Identification of an Organic Acid

Identification of an Organic Acid

1211K Lab

Drawer #H33

December 3, 2016

Thursday 1:00 pm-4:00 pm

Unknown # 8122361-HF-15

Introduction: The purpose of this experiment is to identify an unknown organic acid by conducting several experiments to determine the acid’s unique properties (GSU Lab Manual pg. 21). An organic acid is an organic compound with acidic properties that contains carbon, hydrogen, and at least one carboxyl group (–COOH). They have a low molecular weight and are water-soluble acids that produce H+ ions in an aqueous solution. A base reacts with acids to form salts; therefore, base titrations are used to determine the concentration of unknown substances. In the titration used in this experiment, a KHP experiment is performed to determine the molarity of NaOH and the HCl titrations are used to check technique and used to verify the molarity of NaOH solution. The experiments used in order to determine the unknown organic acid were recrystallization, melting point range determination, equivalent weight determination, solubility of the unknown acid, finding the pKa values and the titrations of KHP and HCl. Once these experiments were completed, a computer search was done using the equivalent weight value and the melting point range to provide a list of possible organic acids were given. A sodium fusion test may also be needed to narrow down the results if the organic acid contained sodium, nitrogen, sulfur, or halogens. Based on the melting point, equivalent weight, and pKa obtained from the experiments, the unknown organic acid (Unknown# 8122361-HF-15) can be identified as 2,4-dichlorobenzoic acid, Cl2C6H3COOH.

Procedures: To find the melting point, two small samples of the unknown acid were each placed into two separate capillary tubes. To determine the melting point of the unknown acid sample, the melting point apparatus temperature is set to a plateau temperature which is 10-15 degrees below the expected melting point of the substance. Once the apparatus reaches the set temperature, the capillary tube is then placed into the apparatus. When the sample started to bubble the temperature that corresponded with the bubbling is recorded. When the sample dissolved the final temperature is recorded. The slow melting point is then determined.

To find the equivalent weight, sodium hydroxide (NaOH) and HCL solutions must be prepared. Solid NaOH is weighed out on a balance and mixed well with 800mL of distilled (DI) water to make a solution of about 0.1M NaOH. A beaker of 60mL of 0.2M HCL is obtained from the fume hood.

 Three samples of KHP were weighed out on an analytical balance ranging from 0.25-0.30g. Each placed into three separate Erlenmeyer flasks and dissolved in DI water. The NaOH solution was poured into a buret after rinsing the buret with NaOH. Two drops of phenolphthalein indicator solution was added to each flask. The samples was then slowly titrated until there was a light, pink color change. The amount of NaOH used was recorded after each trial.

Three samples of 10mL of HCL were taken from a pipet and each sample was placed into a flask. The buret was filled with the NaOH solution.  Two drops of indicator solution was added to each flask. Sample was slowly titrated until the clear liquid changed to a light, pink color. The amount of NaOH used for each sample trial was recorded.

Three samples of the unknown acid of 0.01g were weighed out into separate flasks and each mixed with 2mL of DI water. A 2:1 ratio of ethanol and water solvent is made. Each unknown acid sample is dissolved in 40-50mL of the ethanol/water solvent. The mixture is heated until the sample fully dissolves. A NaOH solution was placed into the buret and a drop of indicator solution was added to the flask containing the samples. The sample was then slowly titrated with NaOH until there was a light, pink color change.

        A computer program called “Chemistry 1211 Organic Acid Search Engine” is accessed. The melting point found in the previous procedure is entered with a +/-7 parameter, as well as the equivalent weight with a +/- 10% parameter. After pressing “SEARCH,” a list of possible organic acids is produced.

For the pKa titration curve, three samples of the unknown acid of 0.39292g are weighed out. One sample was placed into a beaker with 100mL of ethanol and 100mL of DI water. The second sample was placed into a beaker with 90mL of ethanol and 110mL of DI water. The third sample was placed into a beaker with 80mL of ethanol and 120mL of DI water. The solutions was heated until the sample dissolved. A pH meter was calibrated and taped into the beaker. A NaOH solution was added to the buret. For the titration, 1mL of NaOH was added each time, and the pH was recorded after each drop of NaOH was added. 0.1mL of NaOH was then added when the pH of the solution jumped by at least .30.  

For the sodium fusion test, two test tubes were taken to the fume hood. The solution containing a sample of the unknown acid and prepared by the TAs, filtered in to a beaker and divided into the two test tubes brought. 5 drops of ferrous sulfate, 5 drops of potassium fluoride, and 5 drops of ferric chloride, and 5 drops of sulfuric acid were added to the first test tube and stirred. Solution would turn blue if there was nitrogen present or clear if there was no nitrogen present. In the second test tube, 5 drops of nitric acid was added and tested with a litmus paper. 2 to 5 drops of silver nitrate is added and stirred. Solution would have a white, tan or yellow precipitate if a halogen is present, and a black, clear, or dark solution when no halogen is present.

Results and Discussion:

Melting point:

Discussion: The experimental melting point of the unknown acid was found to be 168˚C - 171˚C.        

Equivalent Weight:

Titration of KHP with NaOH

Calculations: