Introduction
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Waltz et al. used the conductometric titration to calculate the amount of amine end-group in nylon 66 (polyhexamethylene adipamide). The pre-treatment of the sample under study, consisted of dissolving the polymer using purified phenol and shaking the system. Then 95% ethanol and distilled water were added. After that, conductometric titration was carried out using, 0.1 N hydrochloric acid and slow stirring. As phenol was not a good solvent for the determination of the carboxyl end groups because after equivalent point, it reacts with the base, the benzyl alcohol is used instead. Even though the conductance found was lower than that expected for the solvent previously used (phenol–ethanol–water) and the cut point of the two straight lines was not so sharp, the results obtained by conductometric titration were in agreement with the ones obtained by titration using an indicator [7]. In this research work to analyze the amine and carboxyl end-group is necessary to dissolve the sample of polymer under study, which considerably increases the time needed for the determination. Another important statement presented, is to find the correct solvent or mixture of solvents that do not incorporate an error in the interpretation of the results obtained.
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Erbil et al. determined the copolymer composition and monomer reactivity ratios by conductometric titration of acrylamide and itaconic acid. There, the pre-treatment consisted of dissolving 0.1 g of solid polymer with 30 mL of 0.1 N sodium chloride using a magnetic stirrer. 0.1 N sodium hydroxide was used as titrant. From established mixtures of homopolymers (of acrylamide and itaconic acid), the equivalent point was obtained and the data were used to construct a calibration curve that allowed estimating the acidic comonomer content and calculating the copolymer composition [8]. In this case, it is also necessary to dissolve the polymer to obtain the parameters above expressed by the inflection points of the titration curves, increasing the time needed to carry out the experiment and added ion species in the system.
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Bochek et al. suggested the conductometric over the potentiometric titration to calculate the esterification degree of polygalacturonic acid. Usually, the procedure consists of determining the number of free carboxy groups, with phenolphthalein as indicator and with the same solution, obtains the number of esterified carboxy groups by back titration. This technique needs a pre-treatment to dissolve the pectin under study. The pectin was dissolved by wetting with ethanol and with the addition of distilled water heated at 40 °C. The system was stirred for 2 h. They emphasized that the color turn of the indicator occurs in a relatively wide range of pH so this fact can result in a considerable error in the determination. Their results showed that conductometric titration is the technique that offers more similar results to those published in the literature, if compared with the results obtained by potentiometric titration [6]. Two important conclusions can be remarked from this research work. The first one is the benefit of not using an indicator in the conductometric titration technique, so the error of this kind can be disregarded and the second one, which was related to the first one, is the advantages of implementing this technique over the potentiometric because it offers more accurate results.
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dos Santos et al. presented two different methods to obtain the degree of deacetylation of the linear polyaminosaccharide, chitosan: CHN elemental analysis and conductometric titration. After a purification procedure the sample of chitosan was dissolved using 0.05 M HCl and it was stirred for 18 h at room temperature. Titration resulted in a secure and inexpensive method if compared with the equipment-dependent and more expensive CHN elemental analysis [9]. The time needed for purification is long and once again the use of HCl may incorporate ion species that can alter the reading of the results.
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Okubo et al. applied conductometric titration to study the relative distribution of carboxyl groups in a polymer emulsion of styrene/butyl acrylate/methacrylic acid in serum, at surface and inside particle. All the samples were pre-treated and conductometric titration was carried out with 0.02 N potassium hydroxide, at room temperature. The pre-treatment of the sample takes around 9 h where it is necessary first, adjusting the pH to 2 by the addition of 0.2 M HCl and then stirring for 2 h. The resulting emulsion was ultra-centrifuged for 2 h with the purpose of separating the serum and polymer particles and then the polymer particles were redispersed in distilled deionized water. This process was repeated three times and the supernatants were collected in each step to measure carboxyl groups in serum [10]. The methodology used in this work although simple, required a long time to obtain good results and the necessity of incorporating HCl to adjust the pH, may alter the results of the determination as it was mentioned in the above examples where it was used.
Materials and methods
Practical theoretical framework of conductometric titration
Reaction rate
Reaction rate with magnetic stirrer
Reaction rate with partial magnetic stirrer
Reaction rate with bubble stirring
Conductometric titrations
Conductometric titration with magnetic stirrer
Conductometric titration with partial magnetic stirrer
Conductometric titration with bubble stirring
Results and discussion
Reaction rates
Conductometric titrations
Figure 3
| a | b |
---|---|---|
Membrane mass (g) | 0.2195 | 0.209 |
NaOH Concentration (M) | 0.1829 | 0.1807 |
Temperature (°C) | 19.0 ± 0.1 | 19.0 ± 0.1 |
Agitation system | Partial magnetic | Nitrogen bubbles |
Time between determinations (min) | 1440 | 1440 |
Total acid capacity (meq/g) | 0.924 | 0.917 |
Absolute error (%) | 0.056 | 0.062 |
Figure 4
| a | b |
---|---|---|
Membrane mass (g) | 0.2079 | 0.2059 |
NaOH concentration (M) | 0.1784 | 0.1807 |
Temperature (°C) | 17.6 ± 0.1 | 19.0 ± 0.1 |
Agitation system | Magnetic | Nitrogen bubbles |
Time between determinations (min) | 45 | 30 |
Total acid capacity (meq/g) | 1.14 | 1.07 |
Absolute error (%) | 0.16 | 0.09 |
Figure 5
| a | b |
---|---|---|
Membrane mass (g) | 0.2126 | 0.2268 |
KOH concentration (M) | 0.1764 | 0.1905 |
Temperature (°C) | 20.2 ± 0.1 | 19.0 ± 0.1 |
Agitation system | Magnetic | Partial magnetic |
Time between determinations (min) | 30 | 29 |
Total acid capacity (meq/g) | 1.018 | 0.96 |
Absolute error (%) | 0.038 | 0.02 |
Figure 7
| 23 min | 30 min | 1440 min |
---|---|---|---|
Membrane mass (g) | 0.2039 | 0.2059 | 0.209 |
NaOH concentration (M) | 0.1807 | 0.1807 | 0.1807 |
Temperature (°C) | 19.0 ± 0.1 | 19.0 ± 0.1 | 19.0 ± 0.1 |
Agitation system | Nitrogen bubbles | Nitrogen bubbles | Nitrogen bubbles |
Time between determinations (min) | 23 | 30 | 1440 |
Total acid capacity (meq/g) | 0.967 | 1.07 | 0.9173 |
Absolute error (%) | 0.013 | 0.09 | 0.06 |