ASTM D4928 for water in crude oils by Coulometric Karl Fischer titration
3. Summary of Test Method
3.1 After homogenizing the crude oil with a mixer, an aliquot is injected into the titration vessel of a Karl Fischer apparatus in which iodine for the Karl Fischer reaction is generated coulometrically at the anode. When all the water has been titrated, excess iodine is detected by an electrometric end-point detector and the titration is terminated. Based on the stoichiometry of the reaction, one mole of iodine reacts with one mole of water, thus the quantity of water is proportional to the total integrated current according to Faraday's Law.
3.2 The precision of this test method is critically dependent on the effectiveness of the homogenization step. The efficiency of the mixer used to achieve a homogeneous sample is determined by the procedure given in Practice D5854 (API MPMS Chapter 8.3).
3.3 Two procedures are provided for the determination of water in crude oils. In one procedure, a weighed aliquot of sample is injected into the titration vessel and the mass % of water is determined. The other procedure provides for the direct determination of the volume % of water in the crude oil by measuring the volume of crude oil injected into the titration vessel.
4. Significance and Use
4.1 A knowledge of the water content of crude oil is important in the refining, purchase, sale, or transfer of crude oils.
5. Interferences
5.1 A number of substances and classes of compounds associated with condensation or oxidation-reduction reactions interfere in the determination of water by Karl Fischer. In crude oils, the most common interferences are mercaptans and sulfides (not total sulfur). At levels of less than 500 µg/g (ppm) (as sulfur), the interference from these compounds is insignificant. Most crude oils, including crude oils classified as "sour crude", have mercaptan and sulfide levels of less than 500 µg/g (ppm) as sulfur. For more information on substances that interfere in the determination of water by Karl Fischer titration method (see Test Method E 203).
5.2 The significance of the mercaptan and sulfide interference on the Karl Fischer titration for water levels in the 0.005 to 0.02 mass % range has not been determined experimentally. At these low water levels, however, the interference may be significant for mercaptan and sulfide levels of less than 500 µg/g (ppm) (as sulfur).
