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Molybdic acid and its organic complexes—IV Complexes of molybdate with polyhydric phenols— A spectrophotometric study
J. Inorg. Nucl. Chem., 1960. Vol. 15. pp. 133 to 139. Pergamon Press Ltd.
MOLYBDIC
ACID
AND
ITS O R G A N I C
COMPLEXES--IV
COMPLEXES OF MOLYBDATE WITH POLYHYDRIC PHENOLS-A SPECTROPHOTOMETRIC STUDY H . BUCHWALD a n d E . RICHARDSON
Department of Chemistry, Whitehaven College of Further Education Cumberland (Received 26 October 1959)
Abstract--The molybdate ion in aqueous solution forms intense yellow to orange-brown complexes with polyhydric phenols. Using a spectrophotometric method it is shown that at p H 7 the ratio of polyhydric phenol to molybdate in the complex is 2 : 1. A t lower p H values the composition of the complexes becomes variable, and changes according to the other substituent groups on the aromatic nucleus. Only the catechol complex retains a constant composition over the pH range studied. The complexes of six phenols are examined: catechol, protocatechuic acid, 4-methylcatechol, Tiron, pyrogallol and gallic acid. The complex formed with salycilic acid is examined for comparison.
THE reaction between acidified molybdate solutions and phenols has been known for a long time31, ~ Catechol and related compounds (in which there are at least two ortho hydroxyl groups) yield yellow to orange-brown complexes with molybdate solutions. This reaction does not take place with polyhydric phenols in which the hydroxyl groups are placed in meta or para positions to each other.tO,4~ Since aliphatic polyhydroxy compounds, with the hydroxyl groups on adjacent carbon atoms form colourless complexes with molybdate, ~5~ it appears that the formation of eoloured complexes is due to the aromatic nucleus. Some quantitative work on the nature and empirical formulae of the complexes formed by polyhydric phenols with other ions has been reported,t~, 7~and particularly noteworthy are the contributions of Yo~ c8~and SOMr,tER~s~ on the complexes formed by Fe s+ and Ti~- with Tiron (disodium 1,2-dihydroxybenzene-3,5-disulphonate ) and catechol. Apart from a short note, c9~various analytical applicationsClO,n, a~~ and a recent communication by us, txa~very little quantitative data seems to be available for the molybdenum complexes. The situation with molybdenum is more complicated since the reaction is with the molybdate anion and not with a cation as in the other complexes. The orange-yellow colours produced, however, are ~1~ D. H. KIH-~FER and A. LINZ, Molybdenum Compounds pp. 198-199. lnterscience, New York (1952). ~z~ L. FmtNAND~, Gazz. Chim. Ital. 55, 424 (1925). ts~ L. S o ~ , Coll. Czech. Chem. Comm. 22, 414, 1793 (1957). t4) A. T. C~srl and A. G. MADDOCK, I. Inorg. NucL Chem. 10, 58 (1959). ~6~ E. RICrlAgDSOIq, J. Inorg. N u d . Chem. 9, 273 (1959). ~6J D. D. P~glUN, Nature, Land. 182, 741 (1958). ~ J. B~RRUM, G. SCHWAgZgNI~CH and L. G. SILLI~N, Stability Constants--Part L Organic Ligands. Chemical Society Special Publication No. 6, London (1957). ~a~j. H. YoE and A. L. Jab'D, Industr. Engng. Chem. (Anal.) 16, 111 (1944). c,~ j. C. M c G o w ~ and P. W. BRIAN, Nature, Land. 159, 373 (1947). tle~ S. Smrrmt and B. Novlc, Analyt. Chem. 23, 188 (1951). tm F. WIta. and J. H. YOE, Analyt. Chlm. Acta 8, 546 (1953). txz~ S. YA SCHNAIDmtt~N and I. B. Rom~aovx Analyt. Abstr. 6, No. 18 (1959). txs~ H. BUCHWM.D and E. Racl-mgDSOrq, Chem. & Ind. 753 (1959). 133
134
H. BUCHWAI.D and E. RtCHAgDSON
