1、 Experimental Principles and Methods

Using a potentiometric titrator and acid-base titration, the potentiometric titration method was added in Part II of the 2010 edition of the Chinese Pharmacopoeia, page 1226 (Appendix VII A).

【 Content determination 】 Take about 1g of this product, weigh it precisely, place it in a conical flask, add 20ml of sodium hydroxide titrant (1mol/L) precisely, and take a water bath at 70 ℃ for 1 hour. Immediately put it in an ice bath and let it cool. According to the potentiometric titration method (Appendix VII A), titrate with sulfuric acid titrant (0.5mol/L) until the second endpoint; And correct the titration results with a blank test. Every 1ml of sodium hydroxide titrant (1mol/L) is equivalent to 152.1mg of C8H8O3.

2、 Experimental instrument: ZDJ-5 automatic titrator

Electrode: 231- (01) pH glass electrode as indicator electrode

212-01 reference electrode

3、 Reagent:

1) Sodium hydroxide standard solution (1mol/l).

2) Prepare sulfuric acid standard solution (0.5mol/l) and calibrate with sodium hydroxide standard solution (1mol/l).

4、 Sample: Methyl hydroxybenzoate (Nipagin methyl ester, Nipagin M, p-hydroxybenzoate methyl ester), C8H8O3152.15. This product is a white or almost white crystalline or crystalline powder.

5、 Experimental steps

1) Instrument preparation, refer to ZDJ-5 manual

2) Parameter settings:

3) Take about 1g of methyl hydroxybenzoate, weigh it accurately, and place it in a conical flask. Add 20ml of sodium hydroxide titrant (1mol/L) accurately, and take a water bath at 70 ℃ for 1 hour. Immediately place it in an ice bath and let it cool. Refer to the potentiometric titration method (Appendix VII A) and titrate with sulfuric acid titrant (0.5mol/L) to the second endpoint

6、 Titration results:

Sodium hydroxide standard solution (1mol/l) is used to calibrate sulfuric acid.

Sulfuric acid solution concentration: 20 * 1/(22.013+21.954)=0.4548mol/L

Calculate according to the following formula:

Hydroxybenzene methyl ester content=152.1 * (20-C * V/0.5)/(M * 1000) * 100%

Concentration of C-sulfuric acid standard solution (mol/L)

Volume of sulfuric acid standard solution consumed during V-titration (ml)

M - Sample mass (g)

Titration curve of 1.025g hydroxybenzyl methyl ester

End point potential: -26.3mV Consumption volume: 14.449ml

Hydroxybenzene methyl ester content=152.1 * (20-C * V/0.5)/(M * 1000) * 100%

=152.1 * (20-14.449 * 0.4548/0.5)/(1.025 * 1000) * 100%

=101.75%