- Email: [email protected]
Determination of purity of o-sulphobenzoic anhydride
Short communications
1592
Talanta. 1966, Vol. 13, pp. 1592 to 1595. Pergamon Press Ltd. Primed
Determination
in NorthernIreland
of purity of o-sulphobenzoic anhydride
(Received 26 April 1966. Accepted 24 July 1966) RECENTLY a method for determination of organic hydroxyl and primary and secondary amino corn__ pounds by esteriflcation with o-sulphobenzoic anhydride (SBA) was reported from these laboratories.%2 Since the presence of free acids in the anhydride samples is known to retard the rate of esterificationa.4 it was considered necessary to determine the purity of the SBA used. SBA is also an important intermediate in the synthesis of sulphonephthalein indicators. A survey of the literature on functional group analysis6 revealed that although several methods have been reported for determining the carboxylic anhydrides in the presence of the corresponding free acids, no methods have been reported for determining the purity of mixed sulphonic-carboxylic anhydrides such as SBA. One reason for this appears to be that except for SBA such mixed anhydrides are quite uncommon. The mixed anhydrides derived from open chain sulphonic and carboxylic acids are unstable and they easily disproportionate into the carboxylic and the sulphonic anhydrides. In the present communication some methods for determining the purity of SBA samples are described. They are similar to those reported for the determination of carboxylic anhydrides, and hence the experimental details have been omitted, but the limitations of each method are discussed. Determination of anhydrides is usually based on their reaction with compounds containing active hydrogen, i.e., amines or alcohols. The resulting compounds (amides and esters) are neutral, and the equivalent amount of acid formed in the reaction is a measure of the anhydride content of the sample. Since the anhydride samples invariably contain some free acid, it is generally desirable to determine the total acidity of the sample after hydrolysing the anhydride. In a recent method due to Sig@a and FloramoE the free acid in the anhydride sample has been determined directly by titration with a tertiary amine in a non-aqueous medium, and the anhydride determined by difference. This method is quite suitable for determining the purity of anhydrides containing only the parent acids as impurity.
Esterification-hydrolysis method’ Tf& method involves titration of the SBA sample in dry acetone with methanolic sodium methoxide, phenolphthalein being used as indicator. Like the carboxylic anhydrides, SBA also reacts to form the carboxylic ester-sodium sulphonate, whereas the free acid gives a disodium salt. The total acidity of the sample is determined by a separate titration with sodium hydroxide after a pyridinecah1yse.d hydrolysis. Alternatively the sodium methoxide solution can be. used for determining total acidity, and then only one standard solution is needed.
FIG. 1. Potentiometric
titration curve of free acid in SBA sample.
1593
Short communications 0
II
’\ I’“‘, \s/
+ CH,ONa
4
(1)
0
/\
0
0
0
/COOH ‘I 0\
+ 2 CH,ONa +
+ 2 CH,OH
‘I \
‘\SOBH
The purity of the anhydride
/COONa (2)
SOIINa
is calculated as usual. The method is quick and gives satisfactory results.
Methods based on anilide formation Methods based on the reactions of amines (aniline8 or morpholineg) to give the corresponding carboxylic acid amide, followed by the determination of excess of amine by non-aqueous acidimetric titration, were found not to be applicable, because of interference by the strongly acidic sulphonic acid group. Even Critchfield’s morpholine-carbon disulphide methodlo was found unsuitable because of the interference of the sulphonic acid group.
