The use of potentiometric titration for the analysis and characterization of native and modified starches is highlighted. The polyelectrolytic behavior of oxidized starches (thermal and thermal-chemical oxidation), a graft copolymer of itaconic acid (IA) onto starch, and starch esters (mono- and diester itaconate) was compared with the behavior of native starch, the homopolymer, and the acid employed as a graft monomer and substituent. Starch esters showed higher percentages of acidity, followed by graft copolymer of itaconic acid and finally oxidized starches. Analytical techniques and synthesis of modified starches were also described.
Titration is an analytical technique commonly used in many research and industrial chemistry applications. This involves the measured addition of a solution of known concentration of chemical (titrant) to determine the concentration of another chemical (analyte) in a second solution. The chemical in the titrant reacts in a known manner with the analyte material. When the reaction of these chemicals/materials is complete, a surplus of the titrant is detected as a specific end point marking the end of titration. The end point can be determined by several methods: indicators of pH, redox indicators, potentiometry, conductometry, isothermal calorimetry, spectrophotometry, and amperometry [
Analytical techniques for this research included potentiometric titration. Potentiometric titration, based on the measurement of pH changes, is a versatile technique with a wide range of applications. It is a well-established analytical method always effective for simple acid-base systems [
The soluble natural polymers include polynucleotides, polypeptides, and polysaccharides such as starch, cellulose, and chitosan. Due to increased interest in the use of polysaccharides for a wide range of practical applications, potentiometric titration has become a standard method to analyze specific properties of polyelectrolytes in this group. The technique has been widely used to determine the amylose content in the starch [
Starch is the main storage carbohydrate in plants. It is stored as granules in most plant cells and in this state is called native starch. Native starches from different botanical sources vary widely in structure and composition, but all granules are mainly formed by two molecular components, amylose (20–30%) and amylopectin (70–80%) [
However, this polysaccharide has unfavourable properties such as low shear strength, ease of thermal decomposition, and high tendency for retrogradation (crystallization and aging of gels), limiting its use in other applications. These properties can be overcome by chemical and/or physical modification [
Starch oxidation using KMnO4, grafting, or esterification with organic acids, such as itaconic acid, generates structural changes in the starch by incorporating carboxyl groups –COOH [
In this study modified starches were synthesized. The polyelectrolytic behavior of oxidized starches (thermal and thermal-chemical oxidation), a graft copolymer of itaconic acid (IA) onto starch, and starch esters (mono- and diester itaconate) was compared with the behavior of native starch, the homopolymer, and the acid employed as a graft monomer and substituent.
Food grade corn starch supplied by Alfonzo Rivas & Cía; hydrochloric acid, HCl (37%); nitric acid, HNO3 (65%); silver nitrate, AgNO3 (>99.9%); potassium permanganate, KMnO4 (99%) from
Oxidation by hydrothermal treatment involved the preparation of an aqueous dispersion of 10% m/v (10 g dry basis corn starch, equivalent to 0.062 moles of anhydroglucose units (AGU) in 100 mL of distilled water). The dispersion was heated to 75°C for 15 min with gentle shaking to promote starch gelatinization. Once formed, the starch paste and 200 mL of distilled water were added and cooled to the reaction temperature (60°C). 300 mL distilled water were added and the starch was oxidized by heat treatment (Ht-St) after 3 h.
For the thermal-chemical oxidation, after cooling the slurry to 60°C, 0.63 g (4 × 10−3 moles) of KMnO4 (oxidizing initiator) and 1.04 g (0.01 moles) of NaHSO3 (reducing activator) were added and kept at 60°C for 10 min in order to preoxidize the starch. The volume of distilled water was immediately made up to 500 mL and allowed to react for 3 h to obtain the oxidized starch by thermal-chemical treatment (Ox-St).
In both oxidations, the product obtained was cooled to room temperature and precipitated with ethanol. The oxidized starch was washed with a mixture of 50% v/v ethanol/water and dried at 40°C to constant weight.
