Abstract:
A technique for estimation of measurement uncertainty of routine pH measurement using pH
meters; Thermo scientific Orion Star and Basic 20 is presented. There are issues associated with
pH measurements, what we actually measure is not usually what we would expect or even intend,
and it is common practice that analysts just take the immediate reading without doing repetitions.
The pH measurements made from one chemical laboratory are not consistent with those made in
a different laboratory and, the pH meter readings generally do not agree closely with the calculated
pH values. The result of first approach, that is single experiment (3 repetitions), uncertainty
evaluation was done according to the guide to Quantifying Uncertainty in Analytical Measurement
(QUAM) and/or the guide to the expression of uncertainty in measurement (GUM) using
uncertainty budget table as a tool. On second approach, we established correspondence between
observed and predicted uncertainties with new derived equations to calculate the total uncertainty.
At acidic pH -2.08 uncertainty was ± 0.02 and basic pH 13.3, the uncertainty was ± 0.01, the
results of calculations similar on both approaches. Results of single experiment over a short period
of time confirm that individual uncertainty values at particular pH values correspond to the
manufacturer’s specification, but not to the expected pH values. Repeatability conditions and
pooled calibration method were used for further assessment. Repeated analysis under similar
measurement conditions and experimental detail were performed to measure pH of numerous
buffers and sample solutions, and then, pooled calibration, the basic statistical calculations, the
Horwitz equation, coefficient of variation (CV%) and the law of propagation of uncertainty (LPU)
in a spreadsheet model were used for the analysis of uncertainty. Pooled calibration created a
satisfactory correspondence between predicted pH values and those observed by experiment. The
Horwitz equation constituted an expert judgment on the performance of the meters, it indicates
poor performance at pH value -2.08. The results correspond to the CV% of [𝐻
+] and CV% of pH
values as well. HorRat ratio showed significant difference between the coefficients of variation at
pH -2.08. At pH 13.3, there was no significant difference between the coefficients of variation
and variances are homogenous.
