Silver perchlorate, potentiometric titration

Until recently, the most popular reference half-cell for potentiometric titrations, polarography, and even kinetic studies has been the saturated aqueous calomel electrode (SCE), connected by means of a nonaqueous salt bridge (e.g., Et4NC104) to the electrolyte under study. The choice of this particular bridge electrolyte in conjunction with the SCE is not a good one because potassium perchlorate and potassium chloride have a limited solubility in many aprotic solvents. The junction is readily clogged, which leads to erratic junction potentials. For these practical reasons, a calomel or silver-silver chloride reference electrode with an aqueous lithium chloride or quaternary ammonium chloride fill solution is preferable if an aqueous electrode is used. 

Lisinoprii can be determined by potentiometric titration with aqueous sodium hydroxide and non-aqueous perchioric acid. The sodium hydroxide titration is carried out by titrating the iisinoprii potentiometrically with carbonate free 0.1 N NaOH to one endpoint using a combination eiectrode. Lisinoprii can aiso be determined by titration potentiometricaiiy with 0.1 N perchloric acid in acetic acid to one endpoint. The electrode system consists of a glass electrode (such as a Metrohm Model EA 107 vs. a silver/silver chloride reference electrode such as a Metrohm Model EA 432 filled with 0.1 N lithium perchlorate in glacial acetic acid. 

For most potentiometric measurements either the saturated calomel reference electrode or the silver/silver chloride reference electrode are used. These electrodes can be made compact, are easily produced, and provide reference potentials that do not vary more than a few millivolts. The discussion in Chapter 5 outlines their characteristics, preparation, and temperature coefficients. The silver/silver chloride electrode also finds application in nonaqueous titrations, although some solvents cause the silver chloride film to become soluble. Some have utilized reference electrodes in nonaqueous solvents that are based on zinc or silver couples. From our own experience, aqueous reference electrodes are as convenient for nonaqueous systems as are any of the prototypes that have been developed to date. When there is a need to rigorously exclude water, double-salt bridges (aqueous/nonaqueous) are a convenient solution. This is true even though they involve a liquid junction between the aqueous electrolyte system and the nonaqueous solvent system of the sample solution. The use of conventional reference electrodes does cause some difficulties if the electrolyte of the reference electrode is insoluble in the sample solution. Hence the use of a calomel electrode saturated with potassium chloride in conjunction with a sample solution that contains perchlorate ion can cause erratic measurements due to the precipitation of potassium perchlorate at the junction. Such difficulties normally can be eliminated by using a double junction that inserts another inert electrolyte solution between the reference electrode and the sample solution (e.g., a sodium chloride solution). 

Assay. Transfer about 450 mg of dipyridamole, accurately weighed, to a 250 mL beaker, and dissolve in 50 mL of glacial acetic acid. Stir for 30 min, add 75 mL of acetone, and then stir for an additional 15 min. Titrate with 0.1 N perchloric acid VS to a potentiometric end point using a glass electrode and a silver-silver chloride reference electrode system. Perform a blank titration, and make any necessary correction. Each mL of 0.1 N perchloric acid is equivalent to 50.46 mg of... 

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