Potassium Dichromate solutions in Sulphuric and Perchloric Acid

The latest revision(s) of the European Pharmacopoeia ( Supplement 2000 and now European Pharmacopoeia 4) has revised the dichromate check, in relation to earlier versions. This allows a tolerance band of 57 - 63 mg, with an associated A 1%1cm calculated band - also revised in EP 4.

The text detailed below is taken directly from a recently revised publication, originally published in 1981 [1].

" P55. The NBS points out that 0.005 M H2SO4 has two disadvantages compared with perchloric acid at pH = 3: sulphuric acid has a greater ionic strength and hence greater salt effects; and there is the possibility of forming mixed chromium (VI)-sulphate complexes.".

As you will see from the above text, sulphuric acid has two disadvantages when compared to perchloric acid. These disadvantages have been confirmed not only by ourselves in the 30 years we've been working with dichromate solutions, but also by other manufacturers. From personal experience, countless laboratories also experience the problems associated with the use of these sulphuric acid based dichromate solutions, not least in the actual preparation of the solution as detailed in the E.P.

The main two advantages of sulphuric acid, availability and safer handling are cancelled out by using sealed reference materials. In addition, certified values on 'sealed cell' reference can be used to qualify the performance of the spectrophotometer in isolation of the laboratory; something which isn't possible when preparing the solutions in-situ?

Tabulated below is the direct comparison using the published calculated molar extinction coefficients in the E.P. and NIST Special Publication 260-54.

As you can see the Absorbance differences are relatively small, and of the same order of magnitude one would expect when measuring the cells on a day to day basis, and still fall within the allowed tolerances if constant weight is assumed.

Calculated difference between 60.06 mg Potassium Dichromate in 0.005 Sulphuric Acid vs 60.06 mg Potassium Dichromate in 0.001M Perchloric Acid

Wavelength Absorbance E.P. Maximum Tolerance (60 mg) Absorbance calc using NIST A1%1cm values Difference
235 nm 0.748 0.7374 - 0.7572 0.742 -0.006
257 nm 0.865 0.8544 - 0.8742 0.870 0.005
313 nm 0.292 0.2820 - 0.3018 0.290 -0.002
350 nm 0.640 0.6294 - 0.6492 0.643 0.003

The key fact is that you are providing 'evidence of control' with a dichromate solution, at European Pharmacopoeia strength i.e. 60 mg/l, where the traceability path to the NIST primary SRM (SRM 935a) is well established by:

· Using SRM 935a to prepare the solutions as directed in NIST Special Publication 260-54.
· Certified values are obtained on a reference spectrophotometer qualified using a range of primary references, and where the calibration process has been accredited by the United Kingdom Accreditation Service (UKAS) to ISO/IEC 17025.

I trust you find this information useful. If you have any other queries, please do not hesitate in contacting me.

John Hammond C.Chem MRSC
Technical & Marketing Manager, Analytical Products
Email:john.hammond

[1] Standards and Best Practice in Absorption Spectrometry - Edited by Chris Burgess and Tom Frost UVSG. ISBN 0-632-05313-5 Blackwell Science