Lab 1: Effect of pH on sodium benzoate, a food preservative
Sheikh M Zakaria
Person no. 36295651
TA: Synthia Gratia
Date of Submission: 05/29/13
Sodium Benzoate is a common food preservative used in food products such as jams and jellies, soft drinks, pickles, condiments etc and in tinned products in the market. This experiment aimed to determine whether benzoic acid is formed from it’s superior soluble form sodium benzoate in stomach acid, which is simulated by HCl (pH=2). It has been seen that at a low optimum pH, i.e. in the presence of sufficient hydrochloric acid, sufficient benzoic acid is yielded. The percentage yield is determined from the calculations of the theoretical (stoichiometric) calculations and the actual yield from the reaction. Also throughout the experiment, some basic laboratory operations such as measuring mass and volume, separating solids from liquids by vacuum filtration and drying solids are familiarized with.
The reaction of sodium benzoate and hydrochloric acid is a typical phenomenon of hydrolysis reaction which conforms to the principles of acid-base chemistry and stoichiometry. In a typical hydrolysis reaction, when a salt dissolves in water, the salt dissociates to its constituent anions and cations (benzoate and sodium ions respectively). Also, when strong acids like hydrochloric acid dissolves in water, chloride ions and hydronium ions are formed. Hence in the reaction involving sodium benzoate and hydrochloric acid solution, in the presence of high concentration of hydronium ions, protons will be transferred from the strong acid H3O+ to the basic benzoate ion. This will yield benzoic acid, C6H5COOH which precipitates from the solution as it has a really poor solubility (0.34g/100ml). The fundamental reaction is sketched out below: As can be seen from the difference in the pKa values, H3O+ is a much stronger acid than benzoic acid. So, the equilibrium of the reaction strongly favours its completion and hence hydrochloric acid which ideally simulates the acidic conditions of the stomach, does convert benzoate to benzoic acid.
Throughout the lab safety goggles, protective gloves and aprons were worn while carrying out the experiments. While transferring the HCl to the reaction vial in the student hood, the head must never be inside the glass hood as vapour inhalation can cause choking hazard and irritations. Also the hosepipes were carefully plugged in and out of the filter flask to avoid breakage or tipping of the Hirsh funnel.
For this experiment and basic operations, calibrated Pasteur pipettes, a 5ml conical reaction vial, watch glass, stirring rod, electronic balance, small test tube, Hirsh funnel, filter flask and filter paper are used. The procedure for this experiment appears on page 7 of the lab manual, Multiscale Organic Chemistry (4th custom UB edition)3.
Table: weight, volume and percentage yield of benzoic acid
Mass of sodium benzoate
Volume of water
Volume of hydrochloric acid
Actual yield of benzoic acid
Theoretical yield of benzoic acid
Percentage yield of benzoic acid
The theoretical mass of benzoic acid can be calculated by the following equation, given the molecular weights of sodium benzoate and benzoic acid (144.1 g/mol and 122.1 g/mol respectively) and the reaction is 1:1 C6H5COONa + HCl
C6H5COOH + NaCl
Theoretical yield of benzoic acid= 0.42144.1x 122.1
The equation can be broken down to derivation of the moles of benzoate which is 0.42/144.1 This is also equal to the moles of benzoic acid owing to the 1:1 reaction stoichiometry. Hence using mass= moles x molecular weight formula, the theoretical yield is derived. The percentage yield can be calculated using the equation
Percentage yield=( Actual yieldTheoretical yield x 100 ) %
As the primary objective of this experiment was to find...
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