Potassium Dichromate - Sulfuric Acid vs. Perchloric Acid

Potassium Dichromate (PD) solutions have been used for decades in the spectroscopy field.  The purpose of PD filters is for the validation of the accuracy of UV-VIS spectrophotometers for measuring photometric absorbance and validating the photometric linearity. 

In regards to the PD, there are two core scientific publications:

https://www.nist.gov/document-10503

and

"Standards and Best Practice in Absorption Spectrometry," UVSG. ISBN 0-632-05313-5, Blackwell Science. 

As it was established in scientific literature decades ago, the performance of potassium dichromate in sulfuric acid vs. perchloric acid does not make any practical difference from the user end. When using PD in a perchloric acid solution rather than using the EP maximum tolerance table for the PD in sulfuric acid, users need to apply NIST SRM 935a guidelines, which is in-line with EP provisions.

It is essential to highlight the following observations:

Per NIST (www.nist.gov), it had been observed that unpredictable and uncontrolled chemical reactions of PD in sulfuric acid were mixed chromium (VI)-sulphate complexes were formed which significantly change the optical performance of Potassium Dichromate. Therefore, it is highly recommended to use PD in perchloric acid instead.  While European Pharmacopoeia still refers to PD in sulfuric acid; they allow using PD in perchloric acid.  US Pharmacopoeia (USP chapter 857) allows using PD in perchloric acid and PD in sulfuric acid as well

Taking into consideration a significant disadvantage of PD in sulfuric acid vs. PD in perchloric acid, we strongly suggest using Potassium Dichromate in perchloric acid only.  PD is a dangerous toxic material. Therefore we strongly recommend using a PD solution only in a hermetically, permanently sealed cuvette.  PD solutions must be recalibrated at least once a year

While PhEuro uses PD solutions only for photometric absorbance validation of spectrophotometers, USP857 uses PD solutions for both - photometric absorbance and photometric linearity of UV-VIS spectrophotometers'.

The EP document describing PD is available from the link below: http://www.drugfuture.com/Pharmacopoeia/EP7/DATA/20225E.PDF.

While NIST935a requires the PD to be diluted in perchloric acid, the EP document provides all PD data in the table for PD in sulfuric acid. Therefore, users cannot apply "Maximum tolerances" PD data from the EP  table for a PD with perchloric acid.

The above EP document allows using another suitable solution as a substitute for PD in sulfuric acid, and the PD in perchloric acid is definitely a  suitable solution as it is endorsed by NIST.  

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