Who am I? Under regular conditions I am a dark-purple/dark-brown solid. I am primarily used in medicines to clean and disinfect wounds, photography, and in dyes. Table salt is often fortified with me. I am rare in the solar system and Earth's crust, however, compounds with me in them are very soluble in water and I am concentrated in seawater. I am necessary for the proper functioning of the thyroid gland in the human body.
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Include the element's chemical symbol in your answer.
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