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Structure of the zirconium-alizarin s complex in relation to pH changes
ANALYTICA
220
STRUCTURE
OF THE
CHIMXCA
ACTA
ZIRCONIUM-ALI~ARIN TO pH CHANGES
S COMPLEX
IN RELATION
The reaction of fluoride ions with the colored complex of zirconium with alizarin St--7 was considcrcd, after preliminary investigations, as the most appropriate spectrophotometric method for the determination of fluoride. However, before a satisfactory procedure for the &termination could be devised, it was considered essential to make a thorough study of the zirconium-alizarin S reaction itself. A study of the cxperimental conditions under which the above reaction occurs gave the following results: (a) The zirconium-alizarin S complex is entirely unstable in certain pw ranges. (It) In other PH ranges small changes in the hydrogen ion concentration cause modification of the structure of the complex. (c) The proper ratio of zirconium to alizarin S in the complex for the fluoride dctermination required further investigation. The purpose of the present work was to establish the proper ratio of zirconium to alizarin S in the complex as well as to investigate the modifications which the complex undergoes even with small changes in PH.
All reagents were obtained from Merck (p. A. grade). %irco~titr~r solutiotz. A IO-" M solution of ZrOCl~~8H20 was prepared, The conccntrntion of the solution was checked by gravimetric determination of zirconium as B-02. Alizavi?r S solution. A x0-4 M solution of sodium alizarin-3-sulfonatc was prepared. For the adjustment of pi, I : r mixtures of llydr~~~l~l(~ric and sulfuric acids 0.3 N, 3 N and IO N, and 0.x and I N sodium hydroxide were used.
Spcctrophotomctric measurcmcnts were made with a Reckman I)U spectrophotometer ancl xo-mm Corcx cells. PH measurements were carried out with a “Radiometer zz” (Radiometer Copenhagen Co.) and a glass electrode 202 A. The flltration was carried out with platinum Ncubauer Heraeus type filters. Methods For obtaining
the curves of Fig. r the following
proccdurc
was used. The alizarin
ZIRCONXUhf-ALIZARIN
s COMPLEX
AND
pH
CHANGES
22x
of acid or alkali: the solution was left for x h in a thermostat at 25” and then the spcctrophotometric measurements were made. For the Job curves, portions of the zirconium and aiizarin S solutions were separately adjusted to the desired pn, within the range o+6.0. From these solutions, x ml and (I --A$ ml were mixed to form the series. The mixtures were placed in a thermostat at z$’ for T h and then measured spectrophotometricaliy. S solution
was
adjusted
to
the
desired
pH
vaIue
with
a few
drops
br
QJ
These were the curves for the unfiltered samples. For the filtered series, the same procedure was followed oxccpt that the samples wcrc filtered before spectrophotomctry. The absence of suspensions was checked by the “I’yndall effect. RESULTS
The visible spectra (Gq-350 rnp) of the pure alizarin S solution (10-4 Al) wcrc examined over the PH range r to 7 (Fig. x), Between these PH limits the alizarin S color changes clearly from yellow to red. It can be seen from Fig. I that when the pn increases a shift of the maximum occurs gradually from 420 rnp to 520 mp. This displacement of the maximum can be explained Ard.
Ciiim.
Aclu,
29
(rgcig)
zzo-22fi
222
G. PARISSAKlS
AND J. KONTOYANNAKOS
as follows. At low PH values, alizarin S exists in the unionized yellow form (Fig. 2,a). As the pw increases, the alizarin S ionizes giving the mcsomeric forms (b) and (c) which correspond to the u-phenoxyquinoid form8 and are responsible for the red color of the alizarin S molecule. The curves shown in Fig. 3 indicate that these mesomcric forms are also responsible for the formation of the colored complex of zirconium and alizarin S. These curves were obtained by leaving a mixture of zirconium and ulizurin S at constant PH to
f-yyj;;-
bob
* d
O-
(b) Fig.
2. (a) Tlw unionizctl
form of aliznrin
0.400-
(cl S; (la) and (c) icn+al
--_-Zr-Alti .
