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Complex formation between α-hydroxy organic acids and molybdic and tungstic acid prepared by ion exchange
J. Inorg. Nucl. Chem., 1960, Vol. 13, pp. 84 to 90. I~rgamon Prem Lid. Printed in Northern Ireland
COMPLEX FORMATION BETWEEN et-HYDROXY ORGANIC ACIDS AND MOLYBDIC AND TUNGSTIC ACID PREPARED BY ION EXCHANGE E. RICHARDSON Dept. of Chemistry, Whitehaven College of Further Education, Whitehaven, Cumberland (Received 15 June 1959)
Abstract--Complex formation is reported between a-hydroxy carboxylic acids and tungstic and molybdic acid prepared by ion exchange. Similar carboxylic acids without hydroxyl groups do not complex. Some properties of these complexes are described. IN an earlier paper a) complex formation is reported between polyhydroxy compounds and molybdic acid at various p H values. Complex formation between some a-hydroxy acids and molybdic and other inorganic acids has been reported by other workersCg,a, 4~ using a variety of techniques. In most cases the optimum conditions for complex formation occurred at p H values greater than seven indicating that the anion of the organic acid was the effective complexing agent. In this paper, complex formation has been investigated between ~-hydroxy acids and pure unbuffered molybdic and tungstic acid prepared by ion exchange, using potentiometric and conductometric techniques. Ionization of the organic acids is inhibited in the presence of the latter acids prepared by ion exchange at the concentrations employed (-00.05 M) since the p H of their solution is less than two. Comparatively little work has been reported on the organic complexes of tungstic acid and tungstate ions and evidence for such complexes has generally been derived by indirect methods.ta, 5~ This is in part due to the low solubility and the instability of tungstic acid in aqueous solution. Byusing the ion exchange technique, tungstic acid in relatively concentrated solution (0.1 M) m a y easily be prepared and this acid is sufficiently stable to allow preparation of the organic complexes. EXPERIMENTAL Zeo Karb 225, a sulphonated polystyrene resin in the hydrogen form, was used in all studies. This resin was activated before use by alternate washings with 2 N NaOH, water, and 2 N HCI. Excess acid was removed from the column after the final regeneration by washing with distilled water until the conductivity of the washings collected was less than 3 pmhos. Analar sodium tungstate dihydrate, Na2WO42HsO and Analar sodium molybdate dihydrate, NasMoO,2H~O, were used to prepare the free acids. Both salts are sufficiently soluble in water to prepare 0-1 M solutions. The molybdic acid obtained from the column is quite stable and may be stored, then diluted to the required concentration before use. Tungstic acid is unstable and must be prepared immediately before use. All conductance measurements were made using a Mullard Conductivity Bridge operating in the most sensitive range, with a dip type cell of constant 1.54. All pH measurements were made using ~1~E. RICHARDSON.J. lnorg. NucL Chem. 9, 273 (1959). t2~E. RI~BACHand P. Lev,Z. Phys. Chem. 100, 393 (IJ22). qs~G. S. RAOand S. N. BANE~EE,Proc. Natl. Acad. $ci., India A 23, 76 (1954). m F. FAmnROTHERand J. B. TAYLOR,J. Chem. $oc. 4946 (1956). ~6~D. A. LAMnlr,Analyst 70, 124 (1945). 84
Complex formation between a-hydroxy organic acids and molybdic and tungstic acid
85
a Cambridge pH meter with a glus calomel electrode system calibrated by buffer solutions complying with modern standards. AIr extinction measurements were made on a Hilger Uvispek Spectrophotometer using fused-silica cells. Appropriate blank solutions were employed. The experiments were conducted at 18°C unless otherwise specified. The tungsten and molybdenum concentrations and ratios have been expressed throughout with respect to WOs or MoOs, and were determined gravimetrically by precipitation with 8-hydroxy quinoline. RESULTS AND DISCUSSION
I. Complexformation between molybdic aeid and a-hvdroxy acids The effect of selected a-hydroxy acids, and similar acids without hydroxyl groups, on the conductivity of molybdic acid at various mole ratios is shown in Fig. I. These
5 "e.
