Acid-Base Extraction: Benzoic Acid Two-Base Extraction

Topics: Sodium hydroxide, Hydrochloric acid, PH Pages: 18 (1889 words) Published: February 24, 2013

Douglas G. Balmer

(T.A. Mike Hall)

Dr. Dailey

Submitted 11 July 2007

Introduction: The purpose of this experiment was to separate a sample of benzoic acid, 2-

naphthol, and naphthalene of unknown proportions using a two-base extraction method. The

three components of the mixture will react differently to sodium bicarbonate and sodium

hydroxide because each of the bases’ conjugate acids has a different pKa. The pKa’s of benzoic

acid and 2-naphthol are 4.17 and 9.5, respectively, while naphthalene is a neutral compound.

Since benzoic acid is much more acidic than 2-naphthol, the weak base, sodium bicarbonate, will

be able to effectively remove benzoic acid’s acidic hydrogen. It will take the much stronger

base, sodium hydroxide, to remove the hydroxide hydrogen from 2-naphthol. Both of the

sodium salts formed from the base extractions will be soluble in water, while naphthalene will

only be soluble in the original solvent, diethyl ether. Introducing the two sodium salts to

hydrochloric acid will effectively replace the original proton benzoic acid and 2-naphthol lost.

In a chilled environment, both compounds will not be soluble in water, because the solubility of

benzoic acid and 2-naphthol in water at 25˚C is 0.34g/100mL and 0.074g/100mL, respectively.

They can be removed using vacuum filtration. After extraction and purification, the percent

recovery and percentage composition of the unknown sample will be able to be calculated.

The unknown sample may not be separated using melting points because both benzoic

acid and 2-naphthol have a melting point of 123˚C. The purity of the samples will be indicated

by their melting points ranges. The purer the sample, the narrower the melting point range.

Experimental Procedure (Fig.1): An unknown sample of benzoic acid, 2-naphthol, and

naphthalene (Table 1) was massed and the unknown number was recorded. The unknown

sample was dissolved in 30mL of diethyl ether in a 125mL Erlenmeyer flask. The solution was

transferred by funnel to a 125mL separatory funnel. The benzoic acid was extracted by adding

20mL of 10% aqueous sodium bicarbonate. The separatory funnel was swirled before inverting

to allow any carbon dioxide gas produced to dissipate. The inverted separatory funnel was

swirled with frequent venting until the fizzing carbon dioxide subsided. Afterwards, the inverted

separatory funnel was shaken with frequent venting until the fizzing carbon dioxide subsided.

The lower aqueous layer containing the benzoic acid was transferred to a labeled 125mL

Erlenmeyer flask. Any remaining benzoic acid in the organic layer was extracted with another

20mL of 10% aqueous sodium bicarbonate following the same procedure as detailed above. The

aqueous contents of both bicarbonate extractions were collected in the same 125mL Erlenmeyer


The 2-naphthol was extracted from the organic layer by adding 20mL of cold 10%

aqueous sodium hydroxide solution to the 125mL separatory funnel. The separatory funnel was

inverted and shaken with frequent venting. The lower aqueous layer was transferred to a second,

labeled 125mL Erlenmeyer flask.

The ether solution was transferred to a labeled 50mL Erlenmeyer flask and 0.7g of

anhydrous sodium sulfate was added as a desiccant. The flask was corked to prevent










benzoic acid

benzoate ion




Unknown Sample

Dissovle unknown sample in ether.
Extract with aqueous sodium bicarbonate.

aqueous phase
2, water, NaHCO3(aq)

Acidify with HCl.
Chill the solution.


Extract with aqueous sodium

aqueous phase
4, water, NaOH(aq)

1, water
Vaccuum filter the solution
and rinse with cold distilled...
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