AS1 BENZOIC ACID IN SOFT DRINKS Last Revised: January 2015 1. REAGENTS & EQUIPMENT 1.1 100 mg/L benzoic acid (aqueous) in 0.01 M HCl 1.2 0.1 M HCl 1.3 0.1 M NaOH 1.4 1 cm quartz cells 1.5 Ultrasonic bath 1.6 Lemonade (containing preservative 210, 211 or 212) 2. PROCEDURE 2A. Sample preparation 2.1 Degas approximately 100 mL of lemonade using an ultrasonic bath. If the lemonade is fresh, remove most of the gas first by pouring from one beaker to another a few times. 2.2 Add approximately 5 mL of the lemonade and 1 mL of 0.1 M HCl into a 20 mL measuring cylinder, and fill to the 10 mL line with distilled water. Mix. This is to get a rough guide for the necessary sample dilution. 2.3 Pour 5 mL of 0.1 M HCl (by measuring cylinder) into a 50 mL volumetric flask and make up the mark with distilled water. This is the blank. 2.4 Record the spectrum of the diluted sample using the blank in the reference cell. Record the peak wavelength(s) and absorbance(s). 2.5 Discuss a suitable sample dilution scheme with your teacher. 2.6 Prepare accurately diluted samples in duplicate, adding 10 mL of 0.1 M HCl (per 100 mL final volume), according to the agreed scheme. 2.7 Prepare another sample diluted as the others, but with 10 mL of 0.1 M NaOH, not HCl. 2B. Standard preparation 2.8 Set the wavelength to the value determined in step 2.4. 2.9 Zero using two blanks. 2.10 Measure the absorbance of the 100 mg/L. Use this to determine the concentrations that give absorbances of 0.2‐0.8. 2.11 Discuss with your teacher the appropriate range of standards for the calibration graph. 2.12 Prepare 100 mL of three standards matching this range from the 100 mg/L standard, incorporating 10 mL of 0.1 M HCl into each. 2C. Analysis 2.13 Measure the absorbance of each standard and sample. 2.14 Estimate the concentration of benzoic acid in the diluted samples – do not use Excel. 2.15 Discuss with your teacher a suitable amount of recovery check to be added to another diluted sample. A recovery check should typically add around 50% extra analyte to the analysed sample. 2.16 Prepare the recovery check, remembering the HCl. 2.17 Measure its absorbance. REPORT Calculations 
plot a calibration graph using the set of standards 
determine the concentration of benzoic acid in all samples and recovery check 
apply the dilution factor to calculate the concentration of analyte in the undiluted lemonade for the three acidified samples only 
calculate the percentage difference between the acidified and alkaline samples 
calculate the average concentration in the sample 
calculate the % recovery check using the provided spreadsheet Discussion 
explain the difference in results between the acidified and alkaline samples. 
explain why this method would not be applicable to soft drinks such as Coca Cola and Fanta. 
Comment on the recovery check result Questions 1.
Look up Method 960.38 in the AOAC Official Methods of Analysis (online – needs password login ‐ see library). Describe how this method differs from the one used in this exercise. 2.
(a) What is the role of benzoic acid as a food additive? (b) What is its code number? (c) Why is it often added as sodium benzoate? 3.
Outline one non‐spectrophotometric method that would be suitable for this analysis. There is a guide to this Exercise, including help with the calculations, on the website. AS1 p2 AS1. ANALYSIS OF BENZOIC ACID IN SOFT DRINKS
RESULTS SHEET
Brand Manufacture ID code Rough dil’n abs. Dilution required 100 mg/L absorbance Standards Abs. Standard 1 Standard 2 Standard 3 Sample 1 Sample 2 Sample (NaOH) Rec. Check Estimated sample concentration Amount of recovery check added