Iodine volts will turn black when they meet mercury. The main component of iodine volts is complexed iodine, and the active iodine will form mercury iodide when it meets mercury, which is a toxic substance. Mercury is rarely used in clinical practice, unless the patient has a superficial wound or a small wound that may be coated with mercury. Iodine volts are not irritating and are widely used. They have a broad spectrum of bactericidal activity and can destroy bacteria, fungi, and spores, and do not cause hyperpigmentation.