The dictionary defines 'sesqui-' as one and a half; half again. Thus
sesquicentennial is 150 years- 100 plus half.
It can also mean a combination, and they give the example of
sesquicarbonate which in not just carbonate or just bicarbonate, but a
mixture of the two.
I imagine, but don't know, that your reference is meaning a mixture of
potassium ferricyanide and potassium ferrocyanide.
John Bordley