What is the Chromyl Chloride Test?

Chromyl chloride is a kind of chemical compound that is represented as CrO2Cl2. They are tetrahedral and appear the same as dark red blood. Mostly they are made of chromium as an initial derivative.

Mostly it works as an oxidising agent and is found to exhibit oxidation properties to oxidise toluene into benzaldehyde. It can easily form fumes in the air and can reach it easily with alcohol and water. It can easily be dissolved in chlorinated and carboxylic acids. Though it is used to detect the chloride ions, there are some exceptions on which this test is not used including the silver and mercuric chlorides. The evidence for this is that these molecules form a covalent bond with chloride ions and hence cannot be detected by Chromyl chloride.

Chromyl chloride test reactions Example

Some of the examples of turmoil chloride are as follows:

• Oxidation of benzene: when chromyl chloride reacts with benzene, it leads to the formation of chromyl compounds with benzenes. The reaction is given as: 

C6H5CH3 + 2CrO2Cl2 → C6H5CH(CrO2Cl2)2 

This further reaction with water gives benzaldehyde. This reaction is termed as Etards reaction.

• Oxidation of alkenes: CrO2Cl2 is also involved in the oxidation of alkenes into aldehydes. It leads to the production of alpha chloro ketones and is also able to react with benzylic methyl groups like stated above in regard to reaction.

Experimentation to carry out Chloride chromyl test:

Requirements: Sulphuric acids, dilute acetic acid, powdered potassium dichromate, sodium hydroxide, salt, lead acetate, spatula, bunsen burner, test tube, test tube holder, dropper, glass rod.

Procedure: 

Follow the given steps to conduct a chromyl chloride test on the given sample.

• Into a small quantity of salt, add a small quantity of potassium dichromate followed by mixing well.
• With the help of a dropper, drop a small amount of sulphuric acid in a test tube
• Heating of test tube carefully
• Heat up till red fumes are visible in the test tube
• Once red fumes start coming out, collect them using another test tube
• Addition of sodium hydroxide to the tube that contains red fumes
• Allow reacting
• The yellow solution will be formed indicating the formation of sodium chromate
• Addition of dilute acetic acid followed the by addition of lead acetate
• Allow reacting 
• Yellow precipitates indicate the reaction is complete.

Precautions:

• Since the experiment involves the handling of fumes so care must be taken to avoid burns
• Avoid contact of chromyl chloride with eyes and sensitive areas.
• It is to be noted that chromyl chloride is highly carcinogenic, so care must be taken.
• Must be stored in a glass jar
• Some reactions may involve the evolution of heat, so be careful during such reactions.

Chromyl chloride test reaction mechanism:

It relies on a simple basic mechanism.

• Potassium dichromate, when reacted with sulphuric card, yields chromate trioxide. Chromate trioxide is a dark reddish-brown coloured compound.

K2Cr2O7 + H­2SO4 → CrO3 + H2O

• Next, sodium chloride is reacted with sulphuric acid to yield sodium bisulphate along with hydrochloric acid.

NaCl +H2SO4 → NaHSO­4 +HCl

• In the final step, Chromate trioxide is allowed to react with hydrochloric acid that yields chromyl chloride.

CrO3 + HCl → CrO2Cl2

Confirmatory test for chromyl chloride test:

For the confirmation purpose, Sodium hydroxide is used to let the red vapour get dissolved into it. This is marked by the conversion and formation of yellow coloured compounds.

CrO2Cl2 + NaOH → Na2CrO4 + NaCl +H2O

This is further treated with lead acetate and dilute acetic acid and forms a yellow precipitate.

CrO42- + Pb(CH3COO)2 → PbCrO4 +CH3COONa

 The formation of yellow precipitate means the test is successful.

Exceptions and alternative tests:

Since the chromyl chloride test does not apply to mercuric, iodide, bromide and silver chlorides, alternative tests are employed to detect the presence of chloride in them.

This can be achieved by preparation of soda extract of the given compounds, and then thereby treating them with silver nitrates which further precipitate out in the form of silver chloride. At the concluding step, Sodium Arsenate is added, and the test tube is well shaken then. The formation of a yellow precipitate indicates that the experiment is accurate and there is a presence of chloride ions.

A consequence of inhaling Chromyl chloride:

Chromyl chloride is very powerful and can irritate the respiratory tract. Inhalation of chromyl chloride can be severe and fatal. It may produce respiratory burns and can be harmful to the kidney and liver, and also promotes lung cancer and gastrointestinal tract tumours. Chromyl chloride rapidly forms flames.

Conclusion :

We can confirm the presence or absence of chloride ions in a given sample by employing the chromyl chloride test. However, some substances still can not be identified and detected by the chrome chloride test. The one exception is the Mercuric chloride. The other elements that cannot be detected for their reaction with chlorine are Hg, Sn, Ag, Pb and Sb. The reason is that these compounds are covalent and hence cannot form chloride ions; hence the test is not acceptable on them. This is to remember that the chromyl chloride test can only be used for the molecules that tend to form ionic bonds.