0 =
CH,-CH, ,‘co>o \soe
+ H,NC,H,
HN i
>o
< CH,-CH,
__f
(3)
1
,
/CONHC,H6
0\
\SO*H
CH,-CH, -N i
<
>O CH,CHs
1
The method based on the reaction of 2&dichloroanitine I1 to give the corresponding anilide, followed by bromometric determination of excess of aniline, can be used, however. Similarlyamethod based on hydrolysis and anilide formation” can also be used. In this method, the total acid produced by hydrolysis of an aliquot is determined, and the anhydride in another ahquot is converted into anilide by reaction with excess of aniline, and the acid in the resulting solution is determined. The difference of the two titrations gives the amount of acid formed in anilide formation, and this is equivalent to the anhydride content of the sample. Method based on determination of free aciak in anhydrides” Si@a and Floramo made an interesting application of non-aqueous titration by using tri-npropylamine (TPA) or N-ethylpiperidine as titrant to determine free acids in certain anhydride samples. The original method could be applied only to maleic and phthalic acid anhydrides because these acids are strong enough (p& < 3) to be determined by direct titration. A recent modification of this method by Whartonl* uses lithium chloride to enhance the acidity of weak acids, and most of the fatty acids can then be determined in mixtures with their anhydrides. Determination of the free acid in SBA samples by TPA titration in dry methyl ethyl ketone medium offered no difficulty because of the strongly acidic sulphonic acid group (PK. < 2) in the molecule. Potentiometric titration can be carried out with a Cambridge wide-range glass electrode and a standard calomel reference electrode with a methanol bridge saturated with sodium chloride. The end-point could also be detected with Crystal Violet as indicator. The end-point corresponds to the neutralisation of the sulphonic acid group. The carboxylic group does not titrate under the conditions used. A typical potentiometric titration curve for free sulphobenzoic acid in SBA is shown in Fig. 1. The acid and anhydride contents of synthetic mixtures prepared from the pure acid and anhydride were determined by this method and the results are presented in Table I. RESULTS
AND
DISCUSSION
The results for the determination of the purity of some SBA samples by these methods are given in Table II. It will be seen that comparable results are obtained by these methods. The esterificationhydrolysis method is particularly quick and simple, but equally satisfactory results are obtained by the
1594
Short communications TABLEI.-ANALYSIS OF SYNTHETIC MD(TURES OF SBA AND THE CORRESPONDING Acid content
1. 2. 3. 4. 5.
FREE
ACID
Anhydride content
taken meq
found mp9
recovery %
taken meq
found meq
recovery %
0448 0.386 0.375 0.409 0.452
0.450 0.385 0.373 0.408 0.454
100.4 99.1 99.5 98.1 100.5
0.335 0.420 0.316 0.283 0.403
0.332 0.418 0.314 0.282 0.401
98.9 99.5 99.3 99.6 99.5
TABLEII.-ANALYSIS OF SBA SAMPLESBY DIFFERENTME-~I-IODS Esterificationhydrolysis method’
Iodometric method using 2,4dichloroanilinell
Hydrolysisanilide formation1a
Method based on determination of free acid0
96.2 91.0 97.0 97.6 85.7
96.6 96.3 97.0 96.1 86.6
96.0 96.2 96.9 97.2 84.7
96.5 96.9 96.9 97.3 86.3
1. 2. 3. 4. 5. (uncrystallised)
titration of the free acid with TPA. The limitations of methods based on reactions with amines have already been discussed. Another methodI for determination of carboxylic anhydrides based on reaction with anhydrous oxalic acid was also found unsuitable because of partial decomposition of the oxalic acid even by the free u-sulphobenzoic acid. A calorimetric method16 based on formation of hydroxamic acids and their iron chelates can also bc used, particularly for the determination of low concentrations of the anhydride, but it is not preferable to other chemical methods when applied to pure samples of the anhydride. V. IYER N. K. MATHUR
Department of Chemistry University qf Jodhpur Jodhpur, India
Summary--A comparison is made of four methods for the determination of the anhydride and free acid content of impure o-sulphobenzoic anhydride. R&an&-On compare quatre methodes de dosage de la teneur en anhydride et acide libre del’anhydride o-sulfobenzoique impur. Zusammenfassung-Vier Methoden zur Bestimmung des Gehaltes von unreinem o-Sulfobenzoeslureanhydrid an Anhydrid und freier Siiure werden verglichen. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8.
V. Iyer and N. K. Mathur, Anal. Chim. Acta, 1965,33, 554. C. K. Narang, V. Iyer and N. K. Mathur, Microchem. J., 1965, 9,408. P. J. Elving and B. Warshowsky, Anal. Chem., 1947, 19,1006. C. L. Ogg, W. L. Porter and C. 0. Willits, Znd. Eng. Chem., Anal. Ed., 1945, 17, 394. C. W. Hammond in J. Mitchell, Organic Analysis, Vol. III, Interscience, New York, 1965. S. Siggia and N. A. Floramo, Anal. Chem., 1953, 25, 797. D. M. Smith and W. M. D. Bryant, J. Am. Chem. Sot., 1936,58,2452. S. Siggia and J. G. Hanna, Anal. Chem., 1951,23, 1717.
