The PHOTOVOLT AQUATEST systems have, for years, used an optimized pyridine-containing Karl Fischer reagent to accurately determine water. PHOTOVOLT pyridine-free Karl Fischer reagent, when used with the AQUATESTS, allows the same precision and accuracy but without the objectionable pyridine odor. The mechanism of the Karl Fischer titration with both types of reagents is illustrated here:     H2O + I2 + SO2 2HI + SO3 From the above equation, it is clear that water reacts with iodine and sulfur dioxide to form hydrogen iodide and sulfur trioxide. The AQUATEST coulometric titrator detects the change in the concentrations of iodine and iodide and quantitatively regenerates iodine. After calculation and compensation for drift, the AQUATEST reports the amount of water in the sample. A titration vessel with fresh reagent solution has the capacity to determine approximately 750 mg of water. With such a large capacity for water, a great many analyses can be performed before new vessel solution is required. Generator solution is occasionally withdrawn to keep the level of the generator solution below the level of the vessel solution; therefore, fresh generator solution may be required for the generator more frequently than fresh vessel solution. Withdrawn generator solution may be added to the vessel solution instead of discarded. Procedures developed with pyridine-containing and pyridine-free reagents may be used interchangeably with the AQUATESTS. Sample size, pre-treatment, and injection techniques should require no changes. However, the prudent analyst should verify this with his or her own samples. Unlike the volumetric Karl Fischer system, for which the titer of the standard solution must be determined, the Karl Fischer coulometric system requires no calibration. An analyst may want to validate that his or her coulometric system is operating properly before beginning a series of analyses. To do this, refer to the PHOTOVOLT TECHNICAL NOTE on instrument validation with water or PHOTOVOLT TECHNICAL NOTE on the use of the Photovolt Water Standard.