Iron determination with permanganate

Ferric Oxide.—The filtrate from the preceding determination is made up, together with the wash water, to a definite volume (e.g., 250 c.c.) and an aliquot part of it (50 or 100 c.c.) precipitated with ammonia in presence of ammonium chloride the predpitate is collected on a filter and washed. If alumina is present only in negligible quantity, the weight of the caldned predpitate gives the ferric oxide. In the contrary case, the washed and still wet predpitate is dissolved in dilute sulphuric acid and the solution made up to 100 c.c. with water 10 c.c. of this solution are reduced with zinc and the ferrous iron titrated with permanganate (see Vol. I, Limestones and Marls, p. 142). 

The above considerations will be illustrated by the simultaneous determination of manganese and chromium in steel and other ferro-alloys. The absorption spectra of 0.001 M permanganate and dichromate ions in 1M sulphuric acid, determined with a spectrophotometer and against 1M sulphuric acid in the reference cell, are shown in Fig. 17.20. For permanganate, the absorption maximum is at 545 nm, and a small correction must be applied for dichromate absorption. Similarly the peak dichromate absorption is at 440 nm, at which permanganate only absorbs weakly. Absorbances for these two ions, individually and in mixtures, obey Beer s Law provided the concentration of sulphuric acid is at least 0.5M. Iron(III), nickel, cobalt, and vanadium absorb at 425 nm and 545 nm, and should be absent or corrections must be made. 

Douglas investigated heats of formation of dimethyl sulphoxide (and also of the sulphone) and proposed in a footnote that it could be determined by 5-min reaction with potassium permanganate/sulphuric acid, then adding excess iron(II) sulphate and finally titrating with permanganate. The same principle was used by Krishnan and Patel to determine dimethyl sulphoxide in various complexes (with perchlorates of titanyl, zirconyl and thorium), and by Krull and Friedmann to determine the same compound but using only dilute sulphuric acid and 5-min reaction. 

Reduction of Ferric Salts.—The reverse reactions, namely conversion of ferric salts into ferrous, are likewise easily effected by means of the usual reducing agents, such as nascent hydrogen, sulphur dioxide, etc. By the introduction of zinc into an acidified solution of a ferric salt, reduction is rapidly caused. Excess of acid slightly retards the reaction.4 This affords a convenient method of volumetrically determining the presence of ferric iron, the solution after reduction being titrated with permanganate. 

In the B.Vet.C. method for determination of metallic iron in reduced iron, the copper is displaced from copper sulphate and the ferrous salt formed is titrated with OTN permanganate. 1 mlO lN = 0 005585 g. The process has been adversely criticised by Hartley, Linnell, Read and RolfeA Persistently high results were obtained and these were attributed to the hydrolysis of copper sulphate in hot solution, reaction between the acid formed and the mixed salts yielding products which react with permanganate. Preference was given to the Wilner-Merck process using mercuric chloride. The modification of the method which was eventually adopted yielded consistent results which reflected the actual content of metallic iron in the sample ... 

This compound is precipitated when an iron(II) salt solution is mixed with oxalic acid. When dissolved in acid and titrated with permanganate, the latter oxidises both iron(II) and oxalate. If then the titrated solution is reduced by zinc amalgam to regenerate Fe(Il), the latter is then determined by titration with the same permanganate solution. 

Discussion. Alkali persulphates (peroxydisulphates) can readily be evaluated by adding to their solutions a known excess of an acidified iron(II) salt solution, and determining the excess of iron(II) by titration with standard potassium permanganate solution. 

It must be emphasised that if hydrochloric acid has been employed in the original solution of the iron-bearing material, the volume should be reduced to ca 25 mL and then diluted to ca 150mL with 5 per cent sulphuric acid. The determination is carried out as detailed above, but 25 mL of Zimmermann-Reinhardt or preventive solution must be added before titration with standard potassium permanganate solution. For the determination of iron in hydrochloric acid solution, it is more convenient to reduce the solution in a silver reductor... 

