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CHAPTER FOUR
Implementation of
Copper (II)-ethylenediaminetetraacetate
Absorbing System for Indirect
Spectrophotometric
Determination of Iron (III)
Determination of Iron by using Copper (II)-ethylenediaminetetraacetate Absorbing System
114
CHAPTER FOUR
Implementation of
Copper (II)-ethylenediaminetetraacetate Absorbing
System for Indirect Spectrophotometric
Determination of Iron (III)
Absorbance quenching capacity of Fe(III) towards the copper(II)-
ethylenediaminetetraacetate absorbing system, is implemented for measurement
of permittance in spectrophotometric determination of iron. As the cupric ion is
one of the serious interfering ions in the quantitative determination of Fe(III)
using ethylenediaminetetraacetic acid reagent, therefore accuracy in this
measurement is increased by employing the [Cu(II)-EDTA]2- absorbing system.
The use of this absorbing system allows the determination of Fe(III) in presence
of Cu(II). Similar to earlier study (of determination of bismuth), the stability
constant (log Kf) difference of [Cu(II)-EDTA]2- and [Fe(III)-EDTA]1- chelates is
also considered valuable for quantitative determination of iron. Stability of the
[Fe(III)-EDTA]1-chelate is more than [Cu(II)-EDTA]2- chelate, consequently the
surplus EDTA in the Fe(III) solution is determined by adding excess of Cu(II).
In this experiment, the solution of Fe(III) buffered at pH=1.5, was
initially treated with fixed and excess of EDTA reagent; after quantitative
chelation of Fe(III) as [Fe(III)-EDTA]1- the surplus EDTA was utilized for
generation of [Cu(II)-EDTA]2- absorbing system via adding fixed quantity of
Cu(II) solution. The sample solution with greater concentration of Fe(III) left the
smaller amount of surplus EDTA for generation of absorbing system and vice a
versa. As a result, the color intensity of [Cu(II)-EDTA]2- chelate was observed
inversely proportional and permittance directly proportional to the concentration
of Fe(III). Validity of this method was tested at 722 nm by measuring clearance
of the [Cu(II)-EDTA]2- system and the linearity of logarithm of clearance
(permittance) versus concentration of Fe(III) was used for determination of iron.
The effects of some important variables (such as pH, wavelength of
measurement, concentration of reagent, dilution of test solutions, role of foreign
ions, etc.) on permittance and permittance coefficients of the absorbing system
Determination of Iron by using Copper (II)-ethylenediaminetetraacetate Absorbing System
115
were studied and the average value of permittance coefficient was used for
determination of iron. The method was successfully applied for quantitative
determination of iron in synthetic mixture of cations and pharmaceutical
preparations.
4.1 Introduction and Review of Literature
Iron, the most abundant element in the earth’s crust (5.6 % by mass), is
immensely important both in human civilization and in living systems [1]. Iron is
so widely diffused in nature, in both divalent and trivalent oxidation states
combined as ferrous and ferric compounds [2]. The ferrous iron has light green
color while the ferric iron is yellow in color, but ferric iron produces the red-
colored complex with thiocynate solution while ferrous iron yields no coloration
[3]. Iron plays an important role in biological processes. In living systems iron is
an essential constituent of numerous biomolecules. The body of a healthy human
adult contains about 4 to 5 g of iron, 65% of which is present in hemoglobin and
muscle hemoglobin [1, 4] that is myoglobin [5]. Ferrous ion is the central
structural unit of hemoglobin and myoglobin, performing the function of binding
of molecular oxygen through transferring an electron by self oxidation to ferric
ion [5]. For the deficiency of iron, the ferrous ion (generally in the form of
ferrous ascorbate or ferrous fumarate) being used for medical purposes and can
be taken internally without danger.
Iron compounds found many applications in medicines and also used for
preparation of analytical regents for spectrophotometric [6-11] analyses.
Chemically modified working electrode of iron compounds finds extensive
applications in voltammetric [12-18] analysis (Table 4.1). As the uses of iron in
medicine increases, it has spread in the environment and the chance of exposure
of organisms to iron has increased. Therefore, determination of iron at ultra-trace
levels in environmental and biological samples is important.
Several methods have been developed for determination of iron (Table
4.2). These include direct spectrophotometric [19-24] assay using specific
reagent, simultaneous spectrophotometric determination with other metal [25-26]
analyte, derivative spectrophotometric methods [27-29], photometric titrations
[30-31], normal flow injection spectrophotometric [32-34] methods for
determination of iron, flow injection simultaneous [35-36] spectrophotometric
determination of Fe(III) along with other analyte, flow injection simultaneous
Determination of Iron by using Copper (II)-ethylenediaminetetraacetate Absorbing System
116
[37-42] spectrophotometric determination of Fe(II) and Fe(III), reverse flow
injection [43-44] spectrophotometric methods, flow injection luminescence
spectrometry [45], voltammetric methods of determination of iron [46-49] and
total iron[50-53], potentiometric methods [54-55], electrothermal inductively
coupled plasma-optical emission spectrometry [56], titrimetric methods [57] and
indirect spectrophotometric method [58]. However, most of these above
mentioned methods are highly sensitive but are economically unsound since
those require costly sophisticated analytical instruments. Some of the methods
(listed in Table 4.2) need costly and poisonous complexing reagents for detection
and determination of iron in the sample.
The disodium salt of ethylenediaminetetraacetic acid, EDTA has found
considerable use as a standard reagent for determination of numerous metals by
photometric titration. Sweeter and Bricker have reported good photometric
titration of ferric iron with EDTA by using salicylic acid as indicator for ferric
ions [30]. The information of stability constant (log Kf) of [Fe(III)-EDTA]1- and
[Cu(II)-EDTA]2- chelates and their absorption spectra are the parameters utilized
by Underwood for simultaneous determination of iron and copper in a single
photometric titration [31]. Although detection of the exact end point by graphical
means is tedious and time consuming route yet photometric titration methods are
consistently used, since the presence of other substances absorbing at the same
wavelength does not necessarily cause the interference, in as much as only the
change in absorbance is significant [59].
