โ Scribed by L.T. Butt; N. Strafford
No coin nor oath required. For personal study only.
CHENG, BI~AY ANL) KUIIT%* determined ferric iron in soils by direct titration with the disodium salt of ethylenediaminetetraacctic acid (Complexone III, Verscnate, subsequently referred to as EDTA) at pn 2-3 using Tiron (disodium-I :z-dihydroxybcnzene-3 :5-disulphonate) or salicylic acid as indicator and claimed that 0.1 g Na,HPO,.r2H,O did not interfere in the direct titration of 3-20 mg iron; they found, however, that if the soil contained an abnormally large amount of phosphate, interference with the end-point was encountered, although good results were obtained by adding excess of EDTA and back titrating with ferric solution. Titrations of iron using cscess EDTA as recommcndcd by CIIENG, RIZAY AND KURTZ were found to be unsatisfactory in the prcscnce of o.G g Na,HPO,.xzH,O; with salicylic acid as indicator no distinct end-point was observed and with Tiron as indicator the colour change at the end-point was poor and the rccovcry of iron low. It has been found, however, that 30 mg of iron can be determined in the presence of as much as I g Na,Hl?O,.IzH,O by titrating with EDTA, using ammonium thiocyanate as indicator and extracting the red ferric thiocyanate into amyl alcohol or ether, provided the pn is 2.0-2.4
when the end-point is reached. If the ~1.1 is too low excess EDTA must be added before all the iron is complexed and no satisfactory end-point is obtained, whilst if the pn is too high the red ferric thiocyanate is not produced bccausc ferric phosphate is formed preferentially.
The recommended procedure is to titrate rapidly with EDTA until the red ferric thiocyanate colour is discharged and then to adjust the I>H to 2.3 and to back titrate with standard ferric alum solution. Altcrnativcly, the PH can be adjusted to 2.4 bcforc starting the EDTA titration, which is then carried out carefully until the red colour just disappears: if the latter procedure is adopted some ferric phosphate may be out of solution at the beginning of the titration --_ _.
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