Colour-producing writing fluid - comprises potassium ferrocyanide and substance which inhibits colour-forming reaction

摘要

A colour-forming writing fluid for use, e.g. in programmed learning systems, in which the colour formation is based on the formation of Prussian-blue comprises potassium ferrocyanide and a substance which inhibits the colour-forming reaction. This substance is pref. Maddrell's Graham's or Kurrol's salt, or NaF, Kf and/or NH4F or a polyphosphate.