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Trifluoromethyl sulphonic acid as a titrant in glacial acetic acid systems
T&ma,
1961, Vol. 8. pp. 849 to 852. Pergamon Press Ltd. Printed in Northern Ireland
TRIFLUOROMETHYL SULPHONIC ACID AS A TITRANT IN GLACIAL ACETIC ACID SYSTEMS E. S. LANE Chemistry Division, U.K.A.E.A., Harwell, England (Receiued 1 May 1961. Accepted 13 May 1961) Summary-Trifluoromethyl sulphonic acid in glacial acetic acid has been compared with perchloric acid in glacial acetic acid as a titrant in a limited number of cases. It does not appear that the reagent is likely to compete seriously with perchloric acid in non-aqueous titrimetry. Although comparable to perchloric acid in acid strength in acetic acid as well as iu instrument response, it is unlikely that the considerably higher cost can be justified on the grounds of freedom from precipitate formation alone and no other advantageous feature has been observed which could justify this. SEVERAL aliphatic and aromatic sulphonic acids have recently been examined1 as titrants for use in glacial acetic acid systems. None of these have proved to be as useful as perchloric acid as a titrant in this medium but it is quite practicable to use them as they possess, as a class, the advantage of ensuring freedom from precipitate or gel formation when certain compounds are titrated. Potassium hydrogen phthalate,
for example, the common primary standard for use in glacial acetic acid systems, persistently forms a precipitate with perchloric acid during titration, and this can lead to difficulties in some instrumental methods of end-point detection. Trifluoromethyl sulphonic acid has recently been prepared2 and described as one of the strongest known sulphonic acids; and it may even be one of the strongest of all known acids. In view of the interest shown in comparatively weak sulphonic acids as titrants, a comparison of the analytical possibilities of trifluoromethyl sulphonic acid with the conventional perchloric acid in glacial acetic acid has been made, using a representative selection of basic substances and experimental techniques. EXPERIMENTAL Reagents and apparatus Perchloric acid, O*lN in glacial acetic acid: B.D.H. commercially available material was used, freshly standardised against potassium hydrogen phthalate. TrifIuoromethyl sulphonic acid, O.lN inglacial acetic acid: Potassium trifluoromethyl sulphonate* (4.5 g) and concentrated sulphuric acid (6 ml) were heated together in a microdistillation apparatus under reduced pressure (2 mm Hg). Distillation was stopped when the oil bath temperature reached 120”. The distillate consisted of 2 ml of a clear, fuming liquid and a white solid (presumably trifluoromethyl sulphonic acid monohydrate,a which can be distilled from sulphuric acid) adhering to the receiver. The trifluoromethyl sulphonic acid was decanted from the solid, weighed, and diluted with glacial acetic acid to the requisite volume. It was standardised against potassium hydrogen phthalate. I Indicators: 0.5 % solutions in glacial acetic acid of Crystal Violet, Oracet Blue B and u-naphtholbenzein were used. The end-point shade obtained with the different compounds titrated was matched against that obtained during standardisation with the same indicator. Potentiometric titrations. A sleeve-type calomel electrode and a glass electrode were used in conjunction with a direct-reading pH meter (Electronic Industries Ltd. Model 23). End-points were determined graphically. * Kindly supplied by The Minnesota Mining and Manufacturing Co. Ltd. a49
E. S.
850
LANE
High-frequency titrations: The simple one-valve instrument4 operating at 15 MC/S modified as previously describe& was used. Owing to the precipitation of potassium perchlorate during titrations with perchloric acid, diphenylguanidine was preferred as the primary standard for use with this method of end-point detection rather than potassium hydrogen phthalate, which was used for the visual and potentiometric standardisations. RESULTS
A comparison of the potentiometric titration curves obtained with trifluoromethyl sulphonic acid and perchloric acid in glacial acetic acid with diphenylguanidine and potassium hydrogen phthalate as standards is given in Fig. 1. The shape of the highq-.x
0.1
N
Perchloric
_
0.1
N
Trtfluoromethylsulphonlc
Volume
acid acid
titrant
FIG. 1.
