HI4101 – Ammonia ISE, Gas Sensing Combination

The HI4101 ammonia ion-selective electrode is a complete potentiometric cell that contains both a silver/silver chloride (Ag/AgCl) reference and a pH measurement element. These elements are housed within a plastic body with a chloride ion-containing electrolyte, and are isolated from the sample by a gas permeable membrane made of polytetrafluoroethylene (PTFE). Ammonia can exist in two different forms: ammonia as a gas (NH3) and ammonium as the ionized form (NH4+). The concentration of each form in a solution is dependent on the solution's pH. The pKa is the pH value at which ammonia and ammonium are in equilibrium. The pKa of the ammonia/ammonium relationship is 9.24; thus, at a pH of 9.24 there will be an equal concentration of ammonia and ammonium. As pH decreases, becoming more acidic, the concentration of ammonium increases and ammonia decreases. The opposite occurs as the pH increases. The actual concentration for either species can be determined by knowing the concentration of ammonia and the pH of the sample. For the HI4101 to measure ammonia accurately, it is important that the pH of the calibration standards and sample be raised with the ammonia ionic strength adjuster (ISA) solution. The ammonia ISA raises the pH of the solution to pH 11. Any ammonium present in the calibration standards and sample will be converted to ammonia, ensuring an accurate measurement. The HI4101 ammonia gas sensing ISE can measure from 1µM (0.02 mg/L) to 1M (17,000 mg/L). Conversion Table for NH3 Multiply moles/L (M) to ppm (mg/L) 1.70 x 104 ppm (mg/L) to M (moles/L) 5.882 x 10-5 Conversion Table for NH3-N Multiply Molarity (moles/L) NH3 to mg/L (ppm) NH3-N 1.40 x 104 Features at-a-glance Replaceable PTFE Membranes Gas sensing ISEs are combination electrodes that detect dissolved gases in a solution. The sensing element is separated from the sample solution by a gas permeable, PTFE membrane that is easy to replace. Durable Delrin® Body The probe body is composed of durable Delrin plastic. BNC Connection The HI4101 has a universal BNC connector for easy connection to any benchtop meter with a female BNC probe input. Theory of Operation A gas sensor works due to the partial pressure of the measured gas in solution. The dissolved gas in the sample diffuses into the membrane and changes the pH in a thin film of unbuffered electrolyte on the surface of the internal pH sensor. Diffusion continues until the partial pressure of the sample and the thin film of electrolyte are the same. The pH change is proportional to the concentration of dissolved gas in the sample solution.