1061-9348/02/5709- $27.00 © 2002

åAIK “Nauka

/Interperiodica”0758

Journal of Analytical Chemistry, Vol. 57, No. 9, 2002, pp. 758–772. Translated from Zhurnal Analiticheskoi Khimii, Vol. 57, No. 9, 2002, pp. 902–917.Original Russian Text Copyright © 2002 by Ivanov, Kochelaeva, Prokhorova.

The position of molybdenum in the periodic table ofthe elements explains a wide variety of reactions andmethods for its determination. It exhibits positive oxi-dation states of II, III, IV, V, and VI in compounds, andsome reagents can stabilize these states. High coordina-tion numbers of molybdenum (up to 9) and its trend tocoordinate oxygen explain its complex ionic states.Thus, molybdenum(V) and molybdenum(VI) oxo

anions can occur as either cationic [

Mo

,

MoO

2

(OH)

+

, MoO

2

Cl

+

,

MoO

3+

] or anionic [

Mo ,

HMo ,

MoO

2

(OH

] species, depending on the acid-ity of solution and on the anion of an acid or a baseadded. Oxygen-free compounds of Mo(VI)

[

Mo(CN

] or Mo(V) and Mo(VI) [

Mo

2

Cl

10

, Mo ,

and

MoF

6

] and isopoly and heteropoly Mo(V) and

Mo(VI) compounds [

Mo

7

,

SiMo

12

] are known;many of these compounds are stable in solutions.Molybdenum compounds with oxygen are most numer-ous. Thus, the following homopolynuclear molybde-num(VI) complexes can be formed as the acidity is

increased:

Mo

3

, Mo

4

, Mo

4

, HMo

6

,

Mo

7

, HMo

7

, H

7

Mo

12

, HMo

24

,

and

Mo

36

. The ionic state of molybdenum is stronglyaffected not only by the acidity of solution and temper-ature but also the concentration of molybdenum. There-fore, the ionic state of molybdenum should be known inthe development of determination methods.

In the determination of molybdenum, the analyte ismost frequently stabilized as molybdenum(VI) in aweakly alkaline medium. All types of reactions such asacid–base, redox, and complexation reactions wereproposed for the determination of molybdenum. Theyformed the basis of gravimetric, titrimetric, and physi-cochemical determination techniques. Very high sensi-tivity procedures based on the catalytic reactions ofmolybdenum and radiochemical techniques in variousversions are well known. Molybdenum forms sparinglysoluble compounds with inorganic and organicreagents; these compounds lie at the basis of not only

O22+

O42–

O4–

)3–

)84–

F82–

O246–

O404–

O114–

O144–

O132–

O215–

O246–

O245–

O413–

O783–

O1128–

determination methods for molybdenum but also pre-concentration and separation methods, including vari-ous versions of extraction and chromatography. Someof the reactions are used in the processing of molybde-num-containing raw materials.

Molybdenum is primarily used for the production ofspecial alloys and steels. They are heat-resistant, corro-sion-resistant in solutions and alkali metal vapors, andrefractory; they exhibit high coefficients of elasticityand shear moduli. Molybdenum is of considerable bio-logical and physiological importance. This is one of theten biologically active elements. It participates in nitro-gen, protein, carbohydrate, and fat metabolism as wellas in other biochemical processes. Molybdenum stimu-lates the biosynthesis of nucleic acids and proteins; itincreases the chlorophyll and vitamin contents of plantorganisms, and it is necessary for plants throughouttheir life. Molybdenum is a constituent of manyenzymes (xanthine oxidase, aldehyde oxidase, andsulfite oxidase). The concentration of molybdenum inhuman blood is 5

µ

g/L. Molybdenum-deficient soilscause leaf blotch and rolling. A deficiency of molybde-num in the human diet inhibits cellular growth andincreases susceptibility to caries, whereas an increasedconcentration of molybdenum in blood is associatedwith the risk of gout and disseminated sclerosis. At thesame time, along with Cu, Cd, Hg, Pb, Cr, and V,molybdenum is grouped with widespread elements thatare potentially hazardous to humans. The abundance ofmolybdenum in the Earth’s crust is about

3

×

10

–4

%

;however, it can be classified with rare elements,because minerals (molybdenite

MoS

2

and molybdateslike wulfenite

PbMoO

4

and

MgMoO

4

are known) andores are almost absent. The molybdenum content ofsoil, such as chernozem, is

2.5

×

10

–4

or

1.9

×

10

–4

%

ata depth of 0–28 or 140–150 cm, respectively (for com-parison, the upper horizon of chernozem contains0.61% Ti,

7.7

×

10

–2

%

Mn, and

3.4

×

10

–2

%

Zn).Among plants, buckwheat, all legumes (from 0.2 to4.7

µ

g/kg), vegetables, and cereals (from 0.12 to1.14

µ

g/kg) concentrate molybdenum.

The human daily requirement for molybdenum is0.075–0.250 mg for adults and 0.06 mg for children.For comparison, the daily requirements (in mg) for

Methods for Determining Molybdenum

V. M. Ivanov, G. A. Kochelaeva, and G. V. Prokhorova

Moscow State University, Vorob’evy gory, Moscow, 119899 Russia

Received September 20, 2001; in final form, November 19, 2001

Abstract

—The data on determination methods for molybdenum published from 1985 to 2001 are surveyed.

REVIEWS

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2002

METHODS FOR DETERMINING MOLYBDENUM 759

other elements are the following: Al, 0.0014–0.008; Cd,0.07–0.3; Cr, 0.01–1.2; Cu, 0.5–6.0; Fe, 6–40; and Mg,250–380. The molybdenum content of food must becurrently monitored.

Molybdenum plays very important roles in engi-neering and biology. New ore deposits should beexplored and mined; the molybdenum contents ofalloys and environmental samples should be monitored;the natural migration of molybdenum, in particular, inplant and animal organisms, should be studied. Thus,we can state that a topical problem is to develop selec-tive and rapid methods for the determination of molyb-denum over a wide concentration range with good met-rological characteristics.

At concentrations of 1–50, 0.5–5, or <0.1%, molyb-denum is determined by gravimetry, titrimetry, or phys-icochemical techniques, often combined with preconcen-tration, respectively. Of spectroscopic and electrochemicaltechniques, spectrophotometry and voltammetry, respec-tively, are used most frequently, although the use ofatomic emission and atomic absorption spectrometry,amperometry, and potentiometry for the determinationof molybdenum is known. High-sensitivity techniques(inductively coupled plasma mass spectrometry, kineticmethods, and neutron activation analysis) are usedmore rarely. These techniques are most frequently usedafter molybdenum preconcentration and separationfrom a matrix. Moreover, these techniques are timeconsuming (neutron activation analysis) or expensivebecause of the instrumentation used, or they requirehighly skilled personnel.

