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CHAPTER 2

REVIEW OF LITERATURE

The requirements of a satisfactory analytical method are usually manifold,

but certainly selectivity is counted amongst the most important. Hence there

has always been interest in procedures that can improve the sensitivity and

selectivity of the measurement methods themselves. Derivative

spectrophotometry in UV – visible region is a useful technique for extracting

qualitative and quantitative information from overlapping bands of the

analyte and interferent due to incompletely resolved peaks. It has been

profitably applied for analytical applications in which a broad, unstructured

background spectrum overlaps the bands of interest. Derivative techniques

have also been applied for the analysis of multicomponent mixtures and for

detecting and if possible correcting for interferences especially when the

irrelevant absorption originally present in the substance sought is of an

indefinite nature [1].

The advantages of derivative spectra are at least partially offset by

the degradation in the signal-to-noise ratio that accompanies the obtained

derivatives. Also the advantages of derivative measurement depend strongly

on the relative widths of the interferent and analyte bands. Relative error

greater than 30% [2] is obtained from the results of derivative

measurements.

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Standard addition method is used in the analysis of real samples

to overcome calibration and matrix interaction biases, by proper diagnosis of

the data. The technique is valuable for the detection and elimination of some

bias error sources. It also provides in situ normalisation of the proportional

error in the method [3]. The problem arises when the matrix of the sample

produces a constant error in the determined analyte concentration, which

can be corrected by determining the true sample blank by the technique

suggested by Youden [4] as the response curve in the standard addition

method includes both the analyte signal and the true sample blank. In order

to obtain an unbiased result this blank has to be corrected.

Any substance that acts as direct interferent will cause a biased

result but the analysis always requires that the response should be only due

to the analyte and nothing else. Saxberg et al. [5] presented the generalized

standard addition method in which they gave a mathematical model for the

analysis of multicomponent mixtures. In this case each analyte need not to

be fully selective as is required in the conventional single analyte method.

The simultaneous determination of toxic metal ions is important in

environmental analytical chemistry because they have important roles in

biological and medical science [6-11]. Also certain metal ions are always

present together in the nature [12]; therefore it is important to determine

these metal ions in the presence of each other. There are many methods for

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simultaneous determination of metal ions such as inductively coupled

plasma atomic absorption spectrometry (ICP-AAS) [13, 14], inductively

coupled plasma mass spectrometry (ICP-MS) [15], inductively coupled

plasma atomic emission spectrometry (ICP-AES) [16, 17], flotation

electrothermal atomic absorption spectrometry [18], high performance liquid

chromatography (HPLC) [19-22], atomic absorption spectrometry (AAS) [23-

25], hydride generation atomic absorption spectrometry (HG-AAS) [26-30],

hydride generation atomic fluorescence spectrometry (HG-AFS) [31], hydride

generation inductively coupled plasma atomic emission spectrometry (HG-

ICP-AES) [32-34], sequential flow injection technique [35], slurry sampling in

electrothermal atomic absorption spectrometry (ET-AAS) [36], cloud point

preconcentration followed by atomic absorption spectrometry [37],

spectrophotometry using chemometrics methods [12, 38-41], derivative

spectrophotometry [42-49], voltammetry [50], cathodic stripping voltammetry

(CSV) [51], differential pulse stripping voltammetry (DPSV) [52], polarography

[53], flame atomic absorption spectrometry (FAAS) [54, 55], ion-exchange

chromatography with post column reaction [56], graphite furnace atomic

absorption spectrometry (GFAAS) [57], and spectrofluorimetry [58, 59]. But

with some methods selectivity is poor, some demand expensive instruments

or reagents [60-62]; and other requires difficult and time-consuming

separation procedures [63].