The log Kf value [60-61] of EDTA complexes of iron and copper
are reported as 24.23 and 18.70 respectively, these values are sufficiently larger
indicates both chelates have satisfactorily stability; however, [Fe(III)-EDTA]1- is
more stable than [Cu(II)-EDTA]2-. The wide difference in these stability
constants permits for iron to react with EDTA first in presence of copper
consequently, copper ions functioning as an indicator [59] in iron titration as well
as allows for simultaneous [59] determination of both metals in a single
photometric titration. The same concept of difference in log Kf values of [Fe(III)-
EDTA]1- and [Cu(II)-EDTA]2- complex was exercised for spectrophotometric
determination of iron through taking the advantage of copper ion as an indicator
for determination of surplus EDTA.
Determination of Iron by using Copper (II)-ethylenediaminetetraacetate Absorbing System
117
TABLE 4.1
Analytical applications of iron and its compounds in spectrophotometric and
voltammetric studies
Characteristic of the method Ref.
Spectrophotometric determination perphenazine is carried out using
potassium ferricyanide-Fe(III) system. In the presence of potassium
ferricyanide, Fe(III) is reduced to Fe(II) by perphenazine at pH 4.0. The
absorbance of this Prussian blue thus formed is measured at 735 nm.
[6]
The ability of negatively charged phosphatidates to form complexes
with Fe(III) ions is used for spectrophotometric assay of phosphatidic
acid, phosphatidic acid extracts Fe(III) ions from purple colored Fe(III)–
salicylate and decreases the absorbance measured at 490 nm.
[7]
Fe(III) is reduced to Fe(II) by captopril, then the in situ formed Fe(II)
reacts with potassium ferricyanide to give the soluble prussian blue at
pH 4, which is measured at 735 nm.
[8]
The magnetic iron oxide nanoparticles used as an adsorbent for
extraction, preconcentration and determination of traces of fluoride ions.
The determination method is based on the discoloration of Fe(III)–SCN
complex with extracted fluoride ions which was subsequently measured
at 458 nm.
[9]
Ascorbic acid reduces Fe(III) to Fe(II), the reaction of the produced
Fe(II) with 1,10-phenanthroline in a weak acidic medium to form a
colored complex.
[10]
The Fe(III) plus 1,10-phenanthroline reagent was used for determination
of organic compounds, via its reaction with phenolic compounds. By a
redox reaction the reagent forms the chelate [Fe(phen)]+2, which is
extractable as ion-association complexes.
[11]
Determination of Iron by using Copper (II)-ethylenediaminetetraacetate Absorbing System
118
An electrochemical approach was applied for detection of the choline
using prussian blue modified iron phosphate nanostructures. A catalytic
property the nanostructure towards electro-reduction of H2O2 used an
amperometric choline biosensor was used for determination of choline.
[12]
Chemically modified carbon paste electrodes based on Fe(III)-schiff
base containing graphite pastes were prepared and evaluated as
electrochemical sensor for the voltammetric determination of
mefenamic acid and indomethacin in aqueous solutions by differential
pulse voltammetry in at pH 3.5.
[13]
A thiolated 19-mercapture probe was attached to gold coated ferric
oxide nanoparticles and a square wave voltammetric procedure was
applied for the detection of specific hybridization processes, using a
disposable magnetic DNA sensor.
[14]
Fe(II) phthalocyanine modified multi-wall carbon nanotubes paste
electrodes were used as voltammetric sensors to selectively detection of
dopamine in the presence of serotonin (5-HT).
[15]
A carbon-paste electrode modified with iron (II)-phthalocyanine was
used for the sensitive voltammetric determination of epinephrine. The
electrochemical response characteristics of the modified electrode
toward epinephrine, ascorbic acid and uric acid were investigated by
cyclic and differential pulse voltammetry.
[16]
Chemically modified electrode is constructed by doping Fe(III) on
zeolite modified carbon paste, which was used for determination of
tryptophan, uric acid and ascorbic acid at pH 3.5 by application of the
differential pulse voltammetry.
[17]
Titrimetric, potentiostatic coulometric and electrodeposition methods
are employed for determination of total iron, Fe(II), Fe(III) in presence
of copper and copper in the slag sample.
[18]
Determination of Iron by using Copper (II)-ethylenediaminetetraacetate Absorbing System
119
TABLE 4.2
Analytical methods of quantitative determination of iron in different types of
samples by various analytical techniques
Characteristic of the method Ref.
A nucleophile formed from Fe(III) oxidized catechol in 0.1 M HCl
couple with o-tolidine, system 1/p-toluidine, system 2 to produce dye
product, measured at 520 nm.
[19]
Fe(II) after complexation with o-phenanthroline and coupled with
picrate anion and extracted by dispersive liquid-liquid microextraction
for spectrophotometric determination of iron in water samples.
[20]
The 2, 4, 6-Trimorpholino 1, 3, 5-triazine gives complex with Fe(III)
which absorbs in the ultraviolet region. The Fe(III) alone or with
cationic surfactants is titrated with this reagent.
[21]
The N, N’-bis (2-hydroxy-5-bromo-benzyl) 1, 2 diaminopropane reacts
with Fe(III) at pH 3–6 to produce a red complex soluble in chloroform
is measured for determination of iron.
[22]
A selective spectrophotometric determination of both Fe(II) and Fe(III),
has been carried out using a melamine-formaldehyde resin for
concentration of Fe(III) at pH 5. The Fe(II) was not retained, which is
oxidised to Fe(III) concentrated on a second resin column. The iron in
both columns was eluted with 1 M HCI solution and separately analyzed
by the 1, 10-phenanthroline-citrate spectrophotometric method.
[23]
Tiron (1, 2-dihydroxybenzene p-3, 5 disulfonic acid) chromophoric
reagent used for determination of Fe(III).
[24]
A multivariate calibration model was developed for the simultaneous
spectrophotometric determination of Al(III) and Fe(III) in
posthemodialysis fluids with pyrocathecol violet as chromogenic
reagent at 580 nm.
[25]
Determination of Iron by using Copper (II)-ethylenediaminetetraacetate Absorbing System
120
Metal ions such as Co(II), Ni(II), Cu(II), Fe(III) and Cr(III), present in
electroplating baths, were analysed simultaneously by a spectrophoto-
metric method by the ethylenediaminetetraacetate (EDTA) solution as a
chromogenic reagent.
