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Formation of colourless molybdate complexes of phosphorus compounds in aqueous solution
; m;az! ,m,i ('hem. t9"7 \oi 39 pp 2 7
2177 Pergamon Press. Primed in Great Britain
FORMATION OF COLOURLESS MOLYBDATE COMPLEXES OF PHOSPHORUS COMPOUNDS IN AQUEOUS SOLUTION TOSHITAKA HORI Deparlment of Chemistry, Faculty of Science, Kyoto University. K,voto. 606 Japan
t Received 30 Novenlber 1976: received for publicatio,q 3 May 1977) Abstract--rhe formation of coiourless molybdate complexes of phosphorus oxyanions in aqueous solution ha~ been examined by ~pectral measurements in the UV. As a result, it was found that orthophosphate and related phosphorus compounds reacted with molybdate to yield colourless complexes which had an absorbance maximum in the vicinity of 250 nm. Such complexes formed predominantly under the restricted conditions such that [molybdatel >-8 y l0 'M, [phosphorus oxyanion]/[molybdate]/> 1.0 (for the orthophosphate complex, the ratio ~1.25), pH 2.0-4.5 (for the orthophosphate complex, pH 3.1)-4.5). Among these complexes, molybdo-orthophospha!e and molvbdo-pho~phite complex were isolated as the tetraethylammonium salts and identified.
INTRODUCTION
It has been pointed out that some colourless molybdophosphate complexes may exist in equilibrium with the dodecamolybdophosphate complex in aqueous solution. Miolati and Pizzighelli[1] reported a lower polymer (P:Mo-2:5i, from the results of conductivity measurements. Souchay and Faucherre[21, using the cryoscopic method, found a similar complex. Ferrari[3] deduced the existence of ~, colourless heteropoly complex from differences in the reduction behaviour. Murata and Kiba[4] deduced the presence of a colourless complex by examining the influence of phosphate or molybdate on formation of the yellow complex. Fujinaga et al.[5] investigated the molybdate complexes formed by phosphite, pyrophosphate md orthophosphate ions and reported on the UV absorption spectra of the lower polymer of the molybdo-orthophosphate complex, which is similar to the phosphite and pyrophosphate complexes. Petterson [61 made a potentiometric study. In parallel with these sludies in solution, colourless molybdate complexes have been investigated in the solid state, e.g.[7-9J. Recently. the crystal structure of pentamolybdodiphosphate, one of the colourless complexes, was determined b~ Strandberg[10]. Although many investigations on these complexes have been performed as briefly reviewed above, an agreed interpretation has not yet been given, In the present work, the conditions of formation of the "'colourless" complexes were specifically investigated and it was confirmed that ,',ome of the complexes can be identified as pentamolybdate complexes. The equilibrium between the pentamolybdate and dodecamolybdate complexes in aqueous elhmol and aqueous media was also studied. EX PERIMENTAL
Preparathm @ standard svtutions and materials. Molybdate and phosphate standard solutions (both 0.1 M) were prepared from Na:MoOa'2H?O and KH2PO4, respectively. Phosphite solution was made from NaeHPO~.5H20. According to Plimer's melhod [111, monomethylphosphate and dimethylphosphate were isolated and purified as the barium salts from a commercially available 1: l-mixture. 25 g of the mixture was diluted to 2h with water and neutralized by addition of saturated barium hydroxide ,,olution. When the solution was neutralized 1o pH ~6.~,, white precipitates of barium ortho2173
phosphate appea:'ed and were filtered off. If orlhophosphate ions were still detected in an aliquot of the filtrate, a small amour1: of barium hydroxide solution was added to the fihrate and the precipitates that appeared were removed. The purified filtrate was evaporated Io dryness by a rotary evaporator under a lower temperature than 40°. From the residue, barium dimethylphosphate was extractable into 200 ml of ethanol-water mixture 14:1 v/v) and crystallized by e'.aporaling the solvent. I'he salt was washed wih pure ethanol, On the uther hand, barium monomethylphosphate was only slightly soluble in the +queous ethanol and remained as a precipitate after the extraction. The precipitate was collected and washed with the aqueous ethanol. The separation procedures were repeated three times on each salt. By Michaelis' procedure112L, methyltfisphenoxyphosphonaml iodide, (C+,H~t)EPICH~, was ~,ynthesized and the compound was converted to monomethylphosphonic acid. ('H~I)C>~H>, by treatment with s,u~dium hs,drnxide solution and then with fuming nitric acid. The crude 'acid was isolated as lhe lead sah[13], md the salt was dissolved into dilute acetic acid recrx~,tallized by addition of ethanol, and then purified. Free acids of the phosphates and phosphomtte were obtained by removing the barium or lead ions with the cation-e,,changer (Dowex 50WXS, 100-20(I mesh. H ' -form L 2-Aminoethylphosphonic acid was also used, which was synthesized according to Kosolapoff's method[ 141. Disodium glucuse-l-phosphate and disodium glucuse-6phosphate were commercially available and it was ensured before and after use that orthophusphate impurih c~,uid nnl be detected by Bernhart's method]lS]. Tetraethylammonium bromide was synthesized from trielhylamine and ethyl bromide in acetonitrile and purified by recr?stallization from ethanol Tetraethylammonium hydh~xide '~ulution was prepared from an aqueous solution of the bromide by anion-exchange ,Amberlite IRA-400, 100-200 mesh, ()It -follnL Mea,suremenls UV spectra were measured with a Shimadzu UV-200 spectrophotometcr. Quartz cells with pathdengths of I. 1,1.2, 0.1 and ().1',1cm were used, according Io the molybdale concentration. The path-lengths were calibrated h~ lhe standard method[161. A rtpid-scan Hitachi spectrnphoum~elcl (SCall-Fale 200-800nm in 100nasec) was also used far iI/eilstli-clllelll'.,of spectra of a pentamolvbdodiphosphite ,,:tit.
