A Question of Chemistry: Why is iodine called an indicator?

A frequently asked chemistry question is, “Why is iodine called an indicator?” This article will give a detailed explanation of it, covering the properties of iodine, the substance iodine is an indicator for, and the scenarios in which iodine can be used as an indicator. Let’s learn together.

Basic properties of iodine

Iodine is a non-metallic chemical element with the chemical symbol I and atomic number 53. In nature, iodine mainly exists in the form of compounds, such as sodium iodide, potassium iodide, etc. Iodine has purple-black crystals and has a high vapor pressure, so it easily sublimates at room temperature. In addition, iodine also has strong oxidizing and reducing properties and can react with many elements and compounds.

Why is iodine called an indicator?

First of all, an indicator is a chemical reagent that can react chemically with the substance being measured, usually by changing its color or properties, thereby indicating the concentration, composition, and other information of the substance being measured. Indicators are commonly used in chemical analysis, biological experiments, environmental monitoring, and other fields. Why is iodine called an indicator? It can show noticeable color changes during certain chemical reactions. This color change can be used to judge the progress or end point of a reaction. In some cases, the color change after iodine reacts with certain substances is so sensitive that the reaction process can be used to detect trace amounts of substances. Therefore, iodine is widely used as an indicator in chemical analysis, biological experiments, and other fields.

What can iodine be used as an indicator for?

Now that you have the answer to the question of 'Why is iodine called an indicator?’, it’s time to find out what substance can iodine be used as an indicator for. The most well-known one is the starch indicator. The iodine-starch test is a famous experiment that you've probably done in school. From the test, you can find that iodine reacts with starch to form a blue-black complex. This color change can be used to detect the presence and concentration of starch. Based on this property, iodine can be used to detect whether banknotes are counterfeit made from paper containing starch. It is also useful in food analysis. Since starch is a major source of carbohydrates in many foods, the starch iodine test is used to determine the quality and quantity of starch in food products.

Iodine can also be used as an indicator in vitamin C determination. One common way to determine the amount of vitamin C in food is to use a redox titration. Compared with an acid-base titration, the redox reaction is better because there are additional acids in the food, but few of them interfere with the oxidation of ascorbic acid by iodine. Iodine is insoluble in water due to its properties. A simple way to solve this is complexing the iodine with iodide to form triiodide. Given that triiodide oxidizes vitamin C to form dehydroascorbic acid, the triiodide is converted to the iodide ion very quickly in the Vitamin C solution. When vitamin C is oxidized completely, iodine and triiodide will be present and react with starch to form a blue-black complex. The blue-black color is the endpoint of the titration.

In addition, iodine can be used as an indicator for metal ions such as copper ions, iron ions, etc. To be specific, the complexes formed by iodine and metal ions usually give obvious color changes. This reaction can be used to detect the presence and concentration of metal ions. For example, when an iodine solution reacts with copper ions, the solution gradually changes from blue to colorless. This color change can be used to determine the titration endpoint of copper ions. In medical diagnosis, iodine can be used as a contrast agent to enhance the effects of imaging examinations such as X-rays and CT to help doctors diagnose diseases. In environmental monitoring, iodine can be used to detect pollutants in water and soil, such as heavy metal ions, organic matter, etc.

Conclusion:

Coming back to the question of “Why is iodine called an indicator?”,  the answer is now clear as crystal. Iodine, due to its propriety which is to give noticeable color changes during certain chemical reactions, can be used as an indicator for starch, certain metal ions, and vitamin C. It has wide applications in the fields of chemistry, the food industry, medicine, and environmental monitoring.