1. Purpose of the experiment

1-. Understand the principle and method of weak acid determination

2-. Master the determination of boric acid content by enhanced method

2. Experimental principle

Boric acid is a very weak acid, its Ka=5.7x10-10, cannot be directly titrated with NaOH standard solution. Boric acid reacts with glycerol to form glycerol boric acid, whose Ka=8.4×10-6, which can be directly titrated with NaOH standard solution.

Using phenolphthalein as indicator, the end point of titration is reddish.

3. Instruments and reagents

Commonly used laboratory instruments
Boric acid sample
1+1 neutral glycerin mixture (take 50mL glycerin, add 50mL water, mix evenly. Add 2 drops of phenolphthalein indicator, drop 0.01molL NaOH solution to reddish) phenolphthalein indicator
0.1moI/L NaOH standard solution
4. Experimental steps

Accurately weigh 0.2~0.3g sample into a conical flask, add 20mL neutral glycerin mixture, heat slightly to dissolve, add 2 drops of phenolphthalein indicator dropwise after cooling, and titrate with NaOH standard solution until reddish. Then add 3mL glycerol mixture, if the reddish color does not disappear, it is the end point, otherwise, continue the titration, add neutral glycerin mixture, repeat the operation until the reddish color does not disappear as the end point.

5. Result Calculation

The content of the industrial boric acid represented by mass fraction is calculated according to the following formula:

w( %) = cV61.8 10 3 m

In the formula, c—concentration of NaOH standard solution, mol/L;

v——consumed volume of NaOH standard solution, mL; m——mass of industrial boric acid, g3

61.8——Molar mass of H3BO3, g/mol.