RESULTS
UV absorption spectra of colourless molybdate complexes of phosphate in aqueous solution The absorption spectra of 6 x 10 ~M molybdate at pH 3.5 in presence of phosphate concentrations ranging
2174
TOSHITAKA HORI
from zero to 1.2 × 10-2 M were measured. At phosphate concentrations lower than 2 x 10-3 M, the characteristic yellow colour of the dodecamolybdophosphate complex was observed. The colour became paler and finally disappeared, as the phosphate concentration was increased, (Fig. 1). Curve 4 in Fig. 1 shows the spectrum of the colourless complex formed when [molybdate]/[phosphate]=0.5. The absorbance maximum is at 251nm. The formation of the colourless complex depends largely on the pH and the concentrations of molybdate and phosphate. Figure 2 shows that formation of the colourless complex occurs at pH 5.5 and is maximal between pH 3 and 4.5. At pH < 3 the concentration of the complex decreases and the solutions are slightly yellow, indicating the formation of the dodecamolybdate complex. Adherence to the Beer-Lambert law was examined at a constant pH of 3.5 and phosphate concentration of 2 × 10-2 M, the molybdate concentration being changed from 1 x 10 5 to 2 × 10-2 M. The absorption spectra were recorded and the absorbances at 5 nm intervals between 225 and 350 nm as well as at 251 nm were plotted against the molybdate concentration. The plots were linear at molybdate concentrations higher than 8x 10 4M, and though there was a little deviation at lower molybdate concentrations, the plots could be extrapolated to pass through the origin. As discussed later, the predominant species is the pentamolybdodiphosphate, the molar ab-
sorptivity for this species is 3.78× 1041mole - ' cm -I at 251 rim. The effect of phosphate concentration was examined with 8 x 10-3 M molybdate at pH 3.5. Figure 3 shows that absorbance at 251 nm increases with phosphate concentration, reaching a constant value at 1 x 10-ZM phosphate. Petterson has already suggested/6] the existence of such complexes [Hn(MOO4)s(HPO4)2] (14-n)-, from potentiometric work with 3M sodium perchlorate media and reported the formation constants. According to his results, the pentamolybdodiphosphate complex (P:Mo = 2:5) was formed predominantly in solutions with compositions such as [molybdate] 1>8 × 10-4 M, [phosphate]/[molybdate]/> 1.25 and 4.5/> pH /> 3.0. Thus, the colourless complex characterized by the absorbance maximum at 251nm is concluded to be the pentamolybdodiphosphate complex and its normalized absorption spectrum is shown in Fig. 4 as curve 1, with that of dodecamolybdophosphate in ethanol shown for comparison.
The molybdate complexes of otherphosphorus oxyanions By methods similar to those above, it was shown that molybdate and phosphite form a complex at pH 2-4.5. The spectrum and Beer's law plots are practically iden-
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~6 4
06 ~
/
~5o .~'
c4o x:l
./
b ~3o
04
20
t
O0 200
250
I 350
300
Wavelength (nm)
Fig. 1. Absorption spectra of colourless and yellow molybdophosphate complexes in aqueous solution. [molybdate]= 6x 10-3M (constant); pH=3.5; 0.0137cm cell; [molybdate]/[phosphate]: (1) 12.0; (2) 3.8; (3) 2.0; (4) 0.5.
II
m
0)2
Fig. 3. Effect of phosphate concentration on the formation of colourless molbydophosphate complex. [molybdate] = 8x 10-3 M; pH = 3.5; 0.0137cm cell. x 104 4.0
1S
0.8
I
0.005 001 01 PhosphGte Concentration (M)
30
2 2.0
0.6
b
~0.4 o
O0 200
0.2
250
300
350
400
Wavelength (nm) 0
I
I
1
I
I
I
I
2
3
4
5
6
pH
Fig. 2. Dependence of formation of colourless molybdophosphate complex on pH. (1) 1 × 10-2M molybdate+ 1.5 × 10-2M phosphate; (2) 1 x 10 :M molybdate; 0.0137 cm cell.
Fig. 4. Absorption spectra of (1) pentamolybdodiphosphate complex in aqueous solution and (2) dodecamolybdophosphate complex in pure ethanol. (1) pH = 3.5, [phosphate] = 1 x 10 2 M, [molybdate]=6x 10-3M, 0.0137cm cell; (2) 2x 10-SM H3PMo1204o.30H20 in ethanol, 1.00 cm cell.
