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Determination of molybdenum in natural waters after selective adsorption on sephadex gel
Anulytica Elsevier
Chimica Actn, 142(1982) !()I--107 Scientific Publishing Company, Amsterdam
-. Printed
in The Netherlands
DETERMINATION OF MOLYBDENUM IN NATURAL SELECI’IVE ADSORPTION ON SEPHADEX GEL
KAZUHISA
YOSHIMUKA*,
Ikpartment of Chemistry, Fukuoka 8 12 (Japan) (Received
“2nd
February
SOICHI
Fuculty
1IIHAOKA
o/Science,
and TOSHIKAZU
Kyushu
Unioersity
WATERS
AFTER
TARIJTANI
33, 1iai:ozat:i.
Higashiku.
1982)
SUMMARY Molybdenum(V1) ions are adsorbed on Sephades G-25 gel at pH 3.5, and are desorhed reversibly with a complcxing agent, EDTA. The adsorbability is much grater than that for boron(III) and vanadium(V). LnQ:e amounts of sodium chloride have little cffcct on
the adsorption. Molybdenum concentrations in natural waters, caspccially in .seawater. can be determined with good precision and accuracy after selective preconcentration of molybdenum by a Sephadcx G-25 gel column. The detection limit for 250.ml water spcctrometry or by spcctrophotomctry samples is about 1 ~g I” by atomic absorption with bromopyrogallol red as reagent.
Selective concentration methods for boron(lII) and vanadium(V) by use of a Sephadex G-25 gel column have already been described [ 1, 21. Borate and vanadate ions are adsorbed on Sephadex G-25 gel from solutions at pH 9.5-11 and 3-7, respectively, and dcsorbed into 0.01-0.1 M hydrochloric acid solution. Complex formation of these oxo-anions with the hydroxyl groups of the dextrdn matrix may be responsible for the reversible adsorption. Molybdenum is one of the important elements for many organisms and its abundance in natural waters is of interest. it is difficult, however, to determine molybdenum directly in natural waters because of its low pg 1’ ’ levels. A simple, rapid, and quantitative method for the determination of molybdenum is needed. It is well known that molybdatc(V1) forms complexes with polyhydroxy compounds [3] _ Adsorption of molybdate on Sephadex gels has been already reported [4, 51 and has been utilized for the separation of molybdenum as matrix element from traces of alkali and alkaline earth metals [Sl . In this study, a separation and concentration method for traces of molybdenum in natural waters has been developed by taking advantage of the reversible adsorption on Sephadex G-25 gel. The molybdenum can be determined by atomic absorption spectrometry or by solution spectrophotomctry with bromopyrogaliol red [ 71.
0003-2670/82/0000-0000/S02.75
a
1982 Elsevier
Scientific
Publishing
Company
102
EXPERIMENTAL Chemicals
All the chemicals used were of analytical grade and the water was deionized. (5000 ppm). Dissolve 5.9 g of LarO, in 50 ml of concentrated hydrochloric acid, evaporate to dryness and add 1 1 of 0.01 M hydrochloric acid solution_ La(ZII) solution
Procedures Separation and concentration of molybdenum(VI) on asephadex G-25 gel column. Filter the water sample at the sampling point through a glass-fiber filter paper (Whatman GF/D) if necessary. Add 7.5 ml of anhydrous acetic
acid to a 250-ml sample and adjust to p1-I 3.5 with ammonia solution. Use an acrylic resin column (16-mm id., 100 mm long) containing 15 ml the column with of Sephades G-25 gel (Medium, Pharmacia). Condition 0.5 M acetate buffer (pH 3.5) and then pass the sample solution through at a rate of 8 ml mix-t”. Wash the column with 25 ml of 0.1 M ammonium acetate tuffer (pH 3.5) and then desorb the molybdenum with 20 ml of 0.01 M EDT-4 (disodium salt) solution_ Reject the first 5 ml of effluent and collect the nest 15 ml in a 50-ml beaker or in a 25-ml volumetric flask. To the evaporated sample containing Determination of molybdenum. 0.5-50 pg of molybdenum in the beaker, add 2 ml of the La(III) solution. After dissolving, determine the molybdenum by atomic absorption spectrometry with a Nippon Jarrell-Ash spectrometer, Model AA-781. Instrumental conditions are as follows: analytical wavelength 313.26 nm, nitrous oxideacetylene flame, 5-cm long burner. Use peak heights relative to 5-ppm molybdenum solution for the calculation. Alternatively, add 5 ml of 1 M hydrochloric acid, 1.5 ml of 0.01 M zephiramine and 1 ml of 0.03% (w/v) bromopyrogallol red (50% ethanolic solution) to the sample containing 0.2-5 pg of molybdenum in the 25-ml volumetric flask. Stand for 20 min after mixing the solutions. Measure the absorbance at 629 nm using a cell uf 5-cm light path against a reagent blank. A Hirama Model 6B spectrophotometer was used. In each case, use a calibration graph obtained by taking standards throughout the gel adsorption procedure. To investigate the adsorption of molybDistribution measurements. denum(V1) on Sephadex G-25, the distribution coefficient K, was measured by the batch technique, using 0.5 g of Sephadex G-25 and 50 ml of solution_ The mixture was stirred overnight and the molybdenum in the equilibrated solution was determined. The detailed experimental procedure was described previously [ 11.
