Spectral Studies of the Thiocyanate Complexes

The well known change of colour of Cr(VI) from yellow in alkaline solution to orange in acid solution is thought to involve the following equilibria  

Although HCr207 has been reported, its significance is questionable (Inorganic Reaction Mechanisms, Pt.II, Ed.J.O.Edwards, Interscience, New York, 1972, p.98 and references therein). 

Prepare the following solutions, adjusting the pH with HCIO4 and record the absorption spectra in 10 mm quartz cells over the range 200 400 nm. 

Locate the absorption maxima and comment on your results. 

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