Acidic potassium dichromate (PDC) solutions are commonly used as ultraviolet measurement standards for photometric accuracy and linearity testing of absorption spectrophotometers. Derived from NIST SRM 935a crystalline potassium dichromate, weighed quantities of SRM 935a are dissolved and acidified to 0.001 normal aqueous perchloric acid and then sealed in fused-silica cuvettes of optical quality. In addition to the different absorbance levels of prepared PDC solutions, a dilute solution of perchloric acid in water (PAW) is sometimes also prepared and similarly sealed in a fused-silica cuvette. The mode of metrological certification of the PDC solutions, which depends on how the PAW solution/cuvette filter is measured, determines how the certified PDC measurement results are reported. Our preliminary studies have revealed that a measurement bias may occur depending on how the PAW solution/cuvette filter is measured and how the PDC values are then corrected for the PAW value. If the PAW measurement value is not properly accounted for in the certified measurement results, interpretation of the outcomes of the performance status of a spectrophotometer may be flawed when PDC solutions are used in qualification testing: (i) the spectrophotometer may be qualified when in fact it should not be, or (ii) the spectrophotometer may be disqualified when in fact it should be qualified. This presentation will feature a discussion of the unexpected results of our investigative studies of this insidious measurement bias. Metrological insight and guidance will be described on how to properly measure PAW solution/cuvette filters and also how to report PDC measurement results. Transparency of the certification process is essential to allow proper laboratory use of a PDC solution calibration measurement standard which includes a PAW filter.

Metrological evaluation of a potential certification bias for potassium dichromate solution measurement standards

249th National Meeting (2015)

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