Short communications 9. 10. 11. 12. 13. 14. 15.
1595
J. B. Johnson-and G. L. Funk, ibid, 195527, 1464. F. E. Critchfield and J. B. Johnson, ibid., 1956, 28,430. H. Roth, Microchim. Acta, 1958, 767. L. G. Radcliffe and S. Mendofski, J. Sot. Chem. Ind, 1917,36,628. H. W. Wharton, Anal. Chem., 1965, 37, 730. C. K. Rosenbaum and J. H. Walton, J. Am. Gem. Sot., 1930,52,3368. R. F. Goddu, N. F. Le Blanc and C. H. Wright, Anal. Chem., 1955, 27,125l.
Talanta.1966.Vol. 13.pp. 1595to 1598. PcrgamottPressLtd. Printedin NorthernIreland
Behaviour of the platinum electrode as a pH electrode (Received 25 April 1966. Accepted 6 June 1966) ALL metal-metal oxide electrodes, in theory, can function as pH electrodes. the potential of such an electrode is E = E,’ + 0.059 log {H+}.
The equation governing
This is derived on the assumption that the metal forms an oxide or hydrous oxide, the solubility of which in the solution to be examined is negligibly small .I Almost all metal electrodes are covered by a layer of an oxide film and can function as pH electrodes. The efficiency of such metal pH electrodes will admittedly depend on, among other things, the solubility of the oxide or hydroxide formed on the surface of the electrode as well as the passivity of the electrode in that particular solution. The chief among the metal-metal oxide pH electrodes studied are antimony-antimony trioxide,* tungsten,8 and molybdenum electrodes.4 The platinum electrode is very extensively used as a redox indicator electrode in potentiometry. However, the platinum is also covered by an oxide layer,& so it is feasible that platinum may behave as a pH eiectrode in aqueous media. This fact is seldom realised. e This electrode has been used as a pH electrode in non-aqueous media.’ A 19 s.w.g. platinum electrode, (length 1.5 cm) in conjunction with a saturated calomel electrode was used for strong acid-strong base titrations, with a Cambridge bench pH meter to measure mV. The glass-calomel electrode pair was used on the same meter to measure pH and mV. The glass electrode was Cambridge Cat. No. 42518. The solution was stirred at constant speed by a magnetic stirrer. The strong acids used were 1.O and 0.1 N sulphuric and hydrochloric acids. They were titrated separately against the equivalent concentration of sodium hydroxide. The curves obtained for O.lN sulphuric acid-sodium hydroxide titrations for platinum~alomel and glasscalomel systems are shown in Fig. 1; and those for O.lN hydrochloric acid-sodium hydroxide titrations are shown in Fig. 2. The platinumcalomel curves are inverted relative to the glass-calomel curves because the polarity of the calomel half-cell is reversed for the pIatinumcalomel system. The results for various titrations are given in Table I. TABLE I.-STRONG
ACID-BASE
TITRATIONS USJNG CALOMEL SYSTEMS
Acid
Base
Pt-calomel system, height of the wave, mV
HCl(l.ON) HCl(O.l N) H,SO,(l.ON) H,SO,(l.ON) HpS04(0.1 N)
NaOH(l.ON) NaOH(O.l N) NaOH(l.ON) NaOH(l.ON) NaOH(O.l N)
220 220 420 420 380
PLATINUM-CALOMEL
Glass-calomel system Change Height in pH for of the titration wave, m V* 8.7 76 9.2 7.2
452 395 480 375
AND
GLASS-
End-point EtGlasscalomel, calomel, ml ml 23.55 23.65 9.60 9.60 9.04
23.50 2360 9.57 9.62 9.01
* Change in pH is converted into mV according to 52 mV = 1 pH unit, which under our experimental conditions was more suitable than the theoretical 59 mV = IpH unit.
