Dear Stefano,

The procedures for assaying sulfur trioxide-pyridine complex:

1-Eastman Kodak uses an infrared method that measures the ratio of absorbance for pyridine sulfate and pyridine-sulfur trioxide as a function of time. Extrapolation of the graph of percent pyridine sulfate vs. time back to t = 0 provided the amount of pyridine sulfate in the sample.

2-Guiseley reported a titrimetric method to determine the active sulfating reagent in sulfur trioxide-dimethylformamide complex. The complex was allowed to hydrolyze in water and react overnight in anhydrous methanol, and the resulting solutions were titrated with sodiym hydroxide. The amount of active sulfating reagent was calculated from the difference between the two titrations.

3-David H. Sieh and John M. Dunham procedure which involves measuring the difference between two titrations but the assay can be completed in 2 h.

When sulfur trioxide-pyridine complex is dissolved in water, 1 h is required for complete hydrolysis. However, when the complex is dissolved in a 0.1% water in pyridine solution, complete hydrolysis takes place in a maximum of 5 min. This enhanced rate of hydrolysis allows the quantitation of the reaction by a Karl Fischer titration.

Determination of sulfur trioxide by Karl Fischer titration is not without precedent. For example, a procedure has been published for the determination of sulfur trioxide in fuming sulfuric acid based on the hydrolysis of the material followed by titration of the excess water with Karl Fischer reagent.

Determination of Active Sulfur Trioxide.

By use of a volumetric pipet, 50.0-mL portions of wet pyridine (1 mg/mL) were transferred into each of two glass-stoppered 125-mL conical flasks. Since sample handling was found to be crucial to this assay, the following weighing procedure was followed. The top 5 mm of sample was discarded and the remaining material mixed well with a spatula for about 15 s. The sample vial was then accurately weighed. Approximately 150 mg of material was transferred to one of the conical flasks, taking care to exclude lumps. The flask and the sample vial were resealed and the sample vial was reweighed. Both flasks were placed in an ultrasonic cleaner for 30 min to ensure complete dissolution of sample. The water content of 3.0-mL portions of the resulting solutions was determined then as described previously. Duplicate titrations from both flasks were carried out.

The active sulfur trioxide was calculated according to

To view the two equations used in the calculations, please use the following link (equations 1 & 2):

http://pubs.acs.org/doi/pdf/10.1021/ac00244a051

where B is the amount of water titrated for the wet pyridine in micrograms, S is the amount of water titrated for wet pyridine plus sulfur trioxide-pyridine complex in micrograms, and W is the weight of the sample in milligrams.

For more on this topic, please see the following:

http://pubs.acs.org/doi/pdf/10.1021/ac00244a051

Hawryiuk, R. M., Laboratory & Specialty Chemicals Division, Eastman Kodak Co., Rochester, NY, personal communication.

Guiseley, K. 8. "Carbohydrate Sulfates"; Schwelger, R. G., Ed.; American Chemlcai Society, Washington, DC, 1978; Chapter 9, pp 152-153; ACS Symp. Ser., No. 77.

Mltchell, J.; Smith, D. H. "Aquametry", 2nd ed.; Wlley: New York, 1980; Part 111, pp 823-824.

Hoping this will be helpful,

Rafik

Thank you Rafik