What Is Potassium Dichromate?

Did you know another name for potassium dichromate is hexavalent chromium? In this lesson, we will learn why potassium dichromate has a second name, understand its formula, and discover where it is used.

What Is Potassium Dichromate?

It’s well known that the chemical element potassium is very important for a wide variety of biological functions in the human body. However, the chemical compound potassium dichromate is quite the opposite – it is very harmful to our health. Potassium dichromate is a chemical compound typically used as an inorganic chemical reagent for industrial and laboratory processes. As we will see, this chemical is used to produce an array of products ranging from shoe polishes and glues to paints and waxes.

Here you can see potassium dichromate in its solid form. Looking at the image, can you guess a physical property of this chemical compound? Yes, that’s correct! The reddish-orange color is a physical property of potassium dichromate. Known to be odorless, its boiling point is 500 degrees Celsius. Potassium dichromate has a melting point of 398 degrees Celsius and molecular weight of 294.18 grams per mole.

Stable under normal temperature and pressure, potassium dichromate is in no way friends with chemicals that are reducing agents. This is due to the fact that potassium dichromate is a strong oxidizer. A strong oxidizer is a compound that can increase the combustible nature of other materials using oxygen. Hence, potassium dichromate must stay far away from these reducing agents and other incompatible materials. Otherwise, you may have one explosive situation to deal with.

Molecular Structure and Formula

The molecular formula for potassium dichromate is K2Cr2O7. Be careful not to confuse potassium dichromate with its cousin potassium chromate, which has the molecular formula K2CrO4. Here is the molecular structure of potassium dichromate:

Did you spot the potassium and dichromate ions in the structure? Surely you did, but as a quick review, an ion is a molecule (or atom) that carries a negative or positive charge. Unlike some compounds, potassium dichromate is a polyatomic ion. This means it’s made from two or more molecules (or atoms) that carry a charge. Potassium carries a positive charge while dichromate has a negative charge.

Occurrence: Potassium dichromate occurs naturally in mineral form as lopezite, a very rare mineral.

Preparation: Potassium dichromate is produced industrially by reacting potassium chloride (KCl) with sodium dichromate (Na₂Cr₂O₇). It is also obtained from its related compound, potassium chromate (K₂CrO₄), which reacts with acids to give the dichromate salt.

Na₂Cr₂O₇ + 2 KCl → K₂Cr₂O₇ + 2 NaCl

Physical properties : It is a bright red-orange crystalline solid with a density of 2.676 g/mL, melting point of 398 °C and boiling point of 500 °C, when it decomposes. It is odorless and highly water soluble.

Chemical properties: Potassium dichromate readily ionizes in water to give chromate (CrO₄^2-) and dichromate (Cr₂O₇^2-) ions in equilibrium. It is a moderate oxidizing agent widely used in organic chemistry. It is a stable solid under normal conditions, but it decomposes upon heating to give potassium chromate (K₂CrO₄) and chromic anhydride (CrO₃) with the evolution of oxygen.

4K₂Cr₂O₇ → 4K₂CrO₄ + 2Cr₂O₃ + 3O₂

It reacts reversibly with bases such as potash (K₂CO₃) to give a yellow solution of chromate salts.

K₂Cr₂O₇ + K₂CO₃ → 2 K₂CrO₄ + CO₂

It reacts with cold dilute acids to give chromic anhydride, and with concentrated acids, it gives chromate salts and oxygen.

Uses: Potassium dichromate is used for preparing strong cleaning solutions for glassware and for etching materials. It also has uses in leather tanning, photographic processing, cement, and wood staining. It is used as an oxidizing agent in many applications, and is also used to prepare various products such as waxes, paints, glues, etc.

Health effects/safety hazards: As a hexavalent chromium compound, potassium dichromate is carcinogenic and highly toxic. It is also very corrosive and skin/eye contact can cause severe irritation and burning sensation, and even lead to blindness. It is also known to affect reproductive health and acts as a mutagenic agent (affects the genetic material and harms unborn children)

SOURCE: https://www.softschools.com/formulas/chemistry/potassium_dichromate_uses_properties_structure_formula/279/