1592
Talanta. 1966, Vol. 13, pp. 1592 to 1595. Pergamon Press Ltd. Primed
Determination
in NorthernIreland
of purity of o-sulphobenzoic anhydride
(Received 26 April 1966. Accepted 24 July 1966) RECENTLY a method for determination of organic hydroxyl and primary and secondary amino corn__ pounds by esteriflcation with o-sulphobenzoic anhydride (SBA) was reported from these laboratories.%2 Since the presence of free acids in the anhydride samples is known to retard the rate of esterificationa.4 it was considered necessary to determine the purity of the SBA used. SBA is also an important intermediate in the synthesis of sulphonephthalein indicators. A survey of the literature on functional group analysis6 revealed that although several methods have been reported for determining the carboxylic anhydrides in the presence of the corresponding free acids, no methods have been reported for determining the purity of mixed sulphonic-carboxylic anhydrides such as SBA. One reason for this appears to be that except for SBA such mixed anhydrides are quite uncommon. The mixed anhydrides derived from open chain sulphonic and carboxylic acids are unstable and they easily disproportionate into the carboxylic and the sulphonic anhydrides. In the present communication some methods for determining the purity of SBA samples are described. They are similar to those reported for the determination of carboxylic anhydrides, and hence the experimental details have been omitted, but the limitations of each method are discussed. Determination of anhydrides is usually based on their reaction with compounds containing active hydrogen, i.e., amines or alcohols. The resulting compounds (amides and esters) are neutral, and the equivalent amount of acid formed in the reaction is a measure of the anhydride content of the sample. Since the anhydride samples invariably contain some free acid, it is generally desirable to determine the total acidity of the sample after hydrolysing the anhydride. In a recent method due to Sig@a and FloramoE the free acid in the anhydride sample has been determined directly by titration with a tertiary amine in a non-aqueous medium, and the anhydride determined by difference. This method is quite suitable for determining the purity of anhydrides containing only the parent acids as impurity.
Esterification-hydrolysis method’ Tf& method involves titration of the SBA sample in dry acetone with methanolic sodium methoxide, phenolphthalein being used as indicator. Like the carboxylic anhydrides, SBA also reacts to form the carboxylic ester-sodium sulphonate, whereas the free acid gives a disodium salt. The total acidity of the sample is determined by a separate titration with sodium hydroxide after a pyridinecah1yse.d hydrolysis. Alternatively the sodium methoxide solution can be. used for determining total acidity, and then only one standard solution is needed.
FIG. 1. Potentiometric
titration curve of free acid in SBA sample.
1593
Short communications 0
II
’\ I’“‘, \s/
+ CH,ONa
4
(1)
0
/\
0
0
0
/COOH ‘I 0\
+ 2 CH,ONa +
+ 2 CH,OH
‘I \
‘\SOBH
The purity of the anhydride
/COONa (2)
SOIINa
is calculated as usual. The method is quick and gives satisfactory results.
Methods based on anilide formation Methods based on the reactions of amines (aniline8 or morpholineg) to give the corresponding carboxylic acid amide, followed by the determination of excess of amine by non-aqueous acidimetric titration, were found not to be applicable, because of interference by the strongly acidic sulphonic acid group. Even Critchfield’s morpholine-carbon disulphide methodlo was found unsuitable because of the interference of the sulphonic acid group.