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2. 1 H-POINT STANDARD ADDITION METHOD (HPSAM)

In 1988, Bosch-Reig et al. [2, 64] presented a new technique referred to as

H-Point Standard addition method (HPSAM) for resolving spectra of two

analytes with strongly overlapping spectra [65]. This method is based on the

dual wavelength spectrophotometry and standard addition method. HPSAM

has been applied to remove the blank bias error [56, 66] caused by the use of

absorbent blank in liquid chromatography [67], in metal speciation [10, 68,

69] for the analysis of kinetic data [70-73] with time as an additional

variable, and in equilibra study in micellar media [74]. Further studies

revealed high versatility and applicability of the method [75]. HPSAM has

also been applied for the study of ternary mixtures [76]. The greatest

advantage of HPSAM is that it transforms the incorrigible error resulting

from the presence of a direct interferent into a constant systematic error and

makes it possible to determine the concentration of analyte in the presence

of a direct interferent and even the concentration of interferent can be

determined [2]. HPSAM can be used for simultaneous determination of two

components in the solution [39, 69, 77-79]. Results obtained from HPSAM

have been found to be comparable with those obtained from the powerful

multivariate statistical Partial Least Square Regression method (PLS) [80-84].

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2.1.1 Binary-HPSAM

HPSAM is a simple two variable chemometric technique. For applying binary

HPSAM consider a sample containing an analyte X and an interferent Y. To

determine the concentration of analyte X in the presence of interferent Y by

binary-HPSAM requires the selection of two wavelengths λ1 and λ2 at which

the interferent species Y should have the same absorbance [2].

It is possible to select several wavelength pairs where the absorbance

is same for the interferent species. But, the wavelength pairs are selected

following the criteria to give high values for the difference of the calibration

slopes because higher the value for the slope increment the smaller the error

for the analyte concentration. So, the wavelength pair that gives the greatest

slope increment has to be selected. After the selection of appropriate

wavelength pair (λ1 and λ2) known amounts of analyte (X) are successively

added to the mixture and the resulting absorbances are measured at the two

wavelengths.

With the standard addition method the unknown concentration of

analyte (X) can be calculated starting from the concentration values of the

pure analyte at the intersection of the line with the abscissa, then for the

first solution , which contains only the sample and no added analyte, the

analytical signal value is the absorbance due to interferent (Y) and this

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absorbance value is constant at the selected wavelengths, all straight lines

obtained at different wavelengths at which interferent (Y) has the same

absorbance by applying standard addition method will have a common point.

This point is the H-point, the abscissa being the unknown analyte (X)

concentration and the ordinate gives the absorbance due to an interferent

(Y). From this the concentration of interferent can be determined when it is

known to be present and when the corresponding calibration graph is also

known. The two straight lines obtained intersect at the so-called H-point

(-CH, AH).

Hence, AH value is only related to the signal of the interferent Y at

the two selected wavelengths. The concentration of interferent can be

determined from its calibration graph. At H-point, CH is independent from

the concentration of interferent and also AH is independent from the

concentration of analyte [39]. Determination of the analyte concentration, X,

in the presence of the unknown interferent by generalized H-Point standard

addition method (GHPSAM) has also been reported [87-89].

2.1.2 Kinetic – HPSAM

Simultaneous determination of binary and sometimes ternary mixtures of

closely related species on the basis of differences in the rates of reaction with

a common reagent (R) is done by kinetic analysis. The evaluation of kinetic

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data obtained by a kinetic method is based on the reaction of a mixture of

substance with a given reagent. It depends on the ratios of the rate

constants of the different reactions involved.

The kinetic determination of the more reactive species in a binary

mixture is subject to a positive error arising from the slow reaction that

takes place in parallel. The magnitude of such an error will depend both on

the rates of the reaction involved and on the concentration ratio of the

reactants in the mixture. For determination of inorganic species in general,

and to transition metal ions and non–metal anions in particular differential

kinetic methods such as logarithmic–extraplotation, single–point, tangent,

proportional equation, and linear graphical methods are generally used.

Among these, proportional equation method appears to be the most flexible

for simultaneous determination however; it is applicable only in the absence

of synergistic effects.