^
--.,...
0
0"2 0.4 0-6 0.8 I'O Mol. fraction of orqonic FIG. l.--The influence of polyhydroxy acids on the conductivity of pure molybdic acid. O Citric acid × Tartaric acid A Malic acid D Mandelic acid • Succinic and adipic acids
mole ratios were obtained by mixing the requisite quantities of 0.0425 M solutions of the two constituents. Similar curves were obtained from pH studies but since both materials were acids the effect noticed was less pronounced. Since the empirical formula of the tartaric acid complex cannot be determined from Fig. 1, a further experiment was performed for this material using various ratios of 0.085 M tartaric acid and 0.0425 M molybdic acid (Fig. 2). A sharp peak in the conductance was obtained when equal quantities of the solutions were mixed, i.e. an MoOs : C4I~O6 ratio of 1 : 2 . Thus from Figs. 1 and 2 it can be seen that citric, tartaric, malic and mandelic acids complex with molybdic acid at mole ratios MoOs : organic of 2 : 1, 1 : 2, 1 : 1 and 1 : 2 respectively, to form acids of greater strength than either constituent. Suecinic acid and adipic acid, non hydroxy acids, which are included in Fig. (1) for comparison, show no tendency to complex with molybdic acid. It seems reasonable to suppose, therefore, that the complexing power of the non-ionized =-hydroxy acids and mandelic acid is due to the presence of the hydroxyl group. The complexes are colourless in solution and show most of the normal reactions of molybdates. They give an immediate precipitate of molybdenum sulphide with hydrogen sulphide even in the absence of mineral acid, but give a coloration with
86
E. RICHARDSON
ammonium thiocyanate only after addition of HCI. A brown precipitate is obtained from all solutions on addition of potassium ferrocyanide. Complex formation between hydroxy acids, in particular tartaric, and molybdic acid sol prepared by dialysis has been studied by BISWAS.~8,v~ Optical rotation studies of the molybdotartaric acid complex in the presence of added acids showed that the tendency to complex lay in the order oxalic > tartaric > citric > malic. In a later study BISWAS¢~' obtained a'peak in conductivity and hydrogen ion concentration when tartaric acid and molybdate were present in equimolecular quantities. 20
5 7
Vol. 0.0850 tortoric ocid 16 8 4
0
/
"°'4 r Q c o
0
4 8 12 16 20 Vol. 0'0425 molybdic ocid
FIG. 2.--The influence o f tartaric acid on the conductivity o f pure molybdic acid.
It is of interest to note that, in the present study using molybdic acid prepared by ion exchange, at a lower pH value (<2), where complex formation must involve the non ionized form of the organic acid, citric acid forms a 1 : 2 complex (Org. 2MoOa) with molybdic acid which has a greater conductivity than the 2 : 1 (20rg. MoO D complex formed by tartaric acid. It has been shown in a previous stud/s~ that molybdic acid prepared by ion exchange, at the concentration used in the present study, is probably present in its simplest form as H2Mo4Ola. Providing degradation of the molybdic acid does not occur during complex formation, it seems possible that the greater complexing power of the citric acid may be due to a more favourable configuration of the molecule for co-ordination to a large molybdi¢ acid molecule. Degradation of molybdic acid to simpler units may occur, however, during the formation of the organo-complexes. Some evidence for this latter process has been obtained from a study of the ultraviolet absorption spectra both for the molybdic acid complexes and the tungstic acid complexes, which will be described later. The evidence is more conclusive in the case of the tungstic acid complexes and for convenience will be described here. The spectra of sodium tungstate (0.005 M) acidified with perchloric acid (which (e) A. B. BISWAS,J. Indian Chem. Soc. 23, 249 (1946). (v~ A. B. BlswAs, J. Indian Chem. Soc. 24, 345 (1947). (s) E. RlCNARDSON,J. lnorg; Nucl. Chem. 9, 267 (1959).
