Acidic potassium dichromate solutions as ultraviolet absorbance standards

The absorbances of five concentrations of potassium dichromate in 0.001 ]l.f perchloric aCid have been determined at eight wavelengths in the ultraviolet on the National Bureau of Standards Institute for Materials Research high-accuracy spectrophotometer. Four of the wavelengths-235, 257, 313, and 3.50 nm-correspond to absorbance maxima or minima in the HCrO,-spectrum and are useful wavelengths for checking the accuracy of the absorbance scale of narrow bandpass speetrophotometers. Although partial dimerization of nCrO,to Cr207-prod~c.e~ small positive deviations from Beer's law at these wayelengths, the apparent absorptlvltles calculated for each concentration arc reproducible to one part in a thousand. The estimated uncertainties in the absorptivity values are 0.7 percent at 0.1 absorbance (A) and 0.2 percent ncar A = 1. These uncertainties include all known sources of possible systematic error and the 95 percent confidence level for the mean. The remaining four wavelengths used for measurement arc ncar two predicted isosbestie points in the HCrO,~/Cr207-sp~ctra . The absorptivities at 345 nm arc sufficiently independent of con-centratIOn that thiS wavelength can be used for checking absorbance linearity to one part in a thousand over the range A = 0.2-1.