Ammonia Nitrogen Detection in Acidic Solutions: An Ammonium ISE Coupled With pH Precompensation

The method for detecting ammonium ions (NH<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${}_{4}^{+}$ </tex-math></inline-formula>) concentrations with ion-selective electrodes (ISEs) offers the benefits of rapid response and no secondary pollution. By calculating the ratio of NH<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${}_{4}^{+}$ </tex-math></inline-formula> to ammonia (NH3) via pH and temperature parameters, the concentration of ammonia nitrogen (NH3–N) can be further calculated. However, H+ can interfere with the membrane potential of ISEs in acidic solutions, especially at lower NH3–N concentration where the interference becomes more pronounced. We investigated the effect of H+ on ISEs at room temperature and proposed a pH precompensation algorithm to enhance the precision of ammonium ISEs. Additionally, we modified the membrane with 5% polytetrafluoroethylene (PTFE) micro powder to improve the hydrophobicity. Consequently, a real-time NH3–N sensor incorporated with a pH precompensation algorithm has been developed. The sensor, which is cylindrical with a diameter of 150 mm and a height of 95 mm, accurately measures NH3–N concentrations from 0.1 to 100 ppm with a relative error of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\le 10$ </tex-math></inline-formula>%.