Ashikhmina Tamara Ya. RussiaInstitute of Biology, Komi Scientific Center, Ural Branch of the Russian Academy of Sciences, st. Kommunisticheskaya, 28, Syktyvkar, Rep. Komi, 167000usr08619@vyatsu.ru Author's articles Improvement of Potentiometric Determination of Ammonium Ions in Surface Waters for the Samples with a High Concentration The prospect of using Shewhart control charts in assessing the quality of ionometric measurements by the method of adding a sample to a standar