In iontophoresis, positive ions are driven into tissues by the positive electrode; which of the following is positively charged?

• A. Iodine
• B. Histamine
• C. Salicylic acid
• D. Brass

Answer: (B)

The main idea here is that only charged particles can be moved by iontophoresis, and a positively charged molecule (a cation) will carry a positive charge at physiological pH due to protonation.

Histamine fits this because its amine group is protonated at body pH, giving the molecule an overall positive charge. Iodine is typically present as iodide (I−), a negative ion. Salicylic acid is a weak acid and is largely deprotonated to salicylate (negative) at physiological pH. Brass is a metal alloy and doesn’t exist as a positively charged organic ion in this context. So histamine is the positively charged option.