Self-test 4.3

1. Absorbance of the sample solution is 0.153AU. The three main uncertainty sources of absorbance in photometric analysis are repeatability, drift of the photometer and uncertainty due to possible interfering effects. The respective uncertainty components were estimated by the analyst as follows: Repeatability standard deviation is 0.004AU, uncertainty due to possible drift is ± 0.001AU and uncertainty due to the possible interfering effects is ± 2% of the absorbance value.

Please calculate the combined standard uncertainty of absorbance. Please provide 4 decimal places after the decimal point.

2. Please calculate the standard uncertainty leaving out the smallest uncertainty component. Please provide 4 decimal places after the decimal point.

3. Is it justified in this case to leave the smallest uncertainty component out (please answer yes or no)?

4. When to use standard deviation of a single measurement and when to use standard deviation of the mean for quantifying repeatability?
During calibration of a pipette the analyst has determined repeatability of pipetting with a 10 ml bulb pipette from 12 repeated pipetting operations and found that the standard deviation (of a single pipetting) was 0.0072 ml. Now she used the same pipette for an analysis and when estimating the uncertainty of the analysis she needs to evaluate the standard uncertainty component of pipetting repeatability. What would be the suitable value of repeatability standard uncertainty of pipetting in this case?

5. When to use standard deviation of a single measurement and when to use standard deviation of the mean for quantifying repeatability?
During photometric measurement of nitrite the analyst determines the absorbance value as the mean from 5 independent measurements of different sample solution portions. The obtained average absorbance value was 0.3264 AU and standard deviation (of single measurement) was 0.0036 AU. The analyst needs a standard uncertainty estimate of repeatability of absorbance measurement. What would be the suitable estimate of repeatability standard uncertainty of photometric measurement? Please give the answer with 3 significant digits.