(Equivalent Test Methods: IP 386 and ISO 10337)
TEST SUMMARY
This test method is applicable to the determination of water in the range 0.02 to 5 mass % in crude oils. Mercaptan and sulfide interfere. At levels of <500 ppm as sulfur, the interference from these compounds is insignificant. The range of this test method can be extended to 0.005 mass %, but the effects of the mercaptan and sulfide interferences at this level have not been determined. However, it can be expected to be significant at these low water levels.
An aliquot of a homogenized crude oil sample is injected into the titration vessel of a Karl Fischer apparatus in which iodine for the 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.
The precision of this test method is critically dependent on the effectiveness of the homogenization step. Follow the procedure given in Annexe A1 of Test Method D4928 to check on the efficiency of the mixer used for homogenization. This test method can use samples taken either gravimetrically or volumetrically.
TEST PRECISION
Where X is the sample mean.
This precision is valid in the water concentration range of 0.005 to 5 mass %, either gravimetrically or volumetrically.
This test method was found not to have a bias.