Complex formation between c(-hydroxy organic acids and molybdic and tungstic acid
87
does not complex with tungstic acid) to form solutions at various pH values is shown in Fig. 3. From this figure it can be seen that decrease in pH of the tungstate solution causes a shift of the absorption edge from the ultra-violet towards the visible region of the spectrum. A similar shift has been found for acidified molybdate solutions by many authors,(', 10) and is believed to be due to an increase in the degree of aggregation of the isopoly acid. Acidification of sodium tungstate by tartaric acid, a typical hydroxy-acid, which does not absorb appreciably in this region of the spectrum,
tl
o
240(
\
2800
3200
3600
4000
Flo. 3.--Absorption spectra of acidified tungstate solutions and tungstic acid prepared by ion exchange. NasWO4
HCIO4
Symbol
6.8 6.7 6"68 6"55 6"35 3"55
® O × • • I'
1 2 3 4 5 6
0-005 M 0.005 M 0-005 M 0.005 M 0.005 M 0"005 M
8 9
0.005 M 0-05(Tartaric) 2.50 0-005 M Tungstic acid ppd. by ion exchange pH 2.7 • 0.005M Tungstic acid ppd. by ion exchange containing 0-05 M tartaric acid [] All 1 cm cell except 10 where 0.5 cm cell used.
7 10
0.OO5M
0 0-00042 M 0-00083 M 0-00166 M 0-00332 M 0-0083 M
pH
0-0415M
].60
caused a much smaller shift. Tungstic acid freshly preparedby ion exchange has an absorption edge in the region of 3600 .~ similar to acidified sodium tungstate solutions of pH < 3.5. Addition of an excess of tartaric acid to ion-exchange tungsfic acid to form a solution of the same tungsten content caused a shift Of the absorption edge to 2700 •. Similar results were obtained with citric acid. It seems reasonable to suppose, therefore, that some depolymerisation of the tungstic acid occurs during complex t,t I. LINDQVIST,Acta. Chem. Scand. S, 568 (1951). txo) p. CANNON, J. lnorg. Nucl. Chem. 9, 252 (1959).
88
E. RICHARDSON
formation with the organic acids. Similar results have been obtained using tartaric acid and molybdic acid prepared by ion exchange.
2. Complex formation between lungstic acid and ~-h)'droxy acids. The effect of selected 0t-hydroxy acids and similar acids without hydroxyl groups on the conductivity of tungstic acid at various mole ratios is shown in Figs. (4) and (5). In Fig. (4) the conductivity of the tartaric acid, tungstic acid mixture has been plotted after various times. Data obtained on the tungstic acid complexes are more
54
~2
0
0.4 0'6 0"8 Mol ftoction of o~onic
1.0
FIG. 4.--The effect of time on the conductivity o f tartaric acid-tungstic acid mixtures. • lhr X 1 day [] 2 days 0 6 days A 13 days
difficult to interpret than those obtained from molybdic acid complexes. Molybdic acid is stable for several weeks but an aggregation process, causing opalescence, occurs in tungstic acid solution before preparation of the organic mixture can be completed,cm The aggregated material probably combines much more slowly with the organic acid. The maximum conductivity is not obtained until several days after preparation even in solutions containing a stoicheiometric excess of tartaric acid, which become clear to the naked eye soon after mixing. This effect has been noted for all of the other hydroxy-acids studied and is quite reproducible under given conditions, although the time required for maximum conductivity varies for the different organic acids. ~tl~ E. RtCHARDSO~4,J. Inorg. Nucl. Chem. 12, 79 (1959).
