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Classical and Thermal Methods

Aldehydes and Ketones – Form acetals and ketals respectively with normal methanol-containing reagents – Water formed in this reaction will then be titrated to give erroneously high water results – With aldehydes a second side reaction can take place, consuming water, which can lead to sample water content being underestimated – Replacing methanol with another solvent can solve the difficulties (commercial reagents are widely available) | 4 30 2012 a W VILLANOVA UNIVERSITY Classical and Thermal Methods Lecture Date March 26th 2008 Classical and Thermal Methods Karl Fischer moisture determination - Representative of a wide variety of high-performance modern analytical titration methods - The only titration discussed in detail during this class Thermal Methods - Thermogravimetry TG - Differential thermal analysis DTA - Differential scanning calorimetry DSC Reading - KF Skoog et al. pgs 707-708 - Thermal methods Skoog et al. Chapter 31 Cazes et al. Chapter 15 1 4 30 2012 Karl Fischer Titration KFT Karl Fischer titration is a widely used analytical technique for quantitative analysis of total water content in a material Applications - Food pharma consumer products - Anywhere where water can affect stability or properties Karl Fischer German chemist developed a specific reaction for selectively and specifically determining water at low levels. - reaction uses a non-aqueous system containing excess of sulfur dioxide with a primary alcohol as the solvent and a base as the buffering agent A modern KF titrator For more information about KFT see US Pharmacopeia 921 Karl Fischer Reaction and Reagents Reaction ester CH3OH SO2 RN---- RNH SO3CH3- RNH SO3CH3- H2O I2 2RN RNH SO4CH3 2 RNH I- Reagents 0.2 M I2 0.6M SO2 2.0 M yridine nmethan J ethanol Pyridine Free e.g. imidazole Endpoint detection bipotentiometric detection of by a dedicated pair of Pt electrodes Detector sees a constant current during the titration sudden drop when endpoint is reached I- disappears and only I2 is around when the reaction finishes 2 4 30 2012 Volumetric Karl Fischer Titration Volumetric KFT recommended for larger samples 50 mg - One component Titrating agent one-component reagent I2 SO2 base Analyte of known mass added - Two component reagents are separated Titrating agent I2 and methanol Solvent containing all other reagents used as working medium in titration cell Columetric of Karl Fischer Titration Coulometric KFT recommended for