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Determination of acetic acid in vinegar using acid-base titration
1. Scope The goal of this analytical procedure is to determine the acetic acid content of two samples of vinegar produced by Kolinska, Ljubljana, Slovenia. These samples are: Sample No. 1: vinegar made from wine, 4% Sample No. 2: vinegar made from alcohol, 9% The smallest quantity of the sample is 100 mL. Interesting results may be obtained by comparing the acidity of both samples of vinegar with factory analysis data. 2. Principle The content of acetic acid in vinegar will be determined by acid – base titration with the standard solution of sodium hydroxide with approximate concentration c(NaOH) = 1 mol/L. This is prepared using the primary standard reagent potassium hydrogen phthalate. The time for one titration is about 10 minutes. 2.1. Acid – base titration An acid – base titration is a procedure used in quantitative chemical analysis to determine the concentration of either an acid or a base. Titration is the slow addition of an acid (or a base) of known concentration from a burette (a narrow graduated cylinder) to a base (or an acid) of unknown concentration in an Erlenmeyer flask. We distinguish between strong and weak acids and also between strong and weak bases. So, four types of acid – base titrations are possible: strong acid – strong base, strong acid – weak base, weak acid – strong base, and weak acid – weak base. The end-point occurs when the stoichiometric amount of base (or acid) has been added to the acid (or the base). The end-point is frequently detected using a visual indicator. An acid – base indicator is a substance, which changes colour with pH. For each type of acid – base titration an appropriate indicator must be chosen. See table 1, below. However, the end-point can also be determined potentiometrically using a pH meter or by a conductometric method. At this point, all the acid has been neutralised and neither excess base nor excess acid is present in the solution. The solution consists of salt and water only. So, acid – base titrations are also called neutralisation titrations. The reagent with known concentration is usually prepared separately. For this preparation, substances called primary standards are frequently used. A primary standard is a reagent that is sufficiently pure and does not react with the ordinary environment (air, humidity). So, the exact concentration of a solution of primary standard could be prepared using only weighing and dissolving in a solvent (usually water). Now, a separate titration with this solution of the primary standard is

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needed to determine the exact concentration of one of the reagents to be used in our basic acid – base titration. 2.2. Weak acid (acetic acid) – strong base (sodium hydroxide) titration We can determine the concentration c(HA) of a weak acid (which we call the total acidity or the stoichiometric concentration) by measuring the amount of a strong base (such as NaOH) that is needed to react completely with the sample of the acid. The NaOH(aq) is dispensed from burette. So, if we know the exact concentration and volume of NaOH(aq) required to neutralise the sample of acid, we can calculate accurately total amount of weak acid in the original sample solution. At the end point of titration of an acetic acid CH3COOH solution with the sodium hydroxide NaOH solution the following chemical equation can be written: CH3COOH(aq) + NaOH(aq) → CH3COONa(aq) + H2O(l) The stoichiometric relationships: n(CH3COOH) = n(NaOH) V(CH3COOH) x c(CH3COOH) = V(NaOH) x c(NaOH) Sodium acetate CH3COONa is completely ionised in water. Since CH3COO-(aq) anion is a weak base, we expect the solution to be weakly alkaline with pH > 7. When we calculate theoretical pH value at the end point of the titration described before we obtain pH = 9.3. Thus we can determine the concentration of acetic acid in a sample of vinegar by titration with NaOH(aq) of known concentration. In the table below, changes of colours and pH intervals of some indicators for alkaline part of the pH scale are given. Table 1: Colour changes and pH intervals of some important acid-base indicators in the alkaline part of pH scale _____________________________________________ Name of indicator pH interval Colour change _______________________________________________ Bromthymol blue 6.0 – 7.6 yellow to blue Neutral red 6.8 – 8.0 red to yellow Thymol blue 8.0 – 9.6 yellow to blue Phenolphthalein 8.0 – 9.6 colourless to red ______________________________________________ In this determination of acetic acid in vinegar phenolphthalein is used.

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3. Apparatus 3.1. Equipment 3.1.1. Instruments • Calibrated analytical balance with sensitivity 0.001 g.

