Iron determination in mineral drinking water is carried out by titrimetric method17. Inversion voltammetry (IVA) is applied to determination of the element in alcoholic products and raw materials for its making18. Current Russian standard offers photometry, atomic absorption spectrometry and titrimetry for iron determination in nickel, nickel alloys and copper-nickel alloys19.
The limitation of atomic absorption method is rather high cost of the equipment and consumables. In IVA, oxygen and organic substances should be removed from solution; the electrodes in use either contain toxic mercury or require surface regeneration at regular intervals. Titrimetry is usually used only for determination of enough high concentration of iron.
Advantage of molecular spectroscopy techniques is relative cheapness of consumables and availability of equipment; fluorimetric ones is much more sensitive, than photometric.
3. Iron determination methods:
Iron can be analyzed by different methods which were covered in the next part of the review one by one.
1. Spectrophotometric determination:
This method is based on number of free metal particles present in solution. Generally there is decrease in the number of particles during complex formation and so this method can be used to detect formation of complex.
Spectrophotometric method for the determination of iron in a biological sample is based on release of bound iron and further the released iron is reduced to Fe (II). A color reagent solution is added to it and the formed color complex is measured photometrically.20 Spectrophotometric determination of trace iron (III) after its preconcentration with a chelating resin is also developed.21
1) Phenanthroline Method:
Phenanthroline reacted with iron (II) with red coloured complex formation which was measured at 510nm.
Total Iron was determined by oxidizing any Iron (II) from Iron (III) with Persulfate. A Thiocyanate complex of the Iron (III) is formed in a dilute acid solution. A direct measurement of the Iron-Thiocyanate is made using a calibrated spectrophotometer at 474 nm. The Iron (II) is determined by adding a sample to deaerated 1, 10-Phenanthroline/Sodium Acetate reagent. Any Iron (II) present in the sample will form a colored complex with the reagent that can be measured on a spectrophotometer at 510 nm. Iron (III) is Calculated by subtracting the g/L Iron (II) from the g/L total Iron.22
A method for the determination of Fe (III) at trace levels was described. Thus, prior to the spectrophotometric determination, a preconcentration of the trace amounts of iron (III) using a chelate forming resin was proposed. A strong base anion-exchange resin (Dowex 2X4) loaded with Ferron (7-iodo-8-hydroxyquinoline-5-sulphonic acid) was used for Fe (III) preconcentration, at pH 2.2. After desorption with 5 % ascorbic acid in 0.5 M HCl, the analyte (converted from Fe (III) to Fe (II) was determined spectrophotometrically at 510 nm as Fe (II)-o-phenanthroline complex. The accuracy of the proposed method was verified by comparing the obtained results with those obtained using AAS with the standard addition method. The sensitivity of the spectrophotometric method (after preconcentration) was 0.01μg Fe (III)/ml. The recovery for iron (III) at the 7 mg/l level was 97 %.22
A procedure has been developed for the determination of iron (III) dimethyldithiocarbamate by converting it into the iron (II) − bathophenanthroline complex, which is then dissolved in acetone-water (1:1), and the absorbance is measured at 534 nm against a reagent blank. Beer's law is obeyed over the concentration range 0.5−20 μg mL-1 in the final solution. The method is sensitive and highly selective and is used for the direct determination of ferbam in a commercial sample and in mixtures with various dithiocarbamates (ziram, zineb, maneb, etc.) and from wheat grains. 23
A method for the simultaneous quantification of the various forms ferrous, ferric, soluble and chelated form of iron added or endogenous to foods has been developed. The total, elemental, and soluble irons are determined with minimal pre-treatment by atomic absorption spectrophotometry. The iron valences and complexed iron are measured spectrophotometrically using the bathophenanthroline reagent. The method was both reproducible and accurate in measuring iron added to plant material and to a formulated instant beverage. The procedure may be applied to determination of possible changes in the iron-fortified processed foods.24
Pizzaro et al (1987) described a quantitative method for determination of iron in stool to monitor consumption of iron-fortified milk in infants. The method is simple, fast, and inexpensive. Stool samples from infants consuming fortified milk or non fortified milk were ashed, and ashes were diluted in hydrochloric acid and reacted with bathophenanthroline disulphonate.25
Kawakubo et al, 2004 developed a visual and micro spectrophotometric methods based on the extraction of a coloured ferroine thiocyanate allowed determinations of trace iron in fresh water samples26. For the visual method, a water sample mixed with a reagent solution containing 1, 10-phenanthroline, sodium thiocyanate and 0.1M HCl. Iron was extracted as pink ferroine thiocyanate with 1ml of 4-methyl-2-pentanone.The sample up to 20ml was added step-by-step, until the colour of the extract was detected visually. Without any special instrument or colour standard, iron down to 0.001 mg-1 (0.025μg) in a sample can be determined. For the micro spectrophotometric method, the extract for 20ml of sample was separated by capillary suction in a column with acrylic fibers. A part of the extract was pushed out into a micro cell for the absorbance measurement at 525nm.The column was reusable after washing with ethanol. This method had a detection limit of 0.001mgl-1 can be determined.
Preconcentration of iron was done by chelate-forming sorbents. Very efficient systems provided by immobilization of 8-hydroxyquinoline on solid support. Also, a sulphonic acid derivative of 8-hydroxyquinoline, namely ferron (7-iodo-8-hydroxyquinoline-s-sulphonic acid loaded on anion exchange resin. Ferron is one of the most selective chelating agents used in the Spectrophotometric determination of Fe (III) at pH 2.After retention on the complexing resin at pH 2, the analyte is desorbed with ascorbic acid, (thereby converting from Fe (III) to Fe (II) and determined spectrophotometrically as the Fe (II)-O-phenanthroline complex at 510nm.21
A simplified method is described by Barry (1968) in which urine is digested and iron content of the residue determined, without need for quantitative transfer. A standard of quantity of acid is used to ash urine to yield a dry residue, which is reduced with thioglycolic acid, and a pink colour is developed with bathophenanthroline sulphonate in the presence of acetic acid-acetate buffer at pH 4-5.The main purpose of ashing to destroy the chelating agents and render all iron available for colour formation.27
Ferrozine (PDT disulfonate; 3-[2-Pyridyl]-5, 6-diphenyl-1, 2, 4-triazine-4, 4´-disulfonic acid. Na-salt)
Method:
Stookey (1970) proposed a method for determination of ferric and total (soluble and insoluble) iron with ferrozine.28
The concentration of iron in urine is of interest in monitoring desferoxamine therapy for acute iron poisoning29 or iron overload. Atomic absorption spectrophotometry (AAS), the reference Method for iron measurement30 is usually not available in most routine laboratories. Routine automated methods for measuring serum iron with use of a chromogen do not detect iron in these samples29, because the complex of desferoxamine and Fe3+ is not dissociated.
Several types of pretreatment of urine samples to dissociate the deferoxamine-Fe3+complex have been described.29, 31 Given the need for a sensitive, (semi) and routine analysis automated procedure to monitor deferoxamine therapy in urine, we combined a ferrozine-based method for iron measurement (kit 1127683/69 1; Boehrunger Mannheim, Mannheim, Germany), implemented on an automated analyzer (Hitachi 747; Hitachi, Tokyo, Japan), with a pretreatment. Verhasselt et al (1991) optimized and compared two types of pretreatment32: thioglycolate trichioroacetate29 and ascorbate citrate.31
To measure the recovery of iron, we added FeC13 to normal urine with or without equimolar concentrations of deferoxamine. Urine samples of patients with proteinuria or in desferoxamine therapy were collected. Each sample was thoroughly mixed, and aliquots were stored at 4°C and analyzed within 4 h. In the thioglycolate-trichloroaoetate pretreatment, the reagent (0.29 mol/L thioglycolic acid and 0.61 mol/L trichloroacetate in water) is added to an equal volume of fresh urine. In the ascorbatecitrate pretreatment, 3.5 volumes of reagents (0.48 mol/L ascorbate and 0.57 mol/L citrate in water) were added to 1 volume of fresh urine. These pretreatments are done at room temperature, and the pretreated samples are analyzed after 15-60 mm on the automated analyzer. A control urine sample is processed the same way for a reagent blank. The linearity was tested of the procedures from 0.41 to 1000 mmol/L iron. In the absence of deferoxamine, recovery of added iron (95%, at 1000mmol/L) and linearity (r2 > 0.9999) were excellent without pretreatment.
In urine containing both desferoxamine and iron, iron could not be detected without pretreatment of the samples.
Pretreatment with thioglycolate - trichioroacetate yielded poor recovery (19%) and linearity (r2 = 0.81). Pretreatment with ascorbate-citrate was superior in both recovery (92%) and linearity (r2 = 0.9996) and was thus adopted for further experiments and routine analysis. The ascorbate citrate reagent solution was stable for at least 3 days at 4°C, although values for the reagent blank tended to increase. The reagent can be stored for longer periods at – 18°C.32
A direct method for determination of iron in blood serum comprising reacting the blood serum with ferrozine in the presence of a reducing agent to bring iron ion into divalent form, thiosemicarbazide and hydrochloric acid at a pH between 1.7 and 2.1, without any addition of buffers and tensioactive agents and determining colorimetrically the iron content of the specimen by means of the colored complex formed between iron ion and ferrozine against a reagent blank.
In a method for direct colorimetric determination of iron in blood serum by reacting the serum iron with ferrozine in the presence of acid and a reducing agent to release serum iron from transferrin and reduce iron ion to divalent form. The iron-ferrozine reaction carrying out the in the presence of sufficient thiosemicarbazide to eliminate significant copper interference, without any addition of buffer and tensioaction agents and at a pH sufficiently low that such elimination of copper interference is obtained without significant reaction of the thiosemicarbazide with iron, said low pH also being such that substantially complete formation of a colored iron-ferrozine complex is obtained.33
There are several possible strategies for enhancing the overall sensitivity fro metal determination when using a metal-chelate technique. In an effort to develop a preconcentration technique for measuring Fe (II) and total iron in natural water samples, investigations were conducted concerning the use of ferroine as preconcentrating agents for iron. These techniques involve solvent sublation, solvent extraction, and ion exchange. According to IUPAC definition solvent sublation is a flotation process in which the material of interest is adsorbed on the surface of gas bubbles on liquid, and collected on an upper layer of an immiscible liquid. Solvent sublation has advantages that the analytes of complexed forms can be directly extracted into a solvent by the flotation and determined in the solvent as a concentrated state by different analytical methods.