[26]
The formation of the binary complexes of iron and ruthenium with 4, 7-
diphenyl-1, 10-phenanthroline (bathophenanthroline) in the presence of
ethyleneglycol and ruthenium and iron in mixture were determined by
second derivative spectrophotometric method.
[27]
The complexes of Al(III) and Fe(III) ions with chrome azurol S are
formed at pH 5.5 and these metals are determined by first-derivative
spectra at 675 and 623.5 nm, respectively.
[28]
Copper and iron in mixtures separated by liquid-liquid extraction as
picrate ion pairs. Iron-picrate, reacts in the organic phase with 5-phenyl-
3-(4-phenyl-2-pyridinyl)-l, 2, 4-triazine. The copper-picrate reacts with
2, 9-dimethyl-4, 7-diphenyl-l, 10-phenantroline. The extracts were
evaluated directly by second derivative spectrophotometry.
[29]
Photometric titration of ferric iron with EDTA has been carried out by
using salicylic acid as indicator for ferric ions.
[30]
The difference in the stability constant of [Fe(III)-EDTA]1- and
[Cu(II)-EDTA]2- chelates and their absorption spectra are the parameters
utilized for simultaneous determination of iron and copper in a single
photometric titration.
[31]
The catalytic effect of Fe(III) on the oxidative coupling of p-anisidine
with N, N-dimethylaniline to form a blue compound measured at 735nm
in the presence of H2O2 and 1,10-phenanthroline as an activator, and the
inhibitory effect of EDTA on the Fe(III)-catalyzed reaction.
[32]
Sodium 3-(2-pyridyl)-5, 6-diphenyl-1, 2, 4-triazine-4%, 4-disulphonate
(ferrozine) used for the determination of iron by normal FIA and
reversed FIA.
[33]
Determination of Iron by using Copper (II)-ethylenediaminetetraacetate Absorbing System
121
The production of I3- from the Fe(II) or Fe(III)-induced perbromate-
iodide reaction is monitored at 353nm, by a FIA spectrophotometry.
[34]
Simultaneous determination of iron and zinc in the human hair with 2-
(5-bromo-2-pyridylazo)-5-diethylaminophenol (5-Br-PADAP) using a
pH gradient technique by FIA method.
[35]
The ferroin complex of iron and azomethine-H reaction product used for
the spectrophotometric determination of available iron and boron in soil
extracts, by computer-controlled multi-syringe flow injection systems.
[36]
The simultaneous spectrophotometric determination of Fe(II) and
Fe(III) at 530 nm has been carried out by sulfosalicylic acid and 1,10-
phenanthroline as indicators for ions respectively. EDTA solution is
injected into a carrier stream (HNO3) and EDTA replaces sulfosalicylic
acid, forming a more stable colourless complex with Fe(III), whereas
Fe(II) remains in a complex with 1,10-phenenthroline.
[37]
The flow injection gradient titration in a system of reversed flow is used
for simultaneous spectrophotometric determination of Fe(III) and Fe(II),
at wavelength at 530nm when radiation is absorbed by both complexes.
After injection EDTA replaces sulfosalicylic acid and forms with Fe(III)
more stable colourless complex.
[38]
The iron in tap and natural water samples is acidified to decompose iron
hydroxide and iron-complexes into free iron, Fe(III) and Fe(II). The
amounts of free iron were detected using a catalytic action of Fe(III) and
Fe(II) on the oxidation of N, N-dimethyl-p-phenylenediamine in the
presence of H2O2 at 514 nm.
[39]
The different kinetic-catalytic behaviour of Fe(II) and Fe(III) in the
redox reaction between leucomalacmte green and peroxodisulphate with
and without the presence of the activator l, l0-phenanthroline, are used
for simultaneous spectrophotometric determination Fe(II) and Fe(III).
[40]
Determination of Iron by using Copper (II)-ethylenediaminetetraacetate Absorbing System
122
The direct spectrophotometric determination of Fe(III) and Fe(II) by
FIA with acetohydroxamic acid and 1, 10-phenanthroline as reagents.
[41]
Fe(III) is determined in a FIA by passage through reduction before
spectrophotometric detection with l, 10-phenanthroline in citrate buffer,
pH 5.0 and then Fe(II) and total iron are determined by splitting the
injected sample so that a portion passes through the reductor column.
[42]
A simple reversed FIA colourimetric procedure for determination of
Fe(III) in which, the reaction between Fe(III) with chlortetracycline,
resulting in an intense yellow complex measured at 435 nm.
[43]
The reaction between Fe(III) with norfloxacin in ammonium sulfate
solution, gives an intense yellow complex measured at 435 nm by a
reversed FIA colorimetric method.
[44]
A FIA technique used for determination of Fe(II) and total-Fe in water
samples via preconcentration on the Amberlite XAD-4 resin,
functionalized by N-hydroxyethylethylenediamine groups, and
chemiluminescence detection, using brilliant sulfoflavine plus H2O2.
[45]
Differential-pulse cathodic stripping voltammetry with a carbon paste
electrode have been used for determination of iron in 5-aminoiso-
phthalic acid.
[46]
Chemically modified carbon paste electrodes containing 1, 10-
phenanthroline and Nafion as modifiers were used for preconcentration
Fe(II) and determined by differential pulse voltammetry.
[47]
Cathodic stripping voltammetric determination of total Fe has been
carried out by a chitosan modified glassy carbon electrode.
[48]
Brilliant green exhibits a sensitive differential polarographic wave in
dilute H2SO4. Fe(III) has a strong catalytic effect on the slow photo-
chemical reaction of brilliant green under ultraviolet irradiation in the
[49]
Determination of Iron by using Copper (II)-ethylenediaminetetraacetate Absorbing System
123
presence of oxalic acid as activator. The changes in concentration of
brilliant green can be monitored by polarography.
Pre-reduction of Fe(III) in the presence of hexacyanoferrate (II) and 1,
10-phenanthroline, to form Fe(II)-1, 10-phenanthroline, on the
unmodified carbon electrode surface and total iron was then determined
voltammetrically by oxidation of the Fe(II)-1,10-phenanthroline.