3.1.2. Glassware and other equipment • • • • • • Burette 20 mL burette stand burette holder beaker 400 mL volumetric flask 1 L 4 Erlenmeyer flasks 250 mL • • • • • • transfer pipette 10 mL. weighing bottles graduated cylinder polyethylene bottles desiccator safety glasses

3.2. Materials and their safety codes • • • • sodium hydroxide p.a., NaOH potassium hydrogen phthalate p.a., KHC8H4O4 phenolphthalein indicator, 1% solute in 70% alcohol distilled water CAS No. 64-19-7 1310-73-2 R/S codes R: 34 S 26-36/37/39-46 R: 34 S: 26-36/37/39-46 Slightly polluting

Reagent Acetic acid Sodium hydroxide Potassium hydrogen phthalate

877-24-7

3.3. Reagent solutions 3.3.1. Sodium hydroxide, 50 mass% solution Dissolve 50 g of sodium hydroxide p.a. in 50 mL distilled boiled water and leave the suspension to settle overnight. Sodium carbonate is insoluble in the solution and precipitates. Store the solution in a tightly sealed polyethylene bottle and handle it gently to avoid stirring the precipitate when liquid is withdrawn.

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3.3.2. Sodium hydroxide solution with exact concentration (standardisation) The solution of sodium hydroxide with approximate concentration 1 mol/L as titrant may be prepared by diluting stock solution of 50 mass% NaOH. Potassium hydrogen phthalate is a weak acid. Sodium hydroxide reacts with it: KHC8H4O4 (aq) + NaOH(aq) → H2O(l) + KNaC8H4O4(aq) n(KHC8H4O4) = n(NaOH) Mr(KHC8H4O4) = 204.223 Using 15 mL c(NaOH) = 1 mol/L weigh: m(KHC8H4O4) = V(NaOH) x c(NaOH) x M(KHC8H4O4) m(KHC8H4O4) = 15 mL x 1 mmol/mL x 204.223 mg/mmol = 3.0633 g For each sample of KHC8H4O4, approximately 3.0633 g must be weighed (3063.3 mg). Add 3 drops of phenolphthalein indicator to each and titrate one of them rapidly to find the approximate end point. During each titration you should periodically tilt and rotate the flask to wash all liquid from the walls into the bulk solution. When very near the end you should deliver less then 1 drop of titrant at a time. To do this, carefully suspend a fraction of drop from the burette tip, touch it onto the inside wall of the flask, wash it into the bulk solution by carefully tilting and swirling the solution. The end point is at the first appearance of a faint pink colour that persists for 15 s. (The colour swirl slowly fades as CO2 from the air dissolves in the solution). Calculate the average concentration in mol/L . 3.3.3. Potassium hydrogen phthalate solution (primary standard) Potassium hydrogen phthalate must be dried for 1 h at 110 oC and stored in desiccator. Weigh four samples of solid potassium hydrogen phthalate and dissolve each in approximately 25 mL distilled water in a 250 ml flask. Each sample should contain enough solid to react with 15 mL c(NaOH) ∼ 1 mol/L. 3.3.4. Phenolphthalein indicator Make 1% solute in 70% alcohol.

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3.3.5. Distilled water Boil 2 L distilled water for 5 min to expel CO2. Pour it into a polyethylene bottle which should be capped as tightly as possible. 4. Preparation 4.1. Preparation of apparatus Set up the burette. Rinse the burette with a small amount of sodium hydroxide standard solution, then fill it with the same solution to the mark zero (0.00 mL). Put an empty Erlenmeyer flask on the pan of the balance and set the balance to zero (or record the mass of the empty flask). 4.2. Preparation of sample Fill 10 mL pipette with the vinegar sample and Erlenmeyer flask on the balance pan. transfer the sample into the