particularly suitable-for spectrophotometric studies, al~ In this paper the spectrophotometric studies with six polyhydric phenols are reported: catechol, 4-methylcatechol protocatechuic acid (3,4-dihydroxybenzoic acid), Tiron, pyrogallol and gallic acid. EXPERIMENTAL Standard solutions of sodium molybdate were prepared from an analysed batch of solid. Stock solutions 0"1 M and 0-02 M in Na2MoO4 were generally used. All the phenols used were obtained commercially, in the analytical reagent grade whenever possible. Where the analytical reagent grade was unavailable the materials were recrystallised from a suitable solvent. Aqueous stock solutions 0'02 M and 0-05 M were prepared as required, but were not kept for any appreciable time since discoloration occurred in most cases after only a few hours. Sodium acetate was used as a semi-buffer solution and to maintain a relatively constant ionic strength. A solution of perchloric acid was generally used to adjust the pH. A stock solution approximately 1.4 M was prepared from the A.R. grade 60 per cent acid. The working solutions on which measurements were carried out were prepared from suitable quantities of the above reagents'and kept in well stoppered flasks, pH measurements were made with a glass-calomel electrode system calibrated to read 4"00 at 20°C with 0.05 M potassium hydrogen phthalate. This calibration was checked at pH 7.00 with a boric acid-sodium borate buffer and at pH 2.54 with 0.01 M potassium tetroxalate. Suitable glass electrodes were available for solutions of low and high pH values. Conductivity measurements were made on a Mullard conductivity bridge operating in the most sensitive range, with a maximum error of ± 2 % . A rigid dip-type cell having a constant 1.54 was used; this cell could be sealed into the reaction vessel where necessary in order to minimize atmospheric oxidation. The spectrophotometric work was carried out in 1 cm cuvettes using Unicam SP 500 and Hilger Uvispek spectrophotometers. All measurements were made at 20 + 2°C. A thermostatic cell carrier was available for the Unicam spectrophotometer but was not used since a preliminary investigation showed that a temperature fluctuation of ± 5 ° C had no significant effect. RESULTS
AND DISCUSSION
Fig. 1 shows the absorption spectra of the complexes obtained with the six phenols under similar conditions. The percentage transmission curve is used since it emphasizes the differences. With all the phenols except Tiron the maximum absorption occurs at a wavelength of about 400 m/~. At wavelengths below 400 m/~ no definite peaks are found in the curves, only a broad absorption band. It will be noted that the absorption spectra of the catechol, 4-methylcatechol and protocatechuic acid complexes are practically identical. At any particular wavelength, the introduction of two strongly deactivating --SO3Na groups into the aromatic nucleus lowers the absorption appreciably. This effect is less marked at the extremities of the curves. The absorption increases on the introduction of a third ortho-hydroxyl group. It would appear therefore, that deactivating groups tend to decrease the optical density whilst activating groups have the opposite effect. Table 1 shows the variation in optical density using increasing concentrations of molybdate in presence of a large excess of the phenols. Fig. 2, which is a typical example, indicates that Beer's law holds for all the systems. A comparison of the results under these conditions (i.e. in sufficient excess of phenol to ensure complete complexing of the molybdate), shows that a variation of the substituent groups on the aromatic nucleus causes little change in the optical density or visual colour of the complexes. This indicates that the transition of electrons which gives rise to the colour
Molybdic acid and its organic complexes--IV
135
o f t h e c o m p l e x is c e n t r e d a r o u n d t h e m o l y b d a t e a n i o n . T h e c o l o u r r e a c t i o n is sensitive t o relatively s m a l l c o n c e n t r a t i o n s o f m o l y b d a t e a n d l e n d s itself ideally to a n a l y t i c a l applications. I00,
._j,,
i
.
/
./ ,
~o
///
,
I
i" .--"'"
/' ,'
./4-/
.# !1
...'.~/;,( /'
,'
/
./
500
600
Wavelength,
m~
l.--Solutions 0-1 M in sodium acetate, 0-01 M in sodium molybdate and 0.001 M in polyhydric phenol. - - . - Tiron -protocatechiuc acid - - -"4-methylcatechol - [- I - gallic acid ........ catechol ....... pyrogallol
TABLE 1. - - - V A R I A T I O N
OF O P T I C A L D E N S I T Y W I T H M O L Y B D A T E C O N C E N T R A T I O N FOR S O L U T I O N S IN SODIUM ACETATE AND 0'015
NaMoO4
- --
../, / ,,"
400
FIG.
/"
/
.-?#/-2 , - " i/
,//
~
(M
x
104)
0
0"2
M
0-02 M
IN POLYHYDRIC PHENOL.