103 RESULTS
AND
DISCUSSION
The adsorption of moly bdenum( VI) The pH dependence of the distribution coefficient (K,), the measure of uptake of molybdenum by the gel, is shown in Fig. 1. The uptake was maximal at pH 3.5. The value of K, was much larger than those for borate and vanadate(V) [ 1, 21. The loss of traces of molybdenum in glass vessels can almost be neglected in solutions at any p)i values [S]. Therefore, glassware was used for all the batch experiments. The mole fraction diagram of each species of molybdatc, also shown in Fig. 1, was calculated using the acidic dissociation constants of MoOf ’ : PKI = 1.0, pK* = 2.0, pK3 = 2.4, and pK4 = 3.8 (1 M NaClO.%) [ 91. The diagram corresponds to conditions under which there is no polymerization of molybdatc [lo] _ More precise pK, values under the same conditions as those of the present study could not be found in the literature. However, the pH dependence curve of K, values agrees fairly well with that of mole fraction of HMoO; species, suggesting that the active species for adsorption is H&loo;. Boume et al. [ 31 examined the paper ionophoresis of a number of cyclic carbohydrates in molybdate solution and showed that molybdate forms complexes more strongly in acidic than in alkaline solution and that the complex formation depends on the configuration of the hydroxyl groups in carbohydrates. Complex formation of molybdate with hydroxyl groups of the dextran matrix of the gel may be responsible for the adsorption. Karajannis et al. [6] suggested the formation of a Mo--NH,-glucose complex in the G-10 gel at pH 2.5. However, such complex formation is unlikely under their conditions. The K, value decreased with increasing concentrations of molybdenum, also in the case of borate and vanadate. Polymerized oxo-anions may be excluded from the gel, or may show lower affinity with the gel. Figure 2 shows the effects of coexisting electrolytes on the adsorption of
Fig. 1. pH-dependence of the adsorption of molybdenum(V1) for a 5O-ml sample of 2 X 10’ M Na,hloO, in 0.1 hl NaCl indicate the mole fraction curves.
on 0.5 g of Sephadcx (HCI-.NaOli). Ijotted
G-25 lines
molybdenum on the gel. The presence of acetate (used for maintaining the ~14 of the sample solution), perchlorate, and calcium ions results in decreased adsorption. Acetate and calcium ions may form complexes with molybdrnum(VI). Perchlorate is known to bc! adsorbed on the gel and thus may exclude molybdenum_
Selective concentration of molybdenum( VI) on a Sephadex gel column When the characteristic adsorption of molybdenum on Sephadcx gel is applied to selective concentration by the column method, an effective agent is necessary for desorption of molybdenum from the column. Hydrochloric acid higher than 0.1 M may hydrolyze the gel matrix. The use of sodium hydroside as eluent caused tailing on the elution peak of molybdenum. Molybdenum(V1) forms fairly stable complexes with aminopolycarboxylates [ 111. It is shown in Fig. 3 that the use of 0.01 M EDTA (disodium salt) makes it possible to desorb molybdenum completely from the column and to concentrate the molybdenum content of a sample into about 10 ml. Therefore,
the
first. 5 ml of effluent
was
rejected
and
the next
15 ml was
collected in a 50-ml beaker or in a 25-ml volumetric flask. The pH of the EDTA solution did not affect the recovery of molybdenum within the range 2-11. The molybdenum in the beaker was determined by atomic absorption spectrometry after the solution had been evaporated to dryness and the residue dissolved in 2 ml of the La(II1) solution, which suppresses the intcr-
.
