4.3 Formula 3

In our study, a ratio had been already proposed [68, 71, 72]. However, choosing an appropriate calculation procedure was not an easy task since the choice of the baseline on FTIR spectra, reference, and the probe bands was difficult. In our work, the average DD was determined by the following formula:

$$\text{DD\%} = \mathbf{100} - \left[ \left( \frac{\mathbf{A\_{1655}}}{\mathbf{A\_{3450}}} \right) \* \frac{\mathbf{100}}{\mathbf{1.33}} \right] \tag{4}$$

5. Optimization of chitosan extraction by IR

DOI: http://dx.doi.org/10.5772/intechopen.89708

Quantitative Analysis by IR: Determination of Chitin/Chitosan DD

influences the reaction rate of DD than temperature.

Table 5.

115

5.2 IR study of the effect of temperature on the chitin deacetylation

Table 6 shows the effect of temperature on deacetylation; the more the temperature increases, the more the deacetylation increases, but the total conversion of

Temperature of DD Concentration of the base (M) Time (mn) DD 25°C 8 180 38

80°C 4 38 35

120°C 8 50 min 40

Evolution of DD as a function of the concentration of the base at different temperatures [3].

10 120 45 12 60 55

8 38 52 10 38 55 12 38 63

12 50 min 50 8 300max 65 12 300max 70

5.1 IR study of the effect of alkaline concentration on the chitin deacetylation

In this deacetylation reaction, it is an interrelation between the following variables: the concentration of the basic solution, the temperature, and the reaction time. The deacetylation reaction in basic medium is summarized in Eq. (5):

For CNaOH = 12 N, the DD values calculated from the FTIR shown in Table 5 do not exceed 55% and stabilize after a critical time t < 60 min. At a low concentration of NaOH, the equilibrium time became higher, which shows that the chitosan DD is low, which means that the deacetylation reaction is highly dependent on the concentration of NaOH, due to the inaccessibility of acetamide groups in the polymer chain. This type of behavior, observed by other authors, is explained firstly by the fact that the N-deacetylation occurs preferably at the level of the amorphous region of chitin and then passes from the edge to the interior of the crystalline region [26, 27]. The second reason concerns the equilibrium of the reaction and the degradation of chitosan. Other authors [10, 22, 28] have assumed that it can be controlled by both reaction and diffusion. The low deacetylation of chitin has also been attributed to the rearrangement of acetyl groups in the monomer unit with respect to the OH hydroxyl group [9]. Analysis of the results of the measurement of the DD (Table 5) shows that this increased by approximately 8% (66–74% DD), at a high concentration of NaOH (10 M) and with a thermal increase of 20°C (100–120°C). This variation of DD seems less important, only +2% (26–28% DD) at low concentration of NaOH (7.5 M). This increase in DD is particularly significant (13%) with a base concentration increase of 2.5 M (10 M to 12.5 M). Thus, the variation in NaOH concentration more significantly

<sup>R</sup>‐NHCOCH3 <sup>þ</sup> OH� ! <sup>R</sup>–NH2 <sup>þ</sup> CH3COO� (5)

where A1655 is the absorbance at 1655 cm�<sup>1</sup> of the amide I band as a measure of the N-acetyl group content and A3430 is the absorbance at 3430 cm�<sup>1</sup> due to hydroxyl group as an internal standard. The value 1.33 represents the ratio of this absorbance for a fully acetylated compound. An appropriate baseline in each spectrum was determined by using origin software.

The baseline problem for reference peaks has been studied and summarized in Table 4 and Figure 2. Table 4 shows that there are other formulas for calculating chitosan DD by FTIR and that these formulas have been approved by other techniques such as NMR.

A synthesis of the study on the effect of the operating conditions to know temperature concentration of the base and the time on the reaction of deacetylation will be presented using IR.


Table 4.

Calibration curves from absorption ratios versus standard DA values [73].

Figure 2. FTIR spectrum of chitin and the baseline [73].
