Test for Cupric Radical

Theoretical Background / Context

• Copper was the first and only known metal for over 5000 years. 
• That was around 10,000 years ago when it was discovered for the first time in Iraq and was used in making weapons. 
• Ancient Egyptians have widely used copper in making ships, water pipelines, etc. owing to its robustness and resistance towards the water.

Abundance of Copper in Nature: 

• Copper is the 25th most abundant element on Earth. It is located in the periodic table among the first row transition metals. 
• Copper possesses a soft reddish appearance. It is naturally found among sulphide ores that consist also of arsenic, zinc, and silver metals. 
• Copper represents around 0.5 – 2 % of the ores. Copper can be extracted from its corresponding ores through either pyrometallurgical or hydrometallurgical methods. 
• In addition, copper is naturally abundant as a prosthetic group of some proteins.

Properties and Uses of Copper: 

• Copper is a reddish soft metal that can be reshaped according to the application. 
• It has been widely used in various industries such as coins, metals, utensils, cooling coils in air conditioners and refrigerators, alloys, etc. 
• In addition, its use in some medical, agricultural and electrical applications has been reported.

Copper Salts: 

• Copper is found as either Cu+ or Cu2+ in chemical compounds. 
• However, copper sulfate is one of the most abundant and used copper compounds. 
• It is considered as an inorganic salt with a cupric salt formula of CuSO4. Its melting and boiling points are 200 ⁰C and 650 ⁰C, respectively.  
• Copper salts possess some toxicity depending on the taken concentration. 
• In large amounts, copper salts can cause nausea, vomiting and damage to the body. 
• Continuous exposure to large quantities of copper could lead to severe damage to body fluids, tissues, blood cells, lung, kidney, liver and consequently death, which is referred to as copper toxicity.

Preparation of Copper Sulfate: 

Copper sulfate can be either prepared through the reaction of copper metal with concentrated sulfuric acid or copper oxide with dilute sulfuric acid as the following reactions:

Cu + 2H2SO4 (conc.) → CuSO4 + SO2 + 2H2O

CuO + H2SO4 (dil) → CuSO4 + H2O 

Properties and Uses of Copper Sulfate

• Copper sulfate is a white powdered salt when dehydrated. 
• Its color turns blue upon its hydration, where it attaches to five water molecules to possess a chemical formula of CuSO4.5H2O.
• Copper sulfate can cause eye irritation, nausea, vomiting, etc.
• Due to its toxicity in certain concentrations, it can be used in killing fungi, bacteria, etc.
• Copper sulfate is used as a mordant in vegetable dyeing.
• Due to its color change according to humidity, it is used as an analytical reagent.
• It has two oxidation states, so it is used in various organic synthesis techniques.
• It is involved in the preparation of catalysts.
• It has been used in various applications such as etching, dyeing, electronics, etc.

Principle of Work

• In the test for cupric radicals, cupric ion in copper sulfate is detected through some identification and confirmatory tests. 
• The cupric radical is among the second group of basic radicals in which hydrogen sulfide is used as group reagents.
• During the cupric ion chemical test, salt solubility in water will be tested. 
• Then the behavior of the salt will be tested with hydrogen sulfide followed by confirmatory tests which will be performed using test for copper ions with sodium hydroxide, ammonia, and potassium iodide tests to confirm the presence of cupric radical in the salt.

First: Physical Appearance Test: In this test, a sample of the cupric salt is tested for its color, odor, texture, etc.

Second: Solubility Test: In this laboratory test for copper radical, a sample of the cupric salt is tested for its solubility in water. 

Third: Hydrogen Sulfide Test: It depends on the fact that sulfide anions can displace sulfate ions in cupric sulfate salt forming insoluble cupric sulfide as a black precipitate which is soluble in hot dilute nitric acid as the following reactions:

H2S + CuSO4 → H2SO4 + CuS ↓ (Black ppt.)

3CuS + 8HNO3 → 3CuNO3 + 4H2O + 2NO + 3S 

Fourth: Sodium Hydroxide Test: Sodium Hydroxide solution is added to aqueous cupric sulfate leading to the precipitation of cupric hydroxide as a blue precipitate due to its low solubility product. 

Then, upon boiling, the precipitate changes its color into black due to its conversion into copper oxide:

 CuSO4 + 2NaOH → Na2SO4 + Cu(OH)2 ↓ (Blue ppt.) 

Cu(OH)2 → H2O + CuO ↓ (Black ppt.) 

Fifth: Ammonia Test: 

• Ammonia solution is added to aqueous cupric sulfate leading to the precipitation of cupric hydroxide as a blue precipitate due to its low solubility product. 
• Then, upon adding excess ammonia, the precipitate dissolves again due to the formation of the soluble tetra-amine cupric sulfate complex:

CuSO4 + 2NH4OH → (NH4)2SO4 + Cu(OH)2 ↓ (Blue ppt.) 

NH4OH + Cu(OH)2 → [Cu(NH4)2]2+ + 2OH-

Sixth: Potassium Iodide Test: Potassium iodide solution is added to cupric sulfate aqueous solution resulting in precipitation of cuprous iodide as a white precipitate due to low solubility product as shown in the following chemical reaction:

2CuSO4 + 4KI → 2K2SO4 + I2 + 2CuI↓ (White ppt.)