--
-
-_ Q.5 ccnccn:ro:ton
_-I 1.0
(3)
8
*
0,
--._I_
I __
._
U
1U Efflcenr
2U volts
tnl)
Fig. 2. Effect of electrolytes on the adsorption of molybdenum(V1) on 0.5 g of Sephadex C-25 from 50-ml samples of 2 x lo-* M Na:MoO,: (a) acetate (pH 3.5); in 0.1 M acetate buffer (pH 3.5). (0) NaCI. (LX) NaCIO,, (A) CaCI,. Fig. 3. Desorption of molybdenum(V1) from the column with 0.01 M EDTA soiution as eluent (duplicate experiments). Column 16-mm i.d., 100 mm long; 15 ml of Sephadex G-25.
ferencc by coexisting EDTA. Relative peak height to 5-ppm molybdenum solution was used for calculations, to obtain higher precision and accuracy. Alternatively, the molybdenum in the volumetric flask was determined spectrophotometrically by the bromopyrogallol red method. Figure 4 shows the absorbance plottid against the volume of solution taken. Molybdenum in up to 600 ml of freshwaters, the pH of the solution being buffered by 0.1 M acetate, could be almost completely recovered into 15 ml of eluate. With seawater, molybdenum could be concentrated into a 15-ml solution from up to 300-ml samples containing 0.5 IM acetate. The recommended procedure was applied to loo-ml solutions containing 1.25 pmol of molybdenum and l--lOOOO-fold amounts of various ions. The molybdenum was determined by atomic absorption spectrometry. The results are summarized in Table 1. Interference from vanadium(V) and tungstcn(VI) is probably due to complex formation with molybdenum. Iron(II1) at a lOO-fold level lowered the recovery, probably because its hydroxides or hydroxo complexes adsorb molybdenum, but the interference could be overcome in the presence of 0.5 M acetate. Similar results were also obtained by the bromopyrogallol red method, except in the case of tungsten(V1). When present in a molar ratio of l:l, tungsten was recovered in part as well as molybdenum, which resulted in positive errors. Its presence in a 1:lO molar ratio to molybdenum hardly interfered with the determination. Determination of molybdenum in natural waters The present method makes it possible to determine molybdenum at ~g 1-l levels. Sugawara et al. [ 121 reported that the molybdenum content of river waters in Japan ranges from 0.2 to 0.6 ug 1-l. .4s an aid to geochemical prospecting, the method should be convenient for routine determinations of
Fig. 4. Effect of sample volume on the recovery of molybdenum(V1): (0) 0.1 M acetate absorbance being measured spectrobuffer (pH 3.5) containing 2 x lo7 M NaziMoO,, photometrically [ 7 ] ; (0) seawater from Wajiro, Fukuoka, containing 0.5 M acetate buffer (pH 3.5), peak height being measured relative to a 5-ppm reference molybdenum solution.
106 TABLE Effects
1 of foreign
Species
hlo found (x lo-‘ M)
species
1000 10000 10 100 100 100 1000 10
1.27 1.81 1.20 1.03 1.26 1.21 1.18 1.25
FE( III)
100
1.20
V(V) Cr(VI) >ln(II) XIn(VI1)
--.-_
-
‘hIolybdenum(V1) on the Sephadex 10% hi NarMoO,. 5-ppm refrrcncc
bSamplc
of molybdenum”
Molar ratio to MO
Al
-
ions on the recovery
solution
--
hIolar
ratio
to MO .-
-.
