Gravimetric Determination of Iron

Weigh out accurately 5 g of purest ammonium iron(Il) sulphate, dissolve in some water, add 50 cm cone. HCI and make up to 500 cm . Pipette 50.0 cm of the solution into a beaker and heat to boiling. Add 2 cm cone, nitric acid and boil for a few minutes. After diluting to 300 cm boil again and add slowly, with constant stirring, 1 3 ammonia solution until excess has been added, indicated by mercuiy(I) nitrate paper. 

Weigh out accurately 6 g of purest ammonium iron(III) sulphate and dissolve in water, acidify with dil. HCl to prevent hydrolysis and make up to 500 cm. Pipette 50.0 cm of the solution into a beaker, add 20 cm 1 1 HCl, dilute to 200 cm and heat to boiling. Add slowly the ammonia solution and continue as above. Repeat using another aliquot of the solution. Calculate the concentration of Fe(III) in g dm.  

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Experiment 7 The Gravimetric Determination of Iron in a Commercial Unknown... 

The classical gravimetric determination of iron now finds little application in pharmaceutical work, partly because of the readiness with which other ions are adsorbed on to the precipitate but principally because of the variety of titrimetric methods which are available. Oxidising titrants are the most widely used and these may be applied directly to ferrous iron or, after suitable reduction, to ferric iron. Reducing titrants also find some application for the direct titration of ferric iron. Chelating titrants such as EDTA may be used but, because of the formation of a highly coloured complex and because other rapid titrimetric methods are already available, these are unlikely to find routine application. 

The widespread occurrence of iron ores, coupled with the relative ease of extraction of the metal, has led to its extensive use as a constructional material with the result that the analysis of steels by both classic wet and instrumental methods has been pursued with vigour over many years.3 Iron complexes are themselves widely used as the basis of convenient analytical methods for the detection and estimation of iron down to parts per million. Familiar tests for iron(III) in aqueous solution include the formation of Prussian blue as a result of reaction with [Fe(CN)6]4, and the formation of the intensely red-coloured [Fe(H20)5SCN]2+ on reaction with thiocyanate ion.4 Iron(II) forms particularly stable red tris chelates with a,a -diimines such as 1,10-phenanthroline or 2,2 -bipyridine that have been used extensively in spectrophotometric determinations of iron and in the estimation of various anions.5 In gravimetric estimations, iron(III) can be precipitated as the insoluble 8-hydroxyquinoline or a-nitroso-jS-naphthol complex which is then ignited to Fe203.6 In many situations the levels of free [Fe(H20)6]3+ may be controlled through complex formation by addition of edta. 

Once a sample is in solution, the solution conditions must be adjusted for the next stage of the analysis (separation or measurement step). For example, the pH may have to be adjusted, or a reagent added to react with and mask interference from other constituents. The analyte may have to be reacted with a reagent to convert it to a form suitable for measurement or separation. For example, a colored product may be formed that wUl be measured by spectrometry. Or the analyte will be converted to a form that can be volatilized for measurement by gas chromatography. The gravimetric analysis of iron as FeaOa requires that all the iron be present as iron(in), its usual form. A volumetric determination by reaction with dichromate ion, on the other hand, requires that all the iron be converted to iron(II) before reaction, and the reduction step will have to be included in the sample preparatioii. 

Some of the most successful and widely used chelating reagents include dimethylglyoxime for the gravimetric determination of nickel 1,10-phe-nanthroline and its derivatives for the colorimetric determination of iron and copper dithizone for the separation and colorimetric determination of a number of metals but particularly lead, silver, zinc, cadmium, and mercury the dithiocarbamates such as diethylammonium diethyldithiocarbamate and ammonium pyrrolidinedithiocarbamate, used for colorimetry but more widely applied now as selective extractants and the most successful titrant, EDTA. 

If the glass contains lead and/or barium oxide, the glass should be treated with a mixture of hydrofluoric and sulfuric acids and the insoluble lead and barium sulfates filtered off before the titration. Lead may also be precipitated with hydrogen sulfide. Small amounts of aluminum and iron may be masked with triethanolamine. Greater amounts of aluminum and iron must be preseparated by precipitation as their hydrated oxides in the procedure for gravimetric determination of alumina. If the content of iron oxide, alumina, and titania is large, they can be separated using a 25% solution of urotropin. 