Complex formation between ~-hydroxy organic acids and molybdic and tungstic acid
89
Interpretation of the data on the tungstic acid complexes is further complicated by the slow aggregation of the tungstic acid present in stoicheiometric excess. It appears from Fig. (4), and from data for the other acids not presented, that the presence of hydroxy-organic acids decreases the rate of the aggregation process. The conductivity of several tungstic acid--organic acid mixtures at various mole ratios after standing for six days has been plotted in Fig. 5. From this figure it can be seen that tungstic acid forms complexes with gluconic, mandelic, malic, tartaric, and citric acid at WO3 : organic acid ratios of 1 : 2, I : 3, I : 3, 1 : 2 and I : 1 respectively. Similar plots
7
h=¢
e 'o
8'
0
,
0
0"2
0.4
1
0-6
0-8
t-O
Mole frocfion orcjonic
Fro. 5.--The effect of polyhydroxy acids on the conductivity of tungstic acid prepared by ion exchange. Gluconic acid • Mandelic acid A Malic acid C) Tartaric acid [] Citric acid • AdJpic } Succinic essentially the same.
after thirteen days produced peaks at the same ratios. After ten weeks peaks were still found at the same ratios except for citric acid where a 1 : 2 ratio was indicated. In view of the time required for maximum conductivity even in solutions containing an dxcess of organic acid, and the possibility of decomposition of an unstable complex before six days, the existence of complexes containing a higher proportion of tungstic acid than above is not precluded. Such complexes with WO4~/organic
90
E. RICHARDSON
ratios of 1 : 1 have been demonstrated by lL~o~8~by conductometric and pH studies on sodium tungstate acidified with the organic acid. These complexes were found to be unstable immediately after preparation. The citric acid complex described by R^o, c3~however, did not have the enhanced conductivity found in the present study. Comparison of Fig. (4) with Fig. (1) again shows that citric acid, in solutions of low pH, has the greatest complexing ability. Since adipic and succinic do not complex with tungstic acid it appears that the complex formation is again due to the presence of the hydroxyl group. Evaporation to dryness of a sample from each solution containing the tungstate complex together with a small excess of organic acid yielded colourless crystalline products for tartaric acid and mandelic acid. Some decomposition of the citric acid and malic acid complexes occurred, however, on evaporation, giving a mixture containing yellow tungstic acid.
COMPLEX FORMATION BETWEEN et-HYDROXY ORGANIC ACIDS AND MOLYBDIC AND TUNGSTIC ACID PREPARED BY ION EXCHANGE E. RICHARDSON Dept. of Chemistry, Whitehaven College of Further Education, Whitehaven, Cumberland (Received 15 June 1959)
Abstract--Complex formation is reported between a-hydroxy carboxylic acids and tungstic and molybdic acid prepared by ion exchange. Similar carboxylic acids without hydroxyl groups do not complex. Some properties of these complexes are described. IN an earlier paper a) complex formation is reported between polyhydroxy compounds and molybdic acid at various p H values. Complex formation between some a-hydroxy acids and molybdic and other inorganic acids has been reported by other workersCg,a, 4~ using a variety of techniques. In most cases the optimum conditions for complex formation occurred at p H values greater than seven indicating that the anion of the organic acid was the effective complexing agent. In this paper, complex formation has been investigated between ~-hydroxy acids and pure unbuffered molybdic and tungstic acid prepared by ion exchange, using potentiometric and conductometric techniques. Ionization of the organic acids is inhibited in the presence of the latter acids prepared by ion exchange at the concentrations employed (-00.05 M) since the p H of their solution is less than two. Comparatively little work has been reported on the organic complexes of tungstic acid and tungstate ions and evidence for such complexes has generally been derived by indirect methods.ta, 5~ This is in part due to the low solubility and the instability of tungstic acid in aqueous solution. Byusing the ion exchange technique, tungstic acid in relatively concentrated solution (0.1 M) m a y easily be prepared and this acid is sufficiently stable to allow preparation of the organic complexes. EXPERIMENTAL Zeo Karb 225, a sulphonated polystyrene resin in the hydrogen form, was used in all