For a two component system consisting of two species X and Y

that react with a common reagent (R), if the reaction of Y is faster than that

of X and the former finishes at a fixed time while the latter continues to take

place or if the difference between the rates of the two reactions is large

enough, component X can be accurately determined in the presence of Y. In

this case, the variables to be fixed are two times t1 and t2 at which the

species Y, which does not evolve with time or over the range between these

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two times, should have the same absorbance, rather than two wavelengths

1 and 2 [2, 64] Applying HPSAM at the two selected times will yield [71] the

two straight lines obtained intersect at the H-point.

The criteria to choose the optimum times were as follows. These

times should give the better slope increment [85] and the time pair should

give a lowest error or a greatest accuracy and also the change in absorbance

related to interferent (Y) should be negligible.

When the two species in a mixture, X and Y, evolve with time,

CX and Ai have been calculated by plotting the analytical signal ΔAt1-t2

against the added concentration of X at two wavelengths 1 and 2, and the

absorbance of the Y component at these two wavelengths should be same

(AY), and so are thus the ΔAt1-t2 values. The basis of HPSAM that applies

under these conditions is the use of two times t1 and t2 and two wavelengths

1 and 2, additivity of absorbances and absence of synergistic effects.

2.2 SIMULTANEOUS DETERMINATION OF TOXIC METAL IONS

2.2.1 Binary-HPSAM using one chromogenic reagent:

Abbaspour et al. [90] used HPSAM for simultaneous determination of Fe(III)

and V(V) for selective determination of Fe(III) in the presence of V(V). The

method is based on the difference in absorbance of complex formed between

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Fe(III) and pyrogallol red at two different wavelengths at pH 6.5. The

absorbance of pyrogallol red complexes of both Fe(III) and V(V) increased

with increasing pH. Since at pHs higher than 6.5 the pyrogallol red was

oxidized rapidly with oxygen, therefore pH 6.5 was selected as optimum pH

for subsequent studies. Effect of the concentration of the pyrogallol red was

also investigated. The optimum concentration was 0.33 M of pyrogallol red.

The spectrum of V(V)-pyrogallol red was broad, so for obtaining good

accuracy only Fe(III) was considered as analyte. Wavelength pair chosen for

applying HPSAM was 518 and 745 nm. The proposed method was

successfully applied for simultaneous determination of Fe(III) and V(V) in

synthetic mixtures with different concentration ratio of Fe(III) and V(V).

Afkhami et al. [91] applied HPSAM in acidic media (1.0 M H2SO4) for

simultaneous determination of Bi(III) and Sb(III) iodide complexes. Bi(III)

formed an orange-yellow complex of BiI4¯ and Sb(III) formed a greenish–

yellow complex of SbI4¯ in acidic media and in the presence of excess

amount of iodide, as follows:-

Bi3+ + 4I¯ BiI4¯ Orange-yellow

Sb3+ + 4I¯ SbI4¯ Greenish–yellow

The spectra of the complexes overlapped, and hence interfered in the

spectrophotometric determination of each other. However, HPSAM has been

found to be quite suitable for simultaneous determination of Bi(III) and

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Sb(III). With Bi(III) as as analyte, best wavelength pair was 416 and 435.6

nm and with Sb(III) as analyte the best wavelength pair was 457.4 and 470

nm. Simultaneous determination with the concentration ratios of Bi(III) and

Sb(III) varying from 15:1 to 1:20 in mixed samples has also been done.

HPSAM has been successfully applied for the determination of Bi(III) and

Sb(III) in several synthetic samples and alloys. A comparison between the

results obtained for simultaneous determination of Bi(III) and Sb(III) in the

solution with high concentration ratios showed a better accuracy for HPSAM

than the derivative spectrophotometric method.