0'4
0'6
0-8
1"0
Optical densities at 450 mt~
Phenol
i
Catechol
0-02
0.09
0-16
0.23
0.30
I
0.37
4-methycatechol
0-02
0.10
0' 18
0.26
0.34
0-41
Protocatechuic acid
0.02
0.09
0.17
0.25
0.33
0.40
Tiron
0.03
0.10
0.18
0-26
0.34
0-42
Pyrogallo!
0.04
0.12
0.19
0.28
0"34
0.42
Gallic acid
0.02
0.09
0.16
024
0.32
0-39
T h e c o l o u r e d c o m p l e x e s are f o r m e d i m m e d i a t e l y , a n d d o n o t c h a n g e with temp e r a t u r e i n the r e g i o n 10°-40°C, a l t h o u g h at t e m p e r a t u r e s n e a r to the b o i l i n g p o i n t there is a reversible b l e a c h i n g effect. T h e o r a n g e - y e l l o w c o l o u r s f o r m e d in n e u t r a l s o l u t i o n s with c a t e c h o l a n d p y r o g a l l o l d e g e n e r a t e w i t h i n a few h o u r s to a m u d d y b r o w n d u e to o x i d a t i o n . W i t h p r o t o c a t e c h u i c acid a n d 4 - m e t h y l c a t e c h o l this o x i d a t i o n
136
H. BUCHWALD a n d E. RICHAgDSON
0"6
/
05
0.4
03
c>2
0-2
0"4
0.6
0.8
I'0
No2MoO4 concenfrofion, exlO4
FiG. 2.--Applicability o f Bcer's Law for the gallic acid-molybdat~-s.gstem. Solutions 0.02 M in sodium acetate. 0.015 M in gallic acid. And containing various concentrations of sodium molybdat¢
P 0 o,J
T
% q x
l!
(J
I//
L!
'
1
02
I
0.4
0,6
0.8
1.0
x
Fie. 3 . - - Q = resorcinol | ..... hydroquinane ~ ]aenucal with simple molybdic acid-water solution. × = pyrogallol A = gallic acid. x = Mole fraction molybdic acid, where [phenol] + [molybdic acid] : 0.0425 molar,
Molybdic acid and its organic c o m p l e x e s - - I V
137
takes somewhat longer. With gallic acid the colour remains unchanged for two or three da2¢swhilst with Tiron the colour is completely stable. All the stock solutions of the phenols, though colourless at first, turn yellow to pink in colour within periods ranging from a few hours to several days, depending on the ease of atmospheric oxidation. !
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:
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.
.
.
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o~ X
.~',
~
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.
! ~ ~i;"I' ! ,o4o6o.8
)•
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I
/I
A i
i~/-
/e'o
I.o
o
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W.-~--'o.i.
0-2
'
<2>
__~_._____+_
•
o8
e" o.
.e. °
o o.z o . 4 o . o
o,8
,o
X
FIG. 4 . - - X -- Mole fraction of polyhydric phenol such that [phenol] + [molybdate] = 0-01 molar. Solutions 0-05 M in sodium acetate. Optical densities at 20°C and various wavelengths. 1--Catechol at 480 m/~. A - - p H 5.0; B---pH 4"0; C - - p H 3"0; D - - p H 2'0. 2--Gallic acid at 530 m/z. A---pH 5.4-6.4; B---pH 4.2; C - - p H 1-8: D - - p H 1.3, 3--Pyrogallol at 550 m/t. A - - p H 7.0; B---pH 4.4; C - - p H 1'9. 4--Protocatechuic acid at 530 m/t. A - - p H 5.3~5.3; B - - p H 1.7, 5--Tiron at 520 m~. A - - p H 6.5-7.5; B---pH 4.1; C - - p H 1-8. 6--4-methylcatechol at 530 nap,. A - - p H 7.0; B---pH 1-8.
In order to determine the composition of the complexes a conductometric method of continuous variations, similar to that used in previous studies, was tried in the first instance. Fig. 3 shows the curves obtained using various phenols in conjunction with pure molybdic acid (prepared by an ion-exchange technique), t6) The curve for the resorcinol-molybdic acid and the hydroquinone-molybdic acid systems is the same as that obtained for molybdic acid on its own. The other curves shown are those for the pyrogallol and gallic acid systems. The curve for catechol (not shown) i~similar to that for pyrogaUol. Although complex formation is indicated for the last three phenols mentioned, no definite information can be gained about the composition of the complexes. JOB'S t14) method of continuous variations for the systems was investigated and the curves obtained are shown in Fig. 4. The spectrophot0metric measurements were (14) p~ JOB, Ann. Chim. 6, 97 (1935).