0.350
-
0.300
-
0.23cJ
-
o-o.-~Zr-Alit -----Zr-All2
-._..m._ AlIz
S S
formu
of (a).
tr
S S
t.4lOh t.: 1-10h
O.?OOto+
T 0.130
-
0.100
*
0.50
-
3’00 Fig.
3. Shift
350
of the maximum
400
absorbance
450
of alizarin
500
550
S after addition Atid.
Ckint.
600
of zirconium Acfa.
29
ions. (SgG3)
220-226
ZIRCONXUM-ALIZARIN
s
COMPLEX
AND
pH
CHANGES
223
for IO h in a t~l~~ostat at 25’ ; the absorbance was measured every hour over the range 625-350 mc/. The first measurement (Fig. 3, curve to) was done z h after mixing the alizarin S solution with the zirconium solution. The absorption curve of atizarin S before the addition of the zirconium solution is also shown. ‘I’hc same shift of the maximum from 420 rnp to 520 rnp is observed with increasing time (Fig. 3, curve to + IO h) as in the case of increasing PH. Therefore, in accordance with the mcsomeric forms shown in Fig. 2, the forms shown in Fig. 4 for the zirconium-alizarin S complex may be accepted. develop
To investigate the stoichiomctric ratio of the complex components, Job’s method of continuous variations was applied, Measurements were made at 420 myc and 520 rnp with pH changes from 0.5 to 6, in steps of o.2 pw units in the range 0.5-2.0 and in steps of 1: pH unit in the range 2.0-0.0, with constant time intervals of I h bctwccn measurements at 25”. When the solutions of each series were prepared above a certain pH value, a suspension was obscrvcd in the solution @; this suspension coagulated and settled after several hours or after centrifugation, but even then the solution was not perfectly clear. ‘fhercforc, the optical de&tics were measured cithcr on the solution of the complex of zirconium and alizarin S obtained after mixing the appropriate amountsof the components as Job’s method rccluires, or after filtration of the solution through a porous platinum filter to remove the suspension formed. ‘fhc results for the filtered series over the pH range 0.5-2.0 arc shown in Fig. 5. Table I shows the resulting proportions for each maximum or minimum (520 rnp and 420 rnp, rcspcctivcly) for filtered or unfiltered solutions from pH 0.5 to 2.0.
From the curves shown in Fig 5 and from Table I it is evidcnl that the formation of the zirconium-alizarin S complex is mcasurablconly in the PN range from I to x.8. For pH vrrlucs below +, no complex formation could be observed. Uetwccrt pw 1.8 and 3, the mixtures of zirconium and aliznrin S formed suspensions which were checked by the Tyndall effect. Above pi 3, voluminous suspensions were formed and caused irregular mcnsuremcnts; for this reason the results in this PH region arc not given in Table I. Table I shows that from per I to x.8 the measurements in the filtered and unfiltered series were identical with regard to the ratio of zirconium to alizarin S, At PH x.8 the proportionality in the Job series changed from 50 : 50 to Go :40 for both series. At pH 2 the measurements on the unfiltered series at 520 rnp showed a ratio of 60 : 40 and no result at 4~0 ml”; for the filtered series at 520 rnp no result was found, at 420 rnp the ratio was Go :40. The above facts can be explained as follows. Above pH 2, Anat.
China. Acta,
29
(sgG3)
220-22G
C. PAI~ISSAKIS
224
AND
J. KONTOYANNAKOS
insoluble hydroxy complexes of zirconium begin to be formed 10,and these complexes adsorb some of the zirconium-alizarin S complex, as well as some unreactcd alizarin S. Therefore, for the unfiltered series, the cliffercncc between the optical densities of the
DHao,5
pHa7
p&l
Atiz
Fig. 5. Job’s ecrics for the zirconium-nlizorin
S
S compound
nt different
Arrut. Ciritu. Ada,
ptr vnluca. ‘rg (lgG3)
220-226
TABLE I MOLAR RATIOS OF ZIRCOSlC’M TO ALIZARIS
S I$ PH R.4SGE0. j-2
-_
0.5
PH
__.-.--.----
0.7
.\klS.~
(-) -
Zr : Aliz. S
1-)
______---~__
I
.-_._
(--)
._I_
f-f
--
_-.
