It niny not bo gcncrrrlly known tlint ferric iron niny bo w r y rapidly cstiinntctl witli fnir nccurncy by on iodoniotrio rnotliotl (which docv not inrolro rciliic:tion) without tnking nny spccinl precnutio~is. To obtnin tho best rcsiilts, i t is doubtless ncccssnry to follow the inctliod of JIoiir ns stntcd b Trcndwcll (Anulytict~l tJicmistry, Trcnd\vcll and Hnl[ ii. pnge 532, niid in Sutton's Volumctric :lnalysis 'I). In this, tlio ferric solution is plnccd in n stoppercil bottlc. tho frcc ncid ncnrly neiitriiliscd, nnd tho nir displaced by n ciirrcnt of cnrbon dioxide. Solid potnssium iodido is tlicn nddd, the bottlo closed, slinken, nnd nllowcd to stniid in tho cold for 20 niinntcs. Tho iibcrnted iodinc is tlicn titrntcd with dccinorinnl tliiosiilplinto. Trcndwcll stntcs tliat tho rcsiilts obtnined by this nictliod tire sntisfnctory. JIodcrnto nccurncy mny, Iiowcrcr, Ijo obtairicd witliont tnking nny of tlieso prccnntions. Tho ferric solution is ncidificd with Iiydrochlorio ncid (tlio nniount nscd does not sccm to bo of any importnnco), n few grnms of potnssium iodido ndded, nnd tho iotlino libcrntcd titrntcd nt oncc with stnndnrd thiosulplintc. If tho subvtnnco undcr analysis is, ns is almost invarinbly tlio cnsc, insoliiblc in wntcr. i t is dissolved in strong Iiydroclilorio ncid, to wliicli n few grams of potnssium olilornto nro nddcd if any ferrons iron is prescnt, in wlicli cnso tho liquid is boilcd almost to dryness bcforo titrntion. Tho thiosnlplinto inny bo stnndnrdiscd with iodine, or with n ferric snlt of known urity such ns iron nluni (dc Koninck, B ~I L soc. Ciiiin. ~c k . , 1909, p. 222). In this Inborntory stnrcli solntlon is norm used ns an indicator in iodoinctric proccsscs iinlcss tho solution I~cing titrntcd contnins n colourccl compound othcr tlinn iodinc (e.g. bicliroinntc). \Vo find tho ciid point just as shnrply by wntcliing tlic disnppcnrnnco of the lnst trnce of ycllow colonr duo to tlic iodine.