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Acetic Acid in Vinegar
by Acid/Base Titration
Background
Vinegar is typically made from the fermentation of alcoholic liquids such as wine. Its main 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 determine the acetic acid content is the direct titration with sodium hydroxide (NaOH).
Reactions
- Titer determination:
- Acetic acid content determination:
Safety
Sodium hydroxide is corrosive! Skin and especially eye contact must be avoided. Always wear a lab coat, gloves and protective goggles during this workshop.
Tasks
- Titer determination:
Perform a titer determination (standardization) for the titrant NaOH,c = 0.1 mol/L,
using pure potassium hydrogen phthalate (KHP,M = 204.23 g/mol,z = 1) as the
titration standard. For this purpose accurately weight 0.08 – 0.12 g of potassium hydrogen phthalate into a titration beaker and add 50 mL of deionized water. If you use an indicator add a small amount of phenolphthalein solution (0.1 % in ethanol) to the solution. Start the titration by adding small amounts of titrant (manual titration) or by starting the automated titration, and stir during the whole titration. Titrate until the color changes from colorless to pink (manual titration) or until the EQP is recognized by the automatic titrator. Calculate the titer from the used volume. Rinse the electrode and dispensing tubes with deionized water between every measurement (only for automated titration).
C 7 H 4 KO2-COOH + NaOH C 7 H 5 O2-COOK + H 2 O std. substance titrant reaction products 204.23 g/mol 40.00 g/mol
CH 3 COOH + NaOH CH 3 COONa + H 2 O analyte titrant reaction products 60.05 g/mol 40.00 g/mol
Workshop 1 2 Powered by METTLER TOLEDO
Determine the titer at least three times and calculate the mean value, standard
deviation (s) and relative standard deviation (srel).
- Acetic acid content determination:
Determine the acetic acid content in a vinegar sample. Accurately weight about 1 g of vinegar in a titration beaker and add 50 mL of deionized water. To perform the titration, follow the same procedure as for the titer determination.
Determine the acetic acid content (in %) at least three times and calculate the mean value and standard deviation.
Waste disposal:
Neutralize (pH 7) the aqueous solutions before final disposal.
Solution
- Titer determination:
Calculation:
t =
m
VEQ ∙ c ∙ C
C =
M
10 ∙ p ∙ z
t: Titer of the titrant (no unit)
m: Weight of standard substance used for the determination (in g, here: KHP)
VEQ: Titrant consumption at the equivalence point (in mL)
c: Nominal concentration of the titrant (in mol/L, here: NaOH,c = 0.1 mol/L)
C: Constant (in g/mmol, see equation above)
M: Molar mass of the standard substance (in g/mol, here: KHP,M = 204.
g/mol) 10: Factor for mg to g and % conversion (in mg/(g·%))
p: purity of the standard substance (in %)
z: equivalent number (no unit, here: 1)
Expected result:
The titer should be around 1 for a fresh titrant solution.
- Acetic acid content determination:
Calculation:
R =
VEQ · c ∙ t ∙ C
m
C =
M
10 ∙ z
R: Result, content (in %, here: acetic acid content)
VEQ: Titrant consumption at the equivalence point (in mL)
c: Nominal concentration of the titrant (in mol/L, here: NaOH,c = 0.1 mol/L)
t: Titer of the titrant, as determined before (no unit)
C: Constant (in g·%/mmol, see equation above)
m: Sample weight (in g)
M: Molar mass of the analyte (in g/mol, here: Acetic acid,M = 60.05 g/mol)
10: Factor for mg to g and % conversion (in mg/(g·%))
z: equivalent number (no unit, here: 1)
Expected result:
The acetic acid content in vinegar is usually around 5 %.
Example
- Titer determination:
Three titer determinations were performed using KHP with a purity of 99.8 %. In the
following table the KHP amount (m), the titrant consumption (VEQ) and the
calculated titer (t) for these three measurements are shown:
Nr. m VEQ t
1 0.087 g 4.265 mL 0. 2 0.092 g 4.501 mL 0. 3 0.105 g 5.135 mL 0.
The titer of the first measurement was calculated as follows:
C =
M
10 ∙p ∙z
204.23 (^) molg
10
mg g·% ∙ 99.8 % ∙
g mmol
t =
m
VEQ ∙c ∙C
0.087 g
4.265 mL∙ 0.1 molL ∙ 0.2046 (^4) mmolg
Based on these three determinations the mean, standard deviation (s) and the
relative standard deviation (srel) were calculated:
Mean: 0.
s: 0.
srel: 0.13 %
- Acetic acid content determination:
After the titer determination three measurements of a vinegar sample were performed. For these three measurements the following sample amounts were used and the following results were obtained:
Nr. m VEQ R
1 0.982 g 7.165 mL 4.37 % 2 0.881 g 6.463 mL 4.40 % 3 0.912 g 6.686 mL 4.39 %
The calculation of the first measurement is shown here in detail:
C =
M
10 ∙z
g mol 10
mg g·% ∙ 1
g∙% mmol
R =
VEQ · c ∙ t ∙ C
m
7.165 mL ∙ 0.
mol L ∙^ 0.9983 ∙^ 6.^
g∙% mmol