0 =
CH,-CH, ,‘co>o \soe
+ H,NC,H,
HN i
>o
< CH,-CH,
__f
(3)
1
,
/CONHC,H6
0\
\SO*H
CH,-CH, -N i
<
>O CH,CHs
1
The method based on the reaction of 2&dichloroanitine I1 to give the corresponding anilide, followed by bromometric determination of excess of aniline, can be used, however. Similarlyamethod based on hydrolysis and anilide formation” can also be used. In this method, the total acid produced by hydrolysis of an aliquot is determined, and the anhydride in another ahquot is converted into anilide by reaction with excess of aniline, and the acid in the resulting solution is determined. The difference of the two titrations gives the amount of acid formed in anilide formation, and this is equivalent to the anhydride content of the sample. Method based on determination of free aciak in anhydrides” Si@a and Floramo made an interesting application of non-aqueous titration by using tri-npropylamine (TPA) or N-ethylpiperidine as titrant to determine free acids in certain anhydride samples. The original method could be applied only to maleic and phthalic acid anhydrides because these acids are strong enough (p& < 3) to be determined by direct titration. A recent modification of this method by Whartonl* uses lithium chloride to enhance the acidity of weak acids, and most of the fatty acids can then be determined in mixtures with their anhydrides. Determination of the free acid in SBA samples by TPA titration in dry methyl ethyl ketone medium offered no difficulty because of the strongly acidic sulphonic acid group (PK. < 2) in the molecule. Potentiometric titration can be carried out with a Cambridge wide-range glass electrode and a standard calomel reference electrode with a methanol bridge saturated with sodium chloride. The end-point could also be detected with Crystal Violet as indicator. The end-point corresponds to the neutralisation of the sulphonic acid group. The carboxylic group does not titrate under the conditions used. A typical potentiometric titration curve for free sulphobenzoic acid in SBA is shown in Fig. 1. The acid and anhydride contents of synthetic mixtures prepared from the pure acid and anhydride were determined by this method and the results are presented in Table I. RESULTS
AND
DISCUSSION
The results for the determination of the purity of some SBA samples by these methods are given in Table II. It will be seen that comparable results are obtained by these methods. The esterificationhydrolysis method is particularly quick and simple, but equally satisfactory results are obtained by the
1594
Short communications TABLEI.-ANALYSIS OF SYNTHETIC MD(TURES OF SBA AND THE CORRESPONDING Acid content
1. 2. 3. 4. 5.
FREE
ACID
Anhydride content
taken meq
found mp9
recovery %
taken meq
found meq
recovery %
0448 0.386 0.375 0.409 0.452
0.450 0.385 0.373 0.408 0.454
100.4 99.1 99.5 98.1 100.5
0.335 0.420 0.316 0.283 0.403
0.332 0.418 0.314 0.282 0.401
98.9 99.5 99.3 99.6 99.5
TABLEII.-ANALYSIS OF SBA SAMPLESBY DIFFERENTME-~I-IODS Esterificationhydrolysis method’
Iodometric method using 2,4dichloroanilinell
Hydrolysisanilide formation1a
Method based on determination of free acid0
96.2 91.0 97.0 97.6 85.7
96.6 96.3 97.0 96.1 86.6
96.0 96.2 96.9 97.2 84.7
96.5 96.9 96.9 97.3 86.3
1. 2. 3. 4. 5. (uncrystallised)
titration of the free acid with TPA. The limitations of methods based on reactions with amines have already been discussed. Another methodI for determination of carboxylic anhydrides based on reaction with anhydrous oxalic acid was also found unsuitable because of partial decomposition of the oxalic acid even by the free u-sulphobenzoic acid. A calorimetric method16 based on formation of hydroxamic acids and their iron chelates can also bc used, particularly for the determination of low concentrations of the anhydride, but it is not preferable to other chemical methods when applied to pure samples of the anhydride. V. IYER N. K. MATHUR
Department of Chemistry University qf Jodhpur Jodhpur, India
Summary--A comparison is made of four methods for the determination of the anhydride and free acid content of impure o-sulphobenzoic anhydride. R&an&-On compare quatre methodes de dosage de la teneur en anhydride et acide libre del’anhydride o-sulfobenzoique impur. Zusammenfassung-Vier Methoden zur Bestimmung des Gehaltes von unreinem o-Sulfobenzoeslureanhydrid an Anhydrid und freier Siiure werden verglichen. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8.
V. Iyer and N. K. Mathur, Anal. Chim. Acta, 1965,33, 554. C. K. Narang, V. Iyer and N. K. Mathur, Microchem. J., 1965, 9,408. P. J. Elving and B. Warshowsky, Anal. Chem., 1947, 19,1006. C. L. Ogg, W. L. Porter and C. 0. Willits, Znd. Eng. Chem., Anal. Ed., 1945, 17, 394. C. W. Hammond in J. Mitchell, Organic Analysis, Vol. III, Interscience, New York, 1965. S. Siggia and N. A. Floramo, Anal. Chem., 1953, 25, 797. D. M. Smith and W. M. D. Bryant, J. Am. Chem. Sot., 1936,58,2452. S. Siggia and J. G. Hanna, Anal. Chem., 1951,23, 1717.
