Petroleum Products
Karl Fischer titration is an excellent method for the measurement of trace moisture in petroleum products. Karl Fischer titration has been the standard test method for the Petroleum Society since 1976, and became a Japan Industrial Standard in 1982.
As moisture is generally present only in trace quantities in petroleum products, coulometric titration is presently in widespread use.
Coulometric titration is carried out by adding a sample, such as insulating oil, refrigerating machine oil, transformer oil, kerosene or diesel oil, to an electrolyte. The sample is added 5-10ml at a time. Though some substances fail to dissolve and may cloud the solution, their moisture content can be measured once they are thoroughly dispersed.
For volumetric titration, the substance is dissolved in a dehydrated solvent formulated for oils. Karl Fischer titration can then be carried out directly. Silicon oils can be titrated directly by adding samples to a ketone-type dehydrated solvent.
Some additives, such as antioxidants, cause interference reactions with the Karl Fischer reagent. If lubricants (e.g. gasoline engine oils and diesel engine oils) and other products contain such substances, Karl Fischer titration must be carried out using an oil-type moisture vaporization system.
Dissolve greases that do not contain interference substances in oil-type dehydrated solvent and titrate directly. For those that do contain interference substances, Karl Fischer titration should be carried out indirectly using a moisture vaporization system.
Petroleum products occasionally contain trace amounts of mercaptan or hydrogen sulfide. These react with the Karl Fischer reagent. They react quantitatively with iodine on a mole for mole basis, provided that the amount of mercaptan or hydrogen sulfide is known, moisture content can be determined accurately through adjustment of the titration result.
2RSH+I2 →2RSSR+2HI
H2S+I2→2S+HI
As shown in the above formulae, 1ppm of sulfur or hydrogen sulfide in mercaptan is equivalent to 0.28ppm and 0.56ppm of moisture, respectively.
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