Learn about the assay of borax through a laboratory experiment or practical.
Aim:
To determine the percentage purity (assay) of borax in a given sample using standard 0.5N HCl
Requirements:
Glasswares: Burette, burette stand, conical flask, pipette, beaker, volumetric flask, funnel, glass rod, wash bottle, etc.
Chemicals: Borax, hydrochloric acid (HCl), sodium carbonate (Na2CO3), methyl red indicator, etc.
Apparatus: Digital/analytical balance.
Theory of assay of borax:
Borax has a molecular weight of 381.4 g/mol and is sodium tetraborate decahydrate Na2B4O7.10H2O. It comes in a crystalline powder or a glassy crystal that’s colorless. Since borax is alkaline in nature, it can be titrated with a standard solution of an acid. On titration with HCl, it converted to boric acid and sodium chloride (NaCl).
The indicator must be carefully chosen so that it is not affected by liberated boric acid to obtain the correct endpoint of the titration. The methyl red indicator, which is employed in the assay of borax, which changes from yellow to pink, is more suitable for this requirement. It is an acid-base titration.
The reaction involved in the assay of borax is as follows:
Preparation of 0.5 M hydrochloric acid solution:
Take 42.50 ml of HCl using a pipette, dilute it with 1000 ml of distilled water in a volumetric flask, and properly mix it.
Preparation of 0.5 M sodium carbonate solution:
Take 52.95 gm of anhydrous sodium carbonate and dissolve it in 500 ml of distilled water in a volumetric flask, and properly mix it. Once it has completely dissolved, make up the volume to 1000 ml.
Procedure:
Standardization of 0.5 M hydrochloric acid solution:
- 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.
- Take 10 ml of 0.5 M sodium carbonate solution and pour it into a conical.
- Add 2-3 drops of methyl red indicator.
- Titrate the sample solution with HCl until the endpoint is reached. The actual endpoint of the titration is indicated by the appearance of red color.
- Properly record the readings of the burette.
- To get accurate results, repeat the titration three times.
- Take their mean and calculate the molarity of 0.5M HCl.
Procedure for the assay of borax:
- Accurately weigh 03.00 gm of borax and pour into a conical and add 75.00 ml of distilled water.
- Add 2-3 drops of methyl red indicator.
- Titrate the sample solution with 0.5 M HCl until the endpoint is reached.
- Properly record the readings of the burette.
- To get accurate results, repeat the titration three times.
- Take their mean and calculate the percentage purity of borax.
Observation table:
|
Sr. No. |
Content in conical flask |
Burette reading |
Volume of titrant used (ml) |
|
|
Initial |
Final |
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1 |
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2 |
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3 |
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|
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Mean: |
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Calculations:
Percentage purity of Na2B4O7.10H2O = V x E x AM X 100 / W x RM
Where,
V is a volume of HCl used
E is an equivalent factor
AM is an actual molarity
RM is a required molarity
W is the weight of the sample
Each mL of 0.5M hydrochloric acid is equivalent to 0.09535 gm of borax.
Result:
The percentage purity of the given sample borax was found to be_____.
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