studies. This resin was activated before use by alternate washings with 2 N NaOH, water, and 2 N HCI. Excess acid was removed from the column after the final regeneration by washing with distilled water until the conductivity of the washings collected was less than 3 pmhos. Analar sodium tungstate dihydrate, Na2WO42HsO and Analar sodium molybdate dihydrate, NasMoO,2H~O, were used to prepare the free acids. Both salts are sufficiently soluble in water to prepare 0-1 M solutions. The molybdic acid obtained from the column is quite stable and may be stored, then diluted to the required concentration before use. Tungstic acid is unstable and must be prepared immediately before use. All conductance measurements were made using a Mullard Conductivity Bridge operating in the most sensitive range, with a dip type cell of constant 1.54. All pH measurements were made using ~1~E. RICHARDSON.J. lnorg. NucL Chem. 9, 273 (1959). t2~E. RI~BACHand P. Lev,Z. Phys. Chem. 100, 393 (IJ22). qs~G. S. RAOand S. N. BANE~EE,Proc. Natl. Acad. $ci., India A 23, 76 (1954). m F. FAmnROTHERand J. B. TAYLOR,J. Chem. $oc. 4946 (1956). ~6~D. A. LAMnlr,Analyst 70, 124 (1945). 84
Complex formation between a-hydroxy organic acids and molybdic and tungstic acid
85
a Cambridge pH meter with a glus calomel electrode system calibrated by buffer solutions complying with modern standards. AIr extinction measurements were made on a Hilger Uvispek Spectrophotometer using fused-silica cells. Appropriate blank solutions were employed. The experiments were conducted at 18°C unless otherwise specified. The tungsten and molybdenum concentrations and ratios have been expressed throughout with respect to WOs or MoOs, and were determined gravimetrically by precipitation with 8-hydroxy quinoline. RESULTS AND DISCUSSION
I. Complexformation between molybdic aeid and a-hvdroxy acids The effect of selected a-hydroxy acids, and similar acids without hydroxyl groups, on the conductivity of molybdic acid at various mole ratios is shown in Fig. I. These
5 "e.
^
--.,...
0
0"2 0.4 0-6 0.8 I'O Mol. fraction of orqonic FIG. l.--The influence of polyhydroxy acids on the conductivity of pure molybdic acid. O Citric acid × Tartaric acid A Malic acid D Mandelic acid • Succinic and adipic acids
mole ratios were obtained by mixing the requisite quantities of 0.0425 M solutions of the two constituents. Similar curves were obtained from pH studies but since both materials were acids the effect noticed was less pronounced. Since the empirical formula of the tartaric acid complex cannot be determined from Fig. 1, a further experiment was performed for this material using various ratios of 0.085 M tartaric acid and 0.0425 M molybdic acid (Fig. 2). A sharp peak in the conductance was obtained when equal quantities of the solutions were mixed, i.e. an MoOs : C4I~O6 ratio of 1 : 2 . Thus from Figs. 1 and 2 it can be seen that citric, tartaric, malic and mandelic acids complex with molybdic acid at mole ratios MoOs : organic of 2 : 1, 1 : 2, 1 : 1 and 1 : 2 respectively, to form acids of greater strength than either constituent. Suecinic acid and adipic acid, non hydroxy acids, which are included in Fig. (1) for comparison, show no tendency to complex with molybdic acid. It seems reasonable to suppose, therefore, that the complexing power of the non-ionized =-hydroxy acids and mandelic acid is due to the presence of the hydroxyl group. The complexes are colourless in solution and show most of the normal reactions of molybdates. They give an immediate precipitate of molybdenum sulphide with hydrogen sulphide even in the absence of mineral acid, but give a coloration with
86
E. RICHARDSON
ammonium thiocyanate only after addition of HCI. A brown precipitate is obtained from all solutions on addition of potassium ferrocyanide. Complex formation between hydroxy acids, in particular tartaric, and molybdic acid sol prepared by dialysis has been studied by BISWAS.~8,v~ Optical rotation studies of the molybdotartaric acid complex in the presence of added acids showed that the tendency to complex lay in the order oxalic > tartaric > citric > malic. In a later study BISWAS¢~' obtained a'peak in conductivity and hydrogen ion concentration when tartaric acid and molybdate were present in equimolecular quantities. 20
5 7
Vol. 0.0850 tortoric ocid 16 8 4
0
/
"°'4 r Q c o
0
4 8 12 16 20 Vol. 0'0425 molybdic ocid
FIG. 2.--The influence o f tartaric acid on the conductivity o f pure molybdic acid.