[50]
The voltammetric characteristics of Fe(III) oxinate at a mercury
electrode, in the presence of 0.2M tributylam monium perchlorate and
0.2M tributylamine as the supporting electrolyte in chloroform have
been used with a two electron quasi-reversible process for the reduction
of Fe(III)-oxinate
[51]
Fe(II) and Fe(III) complexes of pyrophosphate determined simultane-
ously at pH 9 by differential pulse anodic stripping voltammetry.
[52]
A Z-type flow cell is operated simultaneously as an optical flow cell for
the spectrophotometric determination of Fe(III) by monitoring the
absorbance of the sulfosalicylic acid-Fe(III) complex at 505 nm and as
an electrochemical flow cell for amperometric monitoring of the current
due to the oxidation of Fe(II) to Fe(III).
[53]
The potentiometric and voltammetric measurements of Fe(II) and
Fe(III), are performed at ionomer coated glassy carbon electrode.
[54]
Ferroin with a PVC membrane sensor containing ferroin-TPB as an
electroactive component plasticized with 2-nitrophenyl phenyl ether is
used for both batch and FIA of Fe(II) or Fe(III).
[55]
A solid phase extraction technique for the speciation of trace dissolved
Fe(II) and Fe(III) in environmental water samples was used by coupling
micro-column packed with N-benzoyl-N-phenylhydroxylamine loaded
on microcrystalline naphthalene to electrothermal vaporization
inductively coupled plasma-optical emission spectrometry.
[56]
Determination of Iron by using Copper (II)-ethylenediaminetetraacetate Absorbing System
124
The metallic iron, ferrous oxide, and ferric oxide are estimated when
present together. The sample is treated with bromine dissolved in
ethanol, and filtered. Iron in the filtrate is titrated iodometrically, and
corresponds to the metallic iron present in the mixture. The oxide
residue is dissolved in HCl in CO2 atmosphere. The Fe(II) formed,
equivalent to FeO present, is titrated with a standard vanadate solution,
and the total iron(III) (= FeO + Fe2O3) in the titrated solution is then
estimated iodometrically.
[57]
An indirect spectrophotometric continuous-flow method was used for
the determination of trace amounts of complexing agents such as
EDTA, DTPA, CyDTA, EDTA-OH, NTA, citrate and pyrophosphate
[58]
In this method, the sample solution of Fe(III) buffered with chloroacetic
acid was treated first with measured and excess of EDTA reagent; after
quantitative chelation of Fe(III) as [Fe(III)-EDTA]1- the surplus EDTA was
utilized for generation of [Cu(II)-EDTA]2- absorbing system via adding measured
quantity of Cu(II) solution. The solution with greater concentration of Fe(III) left
the smaller amount of surplus EDTA (for generation of absorbing system) and
vice a versa. As a result, the color intensity of [Cu(II)-EDTA]2- chelate was
observed inversely proportional to the concentration of iron. Consequently,
permittance [62] of the absorbing system was found directly proportional to the
concentration of iron. Thus, the absorbance quenching [62] action of iron analyte
on to the [Cu(II)-EDTA]2- absorbing system was worked out in alternative
manner for determination of iron.
4.2 Experimental
4.2.1 Instruments and accessories
1. Shimadzu double beam UV-Visible spectrophotometer with the quartz
rectangular cuvettes was used for measurement of %T.
2. Equiptronics pH-meter (EQ-610) was used to check the pH of solutions.
4.2.2 Reagents and chemicals:
All chemicals used were of analytical reagent grade and were used without
further purification. One liter each of the following reagents were prepared.
Determination of Iron by using Copper (II)-ethylenediaminetetraacetate Absorbing System
125
(1) 0.0179M Fe(III) solution [viz. 1.0 mg ml-1 of iron] was prepared by
dissolving 8.635 g. of FeNH4(SO4)2.12H2O in minimum quantity of conc.
HNO3, followed by dilution with distilled water containing sufficient HNO3
to make the final solution in 0.5M HNO3. This solution of Fe(III) was
standardized against standard EDTA solution using chloroacetate buffer and
salicylic acid in acetone as an indicator.
(2) 0.05M EDTA solution was prepared by dissolving 18.612 g. of
Na2H2EDTA.2H2O in distilled water.
(3) 0.05M Cu(NO3)2 solution was prepared by dissolving 12.080 g. of
Cu(NO3)2.3H2O in distilled water. The solutions of EDTA and Cu(NO3)2 are
termed here as absorbing system’s reagents.
(4) 1.0 M CH2ClCOOH solution was prepared by dissolving 94.500 g. of
monochloroacetic acid in distilled water. The solution of chloroacetate was
used as buffer.
(5) 0.5M HNO3 solution was prepared by diluting 32.0 ml of conc. HNO3 with
distilled water.
(6) The iron tablets, Livogen-z (contains 50mg of elemental iron) manufactured
by Merck India Ltd., Ferium-xt and Orofer-xT (both tablet contains 100mg of
elemental iron) manufactured by Emcure Pharmaceuticals Ltd. were used for
determination of iron.