Mo found (x lo-” M)
---
1.15
100 100” 1 10 1000 10000 100
W(V1) SiO, PO:--
1.24 1.24 0.65 I .27 1.13 1.25
.----
in a IOO-ml sample solution was separated and concentrated to 15 ml column and finally to 2 ml. The sample solution was 100 ml of 1.25 X 0.1 M in acetate (~14 3.5). Peak heights were measured relative to a molybdenum solution, which exhibited about 2.5-cm peak height. contains 0.5 M acetate (pH 3.5).
molybdenum in freshwater samples in which somewhat elevated concentrations are present. The method itself is simple in operation and easily adapted for an automated system. The proposed method is particularly effective for seawater. There are no elements prcscnt which affect the recovery of molybdenum or the selective concentration by the column method [ 13]_ Table 2 shows the recoveries of added molybdenum from 250 ml of a seawater sample. The molybdenum was determined by atomic absorption spectromctry. It is desirable to use a calibration graph prepared by the column method. Recovery was almost complete. The analytical results obtained are presented in Table 3. Standard deviations were within 23% for each sample. Molybdenum concentration ranges were reported as 8.6-11.6 pg 1-l in the western North Pacific Ocean and 8.6-11.6 pg I-’ in the eastern China Sea [ 14]_ The present results are in good agreement with these values. The concentrations in the Hakata Bay, however, were found to decrease in summer. In the eutrophic Bay in the summer season, molybdenum may be coprecipitated with flocculated TABLE Recovery
2 of molybdcnum(V1)’ ----
MO added (~a) Relative peak heightb
0 0.64
2.39 1.18
4.78 1.76
Mo found (JIM) --._._ WoastA seawabr
2.55
3.02
7.46
molybdenum.
-_._-.-_.-.-. from Wajiro.
--.-. Fukuoka
(250
ml).
bReference
-
7.17 2.30 9.73
solution,
5-ppm
107
TABLE
3
Molybdenum Sampling point
contents
in seawaters
Mo content -. February
-. Tsuyazaki’ Shingun Shikanoshimae
--
(1981)
(pg I-“) ---August
10.1 -. 10.2
---.-_---.Sampling point
_----
10.3 10.4 10.2
-.--Wajirot’ Kashiib IIakozakib Okinoerabu-jima’
_--
-__-_-__--
“Coastal seawater from positions from the Hnkata Bay of Fukuoka. Ocean.
-__ -
.______ 310 content --.-.~ February
-_ -_-
10.4 10.2 -
.-. -----_-
(pg I”)
.--_
August
.--
7.8 7.9 9.4 11.3
-
.-..-
facing the open sea near Fukuoka. %oastal seawater CSeawater near the island in the westtrrn North Pacific
hydrated iron or manganese oxide or sulfide, which are supplied from bottom materials under anaerobic conditions, or multiplying organisms such
as plankton
may incorporate
molybdenum.
REFERENCES 1 K. Yoshimura. R. Kariyn and ‘I’. Tarutani, Anal. Chim. Acta, 109 (1979) 115. 2 K. Yoshimura, li. Kaji, E. Yamaguchi and T. ‘I’arutani, Anal. Chim. Acta, 130 (1981) 345. 3 E. J. Boume, D. Ii. Hutson and If. Weigcl, .J. Chem. Sot., (1960) 4252. 4 C. 11. Strculi and L. B. Rogers. Anal. Chem., 40 ( 1966) 653. 5 N. Yoza, 11. hlatsumoto and S. Ohashi, Anal. Chim. Act:, 54 (1971) 538. 6 S. Knrajannis, H. hY. Ortncr and H. Spitzy,‘I’alanta, 19 (1972) 903. ‘7 kl. Dcguchi, hl. lizuka and M. Yashiki, Bunseki Kagaku, 23 (19‘74) 760. 8 J. Burclovn. .J. Prasilova and P. Bcncs, ,J. Inorg. Nucl. Chcm., 35 (1973) 909. 9 B. I. Nabivancts, Russ. .I. Inorg. Chcm.. 14 (1969) 341. 10 C. Y. Bats, Jr. and R. E. hlcsmer,The Iiydrolysis of Cations, Wiley, New York, 1976. 11 K. Zarc, P. Lagrange and J. Lagrange, J. Chem. Sot. Dalton Trans., (1979) 1372. 12 K. Sugawara. S. Okahe and hl. Tanaka, J. Earth Sci.. Nagoya Ilniv., 9 (1961) 114. 13 K. K. Turekian, in K. H. Wedepohl (Ed.), Ifandbook of Geochcmistry,Springrr-Vcrlag, Berlin. 1969. 14 K. Sugaw.ara and S. Okabe, J. Earth Sci., Nagoya Univ., 7 (1959) 422.