frequency titration curves obtained in the titration of diphenylguanidine and decamethylene-bis-pyridinium nitrate with both titrants is shown in Fig. 2. Results obtained with trifluoromethyl sulphonic acid for the determination of equivalent weights of a number of basic compounds of widely different constitution, on a semimicro scale, are recorded in Table I. Five separate determinations of the equivalent weight of a sample of commercial trinonylamine with both perchloric acid and tritluoromethyl sulphonic acid using Oracet Blue B as visual indicator gave the results shown in Table II. DISCUSSION The results show that perchloric and trifluoromethyl sulphonic acids are comparable titrants in glacial acetic acid systems. In potentiometric titrations trifluoromethyl sulphonic acid behaves as a very strong acid, and the height of the potential break and the maximum value of dE/dV (the rate of change of potential with volume of titrant) were almost identical with those obtained using perchloric acid. In this respect trifluoromethyl sulphonic acid is superior to the other sulphonic acids so far tested. As with other sulphonic acids, there was no trace of precipitation or gel formation when using potassium hydrogen phthalate as a primary standard, or indeed with any of the compounds so far examined.
Trifhtoromethyl
sulphonic acid as a titrant in glacial acetic acid systems
X-X
0
IN
M
0.1 N
Perchlorlc
851
ocld
Trlfluoromethylsulphonic
Volume
acid
titrant
FIG. 2. TABLE I T
Equivalent wt Method
Compound
I_
Pound _-
Guanidine carbonate Ephedrine Diphenylguanidine Diphenylguanidine Decamethylene-bis-pyridinium nitrate Decamethylene-bis-pyridinium nitrate o-Phenanthroline hydrate Tri-iso-octylamine
[__
Required
Visual-a-naphthol-benzein Visual-a-naphthol-benzein Visual-Oracet blue B Potentiometric
175 210 211
90 174 211 211
High-frequency
210
211
211 198 3.51
211 200 353
Potentiometric Visual-Oracet Visual-Oracet
92
blue B blue B L_
TABLE II
Titrant Perchloric acid Trifluoromethyl
sulphonic acid
399 396
Standard deviation
Required
3.6 1.5
395 395
Using the high-frequency method of end-point indication, there was a slight advantage to be gained by the use of perchloric acid, as evidenced by the smaller angle between the intersecting lines obtained approaching and retiring from the end-point. Furthermore, whereas with perchloric acid information can often be gained from high-frequency titrations concerning the functionality of the compound being titrated, this effect was not noted when trifluoromethyl sulphonic acid was used. Decamethylene-bis-pyridinium nitrate, for example, gave two distinct end-points with
852
E. S. LANE
perchloric acid, corresponding to the titration of first one, and then both quaternary functions, but only one end-point, that corresponding to two quaternary groups, was detected during titration wfth trifluoromethyl sulphonic acid. With visual indication of the end-points the precision obtained in the titration of trinonylamine with trifluoromethyl sulphonic acid was slightly better than when perchloric acid was used. The new titrant, however, was not as effective as perchloric acid for the titration of quaternary ammonium halides using the mercuric acetate method with Crystal Violet as indicator. When a representative selection of quaternary ammonium and phosphonium halides were titrated with trifluoromethyl sulphonic acid, only diffuse end-points were obtained. Several basic compounds were titrated visually with the new titrant, and satisfactory values of their equivalent weights were obtained. Although new uses of trifluoromethyl sulphonic acid are being developed, it would appear doubtful from this limited assessment whether it will ever seriously