The method for separation and preconcentration of metals by flotation and solvent sublation, followed by their spectrophotometric and/or AAS determination, has increased in popularity recent years. A literature survey revealed that solvent sublation followed by sepctrophotometetric and/or AAS determination is rarely reported. Ferrozine finds its role for a direct spectrophotometric determination of Fe (II),and also gives high performance in the preconcentration of Fe (II) via a solvent sublation as ion pairs of [Fe(FZ)3]4- and tetrabutylammonium ions. Ali Akl et al (2006) reported ferrozine as a complexing agent formed ion pair with tetrabutylammonium(TBA+ ) ion as counter ion, oleic acid (HOL) used as surfactant then (TBA)4[Fe(FZ)3] ion pairs were floated by vigorous shaking in the flotation cell and extracted into methyl isobutyl ketone (MIBK)on the surface of the aqueous solution. The iron collected in the MIBK layer was measured directly by spectrophotometry and/or flame atomic absorption spectrophotometry.3
Carter (1971) proposed the use of ferrozine a commercially available sulfonated ferroin, to the determination of sub microgram levels of iron in human serum.34
Seligman and Schleicher (1999) compared two methods for determination of serum iron in presence of iron gluconate or iron dextran.35 One method uses an acetate buffer (pH 4.5) with 15 g/L hydroxylamine hydrochloride to release the iron from transferrin. The released iron reacts with 8.5 g/L Ferrozine reagent during 15min incubation at 37°C to produce a magenta-colored complex. Another method used an acetate buffer (0.1 mol/L, pH 4.8) containing ascorbic acid (56.8 mM/L) and guanidine (6 mol/L) to release transferrin-bound iron. The absorbance is measured at 595 nm, before and after 5-min incubation at 37°C with 36 mmol/L Ferrene S. In this trial methodology that measure serum iron, particularly those using an ascorbic acid/guanidine buffer method, will cause in vitro dissociation of iron bound to gluconate or dextran complexes.
2) Pyridyl regent:
The invention concerns a method for the interference-free determination of iron in biological samples; in particular in serum in which bound iron is released, the released iron is reduced to Fe (II), a color reagent solution is added and a color complex that forms is measured photometrically and it also concerns a combination of reagents which is suitable for the interference-free determination of iron especially in the presence of high amounts of EDTA.
The reagents used are composition of reagent 1 and reagent 2 containing following components used to determine iron in serum: 20
|
Reagent 1 |
200 mmol/l |
citric acid |
100 mmol/l |
thiourea |
|
|
7% (w/v) |
Alkylpolyethylene glycol ether |
|
|
10 mmol/l |
indium (III) chloride |
|
|
Reagent 2 |
150 mmol/l |
sodium ascorbate |
|
6 mmol/l |
3'(2'-pyridyl)-5,6-bis(4-phenylsulfonic acid)-1,2,3- triazine monosodium salt |
Methods in common use for the determination of complexed iron in compounds such as haemoglobin, ferritin, or ferrioxamine depend on a preliminary treatment to release free iron which is then estimated by a colorimetric reaction. The release of iron may be accomplished either by dry ashing in a furnace or wet ashing with hot concentrated acid.36
Both these procedures were time consuming and potentially hazardous. A simple technique is described for the release of iron from haemoglobin, ferritin, or ferrioxamine in solution by digestion with an acid-permanganate solution at room temperature. Excess permanganate is reduced with ascorbic acid, and iron in the resulting clear solution is estimated by the method of Young and Hicks (1965) using tripyridyl triazine.37, 38
Salicylic acid:
Iron can also be determined by spectrophotometric titration in which salicylic acid and iron (III) form a deep coloured complex with a maximum absorption at 525nm; this complex is used as the basis for the photometric titration of iron (III) with standard EDTA solution. At pH ~2.4 the EDTA-iron complex is much more stable (higher stability constant) than the iron-salicylic acid complex. In the titration of iron-salicylic acid solution with EDTA the iron-salicylic acid colour will therefore gradually disappear as the end point is approached.39
The leaching solutions from hydrometallurgical processing of copper sulphides have a high concentration of several metallic sulphates.
The Fe+2 determination was carried out by 1, 10-phenanthroline method.40 The Fe total determination is realized using potassium dichromate volumetric method and Atomic Absorption Spectrometry (AAS) technique.
In this sense, in all of these cases the determination of Fe+3 is not direct and for instance, a quantitative determination of both oxidation states could be imprecise. Recently, a simple and fast method has been described for the quantitative simultaneous determination of Fe+3 and Fe total with SSA in acid mine drainages (AMD) and other solutions.41
For the direct determination of Fe+3 the method of the SSA was used, and indirectly Fe+3 also was determined by means o-phenanthroline method and potassium dichromate method ([Fe total]–[Fe+2]). For the determination of Fe total the three previous methods were compared with those of Atomic Absorption Spectrometry (AAS). 40
The procedure used in the SSA method was as follows:
Into a 100 mL volumetric .ask, add an adequate volume of the sample. Then add 3 mL of 10% (w/v) sulphosalicylic acid solution and complete volume with Ultrapure Millipore Water. Stir during 2 min and measure de absorbance at 500 nm. Then add 3 mL of ammonia solution 25% (w/v) and stir during 2 min and measure the absorbance at 425 nm.41, 42
Squaric acid (1,2-dihydroxy-3,4-diketo-cyclobutene):
Squaric acid (1, 2-dihydroxy-3, 4-diketo-cyclobutene) is used in a specific reaction with Fe (III) for the spectrophotometric determination of Fe (III) and total iron content. The analytical procedure includes mixing ammonium squarate (40mM), prepared in a phthalate buffer solution of pH 2.7, with the sample and measuring the absorbance at 515nm. The method has been successfully applied to the determination of iron (III) and the total iron content after quantitative oxidation of iron (II).43
Suwansaksri et al (2003) compared 3 different methods of serum iron determination, nonprecipitating ferrozine colorimetric method44, precipitating ferrozine colorimetric method, and iron liquicolor cleaning factor colorimetric agar-based (CAB) method (Human, Taunusstein, Germany). For a setting with limited resources such as Thailand, use of the first analytical technique, nonprecipitating ferrozine colorimetric method, is recommended because of its low cost and ease of performance.
