How to determine the molecular weight of cellulose by precipitation titration

Precipitation titration is a method of calculating the molecular weight of the polymer by reacting the terminal groups of the polymer with a specific reagent, and calculating the amount of precipitate produced by titration. For cellulose , titration assays are usually performed for terminal hydroxyl or carboxy groups. Here are the detailed steps:

1. Sample preparation

Sample dissolution: Cellulose dissolves in solvent to ensure uniform dispersion. For cellulose, solvent systems such as DMAc/LiCl can be used.

2. Chemical reactions

Select a suitable titrator: Select a suitable precipitant according to the properties of the cellulose terminal groups. The titrator commonly used for carboxyl groups is silver ion solution (AgNO ₃ ), which reacts with terminal carboxyl groups to form an insoluble precipitate (such as AgCl).

3. Titration steps

Mix the cellulose solution with a reaction reagent (such as AgNO ₃ ) to observe the process of producing precipitate. Add AgNO dropwise ₃ until no new precipitation is generated (the end point can be determined by indicator or potential titration).

4. Calculate molecular weight

Based on the known precipitant consumption, combined with the mass and stoichiometric relationship of the cellulose sample, the number average molecular weight is calculated using the following formula where:

m is the mass of the cellulose sample,

n is the moles of the titrator consumed during the titration process (can be determined by the chemical equation of the precipitation reaction).

5. Things to note

Solution purity and precipitation completeness: Ensure the reaction is complete, precipitation is fully generated, and avoid errors caused by incomplete reactions.

Standardized titrator: Before titration, the titrator should be standardized to ensure accuracy.

Advantages and limitations

Advantages: The method is relatively simple and suitable for detecting polymers with obvious terminal groups.

Limitations: Suitable for low molecular weight cellulose . High molecular weight samples have relatively few end groups, which may lead to a decrease in sensitivity.

This method is suitable for samples with more significant carboxyl terminus in cellulose, and the determination is more accurate.