HPSAM for simultaneous determination of Co(II) and Ni(II) using 1-(2-

Pyridylazo)-2-naphthol (PAN) in micellar media has been reported by

Afkhami et al. [92]. The chromogenic reagent PAN had been widely used for

the spectrophotometric determination of Co(II) [93-95] and Ni(II) [96]. The

metal complexes [Co(II)-PAN and Ni(II)-PAN] were made water – soluble by

the neutral surfactant Triton X-100 (TX-100). Formation of both the

complexes was complete within 10 min at pH 9.2 (adjusted by ammonia

buffer). Since, the spectrum of Co(II) complex was broad, so, only Ni(II) was

considered as analyte. For accurate results wavelength pair chosen was 560

and 621 nm. Several synthetic samples were analyzed using the HPSAM. A

comparison with derivative spectrophotometric method showed that

accuracy, precision, reproducibility, especially sensitivity and linear range

for HPSAM were better than those for derivative method.

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Method for the determination of total iron and vanadium in different

oxidation states in a cationic micellar media of cetyltrimethyl ammonium

bromide (CTAB) using Gallic acid (GA) as complexing agent [69] has been

reported by Abdollahi et al. [81]. Difference in spectra of both oxidation

states of each metal ion were mostly due to differences in their sensitivities.

CTAB increased the sensitivity of complexes at all wavelengths. Different

oxidation states of iron [Fe(II) – Fe(III)] and vanadium [V(IV) – V(V)] complexes

had the same spectral intensity in CTAB micellar medium so, simultaneous

determination of total concentration of these metal ions was possible. No

significant changes were observed in the spectrum of iron complexes in the

pH range of 2–7, but the spectrum of vanadium complexes showed

significant increase in absorbance at all wavelengths up to pH 5.0. Above pH

7.5 GA get oxidized with dissolved oxygen, but at pH 5.0 it was completely

stable and colorless. So, pH 5.0 was selected as an optimum pH value for

simultaneous analysis of iron and vanadium. A best wavelength pair chosen

for applying HPSAM was 560 and 690 nm for iron and 515 and 590 nm for

vanadium respectively. Simultaneous determination in mixed sample having

iron to vanadium concentration ratio of 10:1 to 1:20 had been reported. The

results obtained from HPSAM were compared with the results obtained from

PLS method [97-99]. There were no significant difference between the

outputs of the PLS and HPSAM. It was found that simpler bi-variate method

HPSAM is comparable with a powerful multivariate PLS method. PLS is a

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principal component analysis (PCA) based method and its deep

understanding is required and it is not simple to use but the concept behind

HPSAM is quite simple to use. HPSAM showed nearly the same accuracy as

PLS which is a full spectrum method.

HPSAM has been applied for simultaneous determination of Co(II)

and Pd(II) by Eskandri et al. [84, 100] using 1-(2-Pyridylazo)-2-naphthol

(PAN) or 4-(2-Pyridylazo)-2-resorcinol (PAR) as a chromogenic reagent in

micellar media. Using PAN at neutral pH levels both Co(II) and Pd(II) formed

green colored complexes with PAN which were soluble in aqueous

sodiumdodecyl sulphate (SDS) micellar medium. In acidic SDS micellar

solutions, Co(II)-PAN complex was formed at a slower rate. Pd(II)-PAN

complex was formed at a high rate but with low sensitivity and showed

spectral changes which were dependent on pH. The two complexes formed

quickly with no changes in their spectral characteristics in the pH range 4.0-

9.0. Different micellizing agents such as TX–100 and Tween–80 (Tw-80) were

also tried as solublizing media for Pd(II)-PAN and Co(II)-PAN complexes. In

Tw-80 micellar media Pd(II)-PAN complex was precipitated immediately and

after 1 h in TX-100. Both the complexes showed high absorbance in SDS

micellar media and were stable for at least 3 days hence SDS concentration

was used for further studies. To achieve selective determination of Co(II) and

Pd(II) with PAN, perchloric acid (0.01M) was used to decompose the other

metal-PAN complexes. Simultaneous determination of Co(II) and Pd(II) by

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HPSAM was done spectrophotometrically under optimum conditions.