3. Analytical Method for Determination of Iron
The method proposed for determination of iron was tested primarily with
the standard solution of Fe(III). The test solutions (TS) of iron in the range of
0.001g to 0.010 g were prepared by adding 1.0 ml, 2.0 ml, …, to 10.0 ml aliquots
of 1.0 mg ml-1 Fe(III) solution sequentially into 50ml graduated flasks each
containing 5.0 ml of 1.0 M CH2ClCOOH as a buffer solution. To equalize the
proton ions concentration, 9.0 ml, 8.0 ml,…, to 0.0 ml aliquots of 0.5M HNO3
were also added in descending order into these volumetric flasks numbered as 2
to 11. After addition of 5.0 ml of 0.05M EDTA solution reaction mixture in the
flask were shaken thoroughly for quantitative chelation of Fe(III) with EDTA and
5.0ml of 0.05M Cu(NO3)2 solution was added for utilization of the surplus EDTA
and generation of [Cu(II)-EDTA]2- absorbing system. The reaction mixtures were
further diluted up to the mark with distilled water. Excluding only Fe(III)
Determination of Iron by using Copper (II)-ethylenediaminetetraacetate Absorbing System
126
solution, the reagent blank (RB) solution was prepared in flask No.1 with 10.0ml
0.5M HNO3. The true blank (TB) or reference solution was also prepared in the
similar way using 14.0 ml of 1.0 mg ml-1 Fe(III) solution. The added 5.0 ml of
0.05M EDTA was completed utilized for chelation of the iron present in this
flask, therefore, [Cu(II)-EDTA]2- absorbing system was not generated in TB
solution, so act as a reference. The visible absorption spectrum of [Cu(II)-
EDTA]2- absorbing system (viz. RB solution) was obtained against the TB
solution, and a wavelength 722 nm was selected for measurement. The %T of RB
as well as each TS was measured at 722 nm against TB as a reference. The %T of
the RB was used for obtaining the clearance value of test solutions. The graph of
logarithm of clearance viz. permittance against the concentration of Fe(III) was
used for determination of iron. An analogous method was employed for
quantitation of iron in Livogen-z, Ferium-xt and Orofer-xt tablets.
4.4 Results and Discussion
The hexadentate chelating reagent, ethylenediaminetetraacetic acid gives
remarkably stable chelates with Fe(III) and Cu(II) in one-to-one stoichiometry
[63]. Reilley and Schmid [64] showed the minimum pH needed for satisfactorily
chelation of various cations with EDTA. The strongly acidic pH (in the range of
1-2) can be accepted by the trivalent metal cation for complexation with EDTA.
Therefore, the quantitative chelation of Fe(III) with EDTA can be attainable at
pH 1-1.5. Wagreich and Harrow [65] also showed that, EDTA has enormous
nucleophilic capability to chelate the cupric ion over a broad pH range and the
stability of [Cu(II)-EDTA]2- chelate is not much affected by the pH of solution.
Even at acidic pH the reaction of Cu(II) with EDTA results with generation of
intensely blue colored [Cu(II)-EDTA]2- chelate; which was employed as the
absorbing system at 745 nm for spectrophotometric determination of copper [30].
The earlier study of quantitative determination of bismuth was carried out at pH
1.0-1.25 via generating the [Cu(II)-EDTA]2- absorbing system and the same
absorbing system was exploited here for spectrophotometric determination of
iron.
4.4.1 Determination of Surplus EDTA
The formation constant (log Kf) value [60-61] of [Fe(III)-EDTA]1- and
[Cu(II)-EDTA]2- chelates are reported 24.23 and 18.70 respectively, which
Determination of Iron by using Copper (II)-ethylenediaminetetraacetate Absorbing System
127
indicates that both chelates have enough stability but iron chelate is much more
stable than copper chelate. The sufficient difference (about 5.53) in these log Kf
values permits to utilized surplus EDTA in the test solution for generation of
[Cu(II)-EDTA]2- absorbing system, without disturbing the stability of iron
chelate. Furthermore, copper chelate exhibits maximum absorption in the visible
region where other species in the test solution exhibits nil absorption.
4.4.2 Effect of [H+] on the Linearity
The stability of [Cu(II)-EDTA]2- absorbing system is not much affected
by small change in pH of the solution [66]. But variable concentration of proton
in test solutions affects the ionization of EDTA that reflects an adverse effect on
the analytical performance of the method. Because variable aliquots (1.0 ml to
10.0 ml) of the standard solution of iron (having concentration 1.0mg ml-1
prepared in 0.5M HNO3) used for preparation test solutions, those carry the
different concentration of proton. Consequently, the solution of 0.5M HNO3 was
added in descending order (from 9.0 ml to 0.0 ml) for compensation of effect of
the H+ ions on the ionization of EDTA. The proposed method involves the
measurement of permittance of test solution (TS) in comparison with permittance
of the reagent blank (RB) solution, so excluding only the sample, the composition
of both solutions was kept essentially identical. Moreover, addition of buffer
species ensures quantitative chelation of both metals through nullification of the
effect of protons released from EDTA during complexation reactions.
4.4.3 Selection of the Wavelength for Measurement
In the proposed method, the quantitative determination of iron was carried
through measurement of permittance of [Cu(II)-EDTA]2- absorbing system. The
optical density of absorbing system indicates the concentration of surplus EDTA
in test solution. Therefore, for attending the greater sensitivity, it was necessary
to carry out measurement at the wavelength to which absorbing system shows
absorption maxima (max). For this purpose, the visible absorption curve of
[Cu(II)-EDTA]2- chelate (viz. RB solution) was obtained against the TB solution
as a reference, both were prepared as described in the method. The spectrum
study showed that, [Cu(II)-EDTA]2- chelate in chloroacetic acid and nitric acid
medium exhibits absorption maxima at 722 nm. Therefore, measurement of %T
Determination of Iron by using Copper (II)-ethylenediaminetetraacetate Absorbing System
128
was carried out at 722 nm to which all other species are transparent, except the
free Cu(II) ions have little absorbancy at this wavelength. Therefore, equivalent
amount of it also added in TB or reference solution for compensating the
background absorbance of unused Cu(II) ions. Though the concentration of
unused/free Cu(II) ions was not identical in all test solutions, but that does not
much affect the analytical linearity of permittance versus concentration of iron.
Only Cu(II) ions have slight absorbancy at 722 nm, therefore, other blank
solution (copper blank) was also prepared simply by diluting with distilled water
(to 50.0 ml) the experimental volume of cupric nitrate, nitric acid and
chloroacetic acid solutions. The absorption spectrum of the RB solution when
obtained against copper blank, then also absorbing system proved its absorption
maxima at 722 nm. The results obtained against true blank do not differ
significantly from those obtained against copper blank. So, all the measurements
were carried out using free copper ions solution as a blank or reference.
