Interferences

A number of substances and classes of compounds associated with condensation or oxidation-reduction reactions interferes in the determination of water by Karl Fischer titration. In petroleum products, the most common interferences are mercaptans and sulfides. At levels of less than 500 mg ⁄kg as sulfur, the interference from these compounds is insignificant for water concentrations greater than 0.02 % by mass. For more information on substances that interfere in the determination of water by the Karl Fischer titration method, see Test Method E203. Some interferences, such as ketones, may be overcome if the appropriate reagents are used.

The significance of the mercaptan and sulfide interference on the Karl Fischer titration for water in the 10 mg ⁄kg to 200 mg ⁄kg range has not been determined experimentally. At these low water concentrations, however, the interference may be expected to be significant for mercaptan and sulfide concentrations of greater than 500 mg ⁄kg as sulfur.

The Aquamax KF PRO Oil shown in Fig.1 is the perfect instrument to measure ppm water in oils and fuels without the worry of interference side reactions caused by additives or S/SH-. The unique “closed loop” concept illustrated in Fig.4 avoids methanol to evaporate from the KF reagent. Meaning no additional carrier gas necessary. Directly injecting the sample in to the oven means no blank value is required, making the Aquamax KF PRO Oil a truly accurate, trace level water in petroleum products titrator.

Figure 3. Closed Loop Carrier Gas design

Preparation of Apparatus

Follow the manufacturer’s directions for preparation and operation of the titration apparatus.
Seal all joints and connections to the vessel to prevent atmospheric moisture from entering the apparatus.
Add the Karl Fischer anode solution to the anode (outer) compartment. Add the solution to the level recommended by the manufacturer.

Add the Karl Fischer cathode solution to the cathode (inner) compartment. Add the solution to a level 2 mm to 3 mm below the level of the solution in the anode compartment.

Turn on the apparatus and start the magnetic stirrer for a smooth stirring action. Allow the residual moisture in the titration vessel to be titrated until the end point is reached. Do not proceed beyond this stage until the background current (or background titration rate) is constant and less than the maximum recommended by the manufacturer of the instrument.

NOTE 4—High background current for a prolonged period may be due to moisture on the inside walls of the titration vessel. Gentle shaking of the vessel (or more rigorous stirring action) will wash the inside with electrolyte. Keep the titrator on to allow stabilization to a low background current.

Calibration and Standardization Procedure

In principle, standardization is not necessary since the water titrated is a direct function of the coulombs of electricity consumed. However, reagent performance deteriorates with use and shall be regularly monitored by accurately injecting a known quantity of water (see 7.2 in D6304) that is representative of the typical range of water concentrations being determined in samples. As an example, one may accurately inject 10 000 µg or 10 µL of water to check reagent performance. Suggested intervals are initially with fresh reagent and then after every ten determinations (see 11.3 in D6304) or D6869 standard.

Test Procedure

The procedures described below are just a generic procedures described in D6304. Users should encouraged to consult ASTM STM D869 and especially the Instruction Manual of ECH Instruments.

Procedure A (by Mass)

Add newly made solvents to the anode and cathode compartments of the titration vessel and bring the solvent to end-point conditions as described in Section 9(D6304).

Add the petroleum product test specimen to the titration vessel using the following method:

Starting with a clean, dry syringe of suitable capacity (see Table 1 and Note 5), withdraw and discard to waste at least three portions of the sample. Immediately withdraw a further portion of sample, clean the needle with a paper tissue, and weigh the syringe and contents to the nearest 0.1 mg. Insert the needle through the inlet port septum, start the titration, and with the tip of the needle just below the liquid surface, inject the test specimen. Withdraw the syringe, wipe clean with a paper tissue, and reweigh the syringe to the nearest 0.1 mg. After the end point is reached, record the micrograms of water titrated.

NOTE 5—If the concentration of water in the sample is completely unknown, it is advisable to start with a small trial portion of sample to avoid excessive titration time and depletion of the reagents. Further adjustment of the aliquot size may then be made as necessary.

When the background current or titration rate returns to a stable reading at the end of the titration as discussed in 9.5(D6304), additional specimens may be added as per 11.2.1(D6304).
Replace the solutions when one of the following occurs and then repeat the preparation of the apparatus as in Section 9(D6304).

Persistently high and unstable background current.

Phase separation in the anode compartment or oil coating the electrodes.

The total oil content added to the titration vessel exceeds one quarter of the volume of solution in the anode compartment.

The solutions in the titration vessel are greater than one week old.

The instrument displays error messages that directly or indirectly suggest replacement of the electrolytes—see instrument operating manual.

The result of a 10 µL injection of water is outside 10 000 µg ± 200 µg.