Formation of colourless molybdate complexes of phosphorus compounds in aqueous solution tical with those for the pentamolybdodiphosphate complex. All molybdenum present is complexed when the [phosphite]/[molybdate] ratio exceeds 1, and, therefore, the exact absorption spectrum of the phosphite complex is obtainable, excluding absorbances due to any isopolymolybdates. Although the favourable pH region for the phosphite complex seems to expand towards the more acidic side, compared to that for the phosphate complex, this can be attributed to the fact that phosphite ion can not form a dodecamolybdate complex. The formation conditions and the absorbance characteristics of the complex are shown in the second row of Table 1. Similar examinations were performed by employing such phosphorus oxyanions as monomethylphosphate, glucose-l-phosphate, glucose-6-phosphate, monomethylphosphonate and 2-aminoethylphosphonate in place of phosphite, and formation of the corresponding molybdate complex was confirmed. The results are summarized in Table 1
2175
aqueous solution. The following method of crystallization was used except when molybdic acid (H2MoO4) was used for the synthesis, in which case the pulverized acid was first dissolved by addition of an equivalent amount of tetraethylammonium hydroxide solution. The solutions were prepared according to the compositions given in the last column of Table 2 and evaporated slowly at ~35 ° in the water-bath. When the volume of the solution had been reduced to less than 100 ml (after one weekl, crystals up to ca. 1 cm. in length were obtained. In the preparation of a salt of the phosphite complex, some white molybdate salts deposited together with the crystal, but they can be removed easily by decantation. The crystals were filtered on a coarse sintered-flass filter and washed quickly by pouring cold water on them. The air-dried crystals were dried further at 110-120 ° to obtain the anhydrous form. As shown in "Fable 2. the composition of these salts is well defined with respect to hydrogen, sodium, tetraethylammonium, molybdate,
Table 1. Formation conditions of the colourless molybdate complexes and the absorbance characteristics Formation conditions Phosphorus oxyanion Orthophosphate (pentamolybdodiphosphate) Phosphite (pentamolybdodiphosphite) Monomethylphosphate Glucose-bphosphate Glucose-6-phosphate Monomethylphosphonate 2-aminoethylphosphonate
Absorbance characteristics e at Areax a ..... (X l 0 g 1mole ' (nm) cm ~1
pH range
Concen. range of molybdate
Soln. composition [P]t[Mo]
3-4.5
8 × 10 4-1.5 x 10 " M+
1.25§
251
3.78
2-4.5 2-4.5 2-4.5 2-4.5 2-4.5 2-4.5
8 x 10 4-1.5 x 10 "-M+ 8 × 10 4 - 1 . 5 X 10 2 M+ 8 x 10 4_1.5 × 10 ~MS 8 × 10 4--l.5× l0 3 M:~ 8 x 10 4-1.5 × 10 : Mf 8 × 10 4-1.5 × 10 3 MS
10§ 1.0} I.(11 1.0'1 1.0§
251 250 25() 2511 251 251
3.65 "~.80~i 3.68~ 3.681! 3.65I 3.65¢
[.O il
~The range is determined in the presence of oxyanion in a concentration of 2 x I11 ~ M and +that determined in the concentratmn of 2 :< 1(1 ~ M. §The composition is determined in the presence of molybdate in a concentration of 8 × 10 ~M and ~lthatdetermined in lhe concentration of I × 10 ~M. CThe molar absorptivity is calculated by assuming that pertamolybdate complex is formed.
Isolation of the colourless molybdophosphate and phosphite complex The conclusion above was further confirmed by isolating the colourless complexes as salts. Unlike the dodecamolybdophosphate, the colourless complexes are not extractable into various organic solvents, but the tetraethylammonium salt can be crystallized from
phosphate and phosphite ions, and the salts are very stable in the open air and even at elevated temperature. The absorption spectra of the crystallized salts of the phosphate complex were measured as follows• A small quantity of finely pulverized crystals (ca. 0.3 mg) was put into a I cm quartz cell containing 4 ml of water, and the cell was quickly shaken• Because the salts decompose
Table 2. Preparation and composition of tetraethylammonium salts of pentamolybdate complexes of orthophosphate and phosphite Formula (temperature range for ! • anhydrous form )~ IIC,H . d~t,Na,H,(PO4I,(MoO4)~ . . . . (110-230°1 [tC~HJaNI~H4(PO4)2(MoO0s 1110-!70°1 [IC~H~)4N ]2Na,(HP()0e(MoOd, (110-230°1 +Confirmed by thermogravimetry.
(Calcd.~ Composition(%) \Found! 3.48 H: ~
15.78 C: ----15.6
N 2.30 * "2..~"
39.38 M°: 39~2 4.54 H: 4~-2
p ~ 09 : 4.12 ..... (,. 16.02 ~" 15.9
3 77 Na: :3.25 4.52 N: 4.6
39.1"~ Mo:~
p. 5.16 5.13
H: ~3.54
.~ ~16.18 t.:
Mo: 40.45 ~
p: ~
Amount of materials used for synthesis 1500 ml of water) Na2Mo()4.2H:O 25 g: 85% H~P()a 10 m[: ((7,Hd4NBr 2(1g H:Mo()4 16 g: (C:Hs)4NOH 1).2mole: 85% H~PO4 l0 ml
N: 2.36 ,l'~a: 3.88
Na~MoO4-2H,O 25 g, Na2HPOv5H,O 25 g: (C:HO4NBr 20g
2176
TOSHITAKA HORI
when dissolved in water in the absence of excess of phosphate, the measurements had to be made as quickly as possible after dissolution (decomposition was complete in about 4 rain). The spectra displayed the characteristic absorbance maximum at 251 nm, with molar absorptivity of (3.7-0.7)× 1041mole Xcm 1, in good agreement with the value determined earlier. Similar results were obtained for the salt of the phosphite complex. The air-dried salt was stable, like as the phosphate complex, but decomposed more rapidly than the phosphate salt when dissolved in water (completely decomposed within 50sec after dissolution). Immediately after dissolution, however, the solution exhibited the same spectrum, displaying an absorbance maximum at around 250nm. The spectra and the decomposition process were measured by a rapid-scan technique, and the result is shown in Fig. 5.
