Determination of ammonium with the ion-selective electrode

Although the known photometric methods for the determination of ammonia are accurate, they require a considerable amount of time (Nessler method 30 min, indophenol method 90 min reaction time). A further disadvantage of these methods is that only clear solutions can be processed. Cloudy solutions must first be clarified by time-consuming procedures. These problems do not exist with the ion-selective ammonia electrode. Measurements can be easily performed in drinking water, groundwater, and surface water as well as in wastewater, soil extracts, and Kjeldahl digestion solutions (without distillation). The determination of ammonia in ammonia salts, of the nitric acid content in nitrate,s and of the nitrogen content of organic compounds is based on the principle that the ammonia ion is released as ammonia gas upon addition of excess sodium hydroxide:

NH4+ + OH → NH3 + H2O

The outer membrane of the electrode allows the ammonia to diffuse through it. The change in the pH value of the inner electrolyte solution is monitored by a combined glass electrode. If the substance to be measured is not present in the form of an ammonia salt, it must first be converted into one. Organic nitrogen compounds, especially amino compounds, are digested according to Kjeldahl by heating with concentrated sulfuric acid. The carbon is oxidized to carbon dioxide in the process while the organic nitrogen is transformed quantitatively into ammonium sulfate. Numerous examples are given. In the appendix various sample preparation methods and an example of a Kjeldahl digestion are described.