compete with perchloric acid for use in non-aqueous titrimetry. Although comparable to perchloric acid in acid strength in acetic acid, as well as in instrument response, it is unlikely that the considerably higher cost can be justified on the grounds of freedom from precipitate formation alone, and no other advantageous feature has been observed which could justify this. Zusammenfassung-Trifluoromethylsulfonsiiure in Eisessig wurde mit Perchlorsiiure in Eisessig als Titrationsmittel verglichen. Es scheint nicht wahrscheinlich, dass das Reagens ernsthaft mit Perchlorslure als Titrationmittel in nichtwlssrigem Medium konkurrieren kann. Das Reagens ist zwar rnit Perchlorslure vergleichbar, soweit es die Starke (in EssigsIiure) und das Ansprechen des Instrumentes betrifft; es ist aber unwahrscheinlich, dass sich die betrachtlich hoheren Kosten lediglich wegen Nichtauftretens von Niederschllgen rechtfertigen lassen. Dies ist der einzige auffindbare Vorteil. R&rn&L’auter a compare l’acide trifluoromethylsulfonique dans l’acide ac&ique glacial rl l’acide perchlorique dans le meme solvant comme agent titrant dans un nombre 1imitC de cas. 11ne semble pas probable que ce reactif concurrence serieusement l’acide perchlorique dans la titrimetrie en milieu non aqueux. Bien que comparable a I’acide perchlorique au point de vue de sa force dans l’acide ac&ique, il est improbable que son cofit tr6s eleve puisse 6tre justifie par la seule raison qu’il ne se forme pas de pr6cipite et qu’aucun autre avantage n’ait Cte observe qui pourrait Iegitimer son utilisation. REFERENCES 1 M. M. Case and M. Cefola, Analyt. Chim. Acta, 1959, 21, 374. 2 R. N. Haszeldine and J. M. Kidd, J. Chem. Sm., 1954,4228. 3 J. Burdon, I. Farazmand, M. Stacey and J. Tatlow, ibid, 1957, 2574. 4 J. P. Dowdall, D. V. Sinkinson and H. Stretch, Analyst, 1955, 80, 491. 5 E. S. Lane, ibid., 1955, 80, 675.
1961, Vol. 8. pp. 849 to 852. Pergamon Press Ltd. Printed in Northern Ireland
TRIFLUOROMETHYL SULPHONIC ACID AS A TITRANT IN GLACIAL ACETIC ACID SYSTEMS E. S. LANE Chemistry Division, U.K.A.E.A., Harwell, England (Receiued 1 May 1961. Accepted 13 May 1961) Summary-Trifluoromethyl sulphonic acid in glacial acetic acid has been compared with perchloric acid in glacial acetic acid as a titrant in a limited number of cases. It does not appear that the reagent is likely to compete seriously with perchloric acid in non-aqueous titrimetry. Although comparable to perchloric acid in acid strength in acetic acid as well as iu instrument response, it is unlikely that the considerably higher cost can be justified on the grounds of freedom from precipitate formation alone and no other advantageous feature has been observed which could justify this. SEVERAL aliphatic and aromatic sulphonic acids have recently been examined1 as titrants for use in glacial acetic acid systems. None of these have proved to be as useful as perchloric acid as a titrant in this medium but it is quite practicable to use them as they possess, as a class, the advantage of ensuring freedom from precipitate or gel formation when certain compounds are titrated. Potassium hydrogen phthalate,
for example, the common primary standard for use in glacial acetic acid systems, persistently forms a precipitate with perchloric acid during titration, and this can lead to difficulties in some instrumental methods of end-point detection. Trifluoromethyl sulphonic acid has recently been prepared2 and described as one of the strongest known sulphonic acids; and it may even be one of the strongest of all known acids. In view of the interest shown in comparatively weak sulphonic acids as titrants, a comparison of the analytical possibilities of trifluoromethyl