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SANDA CORPORTION APPLICATION NOTES FACTSTM 6 Am The Thermotitrimetric Determination of Monosodium Glutamate (MSG) Using Sorensen's Titration Thomas K. Smith, MPhil Antom Technologes Pty Ltd ABSTRACT. Monosodium glutamate (MSG) may be simply and rapidly analyzed for purity using a thermometric acid- base titration. The sample is initially reacted with formaldehyde to form the methylol derivative, thus destroying the amphoteric character of the amino acid and permitting it to be titrated with standard base to a thermometrically determined endpoint. The method is accurate and precise (~0.3% rsd). The titration takes approximately two minutes. The method is suitable for fully automated operation. 1. INTRODUCTION. A standard method that is employed for quality control of MSG involves a non-aqueous potentiometric titration with perchloric acid dissolved in essentially anhydrous acetic acid. The titrant is an unpleasant and corrosive liquid. Perchloric acid is a powerful and sometimes unpredictable oxidizing agent, particularly in the presence of organic compounds. Fires and explosions are attendant hazards of working with perchloric acid. The determination must also be accompanied by sample blank and titrant blank titrations. A titration in aqueous solution is a preferable alternative. Amino acids are not directly titratable with base due to their zwitterionic, inner salt character. "Sørensen's Titration" is well documented in organic chemistry texts as a convenient method for the determination of amino acids (1). When for instance formaldehyde solution is added to a solution of glycine, the methylol derivative of glycine is formed: H2NCH2CO2H + 2HCHO → (CH2OH)2NCH2CO2H The resultant acid may be titrated with a base such as NaOH. It is likely that monosodium glutamate reacts with formaldehyde in a similar manner: NaO2CCH2CH2CH(NH2)CO2H + 2HCHO → NaO2CCH2CH2CH(N(CH2OH)2)CO2H Endpoints in thermometric titrations are revealed by rapid reaction enthalpy rate changes when the reaction of analyte with titrant is complete. This is in contrast to potentiometric titrations of organic acids with bases, which may be problematic. The attenuation of titration curves around the endpoint can make precise location difficult. A thermometric version of "Sørensen's Titration" offered the promise of being simple and easily carried out. The following procedure was developed in response to a client inquiry. 2. METHOD OUTLINE. 2-1. Equipment. SANDA FACTSTM (Fully Automated, Computerized Titration System), SANDA Corporation, Philadelphia, USA. 2-2. Reagents. 2-2.1. 2M NaOH titrant. Dissolve 80g NaOH pellets in water, and make up to 1L in a volumetric flask. 2-2.2. Standard 1M HCl solution (Ajax Chemicals "Univol", Australia). 2-2.3. Concentrated formaldehyde solution, nominally 34 - 38% w/v (Sigma, Australia). 2-3. Sample of MSG. A sample of domestic food grade monosodium glutamate monohydrate, NaO2CCH2CH2CH(NH2)CO2H.H2O, M.W. 187.13, was purchased ("Mong Lee Shang" brand, De Sun Industrial Corporation, Taiwan). 2-4. Conduct of the Determination. (The following is the preferred method, based on experimentation). 2-4.1. Standardization of NaOH Titrant. Provide 200 ml styrofoam beakers with a 40 mm long magnetic spin bar, and prepare aliquots of HCl as indicated in the table. Perform titration of each aliquot size in duplicate. Aliquot of 1M HCl, ml 34 - 38% w/v HCHO solution, ml Added water, ml Total volume, ml 10 10 - 20 5 10 5 20 4 10 6 20 2 10 8 20 A starting volume of approximately 20 ml is preferred for the titrations. Formaldehyde is added in the same quantity as is used in titration of the MSG, to compensate for any residual acidity, and impose the same experimental conditions ("matrix matching"). A linear regression is calculated from the aliquot volumes of HCl and the respective volumes of NaOH titrant. The molarity of the NaOH is calculated from the molarity of the HCl divided by the gradient of the curve. The y-intercept of the linear relationship is a constant required in the FACTSTM calculation. The standardization is only required on make-up of a new batch of NaOH titrant, or opening of a new container of concentrated formaldehyde solution. 2-4.2. SANDA FACTSTM Instrument Parameters. • Titrant delivery rate: 12 µl/sec • Data collection interval: 0.3 sec • Gain: 5 • Fast Fourier Transform smoothing factor: 60 2-4.3. Sample Preparation and Titration. Approximately 0.75 - 0.8 g of MSG is accurately weighed into a 200 ml styrofoam cup. A magnetic spin bar is provided, and 10 ml water added. The MSG is dissolved by gentle stirring on a magnetic stirrer, and 10 ml concentrated formaldehyde solution added. The solution is titrated thermometrically in an identical manner to the standards. 3. RESULTS AND DISCUSSION. 3-1. Standardization of NaOH. The molarity of the NaOH was determined to be 2.0276M. The y- intercept was 0.096. Coefficient of correlation (R2) was 0.99999. 3-2. Purity of MSG. The purity of the MSG was determined by replicate titrations to be 98.0 ± 0.3% (1σ). A valency of 1 was assumed for the calculations. 3-3. Titration thermogram. Figure 1 illustrates a typical titration thermogram for the determination of MSG by Sörensen's Titration. The instrument records the temperature change of the solution during the titration. The FACTSTM software calculates the first and second derivatives of this temperature curve. The first derivative shows the rate of enthalpy change, and the second derivative is used to located the endpoint. 28 27.5 First Direct Degrees Celsius Derivative Temperature 27 Curve Curve endpoint 26.5 26 Second Derivative 25.5 Curve 25 0.5 1 1.5 2 ml NaOH Titrant Figure 1. Titration Thermogram of Monosodium Glutamate with 2M NaOH 3-4. Ratio of Sample Mass to Formaldehyde Addition. During the method development, a number of titrations were performed with sample masses around 1.5 g, but using the same volume (10 ml) of formaldehyde solution. Within statistical error, the same result was obtained for titrations with a lower sample mass of approximately 0.75g, but the precision was slightly less. There was no evidence that the kinetics of the reaction of the formaldehyde with the MSG were at all significant. 4. CONCLUSION. The thermometric adaptation of Sørensen's Titration is a quick, easy, accurate and precise method for the determination of the purity of amino acids such as monosodium glutamate. 5. REFERENCE. Finar, I. L. (1959) Organic Chemistry - Volume One - The Fundamental Principles (Third Edition). Longmans, Green and Co. Ltd, London.
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