Acid conductimetric study

We report spectroscopic and conductimetric studies on the reactions of styrene in the presence of perchloric acid (most in methylene dichloride but some in ethylene dichloride), both during and after the polymerisation. During polymerisation no ions are detectable, but at the end aralkyl (and subsequently allylic) ions are formed. Quantitative results show that ion formation sets in when the styrene concentration has fallen to four times the acid concentration we interpret this finding as showing that the ester (oligo-)styryl perchlorate requires four molecules of styrene for stabilisation in solution. 

As mentioned in Section 2.6, triple ion formation is not limited to low permittivity solvents. It also occurs in high permittivity solvents, if they are of very weak acidity and basicity for example, Kt for the formation of Li2Cl and LiCl2 in AN has been determined by polarography to be 105 M 2 [19]. Li+ and Cl- in AN are only weakly solvated and tend to be stabilized by forming triple ions. For conductimetric studies of triple ion formation in dipolar protophobic aprotic solvents, see Ref. [20]. 

Conductimetric Studies of Acid-Base Equilibria Determination of Autoprotolysis Constants... 

Argiielles-Monala, W., Cahrerah, G., Penichec, C., Rinaudod, M. Conductimetric study of the interpolyelectrolyte reaction between chitosan and polygalacturonic acid. Polymer 2000, 41 (7), 2373-2378. 

Physical-chemical data concerning ionic equilibria can be obtained from conductimetric studies. Con-ductimetric measurement has led to the determination of acid and base dissociation constants, stability constants, and solubility product constants. Further details may be found in modern texts of physical chemistry. 

Kaufman, M. J., Gronert, S., and Streitwieser, A., Jr. "Carbon Acidity. 73. Conductimetric Study of Lithium and Cesium Salts of Hydrocarbon Acids. A Scale of Free Ion Acidities in Tetrahydrofuran. Revision of the Ion Pair Scales." /. Am. Chem. Soc., 110,2829 (1988). 

The kinetics of nitration by nitric acid in nitromethane and acetic acid have been studied in detail40, and interpretation of the data here is perhaps made a little more difficult by the fact that spectroscopic and conductimetric analyses give no evidence for the existence of the nitronium ion in these media57. 

Pepper and Reilly [3] studied in detail the kinetics of the polymerisation of styrene by perchloric acid in 1,2-dichloroethane and in mixtures of 1,2-dichloroethane and carbon tetrachloride. Our investigation of the spectroscopic and conductimetric behaviour of this monomer-catalyst pair was carried out in methylene dichloride [1]. Since the evidence obtained from our work was in conflict with Pepper and Reilly s interpretation of the chemistry of the system, we decided to supplement our results by kinetic studies with methylene dichloride instead of 1,2-dichloroethane, so that mechanistic conclusions could be drawn from a self-consistent group of results, without any assumptions about the similarity of the solvents. 

Pepper and Reilly s views on the mechanism of this polymerisation implied that with the concentrations of perchloric acid used by them, it should be possible to estimate the concentration of polystyryl ions spectrophotometrically and so to test whether initiation did indeed give carbonium ions in concentration equal to that of the acid. When Gandini and Plesch [5, 29-31] carried out the appropriate measurements, they found from spectroscopic, conductimetric and kinetic studies that no ions were present during the polymerisations, but that they were formed once the styrene concentration had been reduced by polymerisation to less than four times the concentration of acid. Addition of more styrene instantly removed the ions, which reappeared once again when polymerisation had reduced the styrene concentration sufficiently. This formation of ions after polymerisation had misled some workers into concluding that they were also present during that reaction. 

Measurements of the dissociation constants of m-nitrobenzoic acid and p-nitrobenzoic acid began with Ostwald in 1889114. However, the first reasonably precise values were obtained by Dippy and coworkers in the nineteen-thirties, as part of an extensive study of the ionization of carboxylic acids by conductimetric methods115. The pKa values in water at 25 °C of m-nitrobenzoic acid and p-nitrobenzoic acid were found to be 3.493 and 3.425, respectively, compared with 4.203 for benzoic acid itself. On the basis of these values Hammett28,116 proposed 0 values of 0.710 for m-NOy and 0.778 for p-NOy (see Section II.A). These a values have commonly been used thereafter, often rounded to 0.71 and 0.78, respectively. 

