Answer:
Rhodanine is used to demonstrate the presence of (a) ferric iron.
Step-by-step explanation:
Rhodanine is a chemical compound that reacts with iron (III) ions to form a colored complex. This complex has a distinctive red-orange color and can be used to detect the presence of ferric iron in a sample. This reaction is often used in analytical chemistry and biochemistry to detect the presence of iron in various samples, including biological fluids, soils, and minerals.
Copper, calcium, and urate crystals do not react with rhodanine to form a colored complex.