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High water content of lubricating oils negatively impacts the operation and longevity of the oils and the mechanical equipment components being lubricated. Water increases the oxidation rate of oils, thus prematurely using up the oils’
oxidation inhibitors. Additionally, water has been known to cause certain oil additives to precipitate out, as well as to chemically attack some additives. Some of the modes by which water exists in lubricating oils can lead to catastrophic equipment failure. These include corrosion, erosion, etching and hydrogen embrittlement.
Water in oils can occur in dissolved, emulsified and free states. Visual indication is reliable for quantifying water content only in the free state, while the hot plate crackle test can be used to detect free and emulsified water. However, neither of these methods can detect dissolved water or reproducibly detect trace levels of emulsified water. Furthermore, neither visual indication nor the hot plate test can be used to reliably quantify the water present. Distillation methods, such as ASTM D95 and D4006 provide better quantitative data in the range of approximately 500 ppm to 25 percent, but require large sample sizes and involve long analysis times, typically 60 to 110 minutes.
Since its invention by German petroleum chemist Dr. Karl Fischer in 1935, Karl Fischer (KF) analysis has progressed from an esoteric laboratory procedure to a widely accepted instrumental method routinely used for water determination in the petrochemical industry. It is estimated that nearly 500,000 KF determinations are performed daily around the world. The method forms the basis of several commonly used ASTM standards for water determination in oils, including ASTM D1533, D1744, D4377, D4928 and D6304. The KF method does not suffer from the same issues and limitations associated with the other techniques described above, and a number of recent advances in titrator instrumentation and reagent formulations have further improved the accuracy and reproducibility of KF analyses.Chemistry and Principles
ROH + SO2 + RN → (RNH)·SO3R
(RNH).SO3R + 2 RN + I2 + H2O + → (RNH)·SO4R + 2 (RNH)I
The end-point determination in KF titration occurs by means of bivoltametric indication. That is, while the iodine in the KF reagent is reacting with water, there is no free iodine present in the titration cell, and a high voltage is required to maintain the set polarization current at the double platinum pin indicator electrode. Once all the water has reacted with the iodine, trace quantities of free iodine appear in the titration cell, causing a drop in voltage necessary to keep the polarization current constant, which in turn signals the end-point of the titration.Types of Titration
The most widely used standard methods based on volumetric KF are ASTM D1533 (Method A), D1744 and D4377. Volumetric KF is most accurate in the range of 500 ppm to 100 percent water.
In coulometric KF, the iodine needed by the KF reaction is not present in the KF reagent, but is instead generated electrochemically in situ from iodide at the anode of the generator electrode, a component of the coulometric titration cell (Figure 2). Corresponding reduction of hydride to hydrogen takes place at the cathode. In coulometry, the quantity of iodine generated corresponding to the amount of water present is calculated by the titrator on the basis of current (mA) and time (sec). Coulometric KF is considered an absolute method because time and current can both be accurately measured. The most widely used standard methods based on coulometric KF are ASTM D1533 (Method B), D4928 and D6304 (Method A). Coulometric KF is most accurate in the range of 1 ppm to 5 percent water.
Oil Sample Challenges
For those oil samples which fail to adequately dissolve even when using KF reagents formulated with organic co-solvents, or those that are suspected of containing interfering compounds, an indirect KF analysis using an oil evaporator is recommended, as described below.
Indirect Titration Using an Oil Evaporator
The procedure involves adding the oil sample to the solvent present in the evaporation chamber where it dissolves in the solvent, and in the process forms a binary azeotrope between the solvent and the sample's water content. The solvent/sample mixture is then heated to near the azeotropic point and kept at that temperature while a dry, inert carrier gas, such as nitrogen, is used to carry the azeotropic vapor into the titration cell of the KF titrator, where the water content is quantified. This method thus combines the best features of both KF titration and distillation techniques, such as ASTM D95, because the oil evaporator is simply a miniaturized Dean & Stark distillation apparatus. The distillation component of the indirect titration method ensures that the hard-to-dissolve oil samples, or those containing potentially interfering compounds, are not introduced to the titration cell, while the KF titration component of the method accurately quantifies the water content of the azeotropic vapor carried into the titration cell by the dry gas.
Additionally, because the oil evaporator contains a miniaturized distillation set-up, only small sample (0.1 to 2.5g) and solvent (10 to 15mL) quantities are required.
Sample Size Selection
KF analysis is a versatile and robust analytical tool for water content analysis in oil samples, and can detect water in any of three states commonly found in oils. The technique also offers other substantial advantages over more conventional hot plate crackle test and distillation methods, and it has been incorporated into numerous ASTM standards. Volumetric or coulometric KF methods, utilizing either direct titration or the oil evaporator technique, can be used to quantify water in oil samples from trace amounts to 100 percent.References>