3) Bis(2-hydroxymethyl-5-hydroxy-4-pyrone-6)ketone:
A method has been developed for the direct spectrophotometric determination of iron by using the reddish-orange coloured complex formed in the interaction of iron with bis (2-hydroxymethyl-5-hydroxy-4-pyrone-6) ketone.45
4) Determination of Iron (III) with Amines and Thiocyanate:
Hayashi et al (1986) studied that the coloration of iron (III)-thiocyanate complexes was enhanced by the addition of amine to acidic solution containing the nonionic surfactant.46 A coexistence of 0.25% Capriquat (trioctylmethylammonium chloride) brought about a 1.5-fold increase in sensitivity (molar absorptivity: 2.95×104mol-1 dm3 cm-1 at 485nm) of the spectrophotometric iron determination when compared to a method containing no amines.
5) Potassium thiocyanate:
Some metals, such as iron, will form highly colored complexes when reacted with the thiocyanate ion which resulted in red coloured complex and measured absorbance at 470nm.
Fe3+(aq)
+ 6 SCN-(aq)
[Fe(SCN)6]3-(aq)
Another simple and rapid calorimetric method is described for the determination of iron in blood by Wong (1928).47 The method described was as follows,
Transfer accurately with an Ostwald pipette 0.5 cc. of blood into a 50 cc. volumetric flask and introduce 2 cc. of iron-free concentrated sulfuric acid. Whirl the flask to agitate the mixture for 1 or 2 minutes. Add 2 cc. of saturated potassium persulfate solution and shake. Dilute to about 25cc. with distilled water and add 2cc. of 10 per cent sodium tungstate solution. Mix, cool to room temperature under the tap and then dilute to volume with distilled water. Stopper the flask and invert two or three times to effect thorough mixing. Filter through a dry filter paper into a clean, dry receiving vessel. Pipette exactly 20cc of the clear filtrate into a large test-tube graduated at 20 and 25cc. Measure into another similar test-tube exactly 1cc of the standard iron solution containing 0.1 mg of Fe per cc. Add with a graduated 1cc. pipette 0.8 cc of iron-free concentrated sulfuric acid and dilute to the 20 cc mark with distilled water. Cool to room temperature under the tap. To both the unknown and standard 1cc of saturated potassium persulfate and 4 cc of 3 N potassium sulfocyanate solutions were added. Insert a clean rubber stopper, mix, and compare in a Duboscq calorimeter. Calculation - As the 20cc. of filtrate taken represent 0.2cc of the original blood, and the quantity of standard solution used contains 0.1 mg of Fe, if the reading is made with the standard set at 20 mm., then 20 divided by the reading (R) of the unknown and multiplied by 50 will give the number of mg of Fe in 100cc of the blood examined.
20/R X 50 = mg of Fe per 100cc. of blood.47
Propylene carbonate (4-methyl-1,3-dioxolane-2-one) simultaneously extracts the 2,9-dimethyl-1,10-penanthroline chelate of copper(I) and the tri(2-pyridyl)-1,3,5-triazine(TPTZ) chelate of iron(II) from acetate-buffered aqueous solutions. Molecular absorption Spectrophotometric quantification is accomplished by measuring the absorbance of the iron (II)-TPTZ chelate at 458nm and that of the copper (I)-NC chelate at 458nm. The iron (II)-TPTZ chelate exhibits an absorbance at 458nm that is 0.123 times its absorbance at 596nm.49
Berman (1918) devised a method of iron determination from blood sample in which iron held is split off by the action of concentrated hydrobromic acid.49 The iron was oxidized and the organic matter was destroyed by potassium permanganate. The resulting solution was mixed with ammonium sulphocyanate in water and acetone and the colour was compared with the standard iron solution similarly treated.
6) AAS:
Tang (1978) used CRA (Carbon rod atomizer) flameless with atomic absorption equipment for determining iron from solid sample.8
A heated quartz cell (HQC) is an attractive atomizer for atomic absorption spectrometry (AAS) in regard to its sensitivity, due to its long optical path in addition to atomization without the use of chemical flame. For low boiling point metal elements, the analytes can be easily vaporized and effectively introduced into HQC and their atomic absorption can be measured. The method required only several microliters of sample solution for the determination at nanogram level. Sugesaka et al (2001) proposed a heated quartz cell atomic absorption spectrometry coupled with in situ butylation/vaporization for the determination of iron and tin.50
Fe (III) could be determined by AAS with a calibration curve or by the standard addition method used from a solution with Fe (III) concentration under 0.1mg/ml.21
An ultramicro method was described by Olsen et al (1973) for determining iron in 50 µl of serum.51 Hot trichloroacetic acid was used to remove protein and hemoglobin iron, and atomic absorption analysis of 10µl of the supernatant fluid was carried out with use of a graphite furnace and a specially grooved graphite tube. Results correlate well with those obtained by a conventional flame atomic absorption method for iron that requires a 1-ml sample of serum. When serum was analyzed with no pretreatment, results were higher, probably because of hemoglobin iron present in serum, even serum with no visible haemolysis.51
Brecque et al (1978) proposed an interference-free atomic absorption method for the determination of total iron in geological samples.52 The method is based on the conventional standard curve technique employing the 386nm atomic absorption line in the fluoboric-boric acid matrix. Interference studies of aluminum and/or silicon on the iron determination conclude that these interferences do not exist under the conditions of this method.
