Effect of Hydrocarbon Composition on the Measurement of Water in

Cliff T. Mansfield and Bhajendra N. Barman , Jane V. Thomas , Anil K. Mehrotra , James M. McCann. Analytical Chemistry 1999 71 (12), 81-108. Abstract ...
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Anal. Chem. 1998, 70, 4264-4270

Effect of Hydrocarbon Composition on the Measurement of Water in Oils by Coulometric and Volumetric Karl Fischer Methods Sam A. Margolis*

Analytical Chemistry Division, National Institutes of Standards and Technology, Gaithersburg, Maryland 20899

The effect of the composition of hydrocarbons and hydrocarbon mixtures such as oils on the measurement of their water content has been examined. This effect has been evaluated by both the coulometric and volumetric Karl Fischer methods. The results demonstrate that the highest water titers are measured by the volumetric method. With the exception of xylene and octanol, the water titers obtained by at least one coulometric method were lower than those obtained by the volumetric method. The water content of xylene and octanol was equivalent by all methods. The solubility of the oils in the titration vessel solvents was evaluated as a function of the hydrocarbon composition and the content of cosolvents. The formation of a heterogeneous solution caused a decrease in the observed water titer, with the exception of several transformer oils. At the transition point from a homogeneous solution to a heterogeneous solution, frequently no water was measured in at least one sample. The reduction of the water titer at the point that the vessel solution became heterogeneous is consistent with the sequestering of