Absorbances at the two pairs of wavelengths, 597 and 650 nm or 566 and

612 nm, were monitored while adding standard solutions of Co(II) and Pd(II)

respectively. The method was able to determine accurately Co(II) to Pd(II)

concentration ratio of 5:1 and 1:30. The accuracy and reproducibility of the

method was checked and the recommended procedure was applied to real

water samples and synthetic sample solutions.

Using PAR, addition of acid after complex formation improved the

selectivity due to decomposition of the other metal-PAR complexes, except

that of Pd(II)-PAR and Co(II)-PAR complexes. Both these complexes are water

soluble, but presence of micellizing agents increased the sensitivity. SDS was

selected as the best micellizing agent for further study. As Pd(II)-PAR

complex spectrum was broad, so for obtaining good accuracy, only Co (II)

was considered as analyte. By considering the criteria that the higher the

value for slope increment, the smaller the error for the analyte

concentration, the best wavelength pair for applying HPSAM was 564 and

626nm. Co(II) and Pd(II) can be determined accurately in the concentration

ratio of 7:1 to 1:8.

HPSAM for simultaneous determination of Cu(II) and Ni(II) using,

PAN as complexing reagent in Tw-80 micellar media has also been done by

Eskandari et al. [101]. The effects of various parameters on the sensitivity

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and spectral characteristics of PAN complexes of Ni(II) and Cu(II) were

investigated. The shape of the absorption spectra, the wavelength of

maximum absorption and the apparent molar absorptivities changed in the

pH range 3.29 to 2.09 for Cu(II) and Ni(II), but no such changes were

observed at pH values below 2.09 whereas the molar absorptivities of Cu(II)-

PAN and Ni(II)-PAN complexes decreased with decrease in the pH. So, pH

1.89 was selected for subsequent determination.

Various surfactants, such as Tw-80, TX-100 and SDS were tried as

solublizing agents. In SDS micellar media, low sensitivity for Cu(II) and Ni(II)

determination and low solubility for Ni(II)-PAN complex was observed. Tw-80

was selected for further studies. Optimization of various parameters, such as

the PAN and Tw-80 concentration, was done for Cu(II) and Ni(II) at 555nm

and 569nm, respectively. Based on the obtained results, PAN and Tw-80

concentration of 0.02% and 3.2% were selected for further studies

respectively. Only Cu(II) was selected as analyte, and the best wavelength

pair was 548 and 579 nm. The method was able to determine accurately

Cu(II) to Ni(II) ratio of 15:1 to 1:10. The effects of diverse ions [102] in the

determination of Ni(II) and Cu(II) to investigate the selectivity of the method

were also studied and the proposed method was applied to assays of Cu(II)

and Ni(II) in their various alloys, binary mixtures, and water samples.

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2.2.2 Binary - HPSAM using mixed chromogenic reagents:

There are several problems with simultaneous determinations of metal ions

by only one photometric reagent such as lack of selectivity i.e. number of

metals may form complexes with very similar absorption spectra. Since

multicomponent systems had a very bad conditioning (in mathematical

sense), these systems were not appropriate for selective determination of

metal ions by Multi Component Analysis (MCA). On the other hand, there

were complex forming agents that form complexes with several metal ions,

but only a few of them absorb strongly in the visible spectral range. This

limits simultaneous determinations of metal ions by using only one reagent.

Mixed organic reagents were proposed [8, 39, 103-106] for

spectrophotometric MCA of metal ions. The difficulty arises from the

necessity of finding a compromise condition with respect to the existence of

different metal complexes. In spite of this, the use of mixed reagents systems

can be considered as an alternative means to extend the applicability of

spectrophotometric MCA of metal ions.