Chimica Actn, 142(1982) !()I--107 Scientific Publishing Company, Amsterdam
-. Printed
in The Netherlands
DETERMINATION OF MOLYBDENUM IN NATURAL SELECI’IVE ADSORPTION ON SEPHADEX GEL
KAZUHISA
YOSHIMUKA*,
Ikpartment of Chemistry, Fukuoka 8 12 (Japan) (Received
“2nd
February
SOICHI
Fuculty
1IIHAOKA
o/Science,
and TOSHIKAZU
Kyushu
Unioersity
WATERS
AFTER
TARIJTANI
33, 1iai:ozat:i.
Higashiku.
1982)
SUMMARY Molybdenum(V1) ions are adsorbed on Sephades G-25 gel at pH 3.5, and are desorhed reversibly with a complcxing agent, EDTA. The adsorbability is much grater than that for boron(III) and vanadium(V). LnQ:e amounts of sodium chloride have little cffcct on
the adsorption. Molybdenum concentrations in natural waters, caspccially in .seawater. can be determined with good precision and accuracy after selective preconcentration of molybdenum by a Sephadcx G-25 gel column. The detection limit for 250.ml water spcctrometry or by spcctrophotomctry samples is about 1 ~g I” by atomic absorption with bromopyrogallol red as reagent.
Selective concentration methods for boron(lII) and vanadium(V) by use of a Sephadex G-25 gel column have already been described [ 1, 21. Borate and vanadate ions are adsorbed on Sephadex G-25 gel from solutions at pH 9.5-11 and 3-7, respectively, and dcsorbed into 0.01-0.1 M hydrochloric acid solution. Complex formation of these oxo-anions with the hydroxyl groups of the dextrdn matrix may be responsible for the reversible adsorption. Molybdenum is one of the important elements for many organisms and its abundance in natural waters is of interest. it is difficult, however, to determine molybdenum directly in natural waters because of its low pg 1’ ’ levels. A simple, rapid, and quantitative method for the determination of molybdenum is needed. It is well known that molybdatc(V1) forms complexes with polyhydroxy compounds [3] _ Adsorption of molybdate on Sephadex gels has been already reported [4, 51 and has been utilized for the separation of molybdenum as matrix element from traces of alkali and alkaline earth metals [Sl . In this study, a separation and concentration method for traces of molybdenum in natural waters has been developed by taking advantage of the reversible adsorption on Sephadex G-25 gel. The molybdenum can be determined by atomic absorption spectrometry or by solution spectrophotomctry with bromopyrogaliol red [ 71.
0003-2670/82/0000-0000/S02.75
a
1982 Elsevier
Scientific
Publishing
Company
102
EXPERIMENTAL Chemicals
All the chemicals used were of analytical grade and the water was deionized. (5000 ppm). Dissolve 5.9 g of LarO, in 50 ml of concentrated hydrochloric acid, evaporate to dryness and add 1 1 of 0.01 M hydrochloric acid solution_ La(ZII) solution
Procedures Separation and concentration of molybdenum(VI) on asephadex G-25 gel column. Filter the water sample at the sampling point through a glass-fiber filter paper (Whatman GF/D) if necessary. Add 7.5 ml of anhydrous acetic
acid to a 250-ml sample and adjust to p1-I 3.5 with ammonia solution. Use an acrylic resin column (16-mm id., 100 mm long) containing 15 ml the column with of Sephades G-25 gel (Medium, Pharmacia). Condition 0.5 M acetate buffer (pH 3.5) and then pass the sample solution through at a rate of 8 ml mix-t”. Wash the column with 25 ml of 0.1 M ammonium acetate tuffer (pH 3.5) and then desorb the molybdenum with 20 ml of 0.01 M EDT-4 (disodium salt) solution_ Reject the first 5 ml of effluent and collect the nest 15 ml in a 50-ml beaker or in a 25-ml volumetric flask. To the evaporated sample containing Determination of molybdenum. 0.5-50 pg of molybdenum in the beaker, add 2 ml of the La(III) solution. After dissolving, determine the molybdenum by atomic absorption spectrometry with a Nippon Jarrell-Ash spectrometer, Model AA-781. Instrumental conditions are as follows: analytical wavelength 313.26 nm, nitrous oxideacetylene flame, 5-cm long burner. Use peak heights relative to 5-ppm molybdenum solution for the calculation. Alternatively, add 5 ml of 1 M hydrochloric acid, 1.5 ml of 0.01 M zephiramine and 1 ml of 0.03% (w/v) bromopyrogallol red (50% ethanolic solution) to the sample containing 0.2-5 pg of molybdenum in the 25-ml volumetric flask. Stand for 20 min after mixing the solutions. Measure the absorbance at 629 nm using a cell uf 5-cm light path against a reagent blank. A Hirama Model 6B spectrophotometer was used. In each case, use a calibration graph obtained by taking standards throughout the gel adsorption procedure. To investigate the adsorption of molybDistribution measurements. denum(V1) on Sephadex G-25, the distribution coefficient K, was measured by the batch technique, using 0.5 g of Sephadex G-25 and 50 ml of solution_ The mixture was stirred overnight and the molybdenum in the equilibrated solution was determined. The detailed experimental procedure was described previously [ 11.