0.8
m
4 0.6
c
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I
I
02
oo
200
250
300
350
400
Wavelength (nm)
Fig. 6. Equilibrium between pentamolybdodiphosphate and dodecamolybdophosphatecomplex in the mixed solvent (waterethanol, 4:1 v/v). [molybdate]= 6 × 10-3 M (constant); 0.0137cm cell; pH = 3.5; [molybdate]/[phosphate]:(1) 12.0; (2) 3.8; (3) 2.0; (4) 0.5.
mula, it could be expected that the X-ion can be replaceable by other kinds of phosphorus anions and analogous complexes formed. Spectral measurements revealed the formation of molybdate complexes when such monosubstituted phosphorus oxyanions as methylphosphate, glucosephosphate, methylphosphonate and 2aminoethylphosphonate ions were employed for the Xion. Although one corner of the tetrahedral oxyanions is occupied by a bulky alkyl group, these anions react with Wavelength(nrn) molybdate to yield the respective complexes. The comFig. 5. Absorption spectrum of [(C2Hs)4N]2Na2(HPO3)2(MoO3)5 position and structure of these complexes were not escrystals and the spectrum changes in water after the dissolution. tablished, but they give almost identical spectra, characscan rate = 3 Hz; 1.00cm cell; measuring duration = 5-45 sec terized by an absorbance maximum at around 250 nm (in after the dissolution; concentration of the crystal = 0.34 mg/4ml the last column of Table 2). If Strandberg's result[10] water. obtained for the structure of the pentamolybdodiphosphate can be extended to these molybdate complexes, both the spectral similarity observed among the comDISCUSSION plexes and the reactivity of the oxyanions with molIt has been reported[17, 18] that the absorption spec- ybdate might be rationalised as follows. In the pentrum of dodecamolybdophosphate is affected by addition tamolbydate complex, three corners of the tetrahedral of certain water-miscible polar organic solvents and that orthophosphate ion (coordinatively functional P-O an absorbance maximum appears at around 310 nm. In bonds) are required to connect them to the surrounding the present work it was found that in a water-ethanol pentamolybdate skelton, the remaining corner being free. mixture (4:1 v/v) the spectrum of the dodecamolybdate This implies that a group of monosubstituted phosphorus complex was indeed changed (compare curve 1 in Figs. 1 compounds, each of which has at least three apical and 6), whereas that of the pentamolybdodiphosphate oxygens forms, could function enough as the central complex was not (curve 4 in Figs. 1 and 6). The pen- anion in the pentamolybdate structure in place of ortamolybdate in the mixed solvent is not converted into thophosphate ion. As shown in Table 2, the absorbance the dodecamolybdate so long as an excess of phosphate seems little dependent on both the oxidation number of is present. The isosbestic points are more definite in the the phosphorus and the kind of the substituting groups of the central anions, but dependent on the number of mixed solvent than in the pure aqueous system, showing molybdenum atoms and the surrounding molybdate that only the two molybdophosphate complexes are structure. present in equilibrium. This explains the fact that in the Fleury[19] reported the existence of a molybdate presence of a constant amount of molybdate the formation of the colourless complex reduces the formation complex of /3-glycerophosphate with a composition of of the yellow complex and vice versa. (MoO3)2.sC3H702PO4, and this also fits the above From the established absorption spectrum for the interpretation. The complex might correspond to a pentamolybdodiphoshate complex, a reasonable inter- type of pentamolybdate complex, (MoO3)5(C3H7PO4)2. pretation could be deduced regarding with the formation Moreover, Rosenheim and Schapiro[20] reported the existence of a pentamolybdodiphsophite complex in the of analogous molybdate complexes by other phosphorus oxyanions. As described above, an almost identical solid state. On the other hand, dimethylphosphate ion in which spectrum was observed for the molybdate complexes of phosphate and phosphite and, in fact, complexes with a only two P-O corners remain unblocked did not affect similar composition, formulated as [(X"-)2(MoO3M 2n-, the spectra of molbydate solutions and no evidence of were isolated as salts with both orthophosphate (X= complex formation was obtained by the present specPO43 ) and phosphite ion (X = HPO32-). With this for- trophotometric method.
Formation of colourless molybdate complexes of phosphorus compounds in aqueous solution Acknowledf,,ements--The authnr wishes to acknowledge the helpful advice and discussions given by Prof. T. Fujinaga and Prof. M. Koyama of this laboratory. The author's thanks are due t~ Dr. M. Sugita for his offering 2-aminoethylphosphonic acid and als~ due to the Instrumental Analysis Research Centre of l,~yolo University for offering facilities. REFERENCES
I ~ Miolati and R. Pizzighelh. J, Pract. Chem. 77, 417 (1908). 2 P Soucha~ and J. Faucherrt~, Bull. Soc. Chim. (France) 355 1951) ~. I ' f'errari. Mikrochim..4ct~ 551 (1956). 4. K. Murata and T. Kiba. J. lnorg. NucL Chem. 32. 1667 (1970). I. Fujinaga, M. Koyama antt T. Hori, Bull. Inst. Chem. Res. Ky~,to Unit. 48, 21() (1970i. 6. [. Petterson. A~ta Chem. Seand. 25, 1967 (1971). 7 (" |riedheim. Z. ,4notre. Alh,,. Chem. 4, 275 (1893). ~. ~ triedheim and J Meschi~irer, Z. Anorg. Allg. Chem. 6, 27 ( 1gq4
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9. J. W. Mellor, A Comprehensive Treaties on Inorganic and Theoretical Chemistry, Vol. 11, pp. 659-672. Longmans, London (1931). 10. R. Strandberg, Acta Chem. Scand. 27, 1004 (1973), 11. R. H. A. Plirner and W. J. N. Burch, J. Chem. Soc. 292 (1929). 12. A. Michaelis and R. Kaehne, Ber. 31, 1048 (1898). 13. A. W. Hofman, Ber. 5, 104 (1872). 14. G. M. Kosolapoff, J. Am. Chem. Soc. 69, 2112 (1947). 15. D. N. Bernhart and A. R. Wreath, Anal. Chem. 27, 440 (1955). 16. T. R. Hogness, F. P. Zscheile, Jr. and A. E. Sidwell, Jr., .L Phys. Chem. 41, 379 (1937). 17. R. A. Chalmers and A. G. Sinclair, Anal. Chim. Acta 33, 384 (1965). 18. T. Fujinaga, M. Koyama and T. Hori, Talanta 18,960 (1971). 19. M. P. Fleury, C. R. Acad. Sci. 193, 1350 (1931). 20. A. Rosenheim and M. Schapiro, Z. Anorg. Allg. Chem. 129, 196 (1923).