Olsen et al (1973) developed an ultramicro method for the determination of iron from in 50μl of serum sample compared with conventional flame atomic absorption method for iron that requires a 1ml sample of serum.51 In this method of analysis hot trichloroacetic acid was used to remove protein and hemoglobin iron, and atomic absorption analysis of 10μl of the supernatant fluid was carried out with use of a graphite furnace and a specially grooved graphite tube.
7) Flame atomic absorption method:
A Flame atomic absorption spectrophotometer (FAAS) has been widely used because of its high specificity, while not being an expensive instrument. However, the direct determination of trace concentrations of metals by FAAS is generally difficult because of matrix interference problems and concentrations below the detection limit of FAAS. These problems can be overcome by using preconcentration and separation procedures. For this purpose, various preconcentration/separation methods, such as solvent extraction, coprecipitation, ion exchange, resin chelation and solid phase extraction have been widely used.
A solid phase extraction (SPE) technique has found increasing applications for the preconcentration of trace metal ion and the elimination of matrix interference prior to AAS analysis. Different solid phase extractors, such as Amberlite XAD resins, activated carbon, polymeric fibers, and silica gel have been used to preconcentrate trace metal ions from various media. Kenduzler and Turker (2002) used Ambersorb 572 as a solid phase extractor where iron, manganese and zinc were preconcentrated from a water matrix as their chelates using Ambersorb 572 as a solid phase extractor.53 Ambersorb 572 was chosen because it has a high surface area, durability and purity.
The low concentration of iron present in natural waters (as μgl-1) necessities the selection of a suitable preconcentration procedure. In recent years, microorganisms such as, yeast, bacteria, fungi have often been proposed for the preconcentration and speciation of trace metals. Bag et al (2001) proposed preconcentration of Fe (II), separation of Fe (II) from Fe (III) and flame atomic absorption spectrophotometric determination of each iron species by using an adsorbent, Aspergillus niger, immobilized on sepiolite.54 Repeated use of the column is possible and relatively stable upto 20 runs.
For the determination of iron in light and heavy petroleum fractions by the use of graphite furnace atomic absorption spectrometry after sample dilution with methyl isobutyl ketone, and the use of the method of standard additions are advised for iron determination by Torre et al (1990).55
A new Schiff base, N,N′-bis-(2-hydroxy-5-bromobenzyl)-2-hydroxy-1,3-diiminopropane, has been synthesized by Pehlivan and Kara (2006) for the very sensitive determination of Fe (III) and Fe (II) in natural water samples.56 The method has also been applied to total iron determination in sediment samples. In the preconcentration system, the Schiff base reagent was mixed with the samples and chelates containing Fe (III).
The complexes were then adsorbed on silica gel within a column system. Elution of the adsorbed chelate from the silica gel was performed with a small volume of acetone containing 2.5% nitric acid. The iron was measured off-line by flame atomic absorption spectrometry. The method can be applied to the preconcentration, separation and speciation of iron. The results were compared statistically with those from the standard 1, 10 phenanthroline method used for iron speciation in water systems. A Student t-test indicated no significant difference between the two methods.
Yurchenko and Kharenko (2007) showed that sodium dodecyl benzenesulfonate used as the modifier in flame atomic absorption determination of iron in real samples make it possible to make the measurements in a low-temperature flame (propane-butane-air type) with increased sensitivity and selectivity and also a lower detection limit for iron.57 The detection limit for iron is Cmin = 0.008 µg/cm3.
UV–Vis spectrophotometric (UV–Vis) and flame atomic absorption spectrometric (FAAS) analysis for iron determination in pharmaceutical product were compared in terms of uncertainty budgets by Jurgens et al (2007).58 Non linearity, instrument drift, volumetric measurements contributes for the uncertainty of the two methods.
Saleh et al (2008) proposes a method to determine iron in samples of fish feed and feces using ultrasound in the extraction of the analyte and in subsequent quantification by flame atomic absorption spectrometry.59 The results obtained with the proposed extraction method allowed calculating the coefficients of apparent digestibility of iron in the food sources, which was not possible when using results obtained from samples mineralized by acid digestion.
8) Solid phase spectrophotometry:
Teixeira et al (2002) developed a multivariate calibration method, in the reduced range of 560-650nm60, in association with solid phase spectrophotometry with the advantage of rapid determination of the analyte in a mixture without previous masking, preconcentration or separation steps.
Fe (II), Ni (II) and Zn (II) are retained on 1-(2-thiazolylazo) 2-napthol (TAN) immobilized in C18 bonded silica, forming complexes allowing in situ analyte separation. The method based on the different reaction/retention ratios of the studied ions on the solid support.
Garcia et al (1995) utilized a cataionic exchanger paper to retain analytes in solution and, after drying, to analyze directly by measuring the UV-Vis absorbance of the paper.61 The method was applied to determination of iron using its known 1, 10-phenanthroline complex. By this way determination of iron at ng/ml level by solid phase spectrophotometry after preconcentration on cation exchange filters was done.
