Carbonate Radical - Carbonate Test

Theoretical Background / Context

• In inorganic chemistry, carbonate (CO32-) is the conjugate base of bicarbonate (HCO3-) which is the conjugate base of the carbonic acid (H2CO3), and carbonate and bicarbonate identification relies on their distinct chemical properties.
• Carbonate anion carries a formal charge of -2 and has a molar mass of 60.01 g/mol. 
• Carbonic acid is a diprotic acid i.e. has two hydrogen atoms. 
• Therefore, it possesses two dissociation constants. 
• The first one is related to the dissociation of carbonic acid into bicarbonate ions as shown below.

 Ka1 = 2.5×10−4, pKa1 = 3.6 at 25 °C.

• On the other hand, the second constant is related to the dissociation of bicarbonate ions into carbonate ions as shown below.

 Ka2 = 4.69×10−11; pKa2 = 10.329 at 25 °C

•  In organic chemistry, the carbonate term refers to the ester group, a functional group consisting of a carbonyl group where the carbon atom of the carbonyl group is linked to two oxygen atoms, one from each side. 
• Then each oxygen atom is linked to R group that could be an aliphatic or aromatic potion of organic compounds as shown below.

•  In inorganic chemistry, carbonate anion, polyatomic ion with the chemical formula of CO32-, is considered to be the simplest abundant oxocarbon anion. 
• It consists of one carbon atom linked to three oxygen atoms in a trigonal planar conformation. 
• The Lewis structure of the carbonate ion suggests the presence of two long single bonds that link the central carbon atom with two negatively charged oxygen atoms, and a shorter double bond that links the central carbon atom to neutral oxygen as shown below.
• However, the previously illustrated suggestion does not match the symmetry of the carbonate ion that has been observed in which the three bonds that link the central carbon atom with the three oxygen atoms are equivalent. 
• This is justified by the resonance among the carbonate structure as shown below:
• This could be summarized by the following resonating model that possesses equivalent fractional charges on the three oxygen atoms:
• Carbonic acid is not commonly used. However, there are various carbonate salts that possess different properties and are used in various applications. 
• For instance, all normal carbonate salts except those of alkali metals (such as sodium and potassium) and ammonia, are insoluble in water.
• One of the most common and widely used carbonate salts is sodium carbonate with a chemical formula of Na2CO3 and a molar mass of 105.98 gm/mol. Sodium carbonate is a white odorless powder that is highly soluble in water. 
• Sodium carbonate has been used as an additive in some cleansing and cosmetics preparations. 

• For example, it has been used as a water softener in laundry products and swimming pools. In addition, it has been used as a foaming agent in kinds of toothpaste. 
• Furthermore, sodium carbonate has been used in different industrial applications such as soda ash, soda crystals, glass, etc. 
• Additionally, it has various chemical applications. 
• For instance, it is used as either standard in acid-base titrations or as an electrolyte in electrolysis.  
• Carbonate ion is classified as a member of acidic radicals of the first group in which hydrochloric acid is used as the group reagent.

• In addition, soluble carbonate salts such as sodium carbonate could be detected through some confirmatory tests using mercuric chloride and magnesium sulfate.

Principle of Work

• In the carbonate test experiment, carbonate ion in sodium carbonate is detected through some carbonate identification test methods and confirmatory test for carbonate.
• In addition, these test(s) can be used to differentiate between the first anionic class and other acid radical classes. 
• Moreover, confirmatory and specific tests are used to differentiate between the presences of different members of the first class of anions.

First: Solubility Test 

• In this test, a sample of sodium carbonate salt is tested for its solubility in distilled water on cold. 
• Most carbonate salts are water insoluble except alkali metals such as sodium and potassium in addition to ammonium carbonates which are soluble in water without the need of heating.

Second: Hydrochloric Acid Test (HCl test for carbonate radical)

• It depends on the fact that hydrochloric acid can displace carbonate ions in its sodium salt forming sodium chloride salt and carbon dioxide gas. 
• The evolved gas (CO2) can pass through lime water causing its turbidity due to the conversion of the soluble calcium hydroxide into the insoluble calcium carbonate.

Step 1: 

Reaction of Sodium Carbonate with Hydrochloric Acid.

Na2CO3 + 2HCl → 2NaCl + H2O + CO2 ↑

Step 2: 

Reaction of CO2 gas with lime water.

CO2 + Ca(OH)2 → H2O + CaCO3 ↓ (White ppt)

Third: Mercuric Chloride Test -  Carbonate confirmation test

• Mercuric chloride solution is added to a solution of sodium carbonate leading to the precipitation of mercuric carbonate salt as a reddish-brown precipitate due to its low solubility product. 
• The reaction of the test is:

Na2CO3 + HgCl2 → 2 NaCl + HgCO3↓ (Red-brown)

Fourth: Magnesium Sulfate Test - Confirmation test for carbonate procedures

Magnesium sulfate solution is added to sodium carbonate solution resulting in precipitation of magnesium carbonate as a white precipitate due to low solubility product as shown in the following chemical reaction:

MgSO4 + Na2CO3 → Na3SO4 + MgCO3 ↓ (White ppt)