103 RESULTS
AND
DISCUSSION
The adsorption of moly bdenum( VI) The pH dependence of the distribution coefficient (K,), the measure of uptake of molybdenum by the gel, is shown in Fig. 1. The uptake was maximal at pH 3.5. The value of K, was much larger than those for borate and vanadate(V) [ 1, 21. The loss of traces of molybdenum in glass vessels can almost be neglected in solutions at any p)i values [S]. Therefore, glassware was used for all the batch experiments. The mole fraction diagram of each species of molybdatc, also shown in Fig. 1, was calculated using the acidic dissociation constants of MoOf ’ : PKI = 1.0, pK* = 2.0, pK3 = 2.4, and pK4 = 3.8 (1 M NaClO.%) [ 91. The diagram corresponds to conditions under which there is no polymerization of molybdatc [lo] _ More precise pK, values under the same conditions as those of the present study could not be found in the literature. However, the pH dependence curve of K, values agrees fairly well with that of mole fraction of HMoO; species, suggesting that the active species for adsorption is H&loo;. Boume et al. [ 31 examined the paper ionophoresis of a number of cyclic carbohydrates in molybdate solution and showed that molybdate forms complexes more strongly in acidic than in alkaline solution and that the complex formation depends on the configuration of the hydroxyl groups in carbohydrates. Complex formation of molybdate with hydroxyl groups of the dextran matrix of the gel may be responsible for the adsorption. Karajannis et al. [6] suggested the formation of a Mo--NH,-glucose complex in the G-10 gel at pH 2.5. However, such complex formation is unlikely under their conditions. The K, value decreased with increasing concentrations of molybdenum, also in the case of borate and vanadate. Polymerized oxo-anions may be excluded from the gel, or may show lower affinity with the gel. Figure 2 shows the effects of coexisting electrolytes on the adsorption of
Fig. 1. pH-dependence of the adsorption of molybdenum(V1) for a 5O-ml sample of 2 X 10’ M Na,hloO, in 0.1 hl NaCl indicate the mole fraction curves.
on 0.5 g of Sephadcx (HCI-.NaOli). Ijotted
G-25 lines
molybdenum on the gel. The presence of acetate (used for maintaining the ~14 of the sample solution), perchlorate, and calcium ions results in decreased adsorption. Acetate and calcium ions may form complexes with molybdrnum(VI). Perchlorate is known to bc! adsorbed on the gel and thus may exclude molybdenum_
Selective concentration of molybdenum( VI) on a Sephadex gel column When the characteristic adsorption of molybdenum on Sephadcx gel is applied to selective concentration by the column method, an effective agent is necessary for desorption of molybdenum from the column. Hydrochloric acid higher than 0.1 M may hydrolyze the gel matrix. The use of sodium hydroside as eluent caused tailing on the elution peak of molybdenum. Molybdenum(V1) forms fairly stable complexes with aminopolycarboxylates [ 111. It is shown in Fig. 3 that the use of 0.01 M EDTA (disodium salt) makes it possible to desorb molybdenum completely from the column and to concentrate the molybdenum content of a sample into about 10 ml. Therefore,
the
first. 5 ml of effluent
was
rejected
and
the next
15 ml was
collected in a 50-ml beaker or in a 25-ml volumetric flask. The pH of the EDTA solution did not affect the recovery of molybdenum within the range 2-11. The molybdenum in the beaker was determined by atomic absorption spectrometry after the solution had been evaporated to dryness and the residue dissolved in 2 ml of the La(II1) solution, which suppresses the intcr-
.