9) Photoacoustic Spectrometry:
Shida and Masuda (1998) developed a sensitive method based on preconcentration on a membrane filter with a finely pulverized anion-exchange resin for the determination of iron (II) in water samples by Photoacoustic spectrometry. Iron reacts with 2-nitroso-5- (N-propyl-N-sulfopropylamino) phenol (Nitroso-PSAP) to form a water-soluble chelate anion, which is adsorbed on the resin at pH 8.0; the resulting resin is filtered through a membrane filter.62 The resin with the membrane filter is inserted into cell, and the intensity of the photoacoustic signal of iron (II) is measured at 826nm. The colour intensity of the green iron (II)-Nitroso-PSAP complex on a membrane filter was measured visually following the standard series method. Iron (II) was determined as total iron after reduction with 0.1M ascorbic acid solution, because the brown iron (III)-Nitroso-PSAP complex was also adsorbed on the anion exchange resin solution (ARS).62
The fortification of foods, such as cereals, with iron has been recognized as a worldwide necessity, since a deficiency of this element produces different metabolic disorders. Therefore the developments of methods to measure low concentrations in foods has become of increasing importance. In order to achieve higher sensitivity and selectivity the use of photothermal (PT) technique is advantageous. The technique based on a photoinduced periodical changes in the thermal state of a sample. Light energy, which is absorbed and not lost by reemission, leads to sample heating, which induces changes in the temperature-dependant sample parameters.
Delgado-Vasallo et al (2003) reported microphone based open Photoacoustic (PA) cell especially designed for spectroscopic measurements in liquid systems.63 It renders same results as optical spectroscopy, but several advantages such as independence of sample thickness, which allows measurements over a wider absorbance range.
10) Nondispersive Atomic fluorescence spectrometer(NAFS)
The analytical potential of the first commercial nondispersive atomic fluorescence spectrometer (NAFS) with a tantalum coil atomizer for monitoring iron in HCl gas has been investigated by Khvostikov. The limit of detection (LOD) of Fe in a mixture of 1% HCl in Ar was found to be 9 ppb (v/v). 64
11) ELAN DRC (Dynamic reaction cell) ICP-MS:
Nixon et al developed an ELAN DRC (Dynamic reaction cell) ICP-MS that made analysis of iron and copper from liver tissues much faster and simpler.65 The DRC eliminate the interfering species through a process called chemical resolution which used a reaction gas to selectively remove the interferences (40Ar16O+,40Ar16OH+,23Na40Ar+). DRC provided lower detection limit therefore use of higher dilution factors was possible. This reduced the amount of sample matrix introduced into the instrument, overall routine maintenance, cleaning the interface cones and changing the pump oil.
12) X-ray fluorescence spectroscopy (XRF) and X-ray absorption near edge structure spectroscopy (XANES):
X-ray fluorescence spectroscopy (XRF) and X-ray absorption near edge structure spectroscopy (XANES), both using synchroton radiation (SR) microbeams can be apply for determination of the chemical state of the elements and also provide the spatial distribution of each element in each oxidative state selectively.66
Bower (2007) proposed a method of ferrous iron determination in which Iron (II) oxide in silicate rocks measured after digestion with HF and HCl by reaction with KI and KIO3.67 The iodine produced during the digestion was trapped in CCl4 and measured at 512 nm spectrophotometrically.
13) Electron probe microanalysis (EPMA):
The analytical task of iron content measurement in the ink writing on the surface of an 18th century manuscript by electron probe microanalysis (EPMA) using a scanning electron microscope equipped with an energy-dispersive X-ray spectrometer (SEM/EDS) was done by Virro et al (2008) and It was concluded that EPMA using a SEM/EDS is suitable for at least semi-quantitative determination of iron in the writing of ink-written manuscripts.68
A catalytic kinetic determination of iron was performed by Khan (2006) by exploiting the reactions of Neutral Red (NR).69 The iron catalyzed oxidation of NR with potassium bromate was studied kinetically by using fixed time method. The reaction was followed by measuring the decrease in absorbance at 535nm. The method was applied successfully to the determination of iron in the pharmaceutical and synthetic samples. The results showed good agreement with those obtained by atomic absorption spectrophotometer (AAS) with a detection limit of 0.019 µgmL-1.
2. Chromatographic method:
Chromatography based on partition coefficient can be used for separation and determination of iron along with ligand. Various ways of chromatographic methods have been used.
For the determination of the chelated Fe content in synthetic Fe3+ fertilizer Garcia Marco et al(2006) reported an ion pair HPLC method that allows the determination of o,p-EDDHA/Fe3+ in EDDHA/Fe3+ commercial fertilizers.70
Total iron measured by AAS analysis to a detection limit of 0.1mg/L. The sample as well as standard should be acidified with HPLC to a final concentration of HCL 1M (10ml solution + 1ml HCL 11M) and chelated iron measured by HPLC.71
A selective determination method for Iron (Fe) ion in tap water has been developed by solvent extraction, followed by reversed phase HPLC with photometric detection in which The Fe (III) ion was quantitatively extracted into chloroform over the pH range of 3.2 to 4.3 as 3, 4-dihydro-3-hydroxy-4-oxo-1, 2, 3-benzotriazine (DHOB) chelate. The extracted Fe-DHOB chelate was then separated on a phenyl column with an eluent of methanol/water/0.05 M DHOB (40:20:40, v/v) and detected at 500 nm. The detection limit of the Fe ion in 5 mL water was estimated at 7 ppb.72
1) Micellar Electrokinetic Chromatography (MEKC):
Simultaneous analysis of two analytes is important not only for biochemistry and environmental chemistry, but also in many scientific and industrial fields. So far atomic absorption spectrometry (AAS), inductively coupled plasma (ICP) and x-ray fluorescent spectrometry (XRF) have been generally known as analytical methods for metal ions; however, these methods can not distinguish any difference in the metal valences.