4.4.4 Analytical Performance of the Method
The test solutions of iron prepared as mentioned in method confirms that,
at the measured and excess concentration of EDTA, the optical density of
[Cu(II)-EDTA]2- system was directly proportional to the concentration of surplus
EDTA and inversely proportional to the concentration of iron. Because, the
extent of chelation reaction that occurred between Fe(III) and excess of EDTA is
governed by only the concentration of Fe(III). Therefore, the concentration of
surplus EDTA left after complexation of Fe(III) was inversely proportional to the
concentration of iron and hence permittance of test solutions was observed
directly proportional to the concentration of iron. After measurement of %T at
722 nm of [Cu(II)-EDTA]2- absorbing system (each TS and RB) against TB as a
reference, the clearance of test solution was calculated and the logarithm of
clearance (permittance) of the absorbing system was determined by using
following eq.(1). It was observed that, permittance of the test solution is linear
function of concentration of iron and was dependent only on the concentration of
iron. Therefore, the linearity of permittance against the concentration of iron was
used for construction of calibration curve. With 1.0 cm optical path of absorbing
system the proportionality constant determined for determination of iron.
Determination of Iron by using Copper (II)-ethylenediaminetetraacetate Absorbing System
129
4.4.5 Permittance and Permittance Coefficient
The permittance (Pr) of the test solution was calculated by using following
equation (1)
%TTSPr = log --------- --- (1)
%TRB
In this equation, %TTS and %TRB designates the percent transmittance of the test
solution and reagent blank solution respectively, measured against the same
reference solution. The relationship between permittance (Pr) of test solution and
concentration (c) of quencher analyte in it was reported in earlier study and was
used here for determination of proportionality constant/permittance coefficient
(a') of 1.0cm path length (b) absorbing system.
Pra' = ------- (concentration of quencher analyte in g L-1) --- (2)
b c
In this experiment, the quantitative determination of iron in the range of
1.0mg to 10.0mg was carried out at 722 nm by using different volumes (5.0ml,
6.0ml, 7.0ml and 8.0ml) of the absorbing system’s reagents. The copper blank
solution used as a reference for these measurements. Permittance value of the test
solutions was observed directly proportional to the concentration of iron (Table
4.3) and even if the concentration of reagents was altered, the permittance is
constant for a fixed concentration of iron. Similarly to earlier the study in this
experiment it is observed that, at a fixed wavelength, permittance is dependent
only on concentration of the analyte and is independent on the concentration of
absorbing system’s reagents. At 722 nm, the permittance coefficient at every
different concentration of Fe(III) as well as the volume of reagents is also nearly
constant.
The values of permittance (Table 4.3) are little bit more at higher
concentration of iron because, the yellow color intensity of ferric-EDTA chelate
fades the blue color intensity copper-EDTA chelate (have low concentration at
higher concentration of iron) through generating the green color to test solution.
This effect also increases value of the permittance coefficient corresponding
more, so the average value of permittance coefficient was considered for
determination of analyte. In these determinations (Table 4.3) the average value of
Determination of Iron by using Copper (II)-ethylenediaminetetraacetate Absorbing System
130
permittance coefficient was found equal to 0.5168 lit.g-1cm-1 and which was for
determination concentration of the stock solution of iron (1.0 mg ml-1) from
which the test solutions were prepared.
4.4.6 Magnitude of Permittance and Permittance Coefficient
The concentration/volume of reagents does not affect the permittance as
well as permittance coefficient (Table 4.3) but dilution (of the test solutions)
factor shows pronounced effect on permittance. This study was carried out
through quantitative determination of iron in the range of 1.0 mg to 10.0 mg
(with the 5.0ml volume of the reagents) at the 25 ml, 50 ml and 100 ml final
dilution of test solutions. The result of these assay (is summarized in (Table 4.4),
elucidates that, the value of proportionality constant [calculated with eq. (2)] was
observed decreasing with increase in final dilution volume of test solutions. This
is because, the number of absorbing species per unit path length in the solution
decreases with dilution. For result reported in Table 4.2, the average
proportionality constants are 20.656 at 25.0 ml, 10.338 at 50 ml and 5.171 at 100
ml final dilution. From this it is clear that, when dilution volume of test solutions
is doubled, the proportionality constant was decrease nearly equal to half but at
one liter dilution is permittance coefficient (measured in lit.g.-1cm-1) was
observed unchanged.
The second important factor which affects the magnitude of permittance
and permittance coefficient is the wavelength selected for measurement. The
wavelength determines the extent of absorption and hence the readings of %T;
consequently, execute the marked effect on the values of these two parameters.
The [Cu(II)-EDTA]2- absorbing system in chloroacetic acid and nitric acid
medium was showed the max at 722nm. Along with max wavelength, when
same test solutions were measured at other wavelengths that generates the
different values for permittance and permittance coefficient. That means the
magnitude of permittance and also permittance coefficient (a') is absolutely
administrated by the wavelength selected for analysis. The magnitude of both of
these parameters was observed maximum at system’s max wavelength.
Therefore, for achieving the greater sensitivity for the method, measurements
were carried out at the absorption maxima (722 nm) of the absorbing system.
Determination of Iron by using Copper (II)-ethylenediaminetetraacetate Absorbing System
131
TABLE 4.3
Effect of concentration of reagents on permittance and permittance coefficient;
results obtained in quantitative determination of iron at 50 ml dilution with
different volume of absorbing system’s reagents
5.0ml 6.0mlIron(g)
Pr Pr. Coeff. Pr Pr. Coeff.
0.000 0.0000 --- 0.0000 ---
0.001 0.0103 0.5153 0.0102 0.5114
0.002 0.0204 0.5108 0.0205 0.5121
0.003 0.0307 0.5125 0.0310 0.5160
0.004 0.0415 0.5183 0.0409 0.5116
0.005 0.0515 0.5152 0.0518 0.5179
0.006 0.0614 0.5117 0.0614 0.5119
0.007 0.0722 0.5154 0.0727 0.5191
0.008 0.0836 0.5225 0.0833 0.5206
0.009 0.0939 0.5216 0.0941 0.5225
0.010 0.1048 0.5241 0.1048 0.5239
Average value: 0.51674 -- 0.51669
TABLE 4.3 Continues…
7.0ml 8.0mlIron(g)
Pr Pr. Coeff. Pr Pr. Coeff.
