Learn the limit test for arsenic through a laboratory experiment or practical.
Aim:
To perform the arsenic limit test for chloride of the given samples as per IP.
Sample: Sodium acetate (CH3COONa)
Reference: Indian Pharmacopoeia (IP) 2010, www.web-formulas.com
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Requirements:
Glasswares and miscellaneous: Measuring cylinder, pipette, glass rod, dropper, rubber stopper, test tube stand, mercuric chloride paper, cotton wool, balance, and arsenic limit test apparatus, etc.
Chemicals: Arsenic trioxide (As2O3), sodium hydroxide (NaOH), sodium acetate (C2H3NaO2), hydrochloric acid (HCL), stannous chloride (SnCl2), potassium iodide (KI), lead acetate (Pb(C2H3O2)2), and zinc (Zn+2), etc
Principle of limit test for Arsenic:
The limit test for arsenic is also known as the Gutzeit test which is based on the reaction of arsenic gas with hydrogen ions to form a yellow stain on paper of mercuric chloride in the presence of reducing agents such as KI is used to determine the limit of arsenic.
Arsenic present in the form of arsenic acid in the sample is reduced to arsenic acid by reducing agents such as KI, SnCl2, zinc, and HCL. Arsenious acid is further reduced to arsine (gas) by hydrogen and reacts with mercuric chloride paper to give a yellow stain.
The depth of yellow stain on mercuric chloride paper will depend upon the quality of arsenic present in the sample.
Preparation of chemicals:
How to prepare 10 ppm arsenic standard solution:
Take 00.33 gm of arsenic trioxide in 05.00 ml of 2 M NaOH and dilute to 250.00 ml of distilled water in a volumetric flask, and properly mixing it. Take 01 volume of this solution and to 100 volumes with distilled water.
How to prepare 02 ppm arsenic standard solution:
Take 02 volumes of prepared 10 ppm arsenic trioxide standard and dilute to 10 volumes with distilled water.
Preparation of stannous chloride solution:
Take 330.00 gm of stannous chloride in 100.00 ml of hydrochloric acid in a volumetric flask, and properly mixing it, add this solution in 500 ml of distilled water, and make up the volume to 1000 ml with water.
How to prepare stannated hydrochloric acid:
Take 01.00 ml of stannous chloride solution and dilute to 100.00 ml of HCL.
Preparation of sodium acetate test solution:
Take 05.00 gm of sodium acetate in 50.00 ml distilled water and add 15.00 ml of stannated hydrochloric acid.
How to prepare 1M potassium iodide solution:
Take 01.66 gm of potassium iodide and dissolve in 50.00 ml of distilled in a volumetric flask and properly mixing it. Once it has completely dissolved, make up the volume to 100 ml.
How to prepare 2M sodium hydroxide solution:
Take 08.00 gm of NaOH and dissolve in 70.00 ml of distilled in a volumetric flask and properly mixing it. Once it has completely dissolved and cool, make up the volume to 100 ml.
Procedure of limit test for arsenic:
Standard:
- Take 01.00 ml of prepared 2 ppm arsenic standard solution using a pipette into a glass bottle flask, dilute to 50.00 ml with water and label it as standard.
- Then add 15.00 ml of stannated hydrochloric acid.
- Add 05.00 ml of prepared 1M potassium iodide solution.
- Then add 10.00 gm of zinc and properly mixed it.
- Assemble the apparatus as soon as possible, and place the bottle in a water bath at a temperature that ensures a uniform gas evolution.
- Allow 40 minutes for the reaction to occur.
Test:
- Take 05.00 gm sodium acetate diluted with 50.00 ml of water in a glass bottle flask and label it as standard.
- Then add 15.00 ml of stannated hydrochloric acid.
- Add 05.00 ml of prepared 1M potassium iodide solution.
- Then add 10.00 gm of zinc and properly mixed it.
- Assemble the apparatus as soon as possible, and place the bottle in a water bath at a temperature that ensures a uniform gas evolution.
- Allow 40 minutes for the reaction to occur, any stain formed by the sample on mercuric chloride paper is compared to that of the standard.
Observations:
The stain produced in the sodium acetate test sample is less/same/more than the stain produced in standard.
Results:
The given sample of sodium acetate compile/does not comply the limit test for arsenic as per the Indian Pharmacopoeia.
FAQ (Frequently Asked Questions):
Why Lead acetate is used in the limit test for arsenic?
In the limit test of arsenic, lead acetate cotton wool is used to form a stain that is used to compare the results.
How to prepare mercuric chloride paper?
Take a smooth white filter paper which has less than 25 mm in width. Soak it in a saturated mercuric chloride solution, squeezed it to remove excess solution, and dried in the dark place at 60°C.
When used in the limit test for arsenic, a paper that has been exposed to sunlight or ammonia vapor produces a lighter stain or no stain at all. Therefore it needs to be store in a stoppered bottle in a dark place.
How to prepare lead acetate paper?
Prepare it with a solution of lead acetate and dry the impregnated paper at 100°.
How to prepare lead acetate cotton?
Immerse absorbent cotton in a mixture of 10 volumes of 10% w/v lead acetate solution and 1 volume of 2 M acetic acid. Remove excess fluid without squeezing the cotton wool, just lay it on several layers of filter paper. Allow time for air drying at room temperature and protect from moisture when storing.
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