Therefore, spectroscopic and electrochemical tech-niques used for determining molybdenum, spectropho-tometry and voltammetry, respectively, are primarilysurveyed in this review. The analytical chemistry ofmolybdenum was considered in a monograph [1]. Theelectrochemical behavior of molybdenum was consid-ered in monographs [2, 3] and reviews [4, 5]. The deter-mination of molybdenum in mineral raw materials,steels, alloys, and soils attracted the greatest interestuntil 1985–1987. More recently, major attention hasbeen focused on methods for the determination ofmolybdenum in plants, human and animal tissues,drinking water, and food products.

This review covers data published from January1985 to 2001; earlier publications are cited only in ahistorical aspect. The use of inorganic reagents isbeyond the scope of this review because of the low sen-sitivity of these determinations. Such reagents wereused before for the determination of large amounts ofmolybdenum and for the standardization of solutions.

SPECTROSCOPIC TECHNIQUES

Spectrophotometry.

Molybdenum(VI) reacts withorganic reagents that primarily contain oxygen as donoratoms. Molybdenum(V) reacts with sulfur-containingreagents, which initially reduce molybdenum(VI) to

molybdenum(V) and then form chelate complexes withthe latter. The interaction with nitrogen-containingreagents is less typical; if the reagents contain

=NH

,

−

NH

2

, or quaternary ammonium groups, ion-associatecomplexes are formed. In this case, the ligand in theinner coordination sphere is responsible for the colorand intensity of absorption.

Of oxygen-containing reagents, the following aremost sensitive to molybdenum(VI) (the absorptionmaximum wavelengths and molar absorption coeffi-cients are given in parentheses after reagent names):6,7-dihydroxy-2,4-diphenylbenzopyrilium chloride

(535; 5.0

×

10

4

)

, 9-(2-nitrophenyl)fluorone (570;

5.60

×

10

4

), and Pyrogallol Red in the presence of dodecanol-trimethylammonium bromide (587;

8.15

×

10

4

). Thefollowing sulfur-containing reagents are most sensitiveto molybdenum(V): thioglycolic acid, toluene-3,4-dithiol, 8-mercaptoquinoline, xanthogenates, dithiocar-bamates, dialkyl dithiophosphates, and diaryl dithio-phosphates [6, 7]. Reactions with molybdenum(V) areless sensitive but more selective, because they occur ina more highly acidic medium. The molar absorptioncoefficients of complexes, which are unstable, are nohigher than

1

×

10

4

(the exception is toluene-3,4-dithiol; the molar absorption coefficient of its complexis equal to

2

×

10

4

).To improve the sensitivity of the determination,

complexes are often extracted.Binary complexes are formed with reagents from

the following classes (Table 1): hydroxamic acids, flu-orones, pyrocatechols, and aldehyde hydrazones.

The reagents were used for the determination ofmolybdenum in soils [9, 10], pharmaceuticals [9, 10],and steels [16]. The complexation of molybdenum(VI)with 2-(2-furyl)-3-hydroxy-4H-chromen-4-one [20]; 3-hydroxy-2-(4-methoxyphenyl)-6-methyl-4H-chromen-4-one [21]; and 3-hydroxyflavone [22] was studied. Inthe majority of cases, homogeneous aqueous organicmixtures were used for dissolving the complexes.

The following

o

,

o

'-dihydroxyazo compounds werefound to be the most promising reagents: 2,2'-dihy-droxyazobenzene, Solochrome Violet R, SolochromeDark Blue B, Solochrome Dark RN, Solochrome BlackAS, Solochrome Green, Acid Chrome Blue 2K [1],Beryllon II IREA [23], Gallion IREA [24], Lumomag-neson IREA, Lumogallion IREA [25, 26], and Magne-son IREA [26]. These reagents contain an

o

,

o

'-dihy-droxyazo functional analytical group, at which com-plexation takes place; the nitrogen atoms of the azogroup are electron-donor atoms. Therefore, thesereagents can be classified with tridentate ligands. Thisis responsible for the valuable analytical properties ofthe reagents and complexes: a large bathochromic shiftupon complexation, a wide range of optimum acidities,and the absence of stepwise complexation because ofthe interaction in a ratio of 1 : 1. At the same time, notall of these reagents are used in actual practice becauseof low selectivity; they are mainly of theoretical interest

760

JOURNAL OF ANALYTICAL CHEMISTRY

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No. 9

2002

IVANOV

et al

.

for the determination of relationships between thestructure of reagents and the properties of the resultingcomplexes. In the past decades, Lumomagneson IREAand Magneson IREA [25, 26] attracted attentionbecause of their ability to form mixed-ligand com-plexes, whose optical, electrochemical, sorption, andcolorimetric properties are interesting from the stand-point of analytical chemistry. The structure of thereagents and the pH range of their interaction withmolybdenum(VI) suggest that the cationic species

Mo

participates in the complexation.

In contrast to Lumomagneson IREA and MagnesonIREA, which react with molybdenum(VI) in the cold,bisazo compounds with analogous analytical functionalgroups do not form binary complexes with molybde-num(VI).

Ternary complexes.

The following two types ofcomplexes can be recognized: ion associates andmixed-ligand complexes.

Ion associates.

Colored organic reagents, most ofthem are summarized in Table 1, occupy the inner coor-dination sphere of these complexes, whereas surfac-tants occur in the outer sphere. Most frequently, theseare cationic surfactants, which compensate for thecharge of the inner coordination sphere or the charge ofdissociated sulfo groups of the reagents. A monograph[27] and a review [28] were devoted to the complex-ation of various metal ions in the presence of surfac-

O22+

tants. Table 2 gives some examples of ion associates[29–42].

It is likely that, of this group, the ion associates offluoronates are most promising. Among these com-plexes, the previously proposed complex of molybde-num with

o

-nitrophenylfluorone in the presence ofdiantipyrilmethane is most sensitive [43, 44]; the molarabsorption coefficient of an associate with this complexis

1.31

×

10

5

(at 530 nm).

Of inorganic reagents, thiocyanate ions are most fre-quently introduced into the inner coordination sphereof complexes.