I.2 - ._. __-._._ --
JIin. jO:jO
Max. jO:jO
Jlin, jO:jO
..---
-
1.4
x.6 -.
_._-
-F-P
‘2
I.8
------..
Min. Max.
Has. jOYjO
_ __.-_-_-
jO:jO
jO:jO
Xin. jO:jO
Max. GO:40
Min. Max. Min. GO:.fO
OO:.iO(-)
____-
_ __
Min. Mas.
--
~r?J,fiffzJ’tdSdJ’iL’I _..__-. ..--
JIin. Jlils. (--) jO:jO
J1in.b hs. (-) f-) -_.-.---
Sfas.
_-
~1
---
Zr : Ah. S
__-.
___
Min. Max. (-)
..-_.--_._--
* Nas. = Jlcasurcmcnts at jr0 mp. b Min. = Slcasuremcnts at $10 mp. (-) = Seither Max. nor Min. could be measured.
jO:jO
Fihred series _p__l.__-_----.
JIin. jO:jO __~__
Max. jO:jO
Yin. jO:jO
_ ---
Max. SOljO
_ __.._ _.__.___
Min. JIax. jO:jO _,...l_
jO:jO -__.
----_I-. Min. jO:jO __-
.Uax. 6O:fO
Sfin, Max. Min. GO:.#O
(-)60:.&O
z2G
C;. I’AKXSSAKIY
AND
J. KONTOYASNhKOS
zirconium-alizarin S complex and the pure alizarin S is positive and a maximum is given at 520 rnp, \vhcrcas for the filtered s&es the values for the complex and the pure due arc almost the same and cancel out. For the filtered series, &Wg to the diminution of the ctrncentrtktion of the srAution with respect to the zirconium-alizarin S complex and the pure dye are quite similar iit 520 rilp the difference in values is larger anti thcreforc ;i minimum occurs. Thus only in the pH range x to I.8 is it possible to measure: ii pure complex of zirconium anti atiXarin S in the ratio s : I. Above 3 psi value of 1.8, some of the I : 1 complex is adsorl,ccl on the surface of the susponclcd hydroxy complexes of zirctmiurn.
220
STRUCTURE
OF THE
CHIMXCA
ACTA
ZIRCONIUM-ALI~ARIN TO pH CHANGES
S COMPLEX
IN RELATION
The reaction of fluoride ions with the colored complex of zirconium with alizarin St--7 was considcrcd, after preliminary investigations, as the most appropriate spectrophotometric method for the determination of fluoride. However, before a satisfactory procedure for the &termination could be devised, it was considered essential to make a thorough study of the zirconium-alizarin S reaction itself. A study of the cxperimental conditions under which the above reaction occurs gave the following results: (a) The zirconium-alizarin S complex is entirely unstable in certain pw ranges. (It) In other PH ranges small changes in the hydrogen ion concentration cause modification of the structure of the complex. (c) The proper ratio of zirconium to alizarin S in the complex for the fluoride dctermination required further investigation. The purpose of the present work was to establish the proper ratio of zirconium to alizarin S in the complex as well as to investigate the modifications which the complex undergoes even with small changes in PH.
All reagents were obtained from Merck (p. A. grade). %irco~titr~r solutiotz. A IO-" M solution of ZrOCl~~8H20 was prepared, The conccntrntion of the solution was checked by gravimetric determination of zirconium as B-02. Alizavi?r S solution. A x0-4 M solution of sodium alizarin-3-sulfonatc was prepared. For the adjustment of pi, I : r mixtures of llydr~~~l~l(~ric and sulfuric acids 0.3 N, 3 N and IO N, and 0.x and I N sodium hydroxide were used.