2177 Pergamon Press. Primed in Great Britain
FORMATION OF COLOURLESS MOLYBDATE COMPLEXES OF PHOSPHORUS COMPOUNDS IN AQUEOUS SOLUTION TOSHITAKA HORI Deparlment of Chemistry, Faculty of Science, Kyoto University. K,voto. 606 Japan
t Received 30 Novenlber 1976: received for publicatio,q 3 May 1977) Abstract--rhe formation of coiourless molybdate complexes of phosphorus oxyanions in aqueous solution ha~ been examined by ~pectral measurements in the UV. As a result, it was found that orthophosphate and related phosphorus compounds reacted with molybdate to yield colourless complexes which had an absorbance maximum in the vicinity of 250 nm. Such complexes formed predominantly under the restricted conditions such that [molybdatel >-8 y l0 'M, [phosphorus oxyanion]/[molybdate]/> 1.0 (for the orthophosphate complex, the ratio ~1.25), pH 2.0-4.5 (for the orthophosphate complex, pH 3.1)-4.5). Among these complexes, molybdo-orthophospha!e and molvbdo-pho~phite complex were isolated as the tetraethylammonium salts and identified.
INTRODUCTION
It has been pointed out that some colourless molybdophosphate complexes may exist in equilibrium with the dodecamolybdophosphate complex in aqueous solution. Miolati and Pizzighelli[1] reported a lower polymer (P:Mo-2:5i, from the results of conductivity measurements. Souchay and Faucherre[21, using the cryoscopic method, found a similar complex. Ferrari[3] deduced the existence of ~, colourless heteropoly complex from differences in the reduction behaviour. Murata and Kiba[4] deduced the presence of a colourless complex by examining the influence of phosphate or molybdate on formation of the yellow complex. Fujinaga et al.[5] investigated the molybdate complexes formed by phosphite, pyrophosphate md orthophosphate ions and reported on the UV absorption spectra of the lower polymer of the molybdo-orthophosphate complex, which is similar to the phosphite and pyrophosphate complexes. Petterson [61 made a potentiometric study. In parallel with these sludies in solution, colourless molybdate complexes have been investigated in the solid state, e.g.[7-9J. Recently. the crystal structure of pentamolybdodiphosphate, one of the colourless complexes, was determined b~ Strandberg[10]. Although many investigations on these complexes have been performed as briefly reviewed above, an agreed interpretation has not yet been given, In the present work, the conditions of formation of the "'colourless" complexes were specifically investigated and it was confirmed that ,',ome of the complexes can be identified as pentamolybdate complexes. The equilibrium between the pentamolybdate and dodecamolybdate complexes in aqueous elhmol and aqueous media was also studied. EX PERIMENTAL
Preparathm @ standard svtutions and materials. Molybdate and phosphate standard solutions (both 0.1 M) were prepared from Na:MoOa'2H?O and KH2PO4, respectively. Phosphite solution was made from NaeHPO~.5H20. According to Plimer's melhod [111, monomethylphosphate and dimethylphosphate were isolated and purified as the barium salts from a commercially available 1: l-mixture. 25 g of the mixture was diluted to 2h with water and neutralized by addition of saturated barium hydroxide ,,olution. When the solution was neutralized 1o pH ~6.~,, white precipitates of barium ortho2173
phosphate appea:'ed and were filtered off. If orlhophosphate ions were still detected in an aliquot of the filtrate, a small amour1: of barium hydroxide solution was added to the fihrate and the precipitates that appeared were removed. The purified filtrate was evaporated Io dryness by a rotary evaporator under a lower temperature than 40°. From the residue, barium dimethylphosphate was extractable into 200 ml of ethanol-water mixture 14:1 v/v) and crystallized by e'.aporaling the solvent. I'he salt was washed wih pure ethanol, On the uther hand, barium monomethylphosphate was only slightly soluble in the +queous ethanol and remained as a precipitate after the extraction. The precipitate was collected and washed with the aqueous ethanol. The separation procedures were repeated three times on each salt. By Michaelis' procedure112L, methyltfisphenoxyphosphonaml iodide, (C+,H~t)EPICH~, was ~,ynthesized and the compound was converted to monomethylphosphonic acid. ('H~I)C>~H>, by treatment with s,u~dium hs,drnxide solution and then with fuming nitric acid. The crude 'acid was isolated as lhe lead sah[13], md the salt was dissolved into dilute acetic acid recrx~,tallized by addition of ethanol, and then purified. Free acids of the phosphates and phosphomtte were obtained by removing the barium or lead ions with the cation-e,,changer (Dowex 50WXS, 100-20(I mesh. H ' -form L 2-Aminoethylphosphonic acid was also used, which was synthesized according to Kosolapoff's method[ 141. Disodium glucuse-l-phosphate and disodium glucuse-6phosphate were commercially available and it was ensured before and after use that orthophusphate impurih c~,uid nnl be detected by Bernhart's method]lS]. Tetraethylammonium bromide was synthesized from trielhylamine and ethyl bromide in acetonitrile and purified by recr?stallization from ethanol Tetraethylammonium hydh~xide '~ulution was prepared from an aqueous solution of the bromide by anion-exchange ,Amberlite IRA-400, 100-200 mesh, ()It -follnL Mea,suremenls UV spectra were measured with a Shimadzu UV-200 spectrophotometcr. Quartz cells with pathdengths of I. 1,1.2, 0.1 and ().1',1cm were used, according Io the molybdale concentration. The path-lengths were calibrated h~ lhe standard method[161. A rtpid-scan Hitachi spectrnphoum~elcl (SCall-Fale 200-800nm in 100nasec) was also used far iI/eilstli-clllelll'.,of spectra of a pentamolvbdodiphosphite ,,:tit.
