High-frequency infrared carbon and sulfur analyzer analysis of the introduction of perchloric acid - Database & Sql Blog Articles

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High-frequency infrared carbon and sulfur analyzer

The high-frequency infrared carbon and sulfur analyzer is a crucial instrument used to measure the content of carbon and sulfur in steel. In some phase analysis, such as SiC, it is necessary to remove free carbon by using sulfuric acid fumes before measuring the silicon carbide content. These fumes are not pure acid vapors but rather fine mist droplets formed from acid vapor and water vapor.

HCl

with

H₂O

Azeotrope compound, boils at 110°C under normal pressure, which is hydrochloric acid.

HNO₃

with

H₂O

Azeotrope compound, boils at 122°C under normal pressure, known as nitric acid fumes. Sometimes, the fumes appear yellow due to the decomposition of HNO₃ into NO₂.

HCl

and

HNO₃

produce fumes that are easier to handle because of their lower temperature. However, during the dissolution of insoluble samples, the main fumes produced are from HClO₄, H₂SO₄, and H₃PO₄. These fumes are white and occur at higher temperatures, making them more challenging to manage. The following section introduces perchloric acid in detail.

When evaporating HClO₄ solution, water is first evaporated, followed by dilute acid. At 476K, the azeotrope of water and perchloric acid can be evaporated. This azeotrope contains 72% HClO₄. During evaporation, HClO₄ may decompose, as shown in the following reactions:

4HClO₄ = 2Cl₂ + 7O₂ + 2H₂O

2HClO₄ = Cl₂ + 3O₂ + H₂O₂

Commercially available HClO₄ typically has a concentration between 70%-72%. When evaporating, the concentration is usually lower than 72%, and the initial evaporation removes water, followed by the formation of the 72% azeotrope. In the pre-furnace analysis of alloy steel, the observed perchloric acid fumes occur at 413K. As the temperature rises, the bottle fills with white smoke until it reaches 476K, where the temperature remains constant. At this point, the bottle is smokeless, and only the mouth emits smoke. This is the standard for identifying the presence of perchloric acid fumes.

In pre-furnace analysis, when using perchloric acid oxidation to determine high-chromium steel, a dissolved acid mixture (H₂O: HCl: HClO₄ = 4:1:20) is used. After low-temperature dissolution, heating continues until smoke appears on the liquid surface, then the bottle fills with white smoke, and finally, only the mouth emits smoke at 476K. This temperature remains constant, and the process is maintained for 30–40 seconds. Sulfuric acid is added to adjust acidity, and the sample is titrated with ammonium ferrous sulfate in the presence of a chromium indicator. Some procedures suggest that the fumes should be at 573–673K, which is incorrect. Perchloric acid forms an azeotrope at 203°C, and the temperature remains constant. According to physical chemistry principles, the final evaporation temperature of HClO₄ and H₂O is 476K, where the concentration of HClO₄ remains at 72% and the temperature does not change until the acid is fully consumed.

Pre-furnace analysis with thiocyanate - When using the ascorbic acid colorimetric method to determine molybdenum in low, medium, and high-alloy steels, the sample is dissolved in perchloric acid. When HClO₄ is evaporated, the smoke rises to the mouth of the bottle, and only the mouth emits smoke. At this point, the temperature is also 476K. Ammonium persulfate is used in the determination of nickel by Ding Erqi colorimetry. The sample is dissolved in HCl and H₂O, and HClO₄ is evaporated to remove interference ions. Then, potassium tartrate is added, and in an alkaline solution, nickel reacts with butyl hydrazine to produce a wine-red solution for colorimetric determination. The requirements for evaporating HClO₄ and the temperature criteria remain the same—only the mouth of the bottle emits smoke at 476K.

High-frequency infrared carbon and sulfur analyzers are widely used in metallurgical industries and play a vital role in improving the quality of steel products. Their accurate measurement capabilities ensure reliable results, supporting the production of high-performance materials.