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Chapter 3: Variations of Karl Fischer Titration
Reagents: Acids & Bases
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Strong bases can increase the pH of the Karl Fischer system if the basicity exceeds the buffering capacity of the reagent solutions. A titration end point will not be reached. Strong bases must be neutralized prior to starting the titration by the addition of an adequate amount of salicylic, benzoic or propionic acid. HYDRANAL®-Buffer Base is a ready-made working medium for water determination in basic samples. It contains salicylic acid and has a buffer capacity of 1 mmol base per mL.
HYDRANAL®-Benzoic Acid is typically used because it is easy to handle and does not have an unpleasant odor. HYDRANAL®-Salicylic Acid is stronger and can reduce the pH to 4-5, which is outside the desirable pH range and can slow down the course of the titration slightly. Salicylic acid is especially useful for water determination in phenols. Propionic acid has similar acidification properties as benzoic acid. However, propionic acid has an unpleasant odor.
Strong acids can reduce the pH of the working medium and cause significant reduction in the Karl Fischer reaction rate. Acids must be neutralized prior to starting the titration by the addition of a suitable base to the working medium. However, the added amount of base should not increase the pH to a level where no end point in the pre-titration can be reached. Pyridine is a commonly used base for this purpose. More suitable however are buffering systems, which not only neutralize but also stabilize the pH of the system. HYDRANAL®-Solvent fulfills this role; 1 mL can buffer approximately 0.6 mmol of acid, i.e. a maximum amount of 12 mmol of acid can be added to the titration using the usual quantity of 20 mL HYDRANAL®-Solvent. We recommend the addition of HYDRANAL®-Imidazole or, preferably, HYDRANAL®-Buffer Acid for larger quantities of acidic samples. Certain carboxylic acids that have a tendency to esterify [1] can be neutralized by using HYDRANAL®-Buffer Acid.
Product range HYDRANAL® additional reagents
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