138
H. Buca~J..D and E. RICR~,.mON
carried out at various pH values ranging from one to seven. In the pH region from four to seven all the phenols form complexes with a phenol to molybdate ratio of 2 : 1. The results at lower pH values are more varied. With catechol there is no 0.7
7
0.6
0.5
4
O4
:Z:
Q.
pH
09 o.
O 02
~i det~ity at
0
oooz
ooo4
490ml.L
ooo6
Molority of acid,
ooo8
o(~o
o olz
HC L04
FIG. 5.--pH and optical density variation with molarity of acid present. Solutions: 0.002 M in sodium molybdate 0.002 M in catechol. 1.0
/pH2.0
>,
0"6
"c ~
pH I-4-
0
02
04
06
0"8
I-0
X
Fzo.
6.--x
= mole fraction of salicyclic acid. [molybdate] -t- [salicyclic acid]= 0.02 molar. Solutions 0.05 molar in sodium acetate. Optical densities at 2 0 ° C .
change in complex composition as pH is lowered, but all the other phenols show considerable variation. At pH 2 the gallic acid and pyrogallol complexes appear to have a phenol to molybdate ratio of 1 : 2, whilst with the remaining phenols the
Molybdic acid and its organic complexes--IV
139
composition varies according to the substituent groups. Comparing the "pH/acid added" and the "optical density/acid added" curves for catechol (Fig. 5), although the composition does not change, it appears that the complex formation is dependent in some way on the state of aggregation of the molybdate anion. Further work is obviously necessary to elucidate the change in composition of the complexes with varying acidity. Colorimetric work in the alkaline range has not been attempted because of the gross interference by other coloured products. In analytical work the interference by coloured oxidation products can be eliminated by the addition of a conventional stabiliser such as sodium sulphite, m) For comparison, Fig. 6 shows similar curves for the molybdate-salicylic acid complex. At pH 7 the solutions are colourless, it is only on acidification that a pale yellow colour is formed. In order to obtain comparable optical densities a wavelength of 375 m/~ was used. Lower wavelengths could not be used due to interference by the molybdate absorption band. It is evident that a different series of complexes is formed. When an 0.1 M solution ofcatechol was added to 50 ml of 0"1 M sodium molybdate there was a steady rise in pH from a value of 6.95 to a maximum of 7.35 when approximately 20 ml of catechol solution had been added. A further addition of 20 ml produced a steady fall in pH of 0.1 unit. This rise in pH on addition of a weak acid indicates that hydroxyl ions are liberated during complex formation, which probably occurs as follows: 2C6H4(OH)2 + Na2MoO 4 ~ (MoO4(C6H~)2)2+ q- 2 Na + q- 4 OHFurther work on the phenols mentioned and on others is in progress and will be reported in due course.
MOLYBDIC
ACID
AND
ITS O R G A N I C
COMPLEXES--IV
COMPLEXES OF MOLYBDATE WITH POLYHYDRIC PHENOLS-A SPECTROPHOTOMETRIC STUDY H . BUCHWALD a n d E . RICHARDSON
Department of Chemistry, Whitehaven College of Further Education Cumberland (Received 26 October 1959)
Abstract--The molybdate ion in aqueous solution forms intense yellow to orange-brown complexes with polyhydric phenols. Using a spectrophotometric method it is shown that at p H 7 the ratio of polyhydric phenol to molybdate in the complex is 2 : 1. A t lower p H values the composition of the complexes becomes variable, and changes according to the other substituent groups on the aromatic nucleus. Only the catechol complex retains a constant composition over the pH range studied. The complexes of six phenols are examined: catechol, protocatechuic acid, 4-methylcatechol, Tiron, pyrogallol and gallic acid. The complex formed with salycilic acid is examined for comparison.
