Estimation of measurement uncertainty in determination of milk acidity using the method of phenolphthalein indicator

The analysis of titrimetric methods for determining milk acidity used to assess the quality of milk and dairy products shows that they do not specify the procedure for estimating the measurement uncertainty. GOST ISO / IEC 17025-2019 requires that the measurement uncertainty be specified in test reports. In this paper, the uncertainty calculation method described in the Guide to the Expression of Uncertainty in Measurement (GUM) is used to estimate the measurement uncertainty. Based on the step-by-step analysis of the procedure for measuring milk acidity using the phenolphthalein indicator, the most significant sources of uncertainty in the measurements were identified: calibration of pipettes, burettes, temperature and repeatability. A mathematical model of the measurement was developed. The values of the input quantities (the influence of uncertainty sources) were estimated and distribution laws were assigned to the input quantities. Threefold determination of milk acidity was carried out. The values of the relative standard uncertainties caused by the influence of the input quantities were calculated. The total standard uncertainty of the measurement by this method was 0,1077 %. The expanded uncertainty is calculated at confidence level of 0,95 and coverage factor of two. A procedure for estimating the measurement uncertainty by this method is described.