Capillary electrophoresis (CE) is a high performance separation technique but there are few reports regarding analysis of oxidation states of metal ions. The use of two selective ligands for Fe (II) and Fe (III) was reported by Pozdniakova et al (1997) in which EDTA and 1, 10-phenanthroline was used with CE. Takagai and Igarashi (2003) considered that, if we could use two selective ligands for Fe (II) and Fe (III, a distinguishable analysis could be made.73 The method was attempted using bathophenanthroline derivative and desferioxamine B derivative as a selective ligands. Thus simultaneous determination of Fe (II) and Fe (III) was achieved. Moreover, when Micellar electrokinetic chromatography (MEKC) was performed under the condition of negligible electroosmotic flow, the Fe-complexes were preconcentrated by micelle sweeping. With this method, the resulting iron analysis was capable not only complete separation, but also a sensitive determination.73
Voltammetry:
Voltammetry is Volt-Am (pero)-Metry where voltage ramp applied to electrode
and current is measured. A voltage ramp is applied and the corresponding current measured. Current flows when a substance is reduced or oxidized on the electrode. In case of no electrochemical reaction, no current is generated.
Stripping voltammetric procedures belong to the most sensitive method of determination of iron traces. The catalytic adsorptive stripping voltammetry (CAdSV) methods, which are based on the adsorptive accumulation of iron-nioxime complex on the hanging mercury drop electrode (HMDE), followed by a stripping voltammetric measurement of the catalytic reduction current of the adsorbed complex in the presence of sodium nitrite. The detection limit obtained for a 20s accumulation time was 0.90nM iron.74
3. Titration
Iron determination by titration can be possible by various ways including direct titration, back titration, potentiometric titration, and complexometric titration.
Salicylic acid and iron (III) ions form a deep colored complex with 525nm as λmax. This complex can be used as the basis for the photometric titration of iron with standard EDTA solution.39
ISO 2597-1:2006 specifies a titrimetric method for the determination of the total iron content of iron ores, using potassium dichromate after reduction of the trivalent iron by tin (II) chloride. The method is applicable to total iron contents between 30 % (mass fraction) and 72 % (mass fraction) in natural iron ores, iron ore concentrates and agglomerates, including sinter products(ISO 2597-1:2006 American National Standards Institute (ANSI).
Kuwabara et al (1999) proposed a potentiometric titration of Fe (II) and Fe (III).75 The method is based on the effect of 1, 10-phenanthroline on the redox reactions of chromium (VI) and iron (II) with cobalt (II). In the presence of phenanthroline the conditional redox potential of the Co (III)/Co (II) system falls below those of the Cr (VI)/Cr (III) and Fe (III)/Fe (II) systems at pH around 1. Therefore, chromium (VI) and iron (III) can be titrated with cobalt (II) in the presence of phenanthroline, alternatively. Firstly, to a sample solution containing iron (II) and iron (III), a known amount of chromium (VI) is added in excess at pH 1, so that Fe (II) can be oxidized to iron (III) in the absence of phenanthroline. After the pretreatment, the sample solution containing the excess of chromium (VI) and iron (III) (total iron) is titrated with a standard cobalt (II) solution in the presence of phenanthroline. The concentrations of iron (II) and iron (III) can be calculated from the first and second potential breaks at the equivalence points for the titration.75
A known volume of a sample solution, containing less than 10 mg of Fe (II), was taken into a 25mL flask. Then, 2.5mL of pH 6.5 buffer solution, 1mL of the masking reagent, and 5.0mL of the CTAB–EBBR (Cetyl trimethyl ammonium bromide - Erichrome blue black R and) aggregate solution were added. The solution was then diluted to 25mL and mixed well. After 20 min, the absorbances were measured at 480 and 610m with a 2-cm cell against a reagent blank without Fe; then, Ac of the ternary complex was calculated. For analysis of a solid sample, first its aqueous solution must be prepared as follows: pulverize 1 g of a sample, add it to a mixture of 5mL of nitric acid and 5mL of 5% hydroxylamine hydrochloride for 2 h, volatile the excessive nitric acid and hydroxylamine hydrochloride by boiling and, finally, neutralized to pH 6–7 with 5% NaOH and filter. Then, the filtrate was diluted to 100mL with deionized water and colored and measured according to the same procedure as given above.76
Starke (1963) proposed a method for determination of basic iron (III) compounds and free acid in iron (III) solutions by iodometric titration with the same sample.77 After the iron has been reduced with potassium iodide and liberated iodine was titrated with thiosulphate and determined, further it was precipitated as iron (II) hexacyanocobaltamate(III) which was almost colorless which did not interfere with the location of the end point of the acid titration.
5. Flow injection analysis
Kas and Ivaska proposed a procedure for determination of concentrations of iron (III) and total iron by sequential injection analysis.78 The method is based on the strong blue-coloured complexes formed between iron (III) and trion. The absorbance of the complexes is measured spectrophotometrically at 635nm.Oxidation of iron (II) and masking of interfering fluoride is simultaneously done by injecting one zone of hydrogen peroxide and one of thorium (IV) between the sample and reagent zones. The sample throughput is approximately 17 samples per hour, including three repetitive determinations of each sample.