Poly(itaconic acid) (PIA) was synthesized thermally using a modification of a classic method developed by Marvel and Shepherd in 1959 [
The same thermal-chemical oxidation procedure was used but after the preoxidation of the starch, the monomer (IA) previously dissolved in 180 mL of distilled water was added. The volume was immediately made up to 500 mL with distilled water. This time was regarded as the initial time of reaction (0.18 moles/L IA). After 3 h, the reaction was stopped, and the product (St-
The esterification reactions were carried out using a combination of procedures in the literature [
The same procedure was used for the itaconate starch diester or starch di-itaconate (DI) but a solution of 150 mL (0.2269 moles/L AGU and 0.7252 moles/L IA) in water was used. Then after the first 4 h of reaction, 18.75 g of starch on a dry basis dispersed in 50 mL of water (0.1875 moles/L AGU) was added and heated at 80°C for another 4 h (0.7015 moles/L AGU and 0.5604 moles/L IA). The reaction product was precipitated with ethanol; the precipitate was filtered and washed with ethanol until the Baeyer test was negative. The samples were dried in an oven at 40°C until constant weight was reached.
In all cases, the formation of the desired starch derivative was confirmed by infrared Fourier transform spectroscopy (FTIR). The spectra were taken on a Shimadzu IR Prestige spectrophotometer in the range of 4000–400 cm−1, using KBr pellets.
In this section the experimental procedure used for the potentiometric titration of different starch derivatives (Ht-St, Ox-St, St-
In Figure
FTIR spectra of the analyzed products.
In the same way, the absence of the signal of the C=C stretch at 1630 cm−1 in the FTIR spectrum of the PIA indicated the formation of the homopolymer by the monomer double bond. The formation of the graft copolymer was confirmed through changes in the tightly bound water signal at 1646 cm−1. Over that one, a shoulder was observed at approximately 1670 cm−1 attributable to the stretching of C=O of –COOH. At 1452 cm−1, another signal was observed that was attributable to a weak interaction in the bending plane of the bound –C–O–H of a carboxylic acid [
Characterization by FTIR of the esters confirmed their formation through the signals at 1669 and 1721 cm−1 observed in spectra of the SI and DI and attributed to the carbonyl group of the itaconate in the substituted starch, while the absorption bands at 1575 and 1514 cm−1 could be attributed to the C=C of the itaconate and the asymmetrical deformation of the carboxylate group (–COO−). In the fingerprint region significant changes were not observed indicating that no anhydroglucose ring opening occurred [
In Table
Content of carbonyl and carboxyl groups of the oxidized starches without leaching compared to the native starch.
Sample | Carbonyl content | Carboxyl content |
---|---|---|
(% CO) | (% COOH) | |
St | 0.7927 ± 0.0326 | 0.0539 ± 0.0059 |
Ht-St | 0.9041 ± 0.0046 | 0.0849 ± 0.0039 |
Ox-St | 0.1502 ± 0.0147 | 0.1141 ± 0.0085 |
It was observed that the hydrothermal treated starch (Ht-St) showed a higher content of carbonyl groups, while the content of acidic groups was greater for the oxidized starch with a redox system. From these results it can be inferred that the reaction conditions employed prolonged heat treatment to produce oxidized starch to aldehyde preferentially; such derivatives are suitable intermediates for further modifications. Meanwhile, the addition of KMnO4 favored oxidation of the hydroxyl groups of the starch to carbonyl and then carboxyl, as outlined in Figure
Schematic representation of the steps in the oxidation of starch in the order of reactivity of the hydroxyl groups.
In Figure
Potentiometric titration curves of native starch (St) and hydrothermal treated (Ht-St) and oxidized (Ox-St) starches.
The shape of the curves coincides with those reported by other researchers [
Titration curves with NaOH for the monomer and homopolymer are shown in Figure
Neutralization curves of itaconic acid (IA) compared to the poly(itaconic acid) (PIA) (a) and of the graft copolymer (St-
Comparing the neutralization curve of the monomer to the homopolymer, it can be observed that the PIA is located above the IA, because the carboxylic acid groups are less exposed than in the monomer, so it is more difficult to neutralize them, thereby generating higher pH and a higher volume of base required to reach the area of basic dominance.