Short communications 9. 10. 11. 12. 13. 14. 15.
1595
J. B. Johnson-and G. L. Funk, ibid, 195527, 1464. F. E. Critchfield and J. B. Johnson, ibid., 1956, 28,430. H. Roth, Microchim. Acta, 1958, 767. L. G. Radcliffe and S. Mendofski, J. Sot. Chem. Ind, 1917,36,628. H. W. Wharton, Anal. Chem., 1965, 37, 730. C. K. Rosenbaum and J. H. Walton, J. Am. Gem. Sot., 1930,52,3368. R. F. Goddu, N. F. Le Blanc and C. H. Wright, Anal. Chem., 1955, 27,125l.
Talanta.1966.Vol. 13.pp. 1595to 1598. PcrgamottPressLtd. Printedin NorthernIreland
Behaviour of the platinum electrode as a pH electrode (Received 25 April 1966. Accepted 6 June 1966) ALL metal-metal oxide electrodes, in theory, can function as pH electrodes. the potential of such an electrode is E = E,’ + 0.059 log {H+}.
The equation governing
This is derived on the assumption that the metal forms an oxide or hydrous oxide, the solubility of which in the solution to be examined is negligibly small .I Almost all metal electrodes are covered by a layer of an oxide film and can function as pH electrodes. The efficiency of such metal pH electrodes will admittedly depend on, among other things, the solubility of the oxide or hydroxide formed on the surface of the electrode as well as the passivity of the electrode in that particular solution. The chief among the metal-metal oxide pH electrodes studied are antimony-antimony trioxide,* tungsten,8 and molybdenum electrodes.4 The platinum electrode is very extensively used as a redox indicator electrode in potentiometry. However, the platinum is also covered by an oxide layer,& so it is feasible that platinum may behave as a pH eiectrode in aqueous media. This fact is seldom realised. e This electrode has been used as a pH electrode in non-aqueous media.’ A 19 s.w.g. platinum electrode, (length 1.5 cm) in conjunction with a saturated calomel electrode was used for strong acid-strong base titrations, with a Cambridge bench pH meter to measure mV. The glass-calomel electrode pair was used on the same meter to measure pH and mV. The glass electrode was Cambridge Cat. No. 42518. The solution was stirred at constant speed by a magnetic stirrer. The strong acids used were 1.O and 0.1 N sulphuric and hydrochloric acids. They were titrated separately against the equivalent concentration of sodium hydroxide. The curves obtained for O.lN sulphuric acid-sodium hydroxide titrations for platinum~alomel and glasscalomel systems are shown in Fig. 1; and those for O.lN hydrochloric acid-sodium hydroxide titrations are shown in Fig. 2. The platinumcalomel curves are inverted relative to the glass-calomel curves because the polarity of the calomel half-cell is reversed for the pIatinumcalomel system. The results for various titrations are given in Table I. TABLE I.-STRONG
ACID-BASE
TITRATIONS USJNG CALOMEL SYSTEMS
Acid
Base
Pt-calomel system, height of the wave, mV
HCl(l.ON) HCl(O.l N) H,SO,(l.ON) H,SO,(l.ON) HpS04(0.1 N)
NaOH(l.ON) NaOH(O.l N) NaOH(l.ON) NaOH(l.ON) NaOH(O.l N)
220 220 420 420 380
PLATINUM-CALOMEL
Glass-calomel system Change Height in pH for of the titration wave, m V* 8.7 76 9.2 7.2
452 395 480 375
AND
GLASS-
End-point EtGlasscalomel, calomel, ml ml 23.55 23.65 9.60 9.60 9.04
23.50 2360 9.57 9.62 9.01
* Change in pH is converted into mV according to 52 mV = 1 pH unit, which under our experimental conditions was more suitable than the theoretical 59 mV = IpH unit.