In iontophoresis, ions with a positive charge are introduced into the tissues with the positive pole of the direct current. Which of the following ions has a positive charge?

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

Answer: (B)

In iontophoresis, charged particles are moved into tissue by the electrical field, with positively charged ions (cations) being repelled from the positive electrode and driven toward the negative electrode. At physiological pH, histamine carries a positive charge because its amine group is protonated, giving it a net positive charge. Iodine in solution is typically present as iodide, which is negatively charged. Salicylic acid behaves as an acid and is deprotonated to form a negatively charged salicylate in body fluids. Brass is an alloy and does not represent a singly charged drug ion. Therefore, histamine is the positively charged ion.