Sodium chromate (\(\text{Na}_2\text{CrO}_4\)) is an inorganic compound that appears as a yellow, crystalline solid at room temperature. It is composed of two sodium atoms, one chromium atom, and four oxygen atoms. Sodium chromate is highly soluble in water.
Solubility Confirmation and Ionic Dissociation
When the solid is introduced to water, it readily dissolves. This dissolution involves ionic dissociation, where the compound separates into its charged particles. Sodium chromate breaks apart into two positively charged sodium cations (\(\text{Na}^+\)) and one negatively charged chromate anion (\(\text{CrO}_4^{2-}\)).
The visual result of this process is the disappearance of the solid crystals as they mix completely with the water. The resulting solution takes on a vivid yellow color, which is characteristic of the chromate ion (\(\text{CrO}_4^{2-}\)). The solubility is high, with approximately 84.5 grams dissolving in 100 milliliters of water at 25 degrees Celsius.
The Governing Principles of Solubility
The high solubility of sodium chromate is predictable based on established chemical principles for ionic compounds. Water is a polar solvent, meaning its molecules have slight positive and negative ends. This polarity makes water effective at dissolving ionic salts, as the attraction between water molecules and the charged ions overcomes the forces holding the crystal lattice together.
The strongest indicator of solubility comes from general solubility rules. These rules state that all compounds containing alkali metal ions, such as sodium (\(\text{Na}^+\)), are soluble. Although chromate compounds (\(\text{CrO}_4^{2-}\)) are often insoluble when paired with other cations, the presence of the sodium ion overrides this tendency.
Key Physical Characteristics and Applications
Sodium chromate is an odorless, yellow crystalline solid. It is hygroscopic, meaning the anhydrous form readily absorbs moisture from the air, often forming various hydrated forms. The bright yellow color is a physical manifestation of the chromate ion, which persists after the compound dissolves in water.
In industry, sodium chromate has several applications due to its chemical properties. It is frequently used as a corrosion inhibitor, particularly in the petroleum industry, to protect metal components from degradation. The compound also serves as a dyeing auxiliary in the textile industry. Furthermore, it is utilized in the formulation of pigments for paints and inks, and plays a central role as an intermediate in the extraction of chromium from its ores.
Safe Handling of Chromate Compounds
While sodium chromate is highly water-soluble, the resulting solution contains hexavalent chromium, abbreviated as \(\text{Cr}(\text{VI})\). This form of chromium is highly toxic and is classified as a known human carcinogen, specifically linked to lung cancer. Exposure to \(\text{Cr}(\text{VI})\) can cause severe irritation and burns to the skin and eyes, and is harmful if inhaled or swallowed.
Due to these health hazards, sodium chromate must be handled with extreme caution and proper protective equipment. Regulatory bodies, such as the Occupational Safety and Health Administration (OSHA), have established strict permissible exposure limits for hexavalent chromium compounds. Proper disposal must follow local hazardous waste regulations, often requiring the reduction of the toxic hexavalent chromium to the less hazardous trivalent form before final containerization.