Barium sulfate (BaSO\(_{4}\)) is a white, odorless crystalline solid that occurs naturally as the mineral barite. Barium sulfate is classified as virtually insoluble in water. This extreme lack of solubility dictates its unique role in science and medicine.
The Chemistry of Barium Sulfate’s Low Solubility
Barium sulfate is highly insoluble; only a minute amount dissolves in water. This is supported by its extremely small solubility product constant (\(K_{sp}\)), which is approximately \(1.1 \times 10^{-10}\) at \(25^{\circ}\text{C}\). The \(K_{sp}\) value reflects the equilibrium between the solid compound and its dissolved ions, where a smaller number indicates lower solubility. At room temperature, only about 2.4 milligrams of barium sulfate dissolve in a liter of water.
The physical reason for this insolubility lies in the balance between two opposing energy forces: lattice energy and hydration energy. Lattice energy is the energy required to break the strong electrostatic forces holding the barium ions (\(\text{Ba}^{2+}\)) and sulfate ions (\(\text{SO}_{4}^{2-}\)) together in the crystal structure. Hydration energy is the energy released when water molecules surround and stabilize the separated ions.
For a salt to dissolve readily, the hydration energy must be greater than the lattice energy. In barium sulfate, the lattice energy holding the ions together is significantly higher than the hydration energy released when the ions interact with water. This imbalance means water cannot supply enough energy to overcome the forces holding the crystal together, resulting in the compound remaining undissolved.
The Role of Insolubility in Medical Imaging
The insolubility of barium sulfate is responsible for its use as a radiocontrast agent in medical imaging, such as a “barium swallow” or barium enema. The procedure makes the soft tissues of the gastrointestinal (GI) tract—the esophagus, stomach, and intestines—visible on an X-ray. Soft tissues do not block X-rays well, appearing as indistinct gray shadows on a standard radiograph.
Barium, an element with a high atomic number of 56, is an excellent blocker of X-ray radiation. When a patient drinks a suspension of barium sulfate, the compound coats the inner lining of the GI tract. This coating creates a stark, white silhouette on the X-ray image, allowing doctors to visualize the shape, contour, and function of the digestive organs.
Because the barium sulfate compound is virtually insoluble, it passes through the entire digestive system without being absorbed into the bloodstream. This ensures the contrast agent blocks X-rays locally within the GI tract. The compound is ultimately excreted from the body unchanged, allowing for safe and effective imaging.
Why Barium Sulfate is Safe Despite Barium’s Toxicity
The safety of ingesting barium sulfate is a consequence of its insolubility, which is crucial because the barium ion (\(\text{Ba}^{2+}\)) itself is highly toxic. Most other soluble barium salts, such as barium chloride or barium carbonate, are potent poisons. Toxicity occurs when free, dissolved barium ions (\(\text{Ba}^{2+}\)) are absorbed from the digestive tract into the systemic circulation.
Once absorbed, the \(\text{Ba}^{2+}\) ion interferes with cellular functions by acting as a competitive blocker of potassium channels. This causes a rapid shift of potassium from the bloodstream into the cells, leading to a dangerous reduction in blood potassium levels, known as hypokalemia. Severe hypokalemia can result in muscle weakness, flaccid paralysis, and cardiac arrhythmias.
Barium sulfate avoids systemic poisoning because its chemical structure is stable, preventing the release of sufficient quantities of the toxic \(\text{Ba}^{2+}\) ion. Its low solubility means the compound remains intact as it travels through the acidic environment of the stomach and the rest of the gastrointestinal tract. Since the toxic ion is not released in an absorbable form, the compound passes through the body without causing systemic effects.