Lab Identification

Carboxylic acids are identified as such in the lab by their acidic properties. How this is done depends on whether the acid is soluble in water.

Soluble Acids

Carboxylic acids will quite often dissolve in water if they have short carbon chains. When they are dissolved in water, some molecules will dissociate and their presence can be detected by litmus paper or pH paper. Actually, what it detected is the presence of hydronium ion which is created by their reaction with water. So, if you are dealing with an unknown compound which is soluble in water, you can test it with litmus paper to see if it is an acid.

Equation showing dissociation of carboxylic acid in water. [64rxn04.JPG]

Insoluble Acids

If it is not soluble in water because the carbon chain is too long, then you have a different problem because you cannot use litmus paper to make that test. If you have something that is not soluble in water but will dissolve in a dilute aqueous base such as sodium hydroxide solution, then that implies that you are dealing with an acid. The reason for this is that bases such as sodium hydroxide will react with the acid and form ionic compounds which are generally soluble in water. So the fact that something that won't dissolve in water but will dissolve in sodium hydroxide solution indicates that that compound is an acid.

Equation showing insoluble carboxylic acid reacting with NaOH. [64rxn05.JPG]

Generating Carbon Dioxide Gas

Another reaction that you can use as a test is shown here (and in Example 6-d). When a carboxylic acid dissolves in an aqueous solution of sodium bicarbonate, the reaction results in the release of carbon dioxide bubbles from the sodium bicarbonate. Because of the visibility of the CO₂ bubbles, this can also be used as a test for the presence of a carboxylic acid.

Equation showing reaction of carboxylic acid with bicarbonate ion. [64rxn06.JPG]

You will be performing all of these tests on carboxylic acids when you come to the lab to do the lab work for this lesson.

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