Sodium Benzoate: Chemical Changes at the pH Level Encountered in the Acidic Environment of the Stomach

Consulting Chemists Institute

Introduction

The Consulting Chemists Institute was contracted by a local food manufacturer to determine if sodium benzoate, a common food preservative, undergoes significant chemical changes at the pH level encountered in the acidic environment of the stomach.

Experimental Method

Sodium benzoate was prepared by addition of an equimolar amount of aqueous sodium hydroxide (3 M, ~ 20 ml) to a benzoic acid (1.256 g, 10.3 mmol) /water mixture (10 ml). Benzoic acid is insoluble in water. The hydroxide base rapidly deprotonates the acid, producing the benzoate anion which is readily soluble in water. The two phase benzoic acid/water system was cleanly converted to a sodium benzoate solution. The preservative, sodium benzoate, is water soluble. To simulate the acidic environment of the stomach, hydrochloric acid (3 M) was added to bring the pH of the benzoate solution to approximately 2. The benzoate was converted to benzoic acid as the pH fell as evidenced by the formation of a white precipitate identical in appearance to the starting acid. The acid was recovered by extraction with dichloromethane (2 x 25 ml). The solvent was striped by simple distillation to recover the starting benzoic acid. Benzoic acid was recovered in 89% (1.123 g; 9.20 mmol)

Discussion / Conclusion

Sodium benzoate will be rapidly protonated by the hydrochloric acid int eh stomach to produce benzoic acid. The relatively high per cent recovery suggest sodium benzoate does not undergo any significant structural change, other than protonation, at low pH. Physical and chemical characterizations were not performed on the product due to water carried by the acid into the solvent. Removal of the water would have allowed melting point and IR data to be collected and compared to literature values.

Report Suggestions –

For the Introduction:

State the nature of the project and the approach to answering the question.

For the Experimental section:

Give a brief summary description in your own words of the procedure, weaving in your observations/results, in as clear and concise a manner as possible. Include masses and millimoles for reactants and products, solvent volumes, etc., with values in parentheses after the material name. Do not copy an experimental procedure from the book.

Include any data from physical or chemical characterization done on your end product. Data should be inserted in the Experimental where it was collected. Data includes MP, BP, IR, NMR, GC data, etc. % yield or % recovery should be included at each point you were concerned about the mass balance.

This section should reflect your attached notebook pages. I don’t care if the notebook pages are messy (poorly written, mark throughs, spills, etc.). Notebooks are not to be rewritten, polished, or “prettied up.” They should accurately reflect the work performed as it is performed, a “real time” narrative.

For the Discussion / Conclusion section:

Give a brief summary of your findings. Include an analysis from comparing your product characterization data to literature values. Did you get the product expected? In good yield? Why or why not? Suggestions for improvements? Follow on work? Wrap your experiment into a cool little package here. Well written, clear conclusion.