The first study of the cationic polymerisation of styrene by triflic acid was undertaken before 1970 by Mathias and Plesch They fixind that very tow catalyst con-centratimis (10 — 10 M) were needed to induce a fairly rapid polymerisation in dichloromethane. These reactions were followed calorimetric y, conductimetrically and spectrophotometrically. The mqor obstacle precluding a stematic study was the poor reproducibility of the results. This problem was finally traced to the ng of the acid solutions contained in sealed phials. It was concluded that a slow reaction between the catalyst and the solvent was the cause of this aging phenomenon ... 

In addition to ion-pair formation, other types of association reactions are important in nonaqueous solvents. Evidence for triple and quadruple ion formation in nonaqueous solvents is obtained from conductimetric, solvent extraction, calorimetric, or cryoscopic measurements. Self-association reactions, that is, equilibria such as 2HA (HA)2, have been reviewed " and have been studied by dififer-ential-vapor-pressure techniques. Frequently the anion A obtained from an acid HA is poorly solvated stabilization then may occur by interaction (homoconjugation) with a second molecule of acid to give HAj . Homoconjugation can be studied by techniques such as spectroscopy or conductimetry. Thus, the homoconjugation... 

Some substances with very weakly basic properties are only partially ionized in sulfuric acid and it is possible to measure their degree of ionization and hence obtain their basicity constants by means of cryo-scopic, conductimetric, and spectroscopic measurements. A number of nitro-compounds have been carefully studied by several different methods. It may be seen in Table IX that the results obtained by the differ-... 

Triblock and random polyampholytes based on DMAEM-MMA-MAA were examined for their phase separation behaviour [52]. Triblock polyampholytes have a much broader phase separation region than the random ones. The specific structure of PMAA-fc-PlM4VPCl with the excess of cationic or anionic blocks at the lEP is close to the structure of non-stoichiometric IPC. It is suggested that its nucleus consists of intraionic IPC surrounded by cationic blocks protecting it from precipitation [53]. ABC triblock copolymers of polystyrene-b/ock-poly(2-(or 4)vinylpyridine)-fc/ock-poly(methacrylic acid) were synthesized by living anionic polymerization [53 a]. Interpolymer complexation of the polyvinylpyri-dine and poly(methacrylic acid) blocks in the micellar solution was studied in relation to pH in solution by potentiometric, conductimetric and turbidimetric titration and in bulk by FTIR spectroscopy. 

Conductimetric SF methods have been used in the study of the decomplexation of [Ca(211)] in the presence of acid through reactions (19) and (20). In... 

Micellar Effects on Chemical Equflibria.—A few studies have been made of acid-base equilibria in micelles. Hydronium ion activity in anionic micelles has been measured conductimetrically using hydrophilic indicators, it being found that a plot of mn+ versus [H ]-t-[Na ] is linear with a slope of 0.82. The quantity mH+ is defined as the number of micellized hydrogen ions per surfactant head group, namely mH = [H ]tot—[H ]w/ [D]tot c.m.c., where [DJtot is the total catalyst concentration. The use of fluorescent indicators (21a) and (21b) in anionic, neutral, and cationic surfactantspermitted the evaluation of the electrical potential at the micellar surface as a function of added electrolytes. Indicator pK values for mixed micelles and pK values of weak... 

Experimental determinations of the conducting properties of electrolyte solutions are important essentially in two respects. Firstly, it is possible to study quantitatively the effects of interionic forces, degrees of dissociation and the extent of ion-pairing. Secondly, conductance values may be used to determine quantities such as solubilities of sparingly soluble salts, ionic products of self-ionizing solvents, dissociation constants of weak acids and to form the basis for conductimetric titration methods. 

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