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
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...
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