Uncertainty of measurement of ionic calcium in milk measured by ISE: a Monte-Carlo insight.

Practically almost all technological processes that affect the milk influence the content of ionic calcium present in it. The most popular method to determine ionic calcium is direct potentiometry using ion-selective electrodes (ISEs). This study proposes an approach to evaluate the measurement uncertainty for the determination of ionic calcium by the ISE method based on JCGM 100 - evaluation of measurement data - guide to the expression of uncertainty in measurement (GUM) with the simulation of the total uncertainty by the Monte Carlo method. The uncertainty of reading and uncertainty of regression coefficients were chosen as the starting input values. Other sources of uncertainty, such as the preparation of calibrators and the incomplete description of the measurand, do not make a significant contribution to the total uncertainty. The type of distribution that describes the results in the best way is the log-normal one. However, deviations from normality are small enough to make the possible use of the coverage factor = 2. It is established that in realistic operating conditions the expanded uncertainty is about 20% ( = 2) and increases sharply simultaneously with increasing uncertainty of reading.