It is of interest to note that, in the present study using molybdic acid prepared by ion exchange, at a lower pH value (<2), where complex formation must involve the non ionized form of the organic acid, citric acid forms a 1 : 2 complex (Org. 2MoOa) with molybdic acid which has a greater conductivity than the 2 : 1 (20rg. MoO D complex formed by tartaric acid. It has been shown in a previous stud/s~ that molybdic acid prepared by ion exchange, at the concentration used in the present study, is probably present in its simplest form as H2Mo4Ola. Providing degradation of the molybdic acid does not occur during complex formation, it seems possible that the greater complexing power of the citric acid may be due to a more favourable configuration of the molecule for co-ordination to a large molybdi¢ acid molecule. Degradation of molybdic acid to simpler units may occur, however, during the formation of the organo-complexes. Some evidence for this latter process has been obtained from a study of the ultraviolet absorption spectra both for the molybdic acid complexes and the tungstic acid complexes, which will be described later. The evidence is more conclusive in the case of the tungstic acid complexes and for convenience will be described here. The spectra of sodium tungstate (0.005 M) acidified with perchloric acid (which (e) A. B. BISWAS,J. Indian Chem. Soc. 23, 249 (1946). (v~ A. B. BlswAs, J. Indian Chem. Soc. 24, 345 (1947). (s) E. RlCNARDSON,J. lnorg; Nucl. Chem. 9, 267 (1959).
Complex formation between c(-hydroxy organic acids and molybdic and tungstic acid
87
does not complex with tungstic acid) to form solutions at various pH values is shown in Fig. 3. From this figure it can be seen that decrease in pH of the tungstate solution causes a shift of the absorption edge from the ultra-violet towards the visible region of the spectrum. A similar shift has been found for acidified molybdate solutions by many authors,(', 10) and is believed to be due to an increase in the degree of aggregation of the isopoly acid. Acidification of sodium tungstate by tartaric acid, a typical hydroxy-acid, which does not absorb appreciably in this region of the spectrum,
tl
o
240(
\
2800
3200
3600
4000
Flo. 3.--Absorption spectra of acidified tungstate solutions and tungstic acid prepared by ion exchange. NasWO4
HCIO4
Symbol
6.8 6.7 6"68 6"55 6"35 3"55
® O × • • I'
1 2 3 4 5 6
0-005 M 0.005 M 0-005 M 0.005 M 0.005 M 0"005 M
8 9
0.005 M 0-05(Tartaric) 2.50 0-005 M Tungstic acid ppd. by ion exchange pH 2.7 • 0.005M Tungstic acid ppd. by ion exchange containing 0-05 M tartaric acid [] All 1 cm cell except 10 where 0.5 cm cell used.
7 10
0.OO5M
0 0-00042 M 0-00083 M 0-00166 M 0-00332 M 0-0083 M
pH
0-0415M
].60
caused a much smaller shift. Tungstic acid freshly preparedby ion exchange has an absorption edge in the region of 3600 .~ similar to acidified sodium tungstate solutions of pH < 3.5. Addition of an excess of tartaric acid to ion-exchange tungsfic acid to form a solution of the same tungsten content caused a shift Of the absorption edge to 2700 •. Similar results were obtained with citric acid. It seems reasonable to suppose, therefore, that some depolymerisation of the tungstic acid occurs during complex t,t I. LINDQVIST,Acta. Chem. Scand. S, 568 (1951). txo) p. CANNON, J. lnorg. Nucl. Chem. 9, 252 (1959).
