Identification and Confirmatory Test for Lead Radical

Theoretical Background / Context

• Lead is a heavy metal whose symbol is Pb. It possesses a higher density than most of the other metals.
• It is characterized by being soft and easily reshaped.  In its pure form, lead has a silvery color with a slight blue tinge. 
• Lead is not a highly reactive post-transition metal, however, it can turn into gray upon its exposure to air for long times due to its oxidation.

The abundance of Lead in Nature: 

• Lead is not commonly abundant in the Earth’s crust. However, it could be refined and used easily.
• In addition, various lead isotopes are liberated at the ending stage of the natural decay of various elements such as actinium, thorium, and uranium.

Properties and Uses of Lead: 

• Once cut, lead possesses a blue-white color. However, it turns into dull grey upon being exposed to air for long times due to its oxidation into Pb2O. 
• Most stable lead compounds are formed of lead in its 2+ oxidation state.  Its main use is in the manufacture of lead-acid storage batteries that are used in automobiles. 
• In pure form, it is too soft to be used for much else.

Lead Salts: 

• Lead nitrate is one of the most known and commonly abundant lead salts. Lead nitrate is an inorganic compound with a chemical formula of Pb(NO3)2.
• Unlike most lead salts, lead nitrate is soluble in water. Lead nitrate has been known as plumb dulics.

 Preparation of Lead Salts: 

Lead(II) nitrate can be prepared on a small scale using either metallic lead or lead oxide and nitric acid. 

Pb + 4 HNO3 → Pb(NO3)2 + 2 NO2 + 2 H2O

In the 19th century, lead(II) nitrate started to be prepared for commercial uses in Europe and the United States.

 Properties and Uses of Lead Nitrate

• Lead nitrate is a white crystalline or powder salt that is soluble in water forming colorless aqueous solutions.
• Lead nitrate is a strong oxidizing reagent.
• Lead nitrate is a toxic compound and is listed as a carcinogenic agent to humans according to the International Agency for Research on Cancer.
• Lead nitrate has been used as a raw material for manufacturing pigments of lead paints. However, titanium based paints have shown less toxicity.
• Lead nitrate solutions are used to improve the leaching process in gold cyanidation.  
• Lead nitrate has been used long ago in stabilizing heat in the industries of nylon, polyesters as well as in photo-thermographic paper coatings.

Principle of Work

•  In this lead nitrate experiment, lead ion in lead nitrate is detected through some identification and confirmatory tests.   
• In the virtual lab, a simulated lead sample was used to demonstrate the chemical reactions involved in the lead ion test experiment.
• The lead radical is among the first group of basic radicals in which hydrochloric acid is used as group reagents.
• During the experiment, salt solubility in water will be tested. 
• Then the behavior of the salt will be tested with hydrochloric acid followed by confirmatory tests which will be performed using sodium hydroxide, potassium iodide, potassium chromate and sulfuric acid reagents to confirm the presence of lead radical in the salt.
• This lead nitrate salt analysis follows standard protocols in lead test chemistry.

First: Solubility Test

In this test, a sample of the lead salt is tested for its solubility in water.

Second: Dilute Hydrochloric Acid Test

It depends on the fact that dil. HCl can displace nitrate ions in lead nitrate salt forming insoluble lead chloride salt that precipitates as a white precipitate to test for lead ions.

Pb(NO3)2 +2HCl → 2HNO3 + PbCl2 ↓ (White ppt.)

Third: Sodium Hydroxide Test

• Sodium Hydroxide solution is added to aqueous lead nitrate leading to the precipitation of lead hydroxide as a white precipitate due to its low solubility product. 
• The reaction of the test is:

Pb(NO3)2+2NaOH →2NaNO3 + Pb(OH)2 (White ppt.)

Fourth: Potassium Iodide Test

Potassium iodide solution is added to lead nitrate aqueous solution resulting in precipitation of lead iodide as a yellow precipitate due to low solubility product as shown in the following chemical reaction:

Pb(NO3)2 + 2KI → 2KNO3 + PbI2↓ (Yellow ppt.)

Fifth: Potassium Chromate Test

• Potassium chromate test is considered as a specific confirmatory test for lead ion for detection of the presence of lead ion as the basic radical of a salt. 
• The test depends on adding potassium chromate solution to aqueous lead nitrate solution leading to the formation of yellow colored precipitate of lead chromate according to the following reaction:

K2CrO4 +  Pb(NO3)2 → 2 KNO3 + PbCrO4↓ (Yellow)

Sixth: Sulfuric Acid Test - during the Confirmatory test for lead

Sulfuric acid test is considered as a specific test for lead salts as they react together forming a white precipitate of lead sulfate according to the following reaction:

  H2SO4 +  Pb(NO3)2 → 2HNO3 + PbSO4↓ (White)