Sakai et al, 2000 developed a new design of a multi-channel micro cell for simultaneous analysis of copper, iron and zinc in the flow injection method.79
Lunvongsa et al (2006) developed a Flow injection analysis method for the sequential determination of iron and copper in the mixture by using oxidation reaction of the N, N-dimethyl-p-phenylenediamine (DPD) with hydrogen peroxide.80 Sample solutions in the absence and in the presence of TFTA were used for sequential determination of iron and copper .One injection is used for the sum of iron and copper contents, and the other for iron contents. For iron determination, TFTA was used as a masking agent for copper. The difference in peak heights was used to calculate the copper concentration.
Several spectrophotometric procedures have been used in flow injection analysis for the determination of Fe (II) and Fe (III) in many kinds of samples. Most of the regents are solid and produce iron complexes with a limited solubility in water. Consequently, mixed solvent systems or acidic buffers have been used as reagent carrier solutions in the flow injection analysis systems. For example, ammonium diisopropyldithiophosphate in an aqueous-alcoholic medium and norfloxacin in ammonium sulfate-sulfuric acid medium have been used for iron determination. Asan et al (2003) described a method for the determination of Fe (III) using dimethylformamide (DMF) as a chelating agent in a flow injection system.81 DMF is a sensitive and selective reagent for Fe (III) for giving a 2:3 (Fe: DMF) complex with a sharp absorption peak at 310nm.Total iron in the range of 5-90ng ml-1 can be determined with this method.
A flow injection method was described by Yaqoob et al (2006) for the determination of iron in fresh water based on potassium permanganate chemiluminescence detection via oxidation of formaldehyde in aqueous hydrochloric acid.82 Total iron concentrations are determined after reducing Fe (III) to Fe (II) using hydroxylamine hydrochloride. The detection limit (three standard deviations of blank) is 1.0 nM, with a sample throughput of 120 h−1.
Asan et al (2008) proposed a highly sensitive and very simple spectrophotometric flow-injection analysis (FIA) method for the determination of iron (III) at low concentration levels81. The method is based on the measurement of absorbance intensity of the red complex at 410 nm formed by iron (III) and diphenylamine-4-sulfonic acid sodium salt (DPA-4-SA). Repeatability of the measurements was satisfactory at the relative standard deviation of 3.5 % for 5 determinations of 10 μg L−1 iron (III).
6. Redox determination:
Iron can be determined by titration of dichromate with iron (II) using indicators like diphenylamine, diphenylbenzidine, diphenyl amine sulphonate. The colour change for all three indicators is green to violet. The method applied as follows:
Prepare a standard solution by dissolving 0.4g in water and make up the volume 100ml.In a flask 0.7g of the iron (II) solid M provided .Add 30ml of diluted sulphuric acid, 100ml of water, 7ml of 85% phosphoric acid and 5 drops of diphenylamine sulfonate indictor. Titrate with dichromate to a purple colour. Calculate the percentage of iron in the solid M.83
Iron can also be determined by direct titration using variamine blue as a redox indicator and titrating with EDTA with end point of the titration from blue to grey just before end point and final to yellow.39 Iron can be determined by redox titration with an oxidant such as KMnO4 or K2Cr2O7 that converts Fe (II) to Fe (III). Potassium dichromate can be used as a primary standard by drying in oven at 150-2000C for two hours to remove any bound water. As the titration precedes the sample solution will turn green due to the presence of Cr3+. The endpoint is reached when the very fine yellow colour of the Cr6+ titrant appears. In oxidation-reduction titration Fe (II) solutions can be titrated with potassium dichromate solution. After all the Fe (II) has been oxidized, the endpoint of the titration can be recognized by the colour change (from green to yellow) when excess dichromate ion oxidizes.Iron can be determined by redox titration in which conversion of Fe (III) to Fe (II) with a suitable reducing agent and a redox indicator (diphenylamine), a colourless compound was used which was oxidized by dichromate to di-phenyl benzidine to form violet coloured compound.84
7. Electrochemical method:
The rising importance of fuel ethanol in economic Brazilian set as an alternative automotive fuel or raw material for alcohol chemical industry requires the identification and control of its contaminants, due to its potentiality for engine corrosion effects and passivation of industrial catalyzers. Several organic and inorganic contaminants have been studied in fuel ethanol by different instrumental methods. These contaminants can be originated from micro-nutrients used on sugar cane production, during the fermentation process, such as transport and storage of final product.
Under the analytical point of view, the use of chemically modified electrodes (CME) coupled to electrochemical methods has presented a considerable increase in the last decade, specially for dosage of metallic species.
Mattos et al (2008) presented a methodology for iron determination in fuel ethanol using a modified carbon paste electrode with 1, 10 Phenantroline/nafion.85 An accumulation time of 5 minutes, such as a 100 mV of pulse magnitude (Esw) and frequency (f) of 25 Hz were used as optimized experimental conditions.
The development of the proposed carbon paste-CME for iron detection employing square wave voltammetry has presented good results for determination of total iron in hydroalcoholic medium, allowing this methodology for iron determination in commercial fuel ethanol samples. When the official Flame atomic absorption spectroscopy (FAAS) method was used for comparison; a good correlation was obtained for iron dosage at fuel sample.85