sulphonic acid with the conventional perchloric acid in glacial acetic acid has been made, using a representative selection of basic substances and experimental techniques. EXPERIMENTAL Reagents and apparatus Perchloric acid, O*lN in glacial acetic acid: B.D.H. commercially available material was used, freshly standardised against potassium hydrogen phthalate. TrifIuoromethyl sulphonic acid, O.lN inglacial acetic acid: Potassium trifluoromethyl sulphonate* (4.5 g) and concentrated sulphuric acid (6 ml) were heated together in a microdistillation apparatus under reduced pressure (2 mm Hg). Distillation was stopped when the oil bath temperature reached 120”. The distillate consisted of 2 ml of a clear, fuming liquid and a white solid (presumably trifluoromethyl sulphonic acid monohydrate,a which can be distilled from sulphuric acid) adhering to the receiver. The trifluoromethyl sulphonic acid was decanted from the solid, weighed, and diluted with glacial acetic acid to the requisite volume. It was standardised against potassium hydrogen phthalate. I Indicators: 0.5 % solutions in glacial acetic acid of Crystal Violet, Oracet Blue B and u-naphtholbenzein were used. The end-point shade obtained with the different compounds titrated was matched against that obtained during standardisation with the same indicator. Potentiometric titrations. A sleeve-type calomel electrode and a glass electrode were used in conjunction with a direct-reading pH meter (Electronic Industries Ltd. Model 23). End-points were determined graphically. * Kindly supplied by The Minnesota Mining and Manufacturing Co. Ltd. a49
E. S.
850
LANE
High-frequency titrations: The simple one-valve instrument4 operating at 15 MC/S modified as previously describe& was used. Owing to the precipitation of potassium perchlorate during titrations with perchloric acid, diphenylguanidine was preferred as the primary standard for use with this method of end-point detection rather than potassium hydrogen phthalate, which was used for the visual and potentiometric standardisations. RESULTS
A comparison of the potentiometric titration curves obtained with trifluoromethyl sulphonic acid and perchloric acid in glacial acetic acid with diphenylguanidine and potassium hydrogen phthalate as standards is given in Fig. 1. The shape of the highq-.x
0.1
N
Perchloric
_
0.1
N
Trtfluoromethylsulphonlc
Volume
acid acid
titrant
FIG. 1.
frequency titration curves obtained in the titration of diphenylguanidine and decamethylene-bis-pyridinium nitrate with both titrants is shown in Fig. 2. Results obtained with trifluoromethyl sulphonic acid for the determination of equivalent weights of a number of basic compounds of widely different constitution, on a semimicro scale, are recorded in Table I. Five separate determinations of the equivalent weight of a sample of commercial trinonylamine with both perchloric acid and tritluoromethyl sulphonic acid using Oracet Blue B as visual indicator gave the results shown in Table II. DISCUSSION The results show that perchloric and trifluoromethyl sulphonic acids are comparable titrants in glacial acetic acid systems. In potentiometric titrations trifluoromethyl sulphonic acid behaves as a very strong acid, and the height of the potential break and the maximum value of dE/dV (the rate of change of potential with volume of titrant) were almost identical with those obtained using perchloric acid. In this respect trifluoromethyl sulphonic acid is superior to the other sulphonic acids so far tested. As with other sulphonic acids, there was no trace of precipitation or gel formation when using potassium hydrogen phthalate as a primary standard, or indeed with any of the compounds so far examined.