RESULTS
UV absorption spectra of colourless molybdate complexes of phosphate in aqueous solution The absorption spectra of 6 x 10 ~M molybdate at pH 3.5 in presence of phosphate concentrations ranging
2174
TOSHITAKA HORI
from zero to 1.2 × 10-2 M were measured. At phosphate concentrations lower than 2 x 10-3 M, the characteristic yellow colour of the dodecamolybdophosphate complex was observed. The colour became paler and finally disappeared, as the phosphate concentration was increased, (Fig. 1). Curve 4 in Fig. 1 shows the spectrum of the colourless complex formed when [molybdate]/[phosphate]=0.5. The absorbance maximum is at 251nm. The formation of the colourless complex depends largely on the pH and the concentrations of molybdate and phosphate. Figure 2 shows that formation of the colourless complex occurs at pH 5.5 and is maximal between pH 3 and 4.5. At pH < 3 the concentration of the complex decreases and the solutions are slightly yellow, indicating the formation of the dodecamolybdate complex. Adherence to the Beer-Lambert law was examined at a constant pH of 3.5 and phosphate concentration of 2 × 10-2 M, the molybdate concentration being changed from 1 x 10 5 to 2 × 10-2 M. The absorption spectra were recorded and the absorbances at 5 nm intervals between 225 and 350 nm as well as at 251 nm were plotted against the molybdate concentration. The plots were linear at molybdate concentrations higher than 8x 10 4M, and though there was a little deviation at lower molybdate concentrations, the plots could be extrapolated to pass through the origin. As discussed later, the predominant species is the pentamolybdodiphosphate, the molar ab-
sorptivity for this species is 3.78× 1041mole - ' cm -I at 251 rim. The effect of phosphate concentration was examined with 8 x 10-3 M molybdate at pH 3.5. Figure 3 shows that absorbance at 251 nm increases with phosphate concentration, reaching a constant value at 1 x 10-ZM phosphate. Petterson has already suggested/6] the existence of such complexes [Hn(MOO4)s(HPO4)2] (14-n)-, from potentiometric work with 3M sodium perchlorate media and reported the formation constants. According to his results, the pentamolybdodiphosphate complex (P:Mo = 2:5) was formed predominantly in solutions with compositions such as [molybdate] 1>8 × 10-4 M, [phosphate]/[molybdate]/> 1.25 and 4.5/> pH /> 3.0. Thus, the colourless complex characterized by the absorbance maximum at 251nm is concluded to be the pentamolybdodiphosphate complex and its normalized absorption spectrum is shown in Fig. 4 as curve 1, with that of dodecamolybdophosphate in ethanol shown for comparison.
The molybdate complexes of otherphosphorus oxyanions By methods similar to those above, it was shown that molybdate and phosphite form a complex at pH 2-4.5. The spectrum and Beer's law plots are practically iden-
0.8
~6 4
06 ~
/
~5o .~'
c4o x:l
./
b ~3o
04
20
t
O0 200
250
I 350
300
Wavelength (nm)
Fig. 1. Absorption spectra of colourless and yellow molybdophosphate complexes in aqueous solution. [molybdate]= 6x 10-3M (constant); pH=3.5; 0.0137cm cell; [molybdate]/[phosphate]: (1) 12.0; (2) 3.8; (3) 2.0; (4) 0.5.
II
m
0)2
Fig. 3. Effect of phosphate concentration on the formation of colourless molbydophosphate complex. [molybdate] = 8x 10-3 M; pH = 3.5; 0.0137cm cell. x 104 4.0
1S
0.8
I
0.005 001 01 PhosphGte Concentration (M)
30
2 2.0
0.6
b
~0.4 o
O0 200
0.2
250
300
350
400
Wavelength (nm) 0
I
I
1
I
I
I
I
2
3
4
5
6
pH
Fig. 2. Dependence of formation of colourless molybdophosphate complex on pH. (1) 1 × 10-2M molybdate+ 1.5 × 10-2M phosphate; (2) 1 x 10 :M molybdate; 0.0137 cm cell.
Fig. 4. Absorption spectra of (1) pentamolybdodiphosphate complex in aqueous solution and (2) dodecamolybdophosphate complex in pure ethanol. (1) pH = 3.5, [phosphate] = 1 x 10 2 M, [molybdate]=6x 10-3M, 0.0137cm cell; (2) 2x 10-SM H3PMo1204o.30H20 in ethanol, 1.00 cm cell.