88
E. RICHARDSON
formation with the organic acids. Similar results have been obtained using tartaric acid and molybdic acid prepared by ion exchange.
2. Complex formation between lungstic acid and ~-h)'droxy acids. The effect of selected 0t-hydroxy acids and similar acids without hydroxyl groups on the conductivity of tungstic acid at various mole ratios is shown in Figs. (4) and (5). In Fig. (4) the conductivity of the tartaric acid, tungstic acid mixture has been plotted after various times. Data obtained on the tungstic acid complexes are more
54
~2
0
0.4 0'6 0"8 Mol ftoction of o~onic
1.0
FIG. 4.--The effect of time on the conductivity o f tartaric acid-tungstic acid mixtures. • lhr X 1 day [] 2 days 0 6 days A 13 days
difficult to interpret than those obtained from molybdic acid complexes. Molybdic acid is stable for several weeks but an aggregation process, causing opalescence, occurs in tungstic acid solution before preparation of the organic mixture can be completed,cm The aggregated material probably combines much more slowly with the organic acid. The maximum conductivity is not obtained until several days after preparation even in solutions containing a stoicheiometric excess of tartaric acid, which become clear to the naked eye soon after mixing. This effect has been noted for all of the other hydroxy-acids studied and is quite reproducible under given conditions, although the time required for maximum conductivity varies for the different organic acids. ~tl~ E. RtCHARDSO~4,J. Inorg. Nucl. Chem. 12, 79 (1959).
Complex formation between ~-hydroxy organic acids and molybdic and tungstic acid
89
Interpretation of the data on the tungstic acid complexes is further complicated by the slow aggregation of the tungstic acid present in stoicheiometric excess. It appears from Fig. (4), and from data for the other acids not presented, that the presence of hydroxy-organic acids decreases the rate of the aggregation process. The conductivity of several tungstic acid--organic acid mixtures at various mole ratios after standing for six days has been plotted in Fig. 5. From this figure it can be seen that tungstic acid forms complexes with gluconic, mandelic, malic, tartaric, and citric acid at WO3 : organic acid ratios of 1 : 2, I : 3, I : 3, 1 : 2 and I : 1 respectively. Similar plots
7
h=¢
e 'o
8'
0
,
0
0"2
0.4
1
0-6
0-8
t-O
Mole frocfion orcjonic
Fro. 5.--The effect of polyhydroxy acids on the conductivity of tungstic acid prepared by ion exchange. Gluconic acid • Mandelic acid A Malic acid C) Tartaric acid [] Citric acid • AdJpic } Succinic essentially the same.
after thirteen days produced peaks at the same ratios. After ten weeks peaks were still found at the same ratios except for citric acid where a 1 : 2 ratio was indicated. In view of the time required for maximum conductivity even in solutions containing an dxcess of organic acid, and the possibility of decomposition of an unstable complex before six days, the existence of complexes containing a higher proportion of tungstic acid than above is not precluded. Such complexes with WO4~/organic
90
E. RICHARDSON
ratios of 1 : 1 have been demonstrated by lL~o~8~by conductometric and pH studies on sodium tungstate acidified with the organic acid. These complexes were found to be unstable immediately after preparation. The citric acid complex described by R^o, c3~however, did not have the enhanced conductivity found in the present study. Comparison of Fig. (4) with Fig. (1) again shows that citric acid, in solutions of low pH, has the greatest complexing ability. Since adipic and succinic do not complex with tungstic acid it appears that the complex formation is again due to the presence of the hydroxyl group. Evaporation to dryness of a sample from each solution containing the tungstate complex together with a small excess of organic acid yielded colourless crystalline products for tartaric acid and mandelic acid. Some decomposition of the citric acid and malic acid complexes occurred, however, on evaporation, giving a mixture containing yellow tungstic acid.