Spcctrophotomctric measurcmcnts were made with a Reckman I)U spectrophotometer ancl xo-mm Corcx cells. PH measurements were carried out with a “Radiometer zz” (Radiometer Copenhagen Co.) and a glass electrode 202 A. The flltration was carried out with platinum Ncubauer Heraeus type filters. Methods For obtaining
the curves of Fig. r the following
proccdurc
was used. The alizarin
ZIRCONXUhf-ALIZARIN
s COMPLEX
AND
pH
CHANGES
22x
of acid or alkali: the solution was left for x h in a thermostat at 25” and then the spcctrophotometric measurements were made. For the Job curves, portions of the zirconium and aiizarin S solutions were separately adjusted to the desired pn, within the range o+6.0. From these solutions, x ml and (I --A$ ml were mixed to form the series. The mixtures were placed in a thermostat at z$’ for T h and then measured spectrophotometricaliy. S solution
was
adjusted
to
the
desired
pH
vaIue
with
a few
drops
br
QJ
These were the curves for the unfiltered samples. For the filtered series, the same procedure was followed oxccpt that the samples wcrc filtered before spectrophotomctry. The absence of suspensions was checked by the “I’yndall effect. RESULTS
The visible spectra (Gq-350 rnp) of the pure alizarin S solution (10-4 Al) wcrc examined over the PH range r to 7 (Fig. x), Between these PH limits the alizarin S color changes clearly from yellow to red. It can be seen from Fig. I that when the pn increases a shift of the maximum occurs gradually from 420 rnp to 520 mp. This displacement of the maximum can be explained Ard.
Ciiim.
Aclu,
29
(rgcig)
zzo-22fi
222
G. PARISSAKlS
AND J. KONTOYANNAKOS
as follows. At low PH values, alizarin S exists in the unionized yellow form (Fig. 2,a). As the pw increases, the alizarin S ionizes giving the mcsomeric forms (b) and (c) which correspond to the u-phenoxyquinoid form8 and are responsible for the red color of the alizarin S molecule. The curves shown in Fig. 3 indicate that these mesomcric forms are also responsible for the formation of the colored complex of zirconium and alizarin S. These curves were obtained by leaving a mixture of zirconium and ulizurin S at constant PH to
f-yyj;;-
bob
* d
O-
(b) Fig.
2. (a) Tlw unionizctl
form of aliznrin
0.400-
(cl S; (la) and (c) icn+al
--_-Zr-Alti .
0.350
-
0.300
-
0.23cJ
-
o-o.-~Zr-Alit -----Zr-All2
-._..m._ AlIz
S S
formu
of (a).
tr
S S
t.4lOh t.: 1-10h
O.?OOto+
T 0.130
-
0.100
*
0.50
-
3’00 Fig.
3. Shift
350
of the maximum
400
absorbance
450
of alizarin
500
550
S after addition Atid.
Ckint.
600
of zirconium Acfa.
29
ions. (SgG3)
220-226
ZIRCONXUM-ALIZARIN
s
COMPLEX
AND
pH
CHANGES
223
for IO h in a t~l~~ostat at 25’ ; the absorbance was measured every hour over the range 625-350 mc/. The first measurement (Fig. 3, curve to) was done z h after mixing the alizarin S solution with the zirconium solution. The absorption curve of atizarin S before the addition of the zirconium solution is also shown. ‘I’hc same shift of the maximum from 420 rnp to 520 rnp is observed with increasing time (Fig. 3, curve to + IO h) as in the case of increasing PH. Therefore, in accordance with the mcsomeric forms shown in Fig. 2, the forms shown in Fig. 4 for the zirconium-alizarin S complex may be accepted. develop
To investigate the stoichiomctric ratio of the complex components, Job’s method of continuous variations was applied, Measurements were made at 420 myc and 520 rnp with pH changes from 0.5 to 6, in steps of o.2 pw units in the range 0.5-2.0 and in steps of 1: pH unit in the range 2.0-0.0, with constant time intervals of I h bctwccn measurements at 25”. When the solutions of each series were prepared above a certain pH value, a suspension was obscrvcd in the solution @; this suspension coagulated and settled after several hours or after centrifugation, but even then the solution was not perfectly clear. ‘fhercforc, the optical de&tics were measured cithcr on the solution of the complex of zirconium and alizarin S obtained after mixing the appropriate amountsof the components as Job’s method rccluires, or after filtration of the solution through a porous platinum filter to remove the suspension formed. ‘fhc results for the filtered series over the pH range 0.5-2.0 arc shown in Fig. 5. Table I shows the resulting proportions for each maximum or minimum (520 rnp and 420 rnp, rcspcctivcly) for filtered or unfiltered solutions from pH 0.5 to 2.0.