Formation of colourless molybdate complexes of phosphorus compounds in aqueous solution tical with those for the pentamolybdodiphosphate complex. All molybdenum present is complexed when the [phosphite]/[molybdate] ratio exceeds 1, and, therefore, the exact absorption spectrum of the phosphite complex is obtainable, excluding absorbances due to any isopolymolybdates. Although the favourable pH region for the phosphite complex seems to expand towards the more acidic side, compared to that for the phosphate complex, this can be attributed to the fact that phosphite ion can not form a dodecamolybdate complex. The formation conditions and the absorbance characteristics of the complex are shown in the second row of Table 1. Similar examinations were performed by employing such phosphorus oxyanions as monomethylphosphate, glucose-l-phosphate, glucose-6-phosphate, monomethylphosphonate and 2-aminoethylphosphonate in place of phosphite, and formation of the corresponding molybdate complex was confirmed. The results are summarized in Table 1
2175
aqueous solution. The following method of crystallization was used except when molybdic acid (H2MoO4) was used for the synthesis, in which case the pulverized acid was first dissolved by addition of an equivalent amount of tetraethylammonium hydroxide solution. The solutions were prepared according to the compositions given in the last column of Table 2 and evaporated slowly at ~35 ° in the water-bath. When the volume of the solution had been reduced to less than 100 ml (after one weekl, crystals up to ca. 1 cm. in length were obtained. In the preparation of a salt of the phosphite complex, some white molybdate salts deposited together with the crystal, but they can be removed easily by decantation. The crystals were filtered on a coarse sintered-flass filter and washed quickly by pouring cold water on them. The air-dried crystals were dried further at 110-120 ° to obtain the anhydrous form. As shown in "Fable 2. the composition of these salts is well defined with respect to hydrogen, sodium, tetraethylammonium, molybdate,
Table 1. Formation conditions of the colourless molybdate complexes and the absorbance characteristics Formation conditions Phosphorus oxyanion Orthophosphate (pentamolybdodiphosphate) Phosphite (pentamolybdodiphosphite) Monomethylphosphate Glucose-bphosphate Glucose-6-phosphate Monomethylphosphonate 2-aminoethylphosphonate
Absorbance characteristics e at Areax a ..... (X l 0 g 1mole ' (nm) cm ~1
pH range
Concen. range of molybdate
Soln. composition [P]t[Mo]
3-4.5
8 × 10 4-1.5 x 10 " M+
1.25§
251
3.78
2-4.5 2-4.5 2-4.5 2-4.5 2-4.5 2-4.5
8 x 10 4-1.5 x 10 "-M+ 8 × 10 4 - 1 . 5 X 10 2 M+ 8 x 10 4_1.5 × 10 ~MS 8 × 10 4--l.5× l0 3 M:~ 8 x 10 4-1.5 × 10 : Mf 8 × 10 4-1.5 × 10 3 MS
10§ 1.0} I.(11 1.0'1 1.0§
251 250 25() 2511 251 251
3.65 "~.80~i 3.68~ 3.681! 3.65I 3.65¢
[.O il
~The range is determined in the presence of oxyanion in a concentration of 2 x I11 ~ M and +that determined in the concentratmn of 2 :< 1(1 ~ M. §The composition is determined in the presence of molybdate in a concentration of 8 × 10 ~M and ~lthatdetermined in lhe concentration of I × 10 ~M. CThe molar absorptivity is calculated by assuming that pertamolybdate complex is formed.
Isolation of the colourless molybdophosphate and phosphite complex The conclusion above was further confirmed by isolating the colourless complexes as salts. Unlike the dodecamolybdophosphate, the colourless complexes are not extractable into various organic solvents, but the tetraethylammonium salt can be crystallized from
phosphate and phosphite ions, and the salts are very stable in the open air and even at elevated temperature. The absorption spectra of the crystallized salts of the phosphate complex were measured as follows• A small quantity of finely pulverized crystals (ca. 0.3 mg) was put into a I cm quartz cell containing 4 ml of water, and the cell was quickly shaken• Because the salts decompose
Table 2. Preparation and composition of tetraethylammonium salts of pentamolybdate complexes of orthophosphate and phosphite Formula (temperature range for ! • anhydrous form )~ IIC,H . d~t,Na,H,(PO4I,(MoO4)~ . . . . (110-230°1 [tC~HJaNI~H4(PO4)2(MoO0s 1110-!70°1 [IC~H~)4N ]2Na,(HP()0e(MoOd, (110-230°1 +Confirmed by thermogravimetry.
(Calcd.~ Composition(%) \Found! 3.48 H: ~
15.78 C: ----15.6
N 2.30 * "2..~"
39.38 M°: 39~2 4.54 H: 4~-2
p ~ 09 : 4.12 ..... (,. 16.02 ~" 15.9
3 77 Na: :3.25 4.52 N: 4.6
39.1"~ Mo:~
p. 5.16 5.13
H: ~3.54
.~ ~16.18 t.:
Mo: 40.45 ~
p: ~
Amount of materials used for synthesis 1500 ml of water) Na2Mo()4.2H:O 25 g: 85% H~P()a 10 m[: ((7,Hd4NBr 2(1g H:Mo()4 16 g: (C:Hs)4NOH 1).2mole: 85% H~PO4 l0 ml
N: 2.36 ,l'~a: 3.88
Na~MoO4-2H,O 25 g, Na2HPOv5H,O 25 g: (C:HO4NBr 20g
2176
TOSHITAKA HORI
when dissolved in water in the absence of excess of phosphate, the measurements had to be made as quickly as possible after dissolution (decomposition was complete in about 4 rain). The spectra displayed the characteristic absorbance maximum at 251 nm, with molar absorptivity of (3.7-0.7)× 1041mole Xcm 1, in good agreement with the value determined earlier. Similar results were obtained for the salt of the phosphite complex. The air-dried salt was stable, like as the phosphate complex, but decomposed more rapidly than the phosphate salt when dissolved in water (completely decomposed within 50sec after dissolution). Immediately after dissolution, however, the solution exhibited the same spectrum, displaying an absorbance maximum at around 250nm. The spectra and the decomposition process were measured by a rapid-scan technique, and the result is shown in Fig. 5.
0.8
m
4 0.6
c
£ o4 a
I
I
02
oo
200
250
300
350
400
Wavelength (nm)
Fig. 6. Equilibrium between pentamolybdodiphosphate and dodecamolybdophosphatecomplex in the mixed solvent (waterethanol, 4:1 v/v). [molybdate]= 6 × 10-3 M (constant); 0.0137cm cell; pH = 3.5; [molybdate]/[phosphate]:(1) 12.0; (2) 3.8; (3) 2.0; (4) 0.5.