N

1

-Hydroxy-

N

1

, N2-diphenylbenzami-dine [45], its chlorine derivative [46], metoclopramide,oxybuprocaine [47], ethyl violet [48, 49], and xanthenedyes [50] can occupy the outer sphere. Many examplesof the complexation and determination of molybdenumas ion associates and micellar or solubilized complexeswith surfactants are given in a monograph [51]. It ismost interesting that, upon the introduction of a surfac-tant, a very large bathochromic shift was observed inthe complexation of molybdenum(VI) with pyrocate-chol violet (PV) in the presence of cetylpyridinium(CP). The binary complex Mo(PV) exhibits an absorp-tion maximum at 560 nm, whereas the ternary complexMo(PV)(CP)2 exhibits an absorption maximum at710 nm [52]; in this case, the molar absorption coefficientof the complex increased from 3.7 × 104 to 15.0 × 104.

Mixed-ligand complexes. Hydroxylamine is com-monly used as the second component of these com-

Table 1. Chemical and analytical characteristics of the binary complexes of molybdenum(VI)

Reagent λmax, nm ε × 10–4 References

Hydroxamic acids

2-Benzylidenaminobenzohydroxamic acid 358 0.55 [8]

2-Salicylidenaminobenzohydroxamic acid 380 0.65 [8]

Hydroxamic acid 375 1.62 [9,10]

Phenylbenzohydroxamic acid 350 7.2 [11]

Mandelohydroxamic acid – – [12]

Fluorones

p-Dimethylaminophenylfluorone 535 12.0 [13]

2,3,7-Trihydroxy-9-dibromohydroxyphenylfluorone 530 11.0 [14]

Azo derivatives of pyrocatechol

Dihydroxyazobenzene 475 1.66 [15]

505 0.16

Thiazolylazopyrocatechol 530 0.54 [15]

p-Nitrobenzeneazopyrocatechol 540 3.30 [15]

Aldehyde hydrazones

3,4-Dihydroxybenzaldehyde guanylhydrazone 550 0.88 [16]

Resacetophenone isonicotinoylhydrazone 440 0.9 [17]

Salicylaldehyde isonicotinoylhydrazone – – [18]

2,4-Dihydroxyacetophenone benzoylhydrazone 402 0.94 [19]

JOURNAL OF ANALYTICAL CHEMISTRY Vol. 57 No. 9 2002

METHODS FOR DETERMINING MOLYBDENUM 761

pounds, whereas monoazo and bisazo derivatives areresponsible for optical characteristics. In 1971, Savvinet al. [53] reported on the interaction of molybdenumwith o,o'-dihydroxyazo compounds in the presence ofhydroxylamine. The formation of mixed-ligand com-pounds with molybdenum, hydroxylamine, and the fol-lowing bisazo derivatives based on chromotropic acidwas reported: Sulfonitrophenol K, SulfochlorophenolK, Picramine M, Sulfochlorophenol M, Sulfonitrophe-nol M, Sulfonitrophenol B, Sulfochlorophenol B, Sul-fonitrophenol S, Picramine S, and Sulfochlorophenol S[53]. The reagents exhibit a single absorption maxi-mum of 540–560 nm, and the complexes exhibit twomaximums of 600–630 and 650–680 nm. The batho-chromic shift on the complexation is 90–135 nm; thecomplexes are formed in an acidic medium (pH ≥ 1.5).The molar absorption coefficients of symmetricalreagents [(2.12–2.94) × 104] are lower than those ofunsymmetrical reagents (a maximum value of 5.0 × 104

for Sulfonitrophenol K). Note that the bisazo deriva-tives of Ó,Ó'-hydroxyazo compounds are the least sensi-tive to the interaction with molybdenum and com-pounds containing a phenol hydroxyl group in the left-hand side and a carboxyl group in the right-hand sideare the most sensitive. The 1 : 1 ratio between compo-nents, which was found with an excess of reagent, sug-gests that molybdenum reacts with only reagents fromthe phenol series, even in the case of symmetricalbisazo reagents.

The mixed-ligand complexes of molybdenum withhydroxylamine and Lumogallion IREA or MagnesonIREA were studied in more detail from the standpoint

of complexation kinetics and thermodynamics [26, 54].The complexes are formed over a wide range of pH1.0–4.5 on heating. Their absorption spectra exhibit asomewhat larger bathochromic shift of absorption max-imums, and the molar absorption coefficients in thepresence of hydroxylamine are higher than those ofbinary complexes by a factor of 2 to 3. Only this factmade it possible to detect the formation of a mixed-ligand complex, because binary complexes are formedin the cold. The activation energies of molybdenumcomplexes with Lumogallion IREA and MagnesonIREA are equal to 26.0 and 19.2 kJ/mol, respectively,and the ratio between components is 1 : 1 : 1 in all cases[54]. That is, a coordinatively saturated complex isformed, in which the coordination number of molybde-num is equal to 8, hydroxylamine is a monodentateligand, and the central atom is molybdenum(VI) as

Mo . The linearity range of a calibration graph is5−50 µg molybdenum in 20 mL of solution.

As well as Ó,Ó'-dihydroxyazo compounds, heterocy-clic azo compounds are tridentate. However, they reactwith many ions that occur in only a cationic form;therefore, they do not form binary complexes withmolybdenum(VI) in a weakly acidic medium [55]. In1969, Lassner et al. [56] reported on the reaction ofmolybdenum(VI) with 4-(2-pyridylazo)resorcinol(PAR) at pH 6–7 in the presence of hydroxylamine. Theresulting mixed-ligand compound with the 1 : 1 : 1 sto-ichiometry exhibited an absorption maximum at530 nm and ε = 2.75 × 104. More recently [54], thekinetics and thermodynamics of complexation in thissystem were studied, and another pyridine azo com-

O22+

Table 2. Chemical and analytical characteristics of the ion associates of molybdenum(VI)

Reagent and surfactant λmax, nm ε × 10–4 References

Fluorones

3,4-Dimethylhydroxyphenylfluorone + Tween 80 526 11.6 [29]

p-Diethylaminophenylfluorone + cetylpyridinium 532 11.0 [30]

Dibromohydroxyphenylfluorone + cetylpyridinium 532 11.9 [31]

Phenylfluorone + dodecylmethylbenzylammonium 550 11.8 [32]

Phenylfluorone + cetylmethylammonium 524 11.0 [33]

o-Nitrophenylfluorone + cetylpyridinium 530 15.5 [34]

531 16.3 [35]

Azo derivatives of pyrocatechol

4-(6-Bromo-2-benzothiazolylazo)pyrogallol + cethyltrimethylammonium 585 5.76 [36]

Azo derivatives of pyrocatechol + cetylpyridinium – – [37]

