Acidic Potassium Dichromate Solutions as Ultraviolet Absorbance Standards

摘要

The absorbances of five concentrations of potassium dichromate in 0.001 perchloric acid have been determined at eight wavelengths in theultraviolet on the National Bureau of Standards Institute for Materials Researchhigh-accuracy spectrophotometer. Four of the wavelengths—235, 257, 313, and 350nm—correspond to absorbance maxima or minima in theHCrO spectrum and are useful wavelengths for checkingthe accuracy of the absorbance scale of narrow bandpass spectrophotometers. Althoughpartial dimerization of HCrO toCr O produces small positive deviations fromBeer’s law at these wavelengths, the apparent absorptivities calculated for eachconcentration are reproducible to one part in a thousand. The estimated uncertainties inthe absorptivity values are ± 0.7 percent at 0.1 absorbance ( )and ± 0.2 percent near = 1. These uncertainties include allknown sources of possible systematic error and the 95 percent confidence level for themean. The remaining four wavelengths used for measurement are near two predictedisosbestic points in theHCr0 /Cr O spectra.The absorptivities at 345 nm are sufficiently independent of concentration that thiswavelength can be used for checking absorbance linearity to one part in a thousand overthe range = 0.2–1.