pulling the ions apart.
The Safety Implications of Insolubility
The insolubility of barium sulfate is the most important factor determining its safety for human use. Highly soluble barium compounds, such as barium chloride, are toxic because ingested \(\text{Ba}^{2+}\) ions are quickly released and absorbed into the bloodstream. These free barium ions interfere with muscle and nerve function, leading to severe poisoning. In contrast, when \(\text{BaSO}_4\) is ingested, its low solubility prevents the toxic \(\text{Ba}^{2+}\) ions from being released in any significant quantity. The compound remains chemically inert and passes safely through the gastrointestinal tract without being absorbed.
This property allows \(\text{BaSO}_4\) to be used safely as a radiocontrast agent in medical imaging, often called a “barium swallow” or “barium meal.” The compound is administered as a fine particle suspension. Because barium is a dense element, it absorbs X-rays strongly, creating a clear outline of the soft tissues of the esophagus, stomach, and intestines on an X-ray image. The safety of this diagnostic tool relies completely on the certainty that the compound will not dissolve and release toxic barium ions.