Other reagents

Quinalizarin + cetylpyridinium 580 12.7 [38]

7,8-Dihydroxy-4-methylcoumarin + cetylpyridinium – 13.2 [39]

2,7-Dihydroxyfluorescein + cetylpyridinium – 13.3 [40]

Rutin + cethyltrimethylammonium 425 3.54 [41]

Bromopyrogallol red + cetylpyridinium 630 6.0 [42]

762

JOURNAL OF ANALYTICAL CHEMISTRY Vol. 57 No. 9 2002

IVANOV et al.

pound, 2-(5-bromo-2-pyridylazo)-5-diethylaminophe-nol (5-Br-PADAP), which is the most sensitive reagentfor all metal ions among the group of heterocyclic azocompounds [57], was added to PAR. The activationenergies are equal to 33.6 and 39.8 kJ/mol for theMo(VI)–PAR–hydroxylamine and Mo(VI)–5-Br-PADAP–hydroxylamine systems, respectively.

Table 3 compares some properties of binary and ter-nary complexes.

The activation energies and van’t Hoff’s tempera-ture coefficients of complexes increase in the followingorder of reagents: Magneson IREA ≤ LumogallionIREA ≤ PAR ≤ 5-Br-PADAP. The molar absorptioncoefficients increase in the same order; however, theyare always lower in binary complexes than in mixed-ligand complexes. This fact also indicates that hydrox-ylamine enters the inner coordination sphere of thecomplex.

The optimum range of pH 4.5–5.3 in the complex-ation with PAR, which was recommended as a reagentfor the determination of molybdenum, was supportedby the following almost identical calibration functions(n = 5; P = 0.95; c is the concentration of molybdenum,M):

A = (2.9 ± 0.1) × 104c + (0.12 ± 0.02) (pH 4.9),

A = (3.0 ± 0.6) × 104c + (0.2 ± 0.1) (pH 5.3).

The calibration graphs were linear at molybdenumconcentrations of 5.0 × 10–6–2.5 × 10–5 M; the detectionlimit of molybdenum was 4.7 × 10–7 å (3s value). Thebest reproducibility in the determination of molybde-num was achieved at pH 4.9.

Although the determination of molybdenum in thepresence of surfactants is more sensitive than the deter-mination as mixed-ligand compounds, the selectivity ishigher in the latter case. The complexes of all ions canbe prepared in both the presence and the absence ofhydroxylamine at equal pH and temperature conditions(on heating). The difference between the absorbancesin the presence and the absence of hydroxylamine cor-

responds to the light absorption of the molybdenumcomplex. In another version, the effect of temperatureon the complexation in the presence of hydroxylaminecan be used: the absorbance should be measured onheating and in the cold; the difference between theabsorbances corresponds to the light absorption of themolybdenum complex. However, such versions remainto be implemented.

There are new possibilities to determine molybde-num with a higher sensitivity, which will be consideredbelow.

In the determination of molybdenum in real sam-ples, extraction–photometric techniques were oftenused for improving both sensitivity and selectivity [59–69]. Flow-injection analysis was used for improvingrapidity [70–73].

Diffuse-reflectance spectroscopy. The linear rela-tionship between the diffuse reflection coefficient (R1)or the Kubelka–Munk function (∆F) and the concentra-tion of an analyte, which was converted into a coloredspecies and sorbed on a solid support, is used in thistechnique. Usually, complex formation and sorptionoccur at the same pH values; therefore, the conditionsof photometric determination in solutions can be usedfor sorption. The concentration factors are no lowerthan 60, because the weight of a sorbent cannot belower than 0.3 g (in the case of Silochromes and ionexchangers) and the volume of a solution with a coloredcompound varies over a range 15–50 mL with goodmetrological characteristics. Ion exchangers (forcharged complexes) or Silochromes (for neutral com-plexes or complexes with a charge compensated by theintroduction of surfactants as counterions) can be usedas sorbents.

Thus, in the case of Lumogallion IREA and Magne-son IREA, the complexes are sorbed by an AV-17 anionexchanger because of the negative charge of a sulfogroup [74]. The concentration factors were no lowerthan 66 at an aqueous phase volume of 20 mL. Thismade it possible to lower the analytical range of molyb-denum to 0.19–1.9 µg in 20 mL and to use diffuse-

Table 3. Properties of molybdenum(VI) complexes with o,o'-dihydroxyazo and heterocyclic azo compounds in the absenceand in the presence of hydroxylamine [54]

Reagentλmax, nm

pHopt ε × 10–4 Ea, kJ/mol Van’t Hoff’s tempe-rature coefficient γ logβ

HR complex

Lumogallion IREA 510 430 1.0–5.0 1.1 17.20

Lumogallion IREA + hydroxylamine 530 430 1.0–4.7 2.1 26.0 1.2 18.00

Magneson IREA 570 490 1.0–4.7 0.9 17.09

Magneson IREA + hydroxylamine 580 490 1.0–5.0 2.9 19.2 1.2 17.66

PAR – 420 No complex formation

PAR + hydroxylamine 530 420 4.6–5.3 2.9 33.6 1.5 11.82

5-Br-PADAP – 440 No complex formation

5-Br-PADAP + hydroxylamine 570 440 4.6–5.3 3.9 39.8 1.6 11.92

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METHODS FOR DETERMINING MOLYBDENUM 763

reflectance spectroscopy. Table 4 compares calibrationfunctions for the determination of molybdenum asbinary and ternary complexes in solutions and in anAV-17 sorbent phase.

In the case of heterocyclic azo compounds, anAV-17 anion exchanger was used for the sorption of acomplex with PAR (a negative charge at the dissociatedp-hydroxyl group of resorcinol) and Silochrome C-120was used for the sorption of a complex with 5-Br-PADAP [75]. The calibration graphs were linear atmolybdenum contents of 1.9–7.7 µg in 20 mL of solu-tion (PAR, AV-17) or 1.0–4.5 µg in 20 mL (PAR or5-Br-PADAP on C-120). In the determination ofmolybdenum as complexes with Lumogallion IREAand Magneson IREA in both the absence and the pres-ence of hydroxylamine, the calibration graphs were lin-ear at 0.5–4.5 µg of molybdenum(VI) in 20 mL of solu-tion.

The sensitivity of the determination of molybdenumas complex sorbates with analytical signal measure-ments by diffuse-reflectance spectroscopy is at least20 times higher than that of the determination in solu-tion.