Thoroughly clean the anode and cathode compartment with xylene if the vessel becomes contaminated with product. Never use acetone or similar ketones. Clogging of the frit separating the vessel compartments will cause instrument malfunction.

For products too viscous to draw into a syringe, add the sample to a clean, dry bottle and weigh the bottle and product. Quickly transfer the required amount of sample to the titration vessel by suitable means, such as with a dropper. Reweigh the bottle. Titrate the sample as in 11.2. 12(D6304).

Procedure B (by Volume)

Follow steps from Procedure A, taking sample by volume instead of mass.

NOTE 6—A volume aliquot of the product is titrated to an electrometric end point using a coulometric Karl Fischer apparatus. The steps described in Procedure A are followed except as noted. The volume injection method is applicable only when the vapor pressure and viscosity of the sample permit an accurate determination of the volume of the sample.

NOTE 7—The referee procedure for determination of water in liquid petroleum products by coulometric Karl Fischer titration is Procedure A, which uses a mass measurement of the product test specimen.

NOTE 8—The presence of gas bubbles in the syringe can be a source of uncertainly. The tendency of product to form gas bubbles is a function of product type and corresponding vapor pressure. Viscous products can prove to be difficult to measure volumetrically with a precision syringe.

Procedure C (Water Evaporator Accessory)

If using the water evaporator accessory for samples difficult to analyze by Procedure A or B due to sample viscosity, matrix interference, or extremely small concentrations of water (for example, <100 mg/kg), add 10 mL of white oil to the evaporator accessory. Bubble dry nitrogen gas at about 300 mL ⁄min through the oil. Heat the oil to the temperature suggested by the instrument manufacturer for a particular product type.

Dissolve 5 g ± 0.01 g of accurately weighed viscous sample in a 10 mL volumetric flask. Make up to volume with dried hexane. Shake the sample until it is completely dissolved in the solvent.

NOTE 10—All parts of the glass assembly must be thoroughly dry before use. The smallest amount of contamination by moisture will cause erroneous results. Perform several preliminary runs with known content standards to determine that the system is operating correctly. Water-inalcohol standards must be capped with rubber septa rather than rubber stoppers.

Inject 1 mL of dissolved sample into the evaporator assembly. Start the operating sequence. Follow steps 11.1 through 11.5 in Procedure A(D6304). After the end point is reached, record the micrograms of water titrated from the digital readout on the instrument.

Calculations

Calculate the water concentration in mg/kg or µL/mL of the sample as follows:

water, mg/kg or µg/g = W1 /W2 or                                                                                   (1)
water, µL/mL = V1/ V2
where:
W1 = mass of water titrated, mg or µg (as appropriate),
W2 = mass of sample used, kg or g (as appropriate),
V1 = volume of water titrated, µL, and
V2 = volume of sample used, mL.
Calculate the water concentration, in mass or volume %, of the sample as follows:
water, mass % = W1/ 10 000 x W2 or                                                                              (2)
volume % = V1 /10 x V2
where W1, W2, V1, and V2 are same as previously defined.
Use the following equations for calculating the water content of the sample in units of volume % from mass %, or of mass % from volume %.
water, volume % = water, mass % x [ density of sample at t/ density of water at t ]        (3)
water, mass % = water, volume %/[ density of sample at t /density of water at t ]           (4)
where:
t = test temperature.
Density may be measured using approved test methods such as Test Method D1298 and Test Method D4052. If the density is measured in units of g/mL and the density of water at test temperature is assumed to be 1 g/mL, Eq 5 and Eq 6 simplify to:
water, volume % = water, mass % x density of sample at t (g/mL)                                 (5)
water, mass % = water, volume %/density of sample at t (g/mL)                                    (6)

Precision and Bias

The precision of this test method as determined by the statistical examination of interlaboratory test results is as follows:

Repeatability—The difference between successive results obtained by the same operator with the same apparatus under constant operating conditions on identical test material would, in the long run, in the normal and correct operation of the test method, exceed the following values only in 1 case in 20.

Reproducibility—The difference between 2 single and independent results obtained by different operators working in different laboratories on identical test materials would, in the long run, exceed the following values in only 1 case in 20.

Volumetric Injection                                           Mass Injection

Repeatability                       0.08852 x ^0.7 volume %                                       0.03813 x ^0.6 mass %

Reproducibility                    0.5248 x ^0.7 volume %                                          0.4243 x ^0.6 mass %

where x is the mean of duplicate measurements.

Bias—This test method has no bias since the coulometric determination can be defined only in terms of this test method.

Keywords

Coulometric Titration, Coulometry, Iodine, Moisture Content, Plastics, Volumetric Karl Fischer Titration