HPSAM for simultaneous determination of Cr(VI) and Fe(III) using

diphenylcarbazide and 1,10-phenanthroline as mixed chromogenic reagent

in a non-ionic micellar solutions of TX-100 has been reported by Abdollahi et

al. [80]. In a solution containing a mixture of these two reagents, reactions

occurring seems to be the simultaneous oxidation of diphenylcarbazide to

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diphenylcarbazone, reduction of Cr(VI) to Cr(III) and that of Fe(III) to Fe(II)

and chelate formation of diphenylcarbazone with Cr(III) and that of 1,10-

phenanthroline with Fe(II). Therefore, the system was appropriate for

simultaneous spectrophotometric determination of Cr(VI) and Fe(III). In the

pH range of 2-6 no significant changes were observed in the spectrum of

Fe(II)-1,10-phenanthroline complex but the spectrum of the Cr(III) complex

showed a decrease in maximum absorbencies at pH higher than 3.0, so

optimum pH for carrying out further study of the system was 2.0. The best

wavelength pair that gave the greatest slope increment was 490 and 520nm

when Cr(VI) was selected as an analyte. Determination of Cr(VI) and Fe(III)

with concentration ratio varying from 15:1 to 1:30 in the mixed sample have

been reported. Several synthetic mixed samples were analyzed by the

suggested method. The accuracy of each result was satisfactory.

Similarly simultaneous determination of Cu(II) and Fe(III) using

mixed organic reagents of Neocuproine and 1,10-penanthroline was done by

Safavi et al. [82]. Neocuproine and 1,10-phenanthroline formed colored

complexes with Cu(I) and Fe(II) respectively. Some of the reducing agents can

be determined by the reduction of a metal ion exhibiting multiple valancies,

followed by treating the lower valency of the metal with chromogenic reagent

[107]. Therefore the Cu (II)–Neocuproine and Fe (III)–1,10-Phenanthroline

system were studied at pH 5.0 in the presence of ascorbic acid which acts as

a good reducing agent for reduction of Cu(II) [107, 108] and Fe(III) [109] in

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the presence of neocuproine and 1,10-phenanthroline respectively. When

Cu(II) was selected as analyte wavelength pair 520 and 492.5nm and when

Fe(III) was selected as analyte wavelength pair 428 and 472.5nm were best.

Several synthetic samples with different concentration ratios of Cu(II) and

Fe(III) were analyzed by the suggested method. The proposed method had

successfully been applied for the determination of Cu(II) and Fe(III) in blood

serum and river water. The results for several analyzed samples were found

to be quite satisfactory and in agreement with those acquired by PLS.

2.2.3 Kinetic- HPSAM using single chromogenic reagent

Kinetic-HPSAM based on the different rate of complex formation of Au(III)

and Pd(II) using 5-(p-dimethylaminobenzylidene) rhodanine (PDR) [110] in

the presence of formate buffer and Brij-35 micellar media [111] was studied

by Pourreza et al. [112] .The kinetics of the system was monitored at 550 nm

as both Au(III)-PDR and Pd(II)-PDR complexes absorbed at this wavelength.

The rate of formation of Au(III)-PDR and Pd(II)-PDR complexes showed

maximum change in the absorbance in the fixed reaction time (75-135s) at

pH 4.0. In the presence of phthalate buffer, there was almost no change in

absorbance of Au(III)-PDR and Pd(II)-PDR complexes. Acetate buffer affected

the rate of complex formation of Au(III)-PDR as the absorbance first

increased and then decreased with time. The presence of citrate buffer

affected the rate of complex formation of Au(III)-PDR and Pd(II)-PDR, as the

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absorbance of Au(III)-PDR decreased and that of Pd(II)-PDR increased. In the

presence of formate buffer there was no change in the absorbance of Pd(II)-

PDR complex, but that of Au(III)-PDR showed an increase in absorbance.