Procedure. Dissolve a weighed portion of the substance in which the amount of iron is to be determined in a suitable acid, and evaporate nearly to dryness to expel excess of acid. Dilute slightly with water, oxidise the iron to the iron(III) state with dilute potassium permanganate solution or with a little bromine water, and make up the liquid to 500 mL or other suitable volume. Take 40 mL of this solution and place in a 50 mL graduated flask, add 5 mL of the thiocyanate solution and 3 mL of AM nitric acid. Add de-ionised water to dilute to the mark. Prepare a blank using the same quantities of reagents. Measure the absorbance of the sample solution in a spectrophotometer at 480 nm (blue-green filter). Determine the concentration of this solution by comparison with values on a reference curve obtained in the same way from different concentrations of the standard iron solution. 

The iron content of ores can be determined by titrating a sample with a solution of potassium permanganate, KMn04. The ore is dissolved in hydrochloric acid, forming iron(II) ions, which react with Mn04 ... 

The concentration of Fe2+ ions in an acid solution can be determined by a redox titration with either KM11O4 or K2Cr207. The reduction products of these reactions are Mir4 and Cr5+, respectively, and in each case the iron is oxidized to Fe3+. In one titration of an acidified Fe2 solution, 25.20 mL of 0.0210 m K2Cr207(aq) was required for complete reaction. If the titration had been carried out with 0.0420 M KMn()4(aq), what volume of the permanganate solution would have been required for complete reaction ... 

Quantitative Determination. — Dissolve 1 gm. of powdered iron in about 50 cc. of dilute sulphuric acid, and dilute the solution to 100 cc. To 10 cc. of this solution add decinonnal potassium permanganate solution until the liquid has a slight red color, and when the liquid becomes decolorized, which may be effected, if necessary, by adding a few drops of alcohol, add 2 gm. of potassium iodide. Allow the mixture to stand one hour in a closed flask at 20° C., and then titrate with decinormal sodium thiosulphate. At least 17.5 cc. should be required to combine with the liberated iodine. 

The usual oxidising media, such as permanganates, bichromates, etc.,4 react instantaneously with ferrous salts, yielding in acid solution the normal ferric salts. Many methods for the quantitative determination of iron are based on these reactions. Thus, for example, with potassium permanganate the oxidation of ferrous sulphate proceeds as follows —... 

Determination of iron The most important applications of dichromate involve either directly or indirectly the titration of Fe(II). An excess of standard Fe(II) can be added to determine oxidants, or an excess of Fe(III) to determine reductants. These determinations usually can be carried out equally well with Ce(IV). For routine applications, however, the low cost and ease of preparation of standard solutions and the great stability of dichromate offer some advantages. Permanganate is at a disadvantage, expecially if hydrochloric acid solutions are to be used. 

Percentages of vanadium(IV) relative to the total vanadium can be determined volumetrically. An acid solution of a sample is titrated with a standard solution of potassium permanganate and then successively titrated with a standard solution of ammonium iron(II) sulfate by the use of diphenylamine as an indicator. The first titration corresponds to the amount of vanadium(IV) and the second to the total amount of vanadium. 

A number of transition metals can be determined conveniently if their cations undergo a definite change of oxidation state see Oxidation Number) on titration with a standard solution of potassium permanganate, potassium dichromate, cerium(IV) sulfate, or ammonium hexanitratocerate(IV). Several visual indicators have been proposed, including diphenylamine and its derivatives, xylene cyanole FF, and especially A-phenylanthranilic acid and tris(l,10-phenanthroline)iron(II) sulfate ( ferroin ). Solutions of have been used in the determination of iron, copper, titanium, vanadium, molybdenum, tungsten, mercury, gold, silver, and bismuth, and standard solutions of and Sn F U, and and Mo have also... 

The concentration of the Wiister cation produced by reaction with FAC can also be determined spectrophotometrically at 515 nm. The absorbance should be measured within Imin following the addition of DPD reagent to avoid interference of combined chlorine. Calibration standards for the spectrophotometer can be prepared from previously standardized chlorine solutions (by titration with ammonium iron(II) sulfate) or by using standardized potassium permanganate (KMn04) solutions to develop the DPD color. 

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