THE reaction between acidified molybdate solutions and phenols has been known for a long time31, ~ Catechol and related compounds (in which there are at least two ortho hydroxyl groups) yield yellow to orange-brown complexes with molybdate solutions. This reaction does not take place with polyhydric phenols in which the hydroxyl groups are placed in meta or para positions to each other.tO,4~ Since aliphatic polyhydroxy compounds, with the hydroxyl groups on adjacent carbon atoms form colourless complexes with molybdate, ~5~ it appears that the formation of eoloured complexes is due to the aromatic nucleus. Some quantitative work on the nature and empirical formulae of the complexes formed by polyhydric phenols with other ions has been reported,t~, 7~and particularly noteworthy are the contributions of Yo~ c8~and SOMr,tER~s~ on the complexes formed by Fe s+ and Ti~- with Tiron (disodium 1,2-dihydroxybenzene-3,5-disulphonate ) and catechol. Apart from a short note, c9~various analytical applicationsClO,n, a~~ and a recent communication by us, txa~very little quantitative data seems to be available for the molybdenum complexes. The situation with molybdenum is more complicated since the reaction is with the molybdate anion and not with a cation as in the other complexes. The orange-yellow colours produced, however, are ~1~ D. H. KIH-~FER and A. LINZ, Molybdenum Compounds pp. 198-199. lnterscience, New York (1952). ~z~ L. FmtNAND~, Gazz. Chim. Ital. 55, 424 (1925). ts~ L. S o ~ , Coll. Czech. Chem. Comm. 22, 414, 1793 (1957). t4) A. T. C~srl and A. G. MADDOCK, I. Inorg. NucL Chem. 10, 58 (1959). ~6~ E. RICrlAgDSOIq, J. Inorg. N u d . Chem. 9, 273 (1959). ~6J D. D. P~glUN, Nature, Land. 182, 741 (1958). ~ J. B~RRUM, G. SCHWAgZgNI~CH and L. G. SILLI~N, Stability Constants--Part L Organic Ligands. Chemical Society Special Publication No. 6, London (1957). ~a~j. H. YoE and A. L. Jab'D, Industr. Engng. Chem. (Anal.) 16, 111 (1944). c,~ j. C. M c G o w ~ and P. W. BRIAN, Nature, Land. 159, 373 (1947). tle~ S. Smrrmt and B. Novlc, Analyt. Chem. 23, 188 (1951). tm F. WIta. and J. H. YOE, Analyt. Chlm. Acta 8, 546 (1953). txz~ S. YA SCHNAIDmtt~N and I. B. Rom~aovx Analyt. Abstr. 6, No. 18 (1959). txs~ H. BUCHWM.D and E. Racl-mgDSOrq, Chem. & Ind. 753 (1959). 133
134
H. BUCHWAI.D and E. RtCHAgDSON
particularly suitable-for spectrophotometric studies, al~ In this paper the spectrophotometric studies with six polyhydric phenols are reported: catechol, 4-methylcatechol protocatechuic acid (3,4-dihydroxybenzoic acid), Tiron, pyrogallol and gallic acid. EXPERIMENTAL Standard solutions of sodium molybdate were prepared from an analysed batch of solid. Stock solutions 0"1 M and 0-02 M in Na2MoO4 were generally used. All the phenols used were obtained commercially, in the analytical reagent grade whenever possible. Where the analytical reagent grade was unavailable the materials were recrystallised from a suitable solvent. Aqueous stock solutions 0'02 M and 0-05 M were prepared as required, but were not kept for any appreciable time since discoloration occurred in most cases after only a few hours. Sodium acetate was used as a semi-buffer solution and to maintain a relatively constant ionic strength. A solution of perchloric acid was generally used to adjust the pH. A stock solution approximately 1.4 M was prepared from the A.R. grade 60 per cent acid. The working solutions on which measurements were carried out were prepared from suitable quantities of the above reagents'and kept in well stoppered flasks, pH measurements were made with a glass-calomel electrode system calibrated to read 4"00 at 20°C with 0.05 M potassium hydrogen phthalate. This calibration was checked at pH 7.00 with a boric acid-sodium borate buffer and at pH 2.54 with 0.01 M potassium tetroxalate. Suitable glass electrodes were available for solutions of low and high pH values. Conductivity measurements were made on a Mullard conductivity bridge operating in the most sensitive range, with a maximum error of ± 2 % . A rigid dip-type cell having a constant 1.54 was used; this cell could be sealed into the reaction vessel where necessary in order to minimize atmospheric oxidation. The spectrophotometric work was carried out in 1 cm cuvettes using Unicam SP 500 and Hilger Uvispek spectrophotometers. All measurements were made at 20 + 2°C. A thermostatic cell carrier was available for the Unicam spectrophotometer but was not used since a preliminary investigation showed that a temperature fluctuation of ± 5 ° C had no significant effect. RESULTS
