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Determination of ionization constant of hydroxypropyl methylcellulose trimellitate by ultraviolet spectrophotometry

Views: 0     Author: Site Editor     Publish Time: 2023-06-26      Origin: Site

Abstract: A method for determining the ionization constant of hydroxypropylmethylcellulose trimellitate (HPMCT) by ultraviolet spectrophotometry was established. The test principle, test steps and data processing methods are introduced. The measurement results show that: solvent, solution pH value and measurement wavelength are the key factors affecting the measurement results; the pKa value of the self-synthesized HPMCT with an esterification rate of 32.34% in anhydrous ethanol-water system is 4.77 .

Key words: hydroxy propyl methyl cellulose trimellitate; UV spectrophotometry; ionization constant

Hydroxypropyl methylcellulose trimellitate (HPMCT) is the trimellitic acid half ester of hydroxypropyl methylcellulose (HPMC), which is obtained by esterifying HPMC with trimellitic anhydride. HPMCT is a new type of enteric film coating material with excellent performance. It has the characteristics of good film-forming property, low pH value of dissolution (pH above 4.0-5.0), fast dissolution speed and stable physical and chemical properties. In the molecular structure of HPMCT, the carboxyl group on the trimelliticoyl group can be ionized in aqueous solution, and the degree of ionization is affected by the pH value of the solution and the self-ionization constant Ka of HPMCT. As an enteric coating material, the size of the ionization constant directly affects the disintegration of HPMCT in a specific situation and the release rate and bioavailability of the coated drug. Therefore, HPM. The determination of CT ionization constant is the premise and key to study its performance in a series of applications. There are potentiometric titration, ultraviolet spectrophotometry, capillary electrophoresis and other methods for the determination of Ka value of small molecular substances, but some of these methods are not suitable for determining the Ka value of HPMCT due to the structural characteristics of long chains of polymers. If the potentiometric titration method is used to measure, it is found that when the titration is carried out to a certain extent, the solution will appear cloudy, indicating that some HPMCT molecules have been precipitated from the solution. This will inevitably lead to inaccurate titration results. The authors explored the use of spectrophotometry to determine the Ka value of HPMCT, which has a simple testing process. The characteristics of convenient data processing.

1. Experimental part

1.1 Experimental principle

1.2 Instruments and reagents

UV757CRT UV-Vis spectrophotometer, ZDJ. 4A type automatic potentiometric titrator, all produced by Shanghai Precision Scientific Instrument Co., Ltd.

HPMCT was synthesized by the author with an esterification rate of 32.34%.

1.3 Experimental steps

Accurately pipette 10 parts of 1 mL stock solution and place them in 10 volumetric flasks respectively, use buffer solutions with different pH values to make up to 50 mL respectively, and use the respective buffer solutions as controls, and scan in the wavelength range of 200-400 am to obtain A series of absorption curves of HPMCT solutions with the same concentration at different pH values. Select the wavelength whose absorbance changes greatly with the change of pH value as the measuring wavelength, and measure the absorbance of HPMCT solution with different pH values. According to the change trend of absorbance, the Ka of HPMCT was calculated by graphic method.

2. Results and Discussion

During the experiment, it was found that to ensure the accuracy of the test results, attention should be paid to the selection and control of the following three factors.

2.1 Selection of solvent

Generally speaking, in addition to good solubility to the solute, the selected solvent also requires that its "transparent" limit wavelength does not interfere with the solute to be measured. The limiting wavelength of ethanol is 210 nm and that of water is 191 nm. The mixed solvent of ethanol-water is a good solvent for HPMCT. The limit wavelength is low, so this mixed solvent is selected as the solvent for determination in this experiment. Because the main measurement parameter in this method is the absorbance of the HPMCT solution, therefore, in order to ensure that the absorbance is not affected by its dissolution. The selected solvent must be a good solvent for HPMCT, so that a uniform solution can be obtained.

2.2 Selection of solution pH range

2.3 Selection of measurement wavelength

2.4 Measure the absorbance of the solution at the selected wavelength

HPMCT absolute ethanol at pH = 7.6, 8.4 and 10.0 was measured at three selected wavelengths. Absorbance of aqueous solution.

2.5 Calculation of Ka value of HPMCT by graphic method

3. Conclusion

A spectrophotometric method for determining the ionization constant of HPMCT was established, and it was shown that the selection and determination of the solvent, the pH range of the solution, and the wavelength of ultraviolet light are the keys to ensure the accuracy and repeatability of the test results. According to the established method, the ionization constants of HPMCT in anhydrous ethanol and water were measured in parallel to be 4.77, 4.76 and 4.78, and the reproducibility of the results was good.