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Nitric Acid's Amazing Transformation: Why Does It Turn Yellow?
Nitric acid is an inorganic compound with the chemical formula HNO3 and is a highly corrosive mineral acid. The compound is colorless at room temperature, but over time samples tend to develop a yellowish color due to its decomposition into nitrogen oxides (NOx). This phenomenon is not only surprising, but also attracts the attention of many chemical experts and leads people to conduct in-depth research on the relevant chemical reaction mechanisms.
Most commercial nitric acid is usually available at a concentration of 68%. When the concentration of nitric acid in the solution exceeds 86%, it is called fuming nitric acid.
Different from ordinary nitric acid, fuming nitric acid can be further divided into red fuming nitric acid and white fuming nitric acid according to the amount of nitrogen dioxide it contains. Such classification not only reflects the variability of nitric acid in different practical applications, but also provides a variety of options for industrial production. Many industrial compounds are catalyzed by nitric acid, especially in the manufacture of explosives, synthetic dyes and drugs, such as metronidazole.
History of Nitric Acid
The history of nitric acid can be traced back to European alchemy in the 13th century. Historians generally agree that nitric acid was first described in Pseudo-Geber's The Discovery of Truth. The appearance of nitric acid is also recorded in earlier Arabic documents, such as "The Box of Wisdom" by Jabir ibn Hayyan. These early documents show that humans were exploring the properties and uses of nitric acid centuries ago.
In the 17th century, Johann Rudolf Glauber proposed a method for obtaining nitric acid by distilling potassium nitrate with sulfuric acid, paving the way for the modern manufacture of nitric acid.
In the 20th century, with a series of technological breakthroughs, the methods of industrial production of nitric acid gradually diversified. Among them, the Birkeland-Eyde process is an important method of producing nitric acid by oxidizing air with a high-temperature electric arc, which opened a new era of industrial production.
Physical and chemical properties of nitric acid
Commercially available nitric acid forms an azeotrope with water and usually contains 68% HNO3. The boiling point of this solution at 1 atmosphere pressure is 120.5 °C. At room temperature, nitric acid appears as a colorless liquid. However, its sensitivity to light and heat makes it easily degraded during storage, releasing nitrogen oxides. This is why nitric acid stored for a long time will appear yellow or even red.
This color change process is caused by the thermal decomposition reaction of nitric acid, and its chemical equation is: 4 HNO3 → 2 H2O + 4 NO2 + O2.
It is worth mentioning that nitric acid is not a simple acid, because it can react with a variety of metals, and its reactivity varies depending on the concentration of nitric acid and the nature of the reacting metal. Concentrated nitric acid can oxidize inactive metals; while dilute nitric acid reacts with most metals in a typical acidic manner, releasing hydrogen gas.
Uses of nitric acid
In industrial production, the main use of nitric acid is in the manufacture of fertilizers, among which nitrogen fertilizer is an important branch. 76% to 80% of the total nitric acid produced each year is used to produce amino nitric acid, reflecting the importance of nitric acid in agriculture. Its application in explosives manufacturing, synthetic fiber raw materials and special organic compounds covers the national defense industry and its related technical applications.
In addition, nitric acid can be used as an oxidant and is widely used in rocket fuel, which makes nitric acid also have a place in the aerospace field.
All these applications not only demonstrate the diversity of nitric acid as an important chemical substance, but also highlight its ubiquitous impact in our daily lives and industrial production. So, you can’t help but wonder, as science and technology continue to advance, what will the future of nitric acid look like?