Taylor PDF, Maeck R, De Bievre P (1992) Determination of the absolute isotopic composition and Atomic Weight of a reference sample of natural iron. Int J Mass Spectrom Ion Processes 121 111-125 Taylor PDF, Maeck R, Hendrickx F, De Bievre P (1993) The gravimetric preparation of synthetic mixtures of iron isotopes. Int J Mass Spectrom Ion Processes 128 91-97 Thirlwall MF (2002) Multicollector ICP-MS analysis of Pb isotopes using a Pb- Pb double spike demonstrates up to 4000 ppm/amu systematic errors in Tl-normalization. Chem Geol 184 255-279... 

In the determination of sulfate, 2 to 5 g of the analysis sample is mixed with HC1 (2 volumes concentrated HC1 + 3 volumes of water), and the mixture is gently boiled for 30 minutes. After filtering and washing, the undissolved coal may be retained for the determination of pyrite sulfur, or it may be discarded and a fresh sample used for pyrite sulfur. Saturated bromine water is added to the filtrate to oxidize all sulfur forms to sulfate ions and ferrous ions to ferric ions. After boiling to remove excess bromine, the iron is precipitated with excess ammonia and filtered. This precipitate must be retained for the determination of nonpyrite iron if a fresh sample of coal was used for the determination of the pyrite iron. The sulfate is then precipitated with ISaCE, and the BaSC>4 is determined gravimetrically. 

Another example is in the separation of sulfate or phosphate from various cations. Samuelson devised a method for sulfur in pyrites that is based on the retention of iron(m) on a cation-exchange resin. The sulfuric add that passes through the colunm can be determined by the usual gravimetric method as barium sulfate. In a similar way, phosphate in phosphate rock can be determined by retention of calcium, magnesium, iron, and aluminum on a cation-exchange resin followed by deterlnination of the phosphate as magnesium pyrophosphate. The metal ions can be eluted from the column with 4 M hydrochloric acid. 

The phosphates and the NaOH produced by hydrolysis reactions were determined by titration with acid. The determination of phosphates was done gravimetrically as 2 2 7 Sulfates were determined gravimetrically as BaSO after removing the phosphates as iron phosphate. The composition of the solid phases was determined from the composition of the phase complex and that of the corresponding saturated solution. Corrections were made for water of evaporation. 

Figure 9-7 shows the iron concentration of test solutions as a function of the HEDP concentration at the end of gravimetric measurements (24 h) (Kalman et al., 1994). The iron content was determined separately in dissolved and precipitated (rust) form. The variation of total iron content is in accordance with the corrosion rate of carbon steel, showing a minimum curve. At lower HEDP concentrations, high amounts of iron exist in the form of rust, indicating a corrosion protection effect of the oxide layer on the carbon steel surface. At higher concentrations (c> 10 M), HEDP keeps the total amount of iron in the solution phase. These results indicate that complex formation between HEDP and iron ions has an important effect on the inhibition effect of HEDP. The... 

Wood and Williams (22) also prefer to use a mixture of bismuth and iron for the analysis of zirconium. Following essentially the same procedure, they determine carbon concentrations between 200 and 2000 /ig/g with an accuracy of 50 /xg/g using a gravimetric determination method. The sample weight is 4 g. For concentrations below 50 /xg/g, a conductgmetric method is used. For one gram samples an accuracy of 10 /xg/g is claimed. 

A sample of an impure iron ore is believed to be approximately 55% w/w Fe. The amount of Fe in the sample is to be determined gravimetrically by isolating it as Fe203. How many grams of sample should be taken to ensure that approximately 1 g of Fe203 will be isolated ... 

Iron was present as Fe " in the calcined precursors. For all the catalysts the reduction procedure described in Sec. 2.1 resulted in incomplete reduction of the Fe to metallic iron. This is in agreement with the findings of previous authors [6,11]. The individual percentage reductions of Fe to Fe°, as determined by the separate gravimetric and volumetric measurements (Sec. 2.2), are shown in Table 1. The values are calculated on the assumption that all the Fe is reduced to Fe prior to the onset of reduction to Fe°. There is good agreement between the two methods. Table 1 also records the actual Fe/(Fe + Mg) ratio in the catalysts as determined by atomic absorption spectroscopy (AAS) on the calcined precursors. 

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