Trifhtoromethyl
sulphonic acid as a titrant in glacial acetic acid systems
X-X
0
IN
M
0.1 N
Perchlorlc
851
ocld
Trlfluoromethylsulphonic
Volume
acid
titrant
FIG. 2. TABLE I T
Equivalent wt Method
Compound
I_
Pound _-
Guanidine carbonate Ephedrine Diphenylguanidine Diphenylguanidine Decamethylene-bis-pyridinium nitrate Decamethylene-bis-pyridinium nitrate o-Phenanthroline hydrate Tri-iso-octylamine
[__
Required
Visual-a-naphthol-benzein Visual-a-naphthol-benzein Visual-Oracet blue B Potentiometric
175 210 211
90 174 211 211
High-frequency
210
211
211 198 3.51
211 200 353
Potentiometric Visual-Oracet Visual-Oracet
92
blue B blue B L_
TABLE II
Titrant Perchloric acid Trifluoromethyl
sulphonic acid
399 396
Standard deviation
Required
3.6 1.5
395 395
Using the high-frequency method of end-point indication, there was a slight advantage to be gained by the use of perchloric acid, as evidenced by the smaller angle between the intersecting lines obtained approaching and retiring from the end-point. Furthermore, whereas with perchloric acid information can often be gained from high-frequency titrations concerning the functionality of the compound being titrated, this effect was not noted when trifluoromethyl sulphonic acid was used. Decamethylene-bis-pyridinium nitrate, for example, gave two distinct end-points with
852
E. S. LANE
perchloric acid, corresponding to the titration of first one, and then both quaternary functions, but only one end-point, that corresponding to two quaternary groups, was detected during titration wfth trifluoromethyl sulphonic acid. With visual indication of the end-points the precision obtained in the titration of trinonylamine with trifluoromethyl sulphonic acid was slightly better than when perchloric acid was used. The new titrant, however, was not as effective as perchloric acid for the titration of quaternary ammonium halides using the mercuric acetate method with Crystal Violet as indicator. When a representative selection of quaternary ammonium and phosphonium halides were titrated with trifluoromethyl sulphonic acid, only diffuse end-points were obtained. Several basic compounds were titrated visually with the new titrant, and satisfactory values of their equivalent weights were obtained. Although new uses of trifluoromethyl sulphonic acid are being developed, it would appear doubtful from this limited assessment whether it will ever seriously compete with perchloric acid for use in non-aqueous titrimetry. Although comparable to perchloric acid in acid strength in acetic acid, as well as in instrument response, it is unlikely that the considerably higher cost can be justified on the grounds of freedom from precipitate formation alone, and no other advantageous feature has been observed which could justify this. Zusammenfassung-Trifluoromethylsulfonsiiure in Eisessig wurde mit Perchlorsiiure in Eisessig als Titrationsmittel verglichen. Es scheint nicht wahrscheinlich, dass das Reagens ernsthaft mit Perchlorslure als Titrationmittel in nichtwlssrigem Medium konkurrieren kann. Das Reagens ist zwar rnit Perchlorslure vergleichbar, soweit es die Starke (in EssigsIiure) und das Ansprechen des Instrumentes betrifft; es ist aber unwahrscheinlich, dass sich die betrachtlich hoheren Kosten lediglich wegen Nichtauftretens von Niederschllgen rechtfertigen lassen. Dies ist der einzige auffindbare Vorteil. R&rn&L’auter a compare l’acide trifluoromethylsulfonique dans l’acide ac&ique glacial rl l’acide perchlorique dans le meme solvant comme agent titrant dans un nombre 1imitC de cas. 11ne semble pas probable que ce reactif concurrence serieusement l’acide perchlorique dans la titrimetrie en milieu non aqueux. Bien que comparable a I’acide perchlorique au point de vue de sa force dans l’acide ac&ique, il est improbable que son cofit tr6s eleve puisse 6tre justifie par la seule raison qu’il ne se forme pas de pr6cipite et qu’aucun autre avantage n’ait Cte observe qui pourrait Iegitimer son utilisation. REFERENCES 1 M. M. Case and M. Cefola, Analyt. Chim. Acta, 1959, 21, 374. 2 R. N. Haszeldine and J. M. Kidd, J. Chem. Sm., 1954,4228. 3 J. Burdon, I. Farazmand, M. Stacey and J. Tatlow, ibid, 1957, 2574. 4 J. P. Dowdall, D. V. Sinkinson and H. Stretch, Analyst, 1955, 80, 491. 5 E. S. Lane, ibid., 1955, 80, 675.