Chrominfo: Assay of boric acid

Learn about the assay of boric acid through a laboratory experiment or practical.

Aim:

To determine the percentage purity (assay) of boric acid in a given sample using standard 0.1 N NaOH.

Reference: Indian Pharmacopeia (IP)

Principle of assay of boric acid:

The alkalimetry method is used to perform an assay of boric acid. Since it is such a weak acid, it cannot be accurately titrated with a strong base. It behaves like a strong monobasic acid when dissolved in a mixture of water and glycerol, and it may then be titrated with a standard alkali solution using phenolphthalein as an indicator.

Requirements:

Glasswares: Burette, burette stand, conical flask, volumetric pipette, beaker, volumetric flask, funnel, glass rod, and wash bottle, etc.

Chemicals: Boric acid (H3BO3), glycerol (C3H8O), phenolphthalein indicator, sodium hydroxide (NaOH), and oxalic acid (C2H2O4), etc.

Preparation of 1N NaOH solution:

Take properly weighed 04.00 gm of sodium hydroxide and dissolve in 50 ml of distilled water in a volumetric flask, and properly mix it. Once it has completely dissolved, make up the volume to 100 ml.

Preparation of 1N oxalic acid solution:

Take properly weighed 06.30 gm of oxalic acid and dissolve in 50 ml of distilled water in a volumetric flask, and properly mix it. Once it has completely dissolved, make up the volume to 100 ml.

Titration procedure:

All glassware should be cleaned and dried according to standard laboratory procedures.
Before filling the burette for the titration, rinse it with distilled water and then pre-rinse it with a portion of the titrant solution. Pre-rinsing is required to make sure that all solution in the burette is the desired solution, not a contaminated or diluted solution.
Take the unknown stock solution of titrant in a clean and dry beaker then fill the burette using the funnel.
Remove air bubbles from the burette and adjust the reading to zero.

For standardization of sodium hydroxide:

Pipette out 10.00 ml of 0.1 N oxalic acid into a conical flask and add 1-2 drops of phenolphthalein indicator.
Titrate the solution against the NaOH solution until it turns pink in color.
Repeat the titration to get concurrent values.

For assay:

Take properly weighed 02.00 gm and dissolved in 50.00 ml water and 100 ml glycerol solution previously neutralized to phenolphthalein solution.
Add 2-3 drops of phenolphthalein indicator.
Titrate the sample solution with 1N NaOH until the endpoint is reached.
The actual endpoint of the titration is indicated by a faint pink color.
Properly record the readings of the burette.
To get accurate results, repeat the titration three times.
Take their mean and calculate the % purity of boric acid.

Observation table:

Sr. No.	Content in conical flask	Burette reading		Volume of titrant used (ml)
Sr. No.	Content in conical flask	Initial	Final	Volume of titrant used (ml)
1
2
3
				Mean:

Calculation:

I.P. factor: Each ml of 1 N NaOH is equivalent to 0.016183 g of H3BO3.

Formula:

Percent purity of H3BO3= Volume of NaOH x I.P. Factor x 100 x N of NaOH (actual) / weight of H3BO3 x N of NaOH (exp)

Result:

The percentage purity of the boric acid (H3BO3) sample was found to be_____.

Frequently Asked Questions (FAQ):

Why Glycerin is used in the assay of boric acid?

Glycerin is used in the assay of boric acid because boric acid is a weak monobasic acid that does not easily react with sodium hydroxide because of its low tendency to give hydrogen ions in its solution. When glycerol and boric acid are combined, the boric acid becomes more acidic and can readily act as a monobasic acid.