Diffuse-reflectance spectroscopy was applied todetermine molybdenum as a complex with pyrocate-chol violet after its sorption on the AV-17 anionexchanger [76]. The following two interesting factswere found: the absorption maximum of the complexsorbate (660 nm) was bathochromically shifted as com-pared to the maximum of the same complex in solution(550 nm), and the maximum in the spectrum of the sor-bate was shifted by 220 nm as compared to the absorp-tion maximum of the reagent in solution (440 nm). Thiseffect of a strong bathochromic shift was observed pre-viously in solutions of this complex in the presence ofcationic surfactants [52, 77] and, more recently, in thesorption of the same complex on silica with chemicallybound propylamine groups [78]. The second anomalyconsists in a shift of complexation pH on sorption to amore acidic medium and in a wider range of complex-ation pH 1.0–4.0 in place of 3.0–4.0 in solutions withno sorption. This shift to lower pH values in solutionswas explained [79] by an increase in the acid propertiesof the reagent upon its interactions with cationic surfac-tants.

The advantages of (I) sorption combined with deter-mination by diffuse-reflectance spectroscopy over (II)determination in solutions are obvious. The linearityrange of a calibration graph in method I is 0–3 µg in20 mL, whereas it is 1.9–57 µg in 20 mL of solution inmethod II; the detection limits are 0.015 and0.32 µg/mL, respectively. The narrowed linearity rangein method I can be explained by a decrease in the con-centration of pyrocatechol violet in a sorbent phase ascompared to its concentration in solution: the totalexchange capacity of AV-17 for the reagent is 6.27 ×10–6 mol/g [76].

Solid-phase spectrophotometry. Solid-phase spec-trophotometry is a combination of the sorption of col-ored complexes on sorbents with the measurement oflight absorption in a thin layer (usually, 0.1 cm) [80,81]. Almost all analytical forms of complexes can beused in solid-phase spectrophotometry. It is difficult tochoose a solid support: it should not only sorb the com-plex but also be transparent in an optimum range ofwavelengths, desirably, in the visible region of thespectrum. In this respect, silica gels and polyurethanefoams are hardly promising, although they were foundto be good sorbents [82, 83]. Studies on the determina-tion of molybdenum by solid-phase spectrophotometryare scarce; this can be explained by difficulties in thefilling of a very narrow cell using a wet technique andby the nonuniformity of the distribution of a complexover a support. Because of this, the reproducibility ofdetermination results is worse than that in diffuse-reflectance spectroscopy. Note that the above technicalproblems of solid-phase spectrophotometry are absentfrom diffuse-reflectance spectroscopy with the use ofthe same sorbates. These techniques are comparable interms of sensitivity.

The advantage of solid-phase spectrophotometryover diffuse-reflectance spectroscopy consists in thepossibility of better monochromatization (1 to 2 nmdepending on the type of the spectrophotometer),whereas it is equal to 10 nm and restricted by the visibleregion of the spectrum in diffuse-reflectance spectros-copy.

Table 4. Calibration functions for the determination of mo-lybdenum by photometry [26] and diffuse-reflectance spec-troscopy [74] (n = 5; P = 0.95)

Reagent cR , M Calibration function

Lumogallion IREA 1 × 10–5 A = (0.111 ± 0.002)c–(0.006 ± 0.001)

Lumogallion IREA + hydroxylamine

1 × 10–5 A = (0.142 ± 0.006)c–(0.014 ± 0.002)

Lumogallion IREA 1 × 10–6 ∆F = (0.295 ± 0.011)c–(0.073 ± 0.007)

Lumogallion IREA + hydroxylamine

1 × 10–6 ∆F = (0.469 ± 0.027)c–(0.032 ± 0.003)

Magneson IREA 1 × 10–5 A = (0.099 ± 0.003)c–(0.007 ± 0.001)

Magneson IREA +hydroxylamine

1 × 10–5 A = (0.193 ± 0.005)c–(0.001 ± 0.001)

Magneson IREA 1 × 10–6 ∆F = (0.247 ± 0.009)c–(0.060 ± 0.001)

Magneson IREA + hy-droxylamine

1 × 10–6 ∆F = (0.781 ± 0.021)c–(0.045 ± 0.004)

Note: A is the absorbance of the solution; ∆F is the Kubelka–Munkfunction for the sorbate; and c is the concentration of molyb-denum(VI) (µg per 20 mL of solution for A or µg/mL for ∆F).

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Solid-phase spectrophotometry in combination withdiffuse-reflectance spectroscopy was used for choosingoptimum conditions for molybdenum complexationwith Lumogallion IREA, Magneson IREA, PAR, and5-Br-PADAP [74, 75, 84]. Calibration functions wereobtained for all of the systems. Solid-phase spectropho-tometry was applied to examine complexation in themolybdenum(VI)–pyrocatechol violet–AV-17 anionexchanger system and to study ionic equilibria of thereagent on the above anion exchanger [76]. At the totalexchange capacity of the sorbent for pyrocatechol vio-let equal to 6.27 × 10–6 mol/g, the exchange capacity forthe molybdenum complex was 7.8 × 10–7 mol/g.

Chromaticity measurements. The chromaticitymeasurement as a science on color measurements andquantification is based on the calculation of color char-acteristics from the available spectral parameters. Itprovides an opportunity to distinguish between sub-stances with spectrally similar characteristics and toobtain additional data on these substances [83]. Thismethod provides information on the color saturation (S,lightness (L), yellowness (G), whiteness (W), and tint(T), as well as on color differences with respect to thesecharacteristics (∆S, ∆L, ∆T, ∆E). Functional relation-ships between the above characteristics and analyteconcentrations can be found with the use of A, B or X,Y, Z chromaticity coordinates. Both complex solutionsand sorbates can be used; the sensitivity of determina-tion is additionally improved in the latter case.

Only a single publication [84] was devoted to thestudy of the chromaticity characteristics of molybde-num complexes. Lumogallion IREA, Magneson IREA,PAR, and 5-Br-PADAP were studied as the reagents,and binary (with Lumogallion IREA and MagnesonIREA) and ternary (with all of the above reagents) com-plexes with the participation of hydroxylamine wereexamined both in solutions and as sorbates on AV-17 orC-120. It was found that the value of any chromaticitycharacteristic of a complex in solution or a complexsorbate is higher than the diffuse reflection coefficient∆F in diffuse-reflectance spectroscopy or the absor-

bance A in spectrophotometry. For example, for theLumogallion IREA complex with molybdenum in aconcentration range of 0.40–1.90 µg per 20 mL, theslopes of calibration graphs (sensitivity factors) of thechromaticity characteristics X, B, ∆S, ∆L, and ∆Tincreased in magnitude by a factor of 10–25 as com-pared to the slope of the Kubelka–Munk plot (∆F) as afunction of molybdenum concentration. Table 5 com-pares the detection limits of molybdenum in the deter-mination by various optical techniques.