This indicated that complex formation of Pd(II)-PDR was faster than that of

Au(III)-PDR in the presence of formate buffer. So formate buffer was selected

for further study. The aqueous micellar medium containing Brij–35 was used

to solublize the complexes and the effect of concentration of Brij–35 was

investigated. The initial absorbance was high at concentration of Brij–35

greater than 0.037 M; therefore, this concentration was selected for further

study. In the presence of formate buffer and Brij–35 the rate of complex

formation of Pd (II) with PDR was faster than that of Au (III). In this case, the

reaction times fixed were, 75 and 335 s. The Pd(II)–PDR complex had the

same absorbance over the range between these two times and also there was

an appropriate difference between the slopes of the calibration lines. The

absorbance of Au(III)-PDR complex at 550nm and at times 75 and 335 s

were different. The absorbance values for the different synthetic samples of

Au(III) and Pd(II) were measured at 75 and 335 s. The intercept of the

straight lines gave the unknown concentration of Au(III) and the analytical

signal of species Pd(II) corresponding to 75 and 335 s in the original

samples. The concentration of Pd(II) was calculated from the analytical signal

from a calibration graph. The method has successfully been used for the

determination of the Au(III) in jewelry and synthetic samples.

101

Based on the difference in the kinetic behaviour of the complexation

reactions of Be(II) and Al(III) with Chrome Azurol S (CAS) in CTAB micellar

media, Afkhami et al. [113] applied kinetic-HPSAM for their simultaneous

analysis. CAS had been applied for the spectrophotometric determination of

both the elements individually [114, 115]. The presence of a large excess of a

cationic surfactant CTAB, increased the sensitivity due to the formation of

ternary complexes between metal ions, CAS and CTAB. Complexation

reaction of Al(III) with CAS in CTAB micellar media was very much faster

than that of Be(II) under the same conditions. CAS concentration (4.0×10-3

mol/l) and CTAB concentration (1.7×10-3 mol/l) was selected for further

study. The rates of reactions were studied in the pH range 1.0–9.0 and it was

observed that for a mixture of Be(II) and Al(III) absorbance increased by

increasing pH up to 5.0 and decreased at higher pHs; so pH 5.0 was selected

for further study. A temperature of 300C was selected as the optimum

temperature as absorbance decreased at a temperature higher than 300C for

the mixture of Be(II) and Al(III). Time pair chosen was 100 and 300s .The

product of the reaction of Al(III) had the same absorbance over the range

between these two times and there was also an appropriate difference

between the slopes of the calibration lines. The wavelength 600 nm was

selected as the fixed wavelength. The proposed method has successfully

been applied for simultaneous determination of Be(II) and Al(III) in alloys,

environmental, and geochemical samples.

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2.3 SPECIATION OF METAL IONS

2.3.1 Binary-HPSAM using mixed chromogenic reagents

Safavi et al. [39] applied HPSAM for the speciation of Fe(II) and Fe(III) with

mixed chromogenic reagents. Different speciation procedures for Fe(II) and

Fe(III) had been proposed [8, 10, 103-106, 116]. HPSAM method has been

used for speciation of Fe (II) and Fe (III) using mixed chromogenic reagents of

1, 10- phenanthroline and salicylic acid. Fe(III)-salicylic acid complex is

highly stable at pH 3.0 so pH 3.0 was selected for selective speciation of iron.

When Fe(II) was selected as analyte, choosen wavelength pair was 535 and

540 nm and when Fe(III) was selected as analyte, best wavelength pair

choosen for applying HPSAM was 460 and 530 nm. The results showed that

Fe(II) and Fe(III) could be determined simultaneously with concentration

ratios of Fe(II) to Fe(III) varying from 10:1 to 1:20 in the mixed sample. The

accuracy and precision of the method were quite satisfactory.