From the curves shown in Fig 5 and from Table I it is evidcnl that the formation of the zirconium-alizarin S complex is mcasurablconly in the PN range from I to x.8. For pH vrrlucs below +, no complex formation could be observed. Uetwccrt pw 1.8 and 3, the mixtures of zirconium and aliznrin S formed suspensions which were checked by the Tyndall effect. Above pi 3, voluminous suspensions were formed and caused irregular mcnsuremcnts; for this reason the results in this PH region arc not given in Table I. Table I shows that from per I to x.8 the measurements in the filtered and unfiltered series were identical with regard to the ratio of zirconium to alizarin S, At PH x.8 the proportionality in the Job series changed from 50 : 50 to Go :40 for both series. At pH 2 the measurements on the unfiltered series at 520 rnp showed a ratio of 60 : 40 and no result at 4~0 ml”; for the filtered series at 520 rnp no result was found, at 420 rnp the ratio was Go :40. The above facts can be explained as follows. Above pH 2, Anat.
China. Acta,
29
(sgG3)
220-22G
C. PAI~ISSAKIS
224
AND
J. KONTOYANNAKOS
insoluble hydroxy complexes of zirconium begin to be formed 10,and these complexes adsorb some of the zirconium-alizarin S complex, as well as some unreactcd alizarin S. Therefore, for the unfiltered series, the cliffercncc between the optical densities of the
DHao,5
pHa7
p&l
Atiz
Fig. 5. Job’s ecrics for the zirconium-nlizorin
S
S compound
nt different
Arrut. Ciritu. Ada,
ptr vnluca. ‘rg (lgG3)
220-226
TABLE I MOLAR RATIOS OF ZIRCOSlC’M TO ALIZARIS
S I$ PH R.4SGE0. j-2
-_
0.5
PH
__.-.--.----
0.7
.\klS.~
(-) -
Zr : Aliz. S
1-)
______---~__
I
.-_._
(--)
._I_
f-f
--
_-.
I.2 - ._. __-._._ --
JIin. jO:jO
Max. jO:jO
Jlin, jO:jO
..---
-
1.4
x.6 -.
_._-
-F-P
‘2
I.8
------..
Min. Max.
Has. jOYjO
_ __.-_-_-
jO:jO
jO:jO
Xin. jO:jO
Max. GO:40
Min. Max. Min. GO:.fO
OO:.iO(-)
____-
_ __
Min. Mas.
--
~r?J,fiffzJ’tdSdJ’iL’I _..__-. ..--
JIin. Jlils. (--) jO:jO
J1in.b hs. (-) f-) -_.-.---
Sfas.
_-
~1
---
Zr : Ah. S
__-.
___
Min. Max. (-)
..-_.--_._--
* Nas. = Jlcasurcmcnts at jr0 mp. b Min. = Slcasuremcnts at $10 mp. (-) = Seither Max. nor Min. could be measured.
jO:jO
Fihred series _p__l.__-_----.
JIin. jO:jO __~__
Max. jO:jO
Yin. jO:jO
_ ---
Max. SOljO
_ __.._ _.__.___
Min. JIax. jO:jO _,...l_
jO:jO -__.
----_I-. Min. jO:jO __-
.Uax. 6O:fO
Sfin, Max. Min. GO:.#O
(-)60:.&O
z2G
C;. I’AKXSSAKIY
AND
J. KONTOYASNhKOS
zirconium-alizarin S complex and the pure alizarin S is positive and a maximum is given at 520 rnp, \vhcrcas for the filtered s&es the values for the complex and the pure due arc almost the same and cancel out. For the filtered series, &Wg to the diminution of the ctrncentrtktion of the srAution with respect to the zirconium-alizarin S complex and the pure dye are quite similar iit 520 rilp the difference in values is larger anti thcreforc ;i minimum occurs. Thus only in the pH range x to I.8 is it possible to measure: ii pure complex of zirconium anti atiXarin S in the ratio s : I. Above 3 psi value of 1.8, some of the I : 1 complex is adsorl,ccl on the surface of the susponclcd hydroxy complexes of zirctmiurn.