Sodium dichromate

Sodium dichromate is a vital inorganic compound with the chemical formula Na2Cr2O7. It is a salt used extensively in various industrial processes. In fact, almost all chromium ore is converted into sodium dichromate first. Most materials and compounds based on chromium are prepared from this specific salt. It is often handled as a dihydrate, which is written as Na2Cr2O7·2H2O. This means the salt contains two molecules of water for every unit of sodium dichromate.

The production of this salt involves a precise, multi-step chemical process. It begins with ores that contain chromium(III) oxides. Workers fuse this ore with a base, such as sodium carbonate. This fusion happens at temperatures around 1000 °C in the presence of air. The air provides the oxygen needed for the reaction. This heat allows the chromium to become soluble so it can be extracted with hot water. During this stage, other components like iron and aluminum compounds remain poorly soluble.

After extraction, the aqueous extract must undergo acidification to create the dichromate. This is done by adding either sulfuric acid or carbon dioxide. The resulting dichromate is then isolated through a process called crystallization. This method allows factories to produce many millions of kilograms of sodium dichromate every year. The salt can exist in several different hydrate forms depending on the temperature. For instance, the decahydrate form exists below 19.5 °C. Other versions include the hexa-, tetra-, and dihydrate forms. If the temperature rises above 62 °C, the salts lose their water spontaneously. This leaves behind the anhydrous material, which contains no water.

Sodium dichromate is highly valued because of its specific chemical properties. It is an oxidizing agent, which means it can cause chemical changes in other substances. It is very similar to potassium dichromate in its appearance and reactivity. However, sodium dichromate has a major advantage in solubility. It is about twenty times more soluble in water than the potassium version. At 0 °C, its solubility is measured at 49 g/L. It also dissolves well in polar solvents like acetic acid. Its lower equivalent weight is also often desirable for industrial use.

In organic synthesis, this compound is used to transform various molecules. It can oxidize benzylic and allylic C-H bonds into carbonyl derivatives. For example, it can oxidize 2,4,6-trinitrotoluene into a corresponding carboxylic acid. It can also turn 2,3-dimethylnaphthalene into 2,3-naphthalenedicarboxylic acid. In other reactions, it turns secondary alcohols into ketones. A common example is the conversion of menthol into menthone. It can also turn dihydrocholesterol into cholestanone.

Beyond laboratory synthesis, sodium dichromate has many practical applications. It is used in the tanning of leather, though it must first be reduced with sulfur dioxide. It is also used for hexavalent chrome plating. To do this, the chromate is converted into chromic acid using sulfuric acid. This chromic acid is essentially chromium trioxide. Additionally, the salt can be used to convert fluorene into fluorenone. These diverse uses make it a cornerstone of chromium chemistry.

Because sodium dichromate contains chromium(VI), it must be handled with extreme caution. This form of chromium is toxic and is classified as a carcinogen. The compound is also corrosive and can cause severe eye damage or blindness. Human exposure can lead to impaired fertility and heritable genetic damage. It can also cause harm to unborn children. Because of these risks, factories must follow very stringent regulations. They must treat effluent, or liquid waste, from refineries. They use reducing agents to change chromium(VI) back into chromium(III). This process makes the waste much less threatening to the environment.