2.3.2 Kinetic- HPSAM using single chromogenic reagent

Simultaneous determination of Fe (II) and Fe (III) has also been done by

Zolgharnein et al. [70] using Kinetic-HPSAM. Here the difference in the rate

of complex formation of iron with Gallic acid (GA) in two different oxidation

states has been reported. The rate of reaction between Fe(II) and GA

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increased with increase in temperature and GA concentration, so a 0.002 M

concentration of GA and a temperature of 300C were selected. Effect of pH on

the rate of Fe (II) – GA complex formation showed that the maximum change

in absorbance in the fixed time period occurred at pH 5.0. The rate of

complex formation at pH 4.0 was very slow and the complexation reaction

was almost completed after 600 s and thus, the change in absorbance

between 50 and 100 s was very small. At pH 7.5 GA was oxidized but at pH

5.0 it was completely stable and colorless. The time pair 30 and 200 s was

selected to obtain the highest accuracy. The method has successfully been

applied for simultaneous determination of Fe(II) and Fe(III) in environmental

and synthetic samples.

Kinetic-HPSAM has been reported for simultaneous determination of

Fe(II) and Fe(III) by Safavi et al. [117] using 1,10-penanthroline and Co(II) as

redicing agent. Difference in kinetic behaviour of two analytes along with

mathematical treatment of data using HPSAM, permitted speciation of two

important species of iron. The reduction of Fe(III) to Fe(II) by Co(II) in the

presence of 1,10-penanthroline and subsequent complexation of Fe(II) with

1,10-penanthroline furnished a sensitive spectrophotometric method for

indirect determination of Fe(III) in the presence of Fe(II). Rate of

complexation of Fe(II) and Fe(III) are different. In fact two processes were

involved; the fast complex formation between Fe(II) and 1, 10-penanthroline

and relatively slow reduction of Fe(III) by Co(II) in micellar media and

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subsequent complex formation of resulted Fe(II) with 1, 10-penanthroline.

So, the kinetic based HPSAM was a good selection for speciation of Fe(II) and

Fe(III). The influence of pH values and concentration of CTAB were

investigated for optimizing the conditions. Addition of CTAB at concentration

above its critical micelle concentration (c.m.c) caused a decrease in the rate

of reduction of Fe(III) by Co(II). Therefore, a CTAB concentration of 2×10-3 M

and pH 3.0 was selected for further study. The best time pair that gave the

greatest slope increment was 30 and 200 s. Under working conditions

determination of Fe(III) was accurate and precise in the presence of excess

Fe(II). The proposed method has successfully been applied for simultaneous

determination of Fe(III) and Fe(II) and for the selective determination of Fe(III)

in the presence of Fe(II) in several synthetic mixtures and some spiked

samples.

Based on the difference in the rate of reaction between pyrogallol red

with Sb(III) and Sb(V), their speciation by Kinetic-HPSAM was done by

Abbaspour et al. [118]. There were a small number of spectrophotometric

methods for Sb speciation [119, 120]. At pH 2.0, Sb(III) formed a red complex

very rapidly as compared to that of Sb(V). Both complexes showed maximum

absorbance at pH 2.0 In this system, Sb(V) was analyte and Sb(III) was

considered as interferent. Differences in rate of complex formation of Sb(III)

and Sb(V) with pyrogallol red and considerable overlapping of their spectra

were the main reasons for applying HPSAM for speciation of antimony. The

105

time pair 100 and 200 s was chosen as optimum values for obtaining the

highest accuracy. The 510 nm wavelength was selected as a fixed wavelength

because both the spectra were linear at this wavelength. The proposed

method was used for determination of Sb(III) and Sb(V) in river and spring

water samples.

Simultaneous determination of toxic metal ions either by binary-HPSAM

or kinetic-HPSAM can be done. In binary-HPSAM both single and mixed

chromogenic reagent can be used. Mixed chromogenic reagent leads to the

selective determination of toxic metals with sensitive spectral characteristics.

In binary-HPSAM point of intersection of two standard addition calibration

lines at the two selected wavelengths gives the concentration of analyte and

the analytical signal corresponding to interferent. In kinetic-HPSAM only use

of single chromogenic reagent has been reported so far. In this two times had

been selected instead of wavelength pair.

ΔAt1-t2

106

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