Starchiodine

Starchiodine refers to the blue‑black complex that forms when starch, particularly its amylose component, interacts with iodine. The term describes the visible coloration and the associated association between iodine molecules and the starch helix, which is widely used in qualitative starch detection. The starch–iodine complex is the basis of the classical starch‑iodine test.

Chemistry and structure: In aqueous iodine solutions, such as Lugol’s solution containing iodide, iodine forms polyiodide chains that can insert into the helical structure of amylose. The resulting starch–iodine complex gives a deep blue to blue‑black color due to charge‑transfer interactions along the polyiodide chain inside the helix. Color intensity depends on starch concentration, iodine concentration, temperature, and pH. The complex is relatively stable under normal conditions but can dissociate with heat or reducing agents; binding is reversible.

Preparation and measurement: To perform a starchodine test, a small amount of sample is treated with iodine solution. A blue‑black coloration indicates the presence of starch. For quantitative purposes, the absorbance of the released or extracted complex can be measured spectrophotometrically, typically with a peak around 620 nanometers, after standardized sample preparation.

Applications and limitations: The starchodine complex is widely used in teaching laboratories, food quality control, and enzymology to monitor starch presence and hydrolysis (e.g., amylase assays). Limitations include interference from other polysaccharides and sensitivity to pH and temperature. Proper handling of iodine solutions and disposal of iodine waste are advised.