0.000 0.0000 --- 0.0000 ---
0.001 0.0102 0.5117 0.0103 0.5136
0.002 0.0206 0.5148 0.0204 0.5096
0.003 0.0307 0.5118 0.0309 0.5143
0.004 0.0414 0.5177 0.0409 0.5117
0.005 0.0515 0.5146 0.0517 0.5171
0.006 0.0617 0.5145 0.0624 0.5202
0.007 0.0724 0.5172 0.0722 0.5158
0.008 0.0835 0.5220 0.0836 0.5222
0.009 0.0941 0.5227 0.0939 0.5217
0.010 0.1045 0.5227 0.1044 0.5222
Average value: 0.51697 -- 0.51684
Determination of Iron by using Copper (II)-ethylenediaminetetraacetate Absorbing System
132
TABLE 4.4
Effect of final dilution of test solutions; results obtained in quantitative
determination of iron in the range of 1.0 mg to 10.0 mg using 5.0 ml of 0.05M
EDTA and 5.0 ml of 0.05M Cu(NO3)2 reagents at different dilutions
25ml dilution 50ml dilutionIron(g)
Pr. Pro.Const. Pr.Coeff. Pr. Pro.Const. Pr.Coeff.0.000 0.000 -- -- 0.000 -- --
0.001 0.0206 20.62 0.5155 0.0103 10.31 0.5153
0.002 0.0413 20.65 0.5163 0.0205 10.25 0.5124
0.003 0.0619 20.64 0.5160 0.0309 10.31 0.5154
0.004 0.0826 20.65 0.5161 0.0415 10.37 0.5183
0.005 0.1030 20.59 0.5148 0.0513 10.26 0.5129
0.006 0.1237 20.61 0.5152 0.0615 10.25 0.5127
0.007 0.1445 20.64 0.5160 0.0722 10.31 0.5154
0.008 0.1653 20.67 0.5166 0.0831 10.39 0.5196
0.009 0.1863 20.70 0.5175 0.0941 10.46 0.5230
0.010 0.2081 20.81 0.5202 0.1048 10.48 0.5241
Average value: 20.656 0.51641 -- 10.338 0.51691
TABLE 4.4 Continues…
100ml dilutionIron(g)
Pr. Pro. Const. Pr. Coeff.0.000 0.000 -- 0.000
0.001 0.0052 5.17 0.5172
0.002 0.0103 5.14 0.5142
0.003 0.0155 5.16 0.5163
0.004 0.0207 5.17 0.5171
0.005 0.0258 5.16 0.5163
0.006 0.0309 5.15 0.5148
0.007 0.0362 5.18 0.5178
0.008 0.0415 5.19 0.5192
0.009 0.0466 5.18 0.5180
0.010 0.0520 5.20 0.5202
Average value: 5.171 0.51712
Determination of Iron by using Copper (II)-ethylenediaminetetraacetate Absorbing System
133
4.4.7 Determination of Iron in Iron Tablets
The proposed method was applied for determination of iron in Livogen-z,
Ferium-xt and Orofer-xT tablets. The sample (two tablets of Livogen-z, one
tablet of Ferium-xt, or one tablet of Orofer-xT) was heated with 15 ml of conc.
HCl followed by addition of 5 ml of conc. HNO3. The organic matter was
destroyed by treatment with 5-6 ml of 70% perchloric acid. (WARNING: Boiling
perchloric acid can result in serious explosions). The solution was slowly heated
in fuming hood for about 20-25 minutes; at this stage maximum of the acid fumes
were ceased. With the addition of 10ml of distilled water, the sample solution
was again boiled for 10 minutes. The solution was cooled and diluted nearly to
90ml with distilled water. With drop wise addition of conc. HNO3, the pH of
sample solution was adjusted equal to pH (=0.48) of standard solution containing
1.0mg ml-1 iron. The sample solution was filtered after dilution to 100ml and the
amount of iron was determined by the recommended method. The results of this
determination are represented in Table 4.5.
The iron tablets contain ferrous iron generally in the form of ferrous
ascorbate or ferrous fumarate. When the tablets were digested with these acids,
the ferrous iron gets oxidized to ferric iron. The concentration of iron in the
sample solutions of tablet (two tablets of Livogen-z, one tablet of Ferium-xt, or
one tablet of Orofer-xT) at 100ml dilution is 1.0mg ml-1. Therefore, 1.0 ml to
10.0 ml of sample aliquots were tested for determination of iron in the range of
1.0 mg to10.0 mg and the value of the permittance coefficient thus obtained was
used for determination of concentration of iron in stock solution of iron tablets.
That is, the accuracy of method was studied with permittance coefficient; values
obtained in the sample analysis are compared with those obtained in the analysis
of standard Fe(III) solution of same strength and same pH.
4.4.8 Interferences in the Determination of Iron
The type of interference can be predicted form the formation constant of
the EDTA chelates of the metal cations. Therefore, the interfering cations can be
classified into two groups. The first group is composed of those cations whose
EDTA chelates in acidic medium are sufficiently stable comparative to iron
chelate. The EDTA chelate of Bi(III) is nearly stable as Fe(III) chelate,
accordingly Bi(III) is the serious interfering cation in determination of iron.
Determination of Iron by using Copper (II)-ethylenediaminetetraacetate Absorbing System
134
TABLE 4.5
Results obtained at 722 nm in the quantitative determination of iron in Livogen-
Z, Ferium-xt and Orofer-xT tablets, permittance coefficient values are compared
for determination concentration of sample
Volume of
0.05M EDTA and
0.05M Cu(NO3)2
Iron sample
analyzed
Obs. Pr.