Record the mass of the sample to within 0.001 g accuracy. Add approximately ∼20 mL of distilled water to the vinegar sample and two drops of phenolphthalein indicator. 5. Procedure Titrate the sample with standard solution of sodium hydroxide until a pink colour is obtained. Constantly swirl the contents of the flask and wash down any liquid that adheres to the side of the flask, with distilled water. • Record the final volume of NaOH titrant, with 0.01 mL accuracy. • Run two more titrations in a similar way and record the results in the data table . • Report the results to two decimal places. 6. Calculations 6.1. Sodium hydroxide solution with exact concentration (standardisation) Calculation of the exact concentration of NaOH(aq): n(KHC8H4O4) = n(NaOH) •

m(KHC 8H4O4 ) = V (NaOH) . c(NaOH) M(KHC 8H4O 4 ) m(KHC 8H4O 4 ) c(NaOH) = M(KHC 8H4O 4 ) . V (NaOH)
Trial 1 m(KHC8H4O4) (mg) Volume of NaOH (ml) c(NaOH) (mol/L) Trial 2 Trial 3 Trial 4

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6.2. Titration of vinegar 6.2.1. Data for the sample Brand name of vinegar: Acid content of vinegar specified by the manufacturer: Concentration of standard NaOH(aq): 6.2.2.Data for each titration: Trial 1 Mass of empty flask (or tare mass) (g) Mass of flask + vinegar (g) Net mass of vinegar (g) Volume of NaOH(aq) titrant (mL) First: for each titration divide the volume of NaOH by the sample mass to calculate V(NaOH) in a 1 g of vinegar: Trial 2 Trial 3

Trial 1

Trial 2

Trial 3

V (NaOH)(mL) mass vinegar (g)

Second: for each titration calculate: - the amount n(CH3COOH) of acetic acid in the sample, - the mass of acetic acid, - percentage of acetic acid for each titration, - average values of the three trials. The mass percentage of acetic acid in vinegar is given by the expression: w =

m(acetic acid )(g ) .100% m(vinegar )(g )

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Trial 1 n(CH3COOH) (mol) m(CH3COOH) (g) mass% acetic acid (%)

Trial 2

Trial 3

7. Expression of results 7.1. Sodium hydroxide solution with exact concentration (standardisation) Average value c(NaOH)(mol/L)

7.2. Titration of vinegar Average value of mass% of acetic acid

Questions

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1. To determine the acetic acid content in vinegar, 10.0 mL of a sample solution was measured and distilled water added. Two drops of phenolphthalein indicator were then added and the sample titrated using sodium hydroxide solution. Which statement is correct? a Up to the end point of the titration the sample solution is red coloured. b At the end point of titration the sample solution is neutral. c Up to the end point of titration the equivalent amount of sodium. hydroxide was added. d At the end point of titration the sample solution become colourless. 2. In a sample of wine vinegar, there is approximately 4 mass% acetic acid. The density of the sample solution is 1.05 g/mL. To determine the acetic acid content of the vinegar a 10.0 mL sample was measured and then titrated with NaOH(aq) c = 1.00 mol/L . The consumption was 7.10 mL. What is the correct concentration of acetic acid in mol/L? a b c d 7.2 .10-4 mol/L 7.2 mmol/L 0.710 mol/L 0.715 mol/L

3. The concentration of acetic acid in a sample of vinegar is 0.715 mol/L. Calculate the mass % of acetic acid. The density of vinegar is 1.05 g/cm3. Mr(CH3COOH) = 60 Select the correct answer. a) 4.1 % b) 0.40 % c) 4.00 % d) 4.0 % 4. A sample of vinegar is titrated with a standard solution of sodium hydroxide. Which pH indicator is appropriate to use? Name of indicator pH interval ___________________________________ a Bromothymol blue 6.0 − 7.6 b Neutral red 6.8 − 8.0 c Thymol blue 8.0 − 9.6 d Phenolphthalein 8.0 − 9.6 5. The primary standard for standardisation of sodium hydroxide solution is potassium hydrogen phthalate. You need to standardise an approximately 1 mol/L sodium hydroxide solution.

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The titration should take about 10mL of the solution. What mass of potassium hydrogen phthalate do you need to weigh out? Mr(KHC8H4O8) = 204.223 Select the correct answer. a b c 204.223 mg 2.0423 g 20.4223 mg

6. Calculate the volume of standard solution of sodium hydroxide c(NaOH) = 1.200 mol/L which should be added to a 10.00 mL sample of vinegar. The sample has 9.06 mass% of acetic acid. The density of the sample solution is 1.0125 g/mL. Select the correct answer. a 14.87 mL b 15.0 mL c 15.2 mL

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