--
-
-_ Q.5 ccnccn:ro:ton
_-I 1.0
(3)
8
*
0,
--._I_
I __
._
U
1U Efflcenr
2U volts
tnl)
Fig. 2. Effect of electrolytes on the adsorption of molybdenum(V1) on 0.5 g of Sephadex C-25 from 50-ml samples of 2 x lo-* M Na:MoO,: (a) acetate (pH 3.5); in 0.1 M acetate buffer (pH 3.5). (0) NaCI. (LX) NaCIO,, (A) CaCI,. Fig. 3. Desorption of molybdenum(V1) from the column with 0.01 M EDTA soiution as eluent (duplicate experiments). Column 16-mm i.d., 100 mm long; 15 ml of Sephadex G-25.
ferencc by coexisting EDTA. Relative peak height to 5-ppm molybdenum solution was used for calculations, to obtain higher precision and accuracy. Alternatively, the molybdenum in the volumetric flask was determined spectrophotometrically by the bromopyrogallol red method. Figure 4 shows the absorbance plottid against the volume of solution taken. Molybdenum in up to 600 ml of freshwaters, the pH of the solution being buffered by 0.1 M acetate, could be almost completely recovered into 15 ml of eluate. With seawater, molybdenum could be concentrated into a 15-ml solution from up to 300-ml samples containing 0.5 IM acetate. The recommended procedure was applied to loo-ml solutions containing 1.25 pmol of molybdenum and l--lOOOO-fold amounts of various ions. The molybdenum was determined by atomic absorption spectrometry. The results are summarized in Table 1. Interference from vanadium(V) and tungstcn(VI) is probably due to complex formation with molybdenum. Iron(II1) at a lOO-fold level lowered the recovery, probably because its hydroxides or hydroxo complexes adsorb molybdenum, but the interference could be overcome in the presence of 0.5 M acetate. Similar results were also obtained by the bromopyrogallol red method, except in the case of tungsten(V1). When present in a molar ratio of l:l, tungsten was recovered in part as well as molybdenum, which resulted in positive errors. Its presence in a 1:lO molar ratio to molybdenum hardly interfered with the determination. Determination of molybdenum in natural waters The present method makes it possible to determine molybdenum at ~g 1-l levels. Sugawara et al. [ 121 reported that the molybdenum content of river waters in Japan ranges from 0.2 to 0.6 ug 1-l. .4s an aid to geochemical prospecting, the method should be convenient for routine determinations of
Fig. 4. Effect of sample volume on the recovery of molybdenum(V1): (0) 0.1 M acetate absorbance being measured spectrobuffer (pH 3.5) containing 2 x lo7 M NaziMoO,, photometrically [ 7 ] ; (0) seawater from Wajiro, Fukuoka, containing 0.5 M acetate buffer (pH 3.5), peak height being measured relative to a 5-ppm reference molybdenum solution.
106 TABLE Effects
1 of foreign
Species
hlo found (x lo-‘ M)
species
1000 10000 10 100 100 100 1000 10
1.27 1.81 1.20 1.03 1.26 1.21 1.18 1.25
FE( III)
100
1.20
V(V) Cr(VI) >ln(II) XIn(VI1)
--.-_
-
‘hIolybdenum(V1) on the Sephadex 10% hi NarMoO,. 5-ppm refrrcncc
bSamplc
of molybdenum”
Molar ratio to MO
Al
-
ions on the recovery
solution
--
hIolar
ratio
to MO .-
-.
Mo found (x lo-” M)
---
1.15
100 100” 1 10 1000 10000 100
W(V1) SiO, PO:--
1.24 1.24 0.65 I .27 1.13 1.25
.----
in a IOO-ml sample solution was separated and concentrated to 15 ml column and finally to 2 ml. The sample solution was 100 ml of 1.25 X 0.1 M in acetate (~14 3.5). Peak heights were measured relative to a molybdenum solution, which exhibited about 2.5-cm peak height. contains 0.5 M acetate (pH 3.5).
molybdenum in freshwater samples in which somewhat elevated concentrations are present. The method itself is simple in operation and easily adapted for an automated system. The proposed method is particularly effective for seawater. There are no elements prcscnt which affect the recovery of molybdenum or the selective concentration by the column method [ 13]_ Table 2 shows the recoveries of added molybdenum from 250 ml of a seawater sample. The molybdenum was determined by atomic absorption spectromctry. It is desirable to use a calibration graph prepared by the column method. Recovery was almost complete. The analytical results obtained are presented in Table 3. Standard deviations were within 23% for each sample. Molybdenum concentration ranges were reported as 8.6-11.6 pg 1-l in the western North Pacific Ocean and 8.6-11.6 pg I-’ in the eastern China Sea [ 14]_ The present results are in good agreement with these values. The concentrations in the Hakata Bay, however, were found to decrease in summer. In the eutrophic Bay in the summer season, molybdenum may be coprecipitated with flocculated TABLE Recovery
2 of molybdcnum(V1)’ ----
MO added (~a) Relative peak heightb
0 0.64
2.39 1.18
4.78 1.76
Mo found (JIM) --._._ WoastA seawabr
2.55
3.02
7.46
molybdenum.
-_._-.-_.-.-. from Wajiro.
--.-. Fukuoka
(250
ml).
bReference
-
7.17 2.30 9.73
solution,
5-ppm
107
TABLE
3
Molybdenum Sampling point
contents
in seawaters
Mo content -. February
-. Tsuyazaki’ Shingun Shikanoshimae
--
(1981)
(pg I-“) ---August
10.1 -. 10.2
---.-_---.Sampling point
_----
10.3 10.4 10.2
-.--Wajirot’ Kashiib IIakozakib Okinoerabu-jima’
_--
-__-_-__--
“Coastal seawater from positions from the Hnkata Bay of Fukuoka. Ocean.
-__ -
.______ 310 content --.-.~ February
-_ -_-
10.4 10.2 -
.-. -----_-
(pg I”)
.--_
August
.--
7.8 7.9 9.4 11.3
-
.-..-
facing the open sea near Fukuoka. %oastal seawater CSeawater near the island in the westtrrn North Pacific
hydrated iron or manganese oxide or sulfide, which are supplied from bottom materials under anaerobic conditions, or multiplying organisms such
as plankton
may incorporate
molybdenum.
REFERENCES 1 K. Yoshimura. R. Kariyn and ‘I’. Tarutani, Anal. Chim. Acta, 109 (1979) 115. 2 K. Yoshimura, li. Kaji, E. Yamaguchi and T. ‘I’arutani, Anal. Chim. Acta, 130 (1981) 345. 3 E. J. Boume, D. Ii. Hutson and If. Weigcl, .J. Chem. Sot., (1960) 4252. 4 C. 11. Strculi and L. B. Rogers. Anal. Chem., 40 ( 1966) 653. 5 N. Yoza, 11. hlatsumoto and S. Ohashi, Anal. Chim. Act:, 54 (1971) 538. 6 S. Knrajannis, H. hY. Ortncr and H. Spitzy,‘I’alanta, 19 (1972) 903. ‘7 kl. Dcguchi, hl. lizuka and M. Yashiki, Bunseki Kagaku, 23 (19‘74) 760. 8 J. Burclovn. .J. Prasilova and P. Bcncs, ,J. Inorg. Nucl. Chcm., 35 (1973) 909. 9 B. I. Nabivancts, Russ. .I. Inorg. Chcm.. 14 (1969) 341. 10 C. Y. Bats, Jr. and R. E. hlcsmer,The Iiydrolysis of Cations, Wiley, New York, 1976. 11 K. Zarc, P. Lagrange and J. Lagrange, J. Chem. Sot. Dalton Trans., (1979) 1372. 12 K. Sugawara. S. Okahe and hl. Tanaka, J. Earth Sci.. Nagoya Ilniv., 9 (1961) 114. 13 K. K. Turekian, in K. H. Wedepohl (Ed.), Ifandbook of Geochcmistry,Springrr-Vcrlag, Berlin. 1969. 14 K. Sugaw.ara and S. Okabe, J. Earth Sci., Nagoya Univ., 7 (1959) 422.