During the neutralization of high molar mass compounds, the progressive ionization caused by the gradual addition of NaOH generates electrostatic repulsions between the charged carboxylate groups (–COO−) and led to the uncoiling of the polymer chains in solution. This conformational change exposes the carboxyl groups not yet loaded, requiring a larger volume of NaOH to be neutralized [
Additionally, an abrupt change was not detected in pH with the addition of small amounts of base, characteristic behavior of polyelectrolyte, showing the largest change of slope in the region near the detectable monomer point of equivalence. Similar behaviors have been reported by several authors [
In Figure
When it is compared with the curve of native starch, a higher slope is observed, corresponding to the sudden change in pH between 4.0 and 9.0, which is due to the dissociation of the acid groups incorporated into the starch. This behavior was also observed in the oxidized starches but with less steep slopes, which suggests that the copolymer showed the best performance in terms of incorporation. The displacement of the copolymer curve indicates that the content of carboxyl groups was larger than the native starch. In general, the order of carboxyl content to the gel fraction obtained was as follows: St-
In order to promote the esterification reactions of starch with IA, high temperatures (80°C) and addition of NaOH were used to achieve starch gelatinization. Despite the addition of alkali reactions were catalyzed by acid, due to the large number of substituents added, to shift the equilibrium of the esterification toward the formation of the ester, which caused the lowering of the pH to values below 2.0. In Figure
Esterification of starch with itaconic acid to obtain (a) a semiester and (b) the diester.
In Table
Degree of substitution and carboxyl content of starch semiester and diester of itaconic acid.
Sample | DSa | (% COOH)b |
---|---|---|
SI | 0.0032 | 0.1395 |
DI | 0.0030 | 0.0703 |
Furthermore, the carboxyl group content for the SI gel fraction was higher, since being a semiester has a greater amount of free acid groups. By contrast, in the DI, the carboxyl group content was lower, because it is disubstituted. The disubstitution produces inter- or intramolecular crosslinking, also leaving less free –COOH groups, and generates products with a lower solubility and the detection of these groups is difficult [
Figure
Potentiometric titration curves of starch itaconate semiester (SI) and the starch diester itaconate (DI) compared to the native starch (St).
Additionally, both the curve of SI and the curve of DI are located below and displaced to the right with respect to starch, a result of an increase in the content of acid groups on these derivatives with respect to the starting material. It is also appreciated that there is a notable difference between the curve of the DI and SI; the disubstitution prevents the acid groups which are available to react with the base; in contrast with the semiester, ionization occurs more sharply because one of the carboxyl groups of the substituent is free and available to react with the base. For that reason, in the curve of St no sudden changes in pH are observed; for the DI this parameter changes from 5 to 9.15 with the addition of 5 mL of NaOH, while for SI 7 mL of titrant was required to raise its pH of 4.5 to 8.57. In summary, the DI had lower ionization than SI but greater than St, which presented the lowest ionization since it is present in most groups –OH.
In Figure
Comparison of potentiometric titration curves of the products obtained by the type of modification.
However, it was observed that the largest displacement of the curve is given to SI, so it follows that this derivative has an increased amount of neutralizable acid groups, followed by St-
Characterization of the native starch and modified starches was achieved by potentiometric titration. By using this technique, each product could be differentiated based on the content of acid groups, thus allowing the selection of the optimal synthetic route to produce modified starches with ionizable groups. The ionization behavior of the itaconic acid and homopolymer was very different, the latter having a pH change much more attenuated as it is a compound of high molar mass. The graft copolymer had a higher acidity than native starch, including oxidized starches. The ester had a greater number of acid groups than the graft copolymer; therefore, the best way to obtain modified starches with ionizable groups was esterification with itaconic acid, one charge of starch.
Importantly, the use of an automatic titrator and optimizing titration methods could allow obtaining better results. Currently the research group is making efforts to improve the method of determining the degree of substitution of esters and application of potentiometric titration to other polysaccharides and their derivatives.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors thank The Council for Scientific and Technological Development (CONDES) and The Graduate Study Division, Faculty of Engineering, Universidad del Zulia, for financial support.