Modern methods of quantitative physical and chemical measurements should necessarily include procedures for quality control of research results. Recently, the procedure for quality control of measurements using control charts is gaining particular popularity in the practice of analytical measurements. To date, the methods of potentiometric studies operating in the territory of the Russian Federation, in most cases, do not include quality control using this control procedure. However, despite the complexity of the implementation and interpretation of its results, it is the use of control charts that allows you to control the quality of ionometric measurements. This is especially important for potentiometric determinations based on the addition method, since the instability of the slope of the electrode function and its deviation significantly contribute to the uncertainty of the measurement results. This article presents the main results of evaluating the use of the Shewhart control chart method for monitoring the repeatability and accuracy of measurement results. The measurement results have been obtained using an improved method of quantitative potentiometric determination of high concentrations (more than 1 gm/cdm) of ammonium ions by the method of adding a sample to the standard. In addition, an experimental substantiation of the fact that the value of the slope of the electrode function significantly affects the accuracy of measurements by the addition method has been presented. It has been revealed that when the angular coefficient of the electrode function is more than 62 mV/pNH4 + at (25±5)°C, deviations of the measurement results from the control standard by more than 50% are observed. It has been found that at (25±5)°С the most accurate results of quantitative determination are achieved when the angular coefficient of the electrode function is from 54 mV/pNH4 + to 62 mV/pNH4 + inclusive.