Acetic Acid in Vinegar by Acid/Base Titration

Transcription

1 Acetic Acid in Vinegar by Acid/Base Titration Background Vinegar is typically ade fro the ferentation of alcoholic liquids such as wine. Its ain constituents are water and acetic acid, usually about 5 %. The acid in vinegar is produced by bacteria, which convert ethanol to acetic acid. An easy and accurate way to deterine the acetic acid content is the direct titration with sodiu hydroxide (NaOH). Reactions 1. Titer deterination: C 7 H 4 KO 2- COOH + NaOH C 7 H 5 O 2- COOK + H 2 O std. substance titrant reaction products g/ol g/ol CH 3 COOH + NaOH CH 3 COONa + H 2 O analyte titrant reaction products g/ol g/ol Safety Sodiu hydroxide is corrosive! Skin and especially eye contact ust be avoided. Always wear a lab coat, gloves and protective goggles during this workshop. Tasks 1. Titer deterination: Perfor a titer deterination (standardization) for the titrant NaOH, c = 0.1 ol/l, using pure potassiu hydrogen phthalate (KHP, = g/ol, z = 1) as the titration standard. For this purpose accurately weight g of potassiu hydrogen phthalate into a titration beaker and add 50 L of deionized water. If you use an indicator add a sall aount of phenolphthalein solution (0.1 % in ethanol) to the solution. Start the titration by adding sall aounts of titrant (anual titration) or by starting the autoated titration, and stir during the whole titration. Titrate until the color changes fro colorless to pink (anual titration) or until the EQP is recognized by the autoatic titrator. Calculate the titer fro the used volue. Rinse the electrode and dispensing tubes with deionized water between every easureent (only for autoated titration). Workshop 1 1 Powered by ETTLER TOLEDO

2 Deterine the titer at least three ties and calculate the ean value, standard deviation (s) and relative standard deviation (srel). Deterine the acetic acid content in a vinegar saple. Accurately weight about 1 g of vinegar in a titration beaker and add 50 L of deionized water. To perfor the titration, follow the sae procedure as for the titer deterination. Deterine the acetic acid content (in %) at least three ties and calculate the ean value and standard deviation. Waste disposal: Neutralize (ph 7) the aqueous solutions before final disposal. Workshop 1 2 Powered by ETTLER TOLEDO

3 Acetic Acid in Vinegar by Acid/Base Titration Equipent anual titration: - 1x Analytical balance - 1x anual burette (10 L) - 6x Titration beakers (e.g. 100 L glass beakers) - 1x agnetic stirrer - 3x agnetic stirrer bars Autoated titration: - 1x Analytical balance - 1x ettler-toledo Easy ph or Easy Pro titrator with 10 L burette and tubing - 1x EG11-BNC ph Electrode - 6x Titration beakers - 3x agnetic stirrer bars Cheicals The quantities below were roughly estiated for 5 titer deterinations and 5 acetic acid deterinations. - 1 L deionized water L sodiu hydroxide solution c = 0.1 ol/l (4 g solid sodiu hydroxide in 1 L solution) - 10 L of phenolphthalein solution (0.1 % in ethanol, 0.01 g phenolphthalein in 10 g solution) - 1 g potassiu hydrogen phthalate of high purity (titration standard) - 10 g vinegar as saple Preparation - Prepare the titrant and indicator solutions. - Rinse the burette of the autoatic titrator at least twice to dispel any air bubbles trapped in the burette and tubing. Coents - Prevent CO 2 uptake of the titrant. For this purpose place a drying tube filled with sodiu hydroxide on a carrier on top of the titrant bottle. - The ethod ay be slightly adapted depending on the vinegar used and its acid content. Teacher Inforation 3 Powered by ETTLER TOLEDO

4 Solution 1. Titer deterination: Calculation: t = VEQ c C 10 p z t: Titer of the titrant (no unit) : Weight of standard substance used for the deterination (in g, here: KHP) VEQ: Titrant consuption at the equivalence point (in L) c: Noinal concentration of the titrant (in ol/l, here: NaOH, c = 0.1 ol/l) C: Constant (in g/ol, see equation above) : olar ass of the standard substance (in g/ol, here: KHP, = g/ol) 10: Factor for g to g and % conversion (in g/(g %)) p: purity of the standard substance (in %) z: equivalent nuber (no unit, here: 1) Expected result: The titer should be around 1 for a fresh titrant solution. Calculation: R = VEQ c t C 10 z R: Result, content (in %, here: acetic acid content) VEQ: Titrant consuption at the equivalence point (in L) c: Noinal concentration of the titrant (in ol/l, here: NaOH, c = 0.1 ol/l) t: Titer of the titrant, as deterined before (no unit) C: Constant (in g %/ol, see equation above) : Saple weight (in g) : olar ass of the analyte (in g/ol, here: Acetic acid, = g/ol) 10: Factor for g to g and % conversion (in g/(g %)) z: equivalent nuber (no unit, here: 1) Expected result: The acetic acid content in vinegar is usually around 5 %. Teacher Inforation 4 Powered by ETTLER TOLEDO

5 Exaple 1. Titer deterination: Three titer deterinations were perfored using KHP with a purity of 99.8 %. In the following table the KHP aount (), the titrant consuption (VEQ) and the calculated titer (t) for these three easureents are shown: Nr. VEQ t g L g L g L The titer of the first easureent was calculated as follows: g 10 p z = ol g 10 g = g % 99.8 % 1 ol t = VEQ c g L 0.1 ol L = g ol Based on these three deterinations the ean, standard deviation (s) and the relative standard deviation (srel) were calculated: ean: s: srel: 0.13 % After the titer deterination three easureents of a vinegar saple were perfored. For these three easureents the following saple aounts were used and the following results were obtained: Nr. VEQ R g L 4.37 % g L 4.40 % g L 4.39 % The calculation of the first easureent is shown here in detail: 10 z = g ol g % 10 g = g % 1 ol R = VEQ c t C = ol L g % L ol = 4.37 % Teacher Inforation 5 Powered by ETTLER TOLEDO

6 As for the titer deterination, the ean, s and srel were calculated: ean: 4.39 % s: 0.02 % srel: 0.35 % Teacher Inforation 6 Powered by ETTLER TOLEDO