AND DISCUSSION
Fig. 1 shows the absorption spectra of the complexes obtained with the six phenols under similar conditions. The percentage transmission curve is used since it emphasizes the differences. With all the phenols except Tiron the maximum absorption occurs at a wavelength of about 400 m/~. At wavelengths below 400 m/~ no definite peaks are found in the curves, only a broad absorption band. It will be noted that the absorption spectra of the catechol, 4-methylcatechol and protocatechuic acid complexes are practically identical. At any particular wavelength, the introduction of two strongly deactivating --SO3Na groups into the aromatic nucleus lowers the absorption appreciably. This effect is less marked at the extremities of the curves. The absorption increases on the introduction of a third ortho-hydroxyl group. It would appear therefore, that deactivating groups tend to decrease the optical density whilst activating groups have the opposite effect. Table 1 shows the variation in optical density using increasing concentrations of molybdate in presence of a large excess of the phenols. Fig. 2, which is a typical example, indicates that Beer's law holds for all the systems. A comparison of the results under these conditions (i.e. in sufficient excess of phenol to ensure complete complexing of the molybdate), shows that a variation of the substituent groups on the aromatic nucleus causes little change in the optical density or visual colour of the complexes. This indicates that the transition of electrons which gives rise to the colour
Molybdic acid and its organic complexes--IV
135
o f t h e c o m p l e x is c e n t r e d a r o u n d t h e m o l y b d a t e a n i o n . T h e c o l o u r r e a c t i o n is sensitive t o relatively s m a l l c o n c e n t r a t i o n s o f m o l y b d a t e a n d l e n d s itself ideally to a n a l y t i c a l applications. I00,
._j,,
i
.
/
./ ,
~o
///
,
I
i" .--"'"
/' ,'
./4-/
.# !1
...'.~/;,( /'
,'
/
./
500
600
Wavelength,
m~
l.--Solutions 0-1 M in sodium acetate, 0-01 M in sodium molybdate and 0.001 M in polyhydric phenol. - - . - Tiron -protocatechiuc acid - - -"4-methylcatechol - [- I - gallic acid ........ catechol ....... pyrogallol
TABLE 1. - - - V A R I A T I O N
OF O P T I C A L D E N S I T Y W I T H M O L Y B D A T E C O N C E N T R A T I O N FOR S O L U T I O N S IN SODIUM ACETATE AND 0'015
NaMoO4
- --
../, / ,,"
400
FIG.
/"
/
.-?#/-2 , - " i/
,//
~
(M
x
104)
0
0"2
M
0-02 M
IN POLYHYDRIC PHENOL.
0'4
0'6
0-8
1"0
Optical densities at 450 mt~
Phenol
i
Catechol
0-02
0.09
0-16
0.23
0.30
I
0.37
4-methycatechol
0-02
0.10
0' 18
0.26
0.34
0-41
Protocatechuic acid
0.02
0.09
0.17
0.25
0.33
0.40
Tiron
0.03
0.10
0.18
0-26
0.34
0-42
Pyrogallo!
0.04
0.12
0.19
0.28
0"34
0.42
Gallic acid
0.02
0.09
0.16
024
0.32
0-39
T h e c o l o u r e d c o m p l e x e s are f o r m e d i m m e d i a t e l y , a n d d o n o t c h a n g e with temp e r a t u r e i n the r e g i o n 10°-40°C, a l t h o u g h at t e m p e r a t u r e s n e a r to the b o i l i n g p o i n t there is a reversible b l e a c h i n g effect. T h e o r a n g e - y e l l o w c o l o u r s f o r m e d in n e u t r a l s o l u t i o n s with c a t e c h o l a n d p y r o g a l l o l d e g e n e r a t e w i t h i n a few h o u r s to a m u d d y b r o w n d u e to o x i d a t i o n . W i t h p r o t o c a t e c h u i c acid a n d 4 - m e t h y l c a t e c h o l this o x i d a t i o n
136
H. BUCHWALD a n d E. RICHAgDSON
0"6
/
05
0.4
03
c>2
0-2
0"4
0.6
0.8
I'0
No2MoO4 concenfrofion, exlO4
FiG. 2.--Applicability o f Bcer's Law for the gallic acid-molybdat~-s.gstem. Solutions 0.02 M in sodium acetate. 0.015 M in gallic acid. And containing various concentrations of sodium molybdat¢
P 0 o,J
T
% q x
l!
(J
I//
L!
'
1
02
I
0.4
0,6
0.8
1.0
x
Fie. 3 . - - Q = resorcinol | ..... hydroquinane ~ ]aenucal with simple molybdic acid-water solution. × = pyrogallol A = gallic acid. x = Mole fraction molybdic acid, where [phenol] + [molybdic acid] : 0.0425 molar,
Molybdic acid and its organic c o m p l e x e s - - I V
137
takes somewhat longer. With gallic acid the colour remains unchanged for two or three da2¢swhilst with Tiron the colour is completely stable. All the stock solutions of the phenols, though colourless at first, turn yellow to pink in colour within periods ranging from a few hours to several days, depending on the ease of atmospheric oxidation. !
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FIG. 4 . - - X -- Mole fraction of polyhydric phenol such that [phenol] + [molybdate] = 0-01 molar. Solutions 0-05 M in sodium acetate. Optical densities at 20°C and various wavelengths. 1--Catechol at 480 m/~. A - - p H 5.0; B---pH 4"0; C - - p H 3"0; D - - p H 2'0. 2--Gallic acid at 530 m/z. A---pH 5.4-6.4; B---pH 4.2; C - - p H 1-8: D - - p H 1.3, 3--Pyrogallol at 550 m/t. A - - p H 7.0; B---pH 4.4; C - - p H 1'9. 4--Protocatechuic acid at 530 m/t. A - - p H 5.3~5.3; B - - p H 1.7, 5--Tiron at 520 m~. A - - p H 6.5-7.5; B---pH 4.1; C - - p H 1-8. 6--4-methylcatechol at 530 nap,. A - - p H 7.0; B---pH 1-8.
In order to determine the composition of the complexes a conductometric method of continuous variations, similar to that used in previous studies, was tried in the first instance. Fig. 3 shows the curves obtained using various phenols in conjunction with pure molybdic acid (prepared by an ion-exchange technique), t6) The curve for the resorcinol-molybdic acid and the hydroquinone-molybdic acid systems is the same as that obtained for molybdic acid on its own. The other curves shown are those for the pyrogallol and gallic acid systems. The curve for catechol (not shown) i~similar to that for pyrogaUol. Although complex formation is indicated for the last three phenols mentioned, no definite information can be gained about the composition of the complexes. JOB'S t14) method of continuous variations for the systems was investigated and the curves obtained are shown in Fig. 4. The spectrophot0metric measurements were (14) p~ JOB, Ann. Chim. 6, 97 (1935).
138
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carried out at various pH values ranging from one to seven. In the pH region from four to seven all the phenols form complexes with a phenol to molybdate ratio of 2 : 1. The results at lower pH values are more varied. With catechol there is no 0.7
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change in complex composition as pH is lowered, but all the other phenols show considerable variation. At pH 2 the gallic acid and pyrogallol complexes appear to have a phenol to molybdate ratio of 1 : 2, whilst with the remaining phenols the
Molybdic acid and its organic complexes--IV
139
composition varies according to the substituent groups. Comparing the "pH/acid added" and the "optical density/acid added" curves for catechol (Fig. 5), although the composition does not change, it appears that the complex formation is dependent in some way on the state of aggregation of the molybdate anion. Further work is obviously necessary to elucidate the change in composition of the complexes with varying acidity. Colorimetric work in the alkaline range has not been attempted because of the gross interference by other coloured products. In analytical work the interference by coloured oxidation products can be eliminated by the addition of a conventional stabiliser such as sodium sulphite, m) For comparison, Fig. 6 shows similar curves for the molybdate-salicylic acid complex. At pH 7 the solutions are colourless, it is only on acidification that a pale yellow colour is formed. In order to obtain comparable optical densities a wavelength of 375 m/~ was used. Lower wavelengths could not be used due to interference by the molybdate absorption band. It is evident that a different series of complexes is formed. When an 0.1 M solution ofcatechol was added to 50 ml of 0"1 M sodium molybdate there was a steady rise in pH from a value of 6.95 to a maximum of 7.35 when approximately 20 ml of catechol solution had been added. A further addition of 20 ml produced a steady fall in pH of 0.1 unit. This rise in pH on addition of a weak acid indicates that hydroxyl ions are liberated during complex formation, which probably occurs as follows: 2C6H4(OH)2 + Na2MoO 4 ~ (MoO4(C6H~)2)2+ q- 2 Na + q- 4 OHFurther work on the phenols mentioned and on others is in progress and will be reported in due course.