LAB 8: DETERMINATION OF ACETIC ACID CONTENT IN VINEGAR

Transcription

1 LAB 8: DETERMINATIN F AETI AID NTENT IN VINEGAR Lab format: This lab is designed for delivery using the Remote Web-based Science Laboratory (RWSL) or a lab kit. INTRDUTIN In an acid-base titration, the point at which both acid and base have been completely consumed can be detected and is known as the equivalence point. The amount of one reactant (the analyte) can be calculated from the known concentration and the volume of reactant in a standard solution (the titrant) using the balanced chemical equation. The end point in this experiment will be detected with an acid/base indicator. An acid/base indictor is a coloured substance with two or more different colors depending on the value of the ph of the solution. The standard solution may be prepared in two ways the direct or indirect method. In the direct method, a precisely weighed quantity of the pure solute (primary standard) is dissolved and diluted to a known volume in a volumetric flask. The concentration of the standard solution is then calculated from the known mass of the solute and the known volume of the solution. If the solute used to prepare the standard solution is pure and the solution is stable (does not decompose), then the compound is referred to as a primary standard. However, often it is not possible to obtain the solute in sufficiently pure form to be suitable as a primary standard. For example, NaH (s) reacts with gases (H 2 and 2 ) in the air which means that NaH (s) is not pure enough to be used as a primary standard. In this case the standard solution is prepared by an indirect method. A solution is prepared at approximately the desired concentration and it is then standardized against another primary standard to determine its exact concentration. BJETIVES The objectives of this experiment are to: Prepare a standard solution Use a prepared standard to standardize a NaH solution Use a standardized NaH solution to measure the acetic acid content of a vinegar sample using an acid-base titration technique

2 EQUIPMENT For RWSL delivery: A high-speed internet connection is recommended for using the RWSL. Students must also have access to a computer with the Windows operating system (XP or newer) and the Internet Explorer browser program. All other equipment will be provided remotely from the RWSL location. If you are using a lab kit, you will find the following chemicals and equipment for this experiment: ~ 2 g KH 8 H 4 4 phenolphthalein indicator solution Distilled water Sample of commercial grade vinegar (may be obtained from home) 50mL beaker 100 ml volumetric flask (with cap) 50 ml burette 250 ml Erlenmeyer flask 25 ml pipette Titration apparatus scales PREDURE Preparation of the Potassium Hydrogen Phthalate Standard For the titration of the vinegar in this experiment the following specific reaction will be used to calculate the acetic acid content of the vinegar sample: H 2 H 3 2 (aq) + NaH (aq) H 2 (l) + Na 2 H 3 2 (aq) 1. Weigh precisely (on an electronic balance) ~ 1.5 grams of pure potassium hydrogen phthalate (KH 8 H 4 4 ) into a 50mL beaker. 2. Dissolve the acid in ~50 ml of water and transfer carefully into a 100 ml volumetric flask. 3. Rinse the beaker several times with small portions of water to ensure quantitative transfer. 4. Make the volume up to the mark with distilled water, cap the flask and mix thoroughly. This solution will be used to standardize a solution of sodium hydroxide.

3 Standardization of the Sodium Hydroxide Solution Potassium hydrogen phthalate, the primary standard, reacts with sodium hydroxide as shown below: - K + H + NaH - K + - Na + + H 2 1. Select a clean 50 ml burette, rinse it with a small portion of the sodium hydroxide solution, and fill it to just below the zero mark. 2. Read and record the initial volume to the nearest 0.01 ml. 3. Rinse a 250 ml Erlenmeyer flask with distilled water to make sure it is clean. 4. Pipette exactly ml of the potassium hydrogen phthalate solution into the flask. 5. Add three drops of phenolphthalein indicator and titrate with the sodium hydroxide solution until a permanent colour change is first detected. The palest pink colour denotes the end point of this reaction. The colour should persist throughout the entire solution when swirled for at least 10 seconds. The colour will fade slowly on standing. 6. Repeat the titration on a second ml aliquot of the primary standard solution. If the titration volumes do not agree within ±0.1 ml continue to repeat the titration. 7. Report your two best titrations. The Determination of Acetic Acid in Vinegar The acetic acid (H 3 H) concentration in commercial vinegar may be easily determined by titrating a suitable sample of the vinegar with the standardized sodium hydroxide solution. 1. Pipette exactly ml of the commercial vinegar sample into a 250 ml Erlenmeyer flask and add ~5 ml of distilled water. 2. Using three drops of phenolphthalein indicator, titrate the acetic acid with the standard base to a pale pink equivalence point. Record the burette readings. 3. Repeat the titration at least once more using a fresh aliquot of vinegar. Results should agree within ±0.2 ml or additional titrations are required. 4. Report your two best titrations.

4 RESULTS Enter your results in the table below: Report Sheet for Assay of Acetic Acid ontent of Vinegar Data Mass of empty 50 ml beaker: g Mass of beaker and KHP: g Standardization of NaH Titration Number Initial Burette Final Burette Volume of NaH used (ml) 1 2 Average Volume(mL) Titration of Vinegar Titration Number Initial Burette Final Burette Volume of NaH used (ml) 1 2 Average Volume(mL) Write the balanced chemical equation describing the reaction of vinegar with the titrant.

5 ALULATINS Show your work: 1. oncentration of the KHP solution: M 2. oncentration of the NaH solution: M 3. oncentration of acetic acid in the vinegar solution: M 4. oncentration of acetic acid in the vinegar solution: % m/m