Note that the maximum permissible concentrationof molybdenum in natural water is 250 µg/L, and evenspectrophotometry is suitable for determining it interms of sensitivity. However, the volume of a test sam-ple can be decreased by a factor of 25 with the use ofsorbates and chromaticity characteristics. TheLumogallion IREA, Magneson IREA, PAR, and 5-Br-PADAP reagents were applied to the determination ofmolybdenum in steels, soils, and seawater by variousversions of optical techniques [85, 86].

Test methods. Although the number of color reac-tions for molybdenum is great, the number of test meth-ods for determining it is limited [82]. The reaction withgallic acid in the presence of hydroxylamine, which isperformed on polymer plates [87], and the reactionwith phenylfluorone in the presence of cetylpyridinium[88], which is performed on paper impregnated with thereagents, were mentioned. Molybdenum concentra-tions are determined visually (0.01–1 mg/L) or fromthe length of a colored zone (0.05–10 mg/L). The pro-cedure was used for the determination of molybdenumin wastewater. Test scales were developed for the deter-mination of molybdenum as complexes with heterocy-clic azo compounds in the presence of hydroxylamine[75, 89], with pyrocatechol violet [76], and with thiocy-anate ions (on polyurethane foam) [90].

Other methods of molecular spectroscopy. Ther-mal lens spectrometry was proposed for the determina-tion of molybdenum by the thiocyanate reaction withthe use of ascorbic acid as a reducing agent. The detec-tion limit of molybdenum was 19 pg/mL; 1000-foldamounts of iron were masked with tartrate. Thismethod was applied to the determination of molybde-num in drinking water and natural water [91].

Molybdenum (0.014–0.149%) in alloys was deter-mined by fluorimetry using the quenching of 2-hydrox-ynaphthoquinone luminescence by molybdenum ionsin the presence of cetylpyridinium [92]. Derivativespectrofluorimetry was used in the determination ofmolybdenum in plant tissues, multivitamin prepara-tions, and food products; hexadecyl red S [93] andalizarin red [94] were used as the reagents. The detec-tion limit of molybdenum was 1.7 ng. Carminic acid[95], thiocyanate, Rhodamine S, and lauryl sulfate [96]were also used as the reagents.

Molybdenum in seawater was determined by X-rayfluorescence analysis with preconcentration as dithio-

Table 5. Detection limits of molybdenum determined as acomplex with 5-Br-PADAP and hydroxylamine using opti-cal techniques [84]

Technique Analyticalsignal

cmin,µg/L

Spectrophotometry A 200

Chromaticity characteristics of solutions for spectrophotometry

∆L 130

Chromaticity characteristics of solutions for sorption

∆L 40

Sorption on C-120 ∆F 30

Chromaticity characteristicsof the sorbate

∆L 7.5

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METHODS FOR DETERMINING MOLYBDENUM 765

carbamates [97–99]; the detection limit was 1 µg/L[98].

ATOMIC SPECTROSCOPY

Atomic absorption spectrometry (AAS). AAS isthe second most important technique for the determina-tion of molybdenum. AAS was used as an investigationtechnique for revealing parameters that affect the deter-mination of molybdenum by flame AAS [100–102] andelectrothermal AAS (with the use of pyrolytic graph-ite and tungsten-coated L’vov platforms) [103, 104].The molybdenum concentrations varied from 0.1 to4.0 µg/g; the absolute amount of molybdenum intro-duced into a graphite furnace was 8–13 µg. The major-ity of researchers used electrothermal atomization inthe determination of molybdenum in alloys [100], bio-logical samples [101, 102, 105–107], plant tissues[108], food products [109–112], drinking water [113],and soils [114]. Table 6 summarizes data on the detec-tion limits of molybdenum in various samples by AAS.

It was proposed to determine low molybdenum con-centrations after coprecipitation with metal chelates[115–118] on various collectors: microcrystallinenaphthalene [119, 120], active carbon [121], chelatingsorbents [122], and anion exchangers [122–124].Extraction was also widely used [125–127]. A methodfor the ultratrace determination of molybdenum in sea-water with preconcentration in a system for flow-injec-tion analysis was proposed [128, 129].

Several procedures for the AAS determination oftrace molybdenum without sample decomposition weredeveloped with the use of hydrazine sulfate [130],hydroxylamine hydrochloride [131], barium hydrofluo-ride [132], and other fluorides [133] as matrix modifi-ers.

Atomic emission spectrometry (AES). Molybde-num was determined by AES in surface water, seawa-ter, mineral water, drinking water [134–136], molybde-num glasses [137, 138], human body fluids (0.001–0.08 µg/mL) [139–141], and soils (0.03–4.09 µg/g)[142–144].

Sample preparation in AES includes wet and drymineralization followed by the preconcentration of agroup of elements by sorption or liquid extraction.Most often, carbamates, 8-hydroxyquinoline, acetylac-etone, N-benzoyl-N-phenylhydroxylamine, and 1-phe-nyl-3-methyl-4-benzoyl-5-pyrazolone were used asextractants and chloroform, carbon tetrachloride, andbenzene were used as solvents [145].

To intensify sample preparation, it was proposed touse autoclaves and microwave [146–148] or ultrasonic[149] digestion.

Inductively coupled plasma mass spectrometry(ICP MS). The hybrid ICP MS technique and othertechniques were applied to routine analyses in waterquality laboratories [150–153]. Problems that appearwhen a method is transferred from research laborato-

ries to industrial laboratories were discussed; an auto-mated system for the purification of water and samplesand for preconcentration to remove the effects of matrixelements was developed. This technique determinesultratrace amounts of molybdenum in blood serum at alevel of n pg/g; in this case, the samples should bediluted by a factor of 5–10 without sensitivity losses[154–157]. The ICP MS technique was used for thedetermination of molybdenum in silicate rocks [158]and wheat and rice flour [159].

NUCLEAR PHYSICAL AND RADIOCHEMICAL METHODS

Neutron activation analysis (NAA). NAA wasused for checking the accuracy of the determination ofmolybdenum in biological materials by AAS [160] orspectrophotometry [161]. This technique is most oftencombined with the radiochemical separation of radio-active molybdenum isotopes from the matrix after irra-diation. The extraction of molybdenum dithiocarbam-ate was used for the separation. NAA was used for thedetermination of molybdenum in scheelite [162], foodproducts [163], seawater [164], human body fluids[165–167], biomaterials [168–170], biological matri-ces [171, 172], and bovine blood serum [173]. The highsensitivity of this technique (≥10–8%) is attractive; how-ever, it is achieved upon the irradiation with a high-power beam of thermal neutrons. At the same time,NAA is a time-consuming technique because of thenecessity of cooling the sample after irradiation for thedecay of short-lived isotopes (up to a month [173]). Theuse of NAA is restricted by cumbersome irradiationfacilities, special requirements on safe practice, and asmall number of extraction systems for separating theradioisotope from the matrix. In contrast to other ele-ments, the sample is usually dissolved for the determi-nation of molybdenum [174–176].

Radiochemical methods. Radiochemical methodsare used for the determination of molybdenum inmedicinal plants [177] and food products [178].

Table 6. Detection limits of molybdenum determined byAAS in various materials

Test material cminRefe-rences

Sheep blood plasma 3 µg/L [102]

Human urine 0.2 µg/L [106]

Human body fluids 0.03 [107]

Food products (bread, meat, eggs, potatoes, and apples)

0.05–3.86 [109]

Baby foods and dried milk 0.063 [111]

Milk 0.28 ng/g [112]

Tap (drinking) water 0.09–2.31 mg/L [113]

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VOLTAMMETRIC DETERMINATION OF MOLYBDENUM

Of the electrochemical methods for determiningmolybdenum in minor amounts, voltammetry is mostwidely used [179–181]. Molybdenum(VI) is electroac-tive on a dropping mercury electrode only in an acidicmedium [182–185]. The solutions of H2SO4, HCl,HNO3, HClO4, and H3PO4 were studied as supportingelectrolytes. The majority of authors believed that thereduction of molybdenum(VI) occurs in the two stepsMo(VI) Mo(V) and Mo(V) Mo(III), and bothof these steps are irreversible.

However, additional waves appeared in some cases;the number of these waves depended on the concentra-tion of a depolarizer and on the nature and acidity ofsupporting solution. As a rule, in the presence of HCland H2SO4, diffusion is the rate-limiting step of Mo(VI)reduction. In a number of cases, diffusion is compli-cated by adsorption and other phenomena, for example,the occurrence of coupled chemical reactions [186].Ionic molybdenum(VI) species in the bulk of solutionand in a near-electrode layer can be different. Becauseof this, it becomes more difficult to determine themolybdenum(VI) species that participates in the elec-trode process. However, classical polarography, whichis based on the measurement of diffusion currents, wasused for the determination of molybdenum in variousnaturally occurring (ores and rocks) and industrial(alloys and steels) materials [5, 187].

A comparative study of the above supporting electro-lytes demonstrated that the height of the Mo(VI) Mo(V) reduction wave in an HClO4 solution was muchgreater than the heights of waves in other acid solutionsbecause of the appearance of catalytic currents. In thiscase, the rate-determining step is not diffusion but achemical reaction of the reduction product, that is,

Mo(III), with the substrate (Cl ) with the regenera-tion of the electroactive substance, that is, Mo(VI).These homogeneous catalytic reactions also occurredin H2SO4 solutions in the presence of oxidizing agents

such as , Cl , Cl , and Br .

The mechanisms of the appearance of catalytic cur-rents were studied in most detail in the following sys-tems: Mo(VI)–KClO3–mandelic acid [188], Mo(VI)–H2O2 at pH 1–5 [189], Mo(VI)–pyrocatechol–Br[190], and Mo–PAR–NaBrO3 [191]. The applicabilityof the catalytic polarographic currents of hydroxy-lamine in the presence of tungsten and molybdenumcomplexes with pyrocatechol was demonstrated. A pro-cedure for the simultaneous determination of n × 10–6 –n × 10–5% tungsten and molybdenum in sedimentaryrocks was developed [192].

Catalytic currents in the presence of oxidizingagents made it possible to considerably lower the detec-tion limit of molybdenum(VI), particularly, with the

O4–

NO3–

O4–

O3–

O3–

O3–

use of such versions as linear sweep voltammetry anddifferential pulse voltammetry.

The complex compounds of molybdenum(VI) withmany organic reagents are surface active; that is, theycan be adsorbed or chemisorbed on the surface of amercury electrode. This makes it possible to concen-trate trace amounts of molybdenum(VI). Such a pre-concentration is impossible in the classical version ofanodic stripping voltammetry with electrolytic concen-tration, which is suitable only for elements that arereduced to metals and are readily soluble in mercury. Itis well known that molybdenum(VI) does not fall intothis category.

The following feasible procedures were proposedfor the adsorption accumulation of metal ions as com-plexes: (1) the complex of a metal ion with a surface-active ligand (L) is formed in the solution and then isadsorbed on an electrode; (2) a ligand undergoesadsorption on an electrode and then reacts with a metalion to form a complex; (3) the metal ion reduced at anelectrode reacts with a ligand in solution, and the result-ing complex is adsorbed on the electrode; and (4) themetal ion reduced at an electrode forms a complex witha ligand adsorbed on the electrode. As a rule, theadsorption of a ligand, the complexation in solution andat the electrode surface, and the adsorption of the com-plex occur simultaneously, and it is difficult to deter-mine which of the mechanisms takes place in a partic-ular case. In a cathodic sweep of the potential, acathodic peak is detected in the voltammogram. Thispeak can result from the following: (1) the reduction ofthe central atom of an adsorbed complex, (2) the reduc-tion of a ligand of an adsorbed complex, and (3) cata-lytic hydrogen evolution.

The adsorption reduction currents of the centralatom or a ligand from an adsorbed complex can be usedfor further decreasing the detection limit, becauseadsorption preconcentration is one of the most efficienttechniques for improving the analytical signal-to-noiseratio in voltammetry.

The high sensitivity of determination with adsorp-tion preconcentration mainly depends on the facts thatthe concentrated element occurs on the electrode sur-face and peaks in the voltammogram are higher that inthe case of concentrating the element as an amalgambecause of the absence of diffusion into an electrodephase. As a rule, monomolecular layers are formed, andthe peak values are independent of the mass-transferrate of the depolarizer to the electrode surface. Thus,more highly sensitive techniques with higher sweeprates of the polarization voltage (linear sweep voltam-metry and high-speed ac voltammetry) can be used inthe measurements of voltammograms. Adsorptionaccumulation can be successfully combined with thecatalytic currents of oxidants (Table 7); this is alsofavorable for an increase in the sensitivity of determina-tion.

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METHODS FOR DETERMINING MOLYBDENUM 767

Table 7. Voltammetric determination of molybdenum in biological materials and environmental samples

ReagentElectrode; Eads; supporting electrolyte;

detection technique (Epeak)cmin (tads) Test sample Refe-

rences

By the catalytic currents of Mo(VI) in the presence of oxidizing agentsHClO4 Hg electrode (hanging drop); DPV (–0.25 V) 0.1 mg/L Glasses [193]After sorptionon Chelex-100; KClO3

Hg electrode (DME); classical polarography 870 pM Seawater [194]

KNO3 Hg electrode (DME); 1 M H2SO4;classical polarography; (E1/2—0.2 V)

1 × 10–7 M Alloys [195]

Ascorbic acid + NH4NO3 Hg electrode (DME); 0.14 M HCl and1.2 × 10–2 M (NH4)2Fe(SO4)2; DPV (–0.56 V)

0.06 mg/kg Leaves, fruits, and soil

[196]

Mandelic acid + Cl Hg electrode (DME); H2SO4; classical po-larography; (E1/2—0.17 V)

0.01 mg/kg H2SO4; quality con-trol

[197]

Benzilic acid + diphe-

nylguanidine + Cl

Hg electrode (DME); H2SO4 classical po-larography; (E1/2—0.17 V)

0.002 mg/L Tap water and spring water

[198]

By the catalytic currents of Mo(VI) in the presence of oxidizing agents after adsorption accumulation8-Hydroxyquinoline + KNO3 Hg electrode (hanging drop); DPV; (–0.7 V) 2 × 10–4% Pb, Pb–Ba glasses,

and biological mate-rials

[193,199]

8-Hydroxyquinoline Hg electrode (hanging drop); –0.2 V; DPV;(–0.59 V)

4 × 10–9 M(120 s)

Seawater [184]

8-Hydroxyquinoline, Br Hg electrode (static); open-circuit line – Formation water [185]

8-Hydroxyquinoline, N Hg electrode (hanging drop); 0.005–0.5 M HNO3; DPV

7 × 10–10 M Biological samplesand plant materials

[179,199]

Mandelic acid Hg electrode (hanging drop); H2SO4; Clthird-order derivative voltammetry

9.4 × 10–11 MDrinking water and rainwater [200]

After adsorption preconcentrationTolidine blue Hg electrode (static); –0.1 V (Ag/AgCl elec-

trode); 0.048 M H2C2O4; LSV (–0.3 V)1 × 10–10 M

(240 s)Natural water and soil

[201]

Cupferon DME (3 s); 0.2 M acetate buffer solution(pH 2.7–3.0); DPV (–0.33 V)

1 × 10–10 M Soils and plants [186, 202]

P Hg electrode (hanging drop); –0.1 V (SCE);5 × 10–3 M NaH2PO4 + 2 × 10–2 M H3PO4(pH 2.4); LSV (–0.39 V)

4 × 10–9 M(120 s)

Biological samples [203]

Eriochrome Blue Black R Hg electrode (hanging drop); pH 1.6;LSV (–0.17 V)

5 × 10–9 M Seawater [204]

Lumogallion IREA DME (5 s); pH 2.0 Seawater [205](–0.20 V)* 3.7 × 10–7 M*(–0.30 V) 3.9 × 10–7 M**

Magneson IREA DME (5 s); pH 2.0; LSV (–0.27 V)* 7.9 × 10–8 M* Seawater [205]Lumogallion IREA + hydrox-ylamine

DME (5 s); pH 2.0; LSV (–0.20 V)* 4.5 × 10–7 M* Seawater [205](–0.40 V)** 5.2 × 10–7 M**

Magneson IREA + hydroxy-lamine

DME (5 s); pH 2.0; LSV (–0.27 V)* 1.5 × 10–7 M* Seawater [205](–0.49 V)** 1.7 × 10–7 M**

PAR + hydroxylamine DME (5 s); pH 2.0; LSV (–0.38 V)* 1.7 × 10–7 M* Seawater [205](–0.49 V)** 2.6 × 10–7 M**

5-Br-PADAP + hydroxy-lamine

DME (5 s); pH 2.0; LSV Seawater [205](–0.42 V)* 2.4 × 10–7 M*(–0.58 V)** 2.6 × 10–7 M**

Note: DPV is differential pulse voltammetry, DME is a dropping mercury electrode, LSV is linear sweep voltammetry, and SCE is a sat-urated calomel electrode.

* Determined using the peak of the free reagent. ** Determined using the peak of the reagent from the complex.

O3–

O3–

O3–

O3–

O3–

O43–

768

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IVANOV et al.

Because a substance is accumulated at the electrodesurface, particular attention is focused on its reproduc-ibility and purity. The renewable surface of a certainsize can be easily produced in mercury electrodes.Therefore, hanging mercury drop electrodes or otherstationary dropping electrodes are most widely used.The deposition potential, the duration of deposition, therate of stirring, the nature and composition of the solu-tion (pH and ionic strength), the temperature, and theconcentration of an organic reagent affect the value ofan analytical signal.

Molybdenum belongs to the elements that areunsuitable for electrolytic preconcentration. Because ofthis, its traces are preconcentrated at an electrode onlyas complex compounds with organic (rarely, with inor-ganic) reagents. Differential pulse stripping voltamme-try and linear sweep voltammetry on a dropping mer-cury electrode or a stationary (hanging drop) electrodeare most frequently used for measuring voltammo-grams. Usually, this technique is used for the determi-nation of molybdenum in biological materials and envi-ronmental samples (Table 7).

CONCLUSIONS

This review demonstrates the predominant develop-ment of optical techniques for the determination ofmolybdenum. This results from not only the advantagesof optical techniques over other techniques but alsofrom the extreme complexationability of molybdenum.The preconcentration of molybdenum is used inincreasing frequency, and extraction methods give wayto more perfect sorption methods with preconcentrationas colored compounds. Concentrates can be immedi-ately used for the determination of molybdenum bynew optical techniques: diffuse-reflectance spectros-copy and the chromaticity measurements of sorbates.The preconcentration of molybdenum complexes onelectrodes combined with the subsequent determina-tion by stripping voltammetry (measuring the reagentcurrent in the complex) considerably increases the sen-sitivity of the determination of molybdenum. In the lastdecade, environmental samples, food products, andbiological materials were added to the materials that areusually analyzed for molybdenum (steels, alloys, andminerals); therefore, new procedures should be devel-oped.

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