Learn the limit test of lead through a laboratory experiment or practical.
Aim:
To perform the limit test for lead of the given samples.
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Requirements:
Glasswares and miscellaneous:
Separating funnel, separating-flask ring, measuring cylinder, pipette, dropper, beaker, volumetric flask, rubber stopper, test tube, glass rod, and balance, etc.
Chemicals:
Ammonium citrate (C6H5+4yFexNyO7), ammonium cyanide(CH4N2), potassium cyanide (KCN), ammonia solution (NH4OH), hydroxylamine hydrochloride (HONH2·HCl), phenol red (C19H14O5S), nitric acid (HNO3), chloroform(CHCl3), and dithizone (C13H12N4S), etc.
Principle of limit test for lead:
The reaction of lead and diphenyl thiocabazone in an alkaline solution produces a lead dithizone complex, which has color-red.
Because dithizone is green in chloroform and the lead-dithizone complex is violet, the final color at the end of the process is red.
Preparation of chemicals:
Preparation of 0.1 % lead standard solution:
In a volumetric flask, dissolve 0.400 gm of lead nitrate in 250.00 ml of distilled water containing 02.00 ml of nitric acid, properly mixing it.
Preparation of 100 ppm lead standard solution:
Take 01 volume of 1% lead standard solution using a pipette and dilute to 10 volume of distilled water in a volumetric flask, and properly mixing it.
Preparation of 10 ppm lead standard solution:
Take 01 volume of 100 ppm lead standard solution using a pipette and dilute to 10 volume of distilled water in a volumetric flask, and properly mixing it.
Preparation of 01 ppm lead standard solution:
Take 01 volume of 10 ppm lead standard solution using a pipette and dilute to 10 volume of distilled water in a volumetric flask, and properly mixing it.
Preparation of 1% v/v nitric acid:
Take 01 volume of nitric acid using a pipette and dilute to 100 volume of distilled water in a volumetric flask, and properly mixing it.
Preparation of ammonium citrate solution:
Take 40.00 gm of citric acid in 90.00 ml of distilled water, add 2 drops phenol red solution, followed by gradual addition of strong ammonia solution until the solution turns a reddish color.
Remove any lead from the solution by extracting it with 30.00 ml increments of dithizone extraction solution until the dithizone solution retains its orange-green color.
Preparation of dithizone extraction solution:
Take 30.00 mg of dithizone in a 1-liter volumetric flask, add 05.00 ml of 95% ethanol, and make up the volume with chloroform. The solution should be kept in the refrigerator. Shake an appropriate volume of the solution with about half the volume of a 1 percent v/v HNO3 before using, and then discard the acid.
Preparation of dithizone standard solution:
Take 10.00 mg dithizone and dissolve to 1000 ml of distilled water in a volumetric flask, and properly mixing it.
Procedure of limit test for lead:
Standard:
- Take the desired amount of lead standard solution in the separating funnel, which is equivalent to the amount of lead allowed in the substance being tested.
- Add 06.00 ml of prepared ammonium citrate solution.
- Add 02.00 ml hydroxylamine hydrochloride and 02.00 ml potassium cyanide.
- Then add 2 drops of phenol red.
- Add ammonia solution to make the solution alkaline.
- Extract instantly with numerous volumes of dithizone extraction solution, each of 05.00 ml, until it becomes green.
- Combine the dithizone extracts with 30.00 ml of nitric acid solution (1 % v/v), shake for 30 seconds, and discard the chloroform layer. (Dithizone remains in the layer of chloroform and lead nitrate is in the aqueous layer)
- Add 05.00 ml of standard dithizone solution to this acid solution.
- Add 04.00 ml of ammonium cyanide solution.
- Shake well for a half-hour, and observe the color of the chloroform layer after separation.
Test:
- Take the desired amount of sample in the separating funnel.
- Add 06.00 ml of prepared ammonium citrate solution.
- Add 02.00 ml hydroxylamine hydrochloride and 02.00 ml potassium cyanide.
- Then add 2 drops of phenol red.
- Add ammonia solution to make the solution alkaline.
- Extract instantly with numerous volumes of dithizone extraction solution, each of 05.00 ml, until it becomes green.
- Combine the dithizone extracts with 30.00 ml of nitric acid solution (1 % v/v), shake for 30 seconds, and discard the chloroform layer. (Dithizone is remains in the layer of chloroform and lead nitrate is in the aqueous layer)
- Add 05.00 ml of standard dithizone solution to this acid solution.
- Add 04.00 ml of ammonium cyanide solution.
- Shake well for a half-hour, and observe the color of the chloroform layer after separation.
Observation:
Test color of the chloroform layer is more/not more intense than the standard color of the chloroform layer.
Results:
The given sample passes/fails the limit test for lead.
FAQ (Frequently Asked Questions):
What is the role of ammonium citrate, potassium cyanide, and hydroxylamine hydrochloride in the limit test for lead?
Ammonium citrate, potassium cyanide, and hydroxylamine hydrochloride are used to adjust the pH to the optimum, removing interference and effects of other impurities.
How many methods are there for the limit test of heavy metals?
The four different methods are used for the limit test of heavy metals; generally, it is based on the reaction of metallic impurities with H2S in an acidic medium to form a colored solution.
Why phenol red is used in the limit test for iron?
Phenol red is used as an indicator, which produces the color at the end of the process.
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