Conc hydroferri/ferrocyanic acid solns - by recycling acid solns of salts over ion exchange resin

Conc. solns. of hydroferricyanic and hydroferrocyanic acids are prepd. by (a) adding concns. solns. of water soluble salts of these acids to an ion exchange resin in an amt. corresp. with its available capacity, (b) terminating the addition of the salt solns., (c) recycling the salt soln. over the resin until the equilibrium between the salt and acid form of the resin is adjusted to >=1.3, (d) discharging the formed concentrated salt soln., and (e) converting the resin into its H form. Gives better yields of the concentrated solns. of the acids e.g. esp. to 95%.