ammonium (NH4+), an ion formed by the reaction of ammonia (NH3) with a hydrogen ion (H+). The ammonium ion is highly soluble in water but does not pass easily through cell membranes, as does the ammonia molecule, and its rate of excretion is influenced in part by the acidity of urine. The lower the pH, the greater the proportion of ammonium ions present, assuming a constant level of ammonia production from amino acid metabolism.