mula, it could be expected that the X-ion can be replaceable by other kinds of phosphorus anions and analogous complexes formed. Spectral measurements revealed the formation of molybdate complexes when such monosubstituted phosphorus oxyanions as methylphosphate, glucosephosphate, methylphosphonate and 2aminoethylphosphonate ions were employed for the Xion. Although one corner of the tetrahedral oxyanions is occupied by a bulky alkyl group, these anions react with Wavelength(nrn) molybdate to yield the respective complexes. The comFig. 5. Absorption spectrum of [(C2Hs)4N]2Na2(HPO3)2(MoO3)5 position and structure of these complexes were not escrystals and the spectrum changes in water after the dissolution. tablished, but they give almost identical spectra, characscan rate = 3 Hz; 1.00cm cell; measuring duration = 5-45 sec terized by an absorbance maximum at around 250 nm (in after the dissolution; concentration of the crystal = 0.34 mg/4ml the last column of Table 2). If Strandberg's result[10] water. obtained for the structure of the pentamolybdodiphosphate can be extended to these molybdate complexes, both the spectral similarity observed among the comDISCUSSION plexes and the reactivity of the oxyanions with molIt has been reported[17, 18] that the absorption spec- ybdate might be rationalised as follows. In the pentrum of dodecamolybdophosphate is affected by addition tamolbydate complex, three corners of the tetrahedral of certain water-miscible polar organic solvents and that orthophosphate ion (coordinatively functional P-O an absorbance maximum appears at around 310 nm. In bonds) are required to connect them to the surrounding the present work it was found that in a water-ethanol pentamolybdate skelton, the remaining corner being free. mixture (4:1 v/v) the spectrum of the dodecamolybdate This implies that a group of monosubstituted phosphorus complex was indeed changed (compare curve 1 in Figs. 1 compounds, each of which has at least three apical and 6), whereas that of the pentamolybdodiphosphate oxygens forms, could function enough as the central complex was not (curve 4 in Figs. 1 and 6). The pen- anion in the pentamolybdate structure in place of ortamolybdate in the mixed solvent is not converted into thophosphate ion. As shown in Table 2, the absorbance the dodecamolybdate so long as an excess of phosphate seems little dependent on both the oxidation number of is present. The isosbestic points are more definite in the the phosphorus and the kind of the substituting groups of the central anions, but dependent on the number of mixed solvent than in the pure aqueous system, showing molybdenum atoms and the surrounding molybdate that only the two molybdophosphate complexes are structure. present in equilibrium. This explains the fact that in the Fleury[19] reported the existence of a molybdate presence of a constant amount of molybdate the formation of the colourless complex reduces the formation complex of /3-glycerophosphate with a composition of of the yellow complex and vice versa. (MoO3)2.sC3H702PO4, and this also fits the above From the established absorption spectrum for the interpretation. The complex might correspond to a pentamolybdodiphoshate complex, a reasonable inter- type of pentamolybdate complex, (MoO3)5(C3H7PO4)2. pretation could be deduced regarding with the formation Moreover, Rosenheim and Schapiro[20] reported the existence of a pentamolybdodiphsophite complex in the of analogous molybdate complexes by other phosphorus oxyanions. As described above, an almost identical solid state. On the other hand, dimethylphosphate ion in which spectrum was observed for the molybdate complexes of phosphate and phosphite and, in fact, complexes with a only two P-O corners remain unblocked did not affect similar composition, formulated as [(X"-)2(MoO3M 2n-, the spectra of molbydate solutions and no evidence of were isolated as salts with both orthophosphate (X= complex formation was obtained by the present specPO43 ) and phosphite ion (X = HPO32-). With this for- trophotometric method.
Formation of colourless molybdate complexes of phosphorus compounds in aqueous solution Acknowledf,,ements--The authnr wishes to acknowledge the helpful advice and discussions given by Prof. T. Fujinaga and Prof. M. Koyama of this laboratory. The author's thanks are due t~ Dr. M. Sugita for his offering 2-aminoethylphosphonic acid and als~ due to the Instrumental Analysis Research Centre of l,~yolo University for offering facilities. REFERENCES
I ~ Miolati and R. Pizzighelh. J, Pract. Chem. 77, 417 (1908). 2 P Soucha~ and J. Faucherrt~, Bull. Soc. Chim. (France) 355 1951) ~. I ' f'errari. Mikrochim..4ct~ 551 (1956). 4. K. Murata and T. Kiba. J. lnorg. NucL Chem. 32. 1667 (1970). I. Fujinaga, M. Koyama antt T. Hori, Bull. Inst. Chem. Res. Ky~,to Unit. 48, 21() (1970i. 6. [. Petterson. A~ta Chem. Seand. 25, 1967 (1971). 7 (" |riedheim. Z. ,4notre. Alh,,. Chem. 4, 275 (1893). ~. ~ triedheim and J Meschi~irer, Z. Anorg. Allg. Chem. 6, 27 ( 1gq4
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9. J. W. Mellor, A Comprehensive Treaties on Inorganic and Theoretical Chemistry, Vol. 11, pp. 659-672. Longmans, London (1931). 10. R. Strandberg, Acta Chem. Scand. 27, 1004 (1973), 11. R. H. A. Plirner and W. J. N. Burch, J. Chem. Soc. 292 (1929). 12. A. Michaelis and R. Kaehne, Ber. 31, 1048 (1898). 13. A. W. Hofman, Ber. 5, 104 (1872). 14. G. M. Kosolapoff, J. Am. Chem. Soc. 69, 2112 (1947). 15. D. N. Bernhart and A. R. Wreath, Anal. Chem. 27, 440 (1955). 16. T. R. Hogness, F. P. Zscheile, Jr. and A. E. Sidwell, Jr., .L Phys. Chem. 41, 379 (1937). 17. R. A. Chalmers and A. G. Sinclair, Anal. Chim. Acta 33, 384 (1965). 18. T. Fujinaga, M. Koyama and T. Hori, Talanta 18,960 (1971). 19. M. P. Fleury, C. R. Acad. Sci. 193, 1350 (1931). 20. A. Rosenheim and M. Schapiro, Z. Anorg. Allg. Chem. 129, 196 (1923).