There are many literature references which are in error.They either use not enough sulfuric acid or they use too much water or not enough. The best article I found was by Richard Adie and Kendall Browning-Journal of the Chemical Society 1903[Decomposition of potassium ferrocyanide with dilute sulfuric acid]. They gave the correct equation as K4Fe[CN]6 +5H2SO4 = FeSO4 +4KHSO4 +6HCN. It is essential that the sulfuric acid added is 4.244 molar as using 96% H2SO4 results in different products. In a garage with a great hood the 22.6% sulfuric is added to the dry ferrocyanide salt. The reaction flask has addition funnel on the vertical neck and a distillation train leading off from the angled neck. The receiver has methanol with 5.9 molar sodium hydroxide[to convert the HCN into NaCN and water]. The interesting observation I made is the reaction pot must be heated to about 85-90C[use boiling stones] and the HCN distils from about 62-67C as an azeotrope with water!The acid can be added in 3 batches with distillation in the middle. After the last acid batch is added,take the bath temp up to 100C, then distil to the end and then heat for 10 minutes longer. It is important to swirl the receiver often as the HCN collects as a layer on top of the methanolic NaOH solution. Mixing forms the crystals of NaCN. After the reaction is done the original pot is cooled and, using great ventilation in a fume hood, bleach solution added to convert any NaCN residue to NaOCN. This will create strong HCl vapors also and oxidize the green Fe+2 ion to the deep blue Fe+3 ion.The receiver chilled 2 hours,crop 1 filtered and the filtrate placed on rotary evaporator until almost dry..pour out of flask into glass tray and dry in an oven outdoors to obtain crop 2... the yield is 95-99% very good.