coefficient
(lit. g-1cm-1)
Conc. of
iron found
(mg ml-1)
Error
in %
1.0mg ml-1 Fe(III) 0.51674 1.0000 0.00
2 Livogen-Z 0.51673 0.9999 0.01
1 Ferium-xt 0.51697 1.0005 0.05
5.0 ml each
1 Orofer-xT 0.51666 0.9998 0.02
1.0mg ml-1 Fe(III) 0.51669 1.0000 0.00
2 Livogen-Z 0.51691 1.0004 0.04
1 Ferium-xt 0.51673 1.0001 0.01
6.0 ml each
1 Orofer-xT 0.51696 1.0005 0.05
1.0mg ml-1 Fe(III) 0.51697 1.0000 0.00
2 Livogen-Z 0.51697 1.0000 0.00
1 Ferium-xt 0.51683 0.9999 0.01
7.0 ml each
1 Orofer-xT 0.51678 0.9996 0.04
The second type of interfering ions includes those cations which do not
compete with iron but with copper for the EDTA. These are the cations which
forms more stable chelate than copper chelate. At such strongly acidic pH 1.15,
no any divalent metal forms more stable chelate than copper chelate. The cations
whose EDTA chelate is less stable than copper chelate, particularly the
aluminum, barium, calcium, cadmium, lead, magnesium, manganese, zinc and as
well the copper does not interferes in iron determination. The interference study
of copper was not carried out in this experiment because that was added in excess
for generation of absorbing system. The interference study was carried out by
adding 10 mg, 20 mg, 30 mg or 40 mg of these metal cations separately in to the
test solutions containing 1.0 mg to 10.0 mg of iron. The interfering cation of
same concentration was also added in true blank solution but not in the reagent
blank solution. The result of the interference study is reported in Table 4.6, in the
Determination of Iron by using Copper (II)-ethylenediaminetetraacetate Absorbing System
135
form of permittance coefficient. The values of permittance coefficient in absence
(only with standard 1.0 mg ml-1 Fe(III)) and in presence (with standard 1.0 mg
ml-1 Fe(III) plus the added cation) of interference are observed nearly same. At
pH 1.15, all of these metal cations up to 40 mg do not interfere in the
determination of iron. When the added metal cations interfere in the
determination which increases the %T reading of test solutions, consequently that
raises the values of permittance as well permittance coefficient. The
concentration of added cation when reaches in excess, that increases the optical
density of the test solutions with respect to regent blank solution. Hence, the %T
readings were observed decreased and that decreases the values of permittance
and permittance coefficient. In the previous literature the inferences study was
also completed.
TABLE 4.6
Effect of cations on determination of iron, results obtained in the determination of
iron from 1.0 mg to 10.0 mg, the value permittance coefficient was determined at
722 nm in absence and in presence of different concentration of cations
Cation
added as
interference
Amount
added
(mg)
Obs. Pr.
coefficient
(lit.g-1cm-1)
Conc. of iron
solution found
(mg ml-1)
Error
in %
Standard 0.00 0.51690 1.0000 0.00
Aluminum 10.0 0.51696 1.0002 0.02
20.0 0.51695 1.0001 0.01
30.0 0.51690 1.0000 0.00
40.0 0.51697 0.9994 0.06
Barium 10.0 0.51679 0.9998 0.02
20.0 0.51705 1.0003 0.03
30.0 0.51684 0.9999 0.01
40.0 0.51683 0.9998 0.02
Calcium 10.0 0.51683 0.9998 0.02
20.0 0.51696 1.0002 0.02
30.0 0.51663 0.9995 0.05
40.0 0.51700 1.0002 0.02
Determination of Iron by using Copper (II)-ethylenediaminetetraacetate Absorbing System
136
TABLE 4.6 Continues…
Cadmium 10.0 0.51676 0.9997 0.03
20.0 0.51704 1.0003 0.03
30.0 0.51683 0.9998 0.02
40.0 0.51688 0.9999 0.01
Lead 10.0 0.51689 0.9999 0.01
20.0 0.51695 1.0001 0.01
30.0 0.51677 0.9997 0.03
40.0 0.51704 1.0003 0.03
Magnesium 10.0 0.51676 0.9997 0.03
20.0 0.51704 1.0003 0.03
30.0 0.51683 0.9998 0.02
40.0 0.51688 0.9999 0.01
Manganese 10.0 0.51696 1.0002 0.02
20.0 0.51695 1.0001 0.01
30.0 0.51690 1.0000 0.00
40.0 0.51677 0.9997 0.03
Zinc 10.0 0.51683 0.9998 0.02
20.0 0.51680 0.9998 0.02
30.0 0.51690 1.0001 0.01
40.0 0.51687 0.9999 0.01
4.5 Conclusions
The method described here for determination iron is based on the
measurement of concentration of surplus EDTA through generating the [Cu(II)-
EDTA]-2 absorbing system. The absorbing system was found excellent (for
determination of iron) because of its formation and stability at strongly acidic pH
1.15 to which quantitatively chelation of trivalent Fe(III) was attained. At this
pH, many divalent metal cations do complexed by EDTA, this makes the process
selective. The sufficient difference in the stability constant of [Fe(III)-EDTA]-1
chelate and [Cu(II)-EDTA]-2 chelate, the stability of iron-EDTA chelate is not
affected because of the addition of relative larger amount of Cu(II)ions. For the
Determination of Iron by using Copper (II)-ethylenediaminetetraacetate Absorbing System
137
same reason that, copper ions does not interferes in the determination of iron. The
linearity between permittance and the concentration of iron was maintained even
at low concentration (1.0 mg) of iron. The lower concentration of iron (less than
1.0 mg) is not determined in this experiment, but it is possible through increasing
the sensitivity of the method by decreasing the volume/concentration of reagents;
since permittance and permittance coefficient are not preside over the
concentration of reagents. The proposed method is very simple, easy to execute
and which proved to be a better method as compared to other methods involve
the step of preconcentration of analyte. The sensitive of the method practiced
here was found up to 1.0mg of iron, when determined with 5.0ml of 0.05M
EDTA and 5.0ml of 0.1M Cu(NO3)2 and produces the reproducible results with
good accuracy. The method also found excellent over the photometric titration of
iron with EDTA since, the graphical method of determination of exact end point
is tedious and time consuming process. When the proportionality constant at
specific dilution (or permittance coefficient) is determined for standard solutions
of known concentration, in this method the concentration analyte can be
determined directly with eq.(1) reported in earlier study. The proposed method
also neglects a step of standardization of reagents, since it involves the
measurement of permittance of TS with respect to RB. Excluding only the
analyte, the composition of the reagent blank must be identical in every respect to
test solutions. This is the only care have to be taken for the good linearity. The
results reported in the Table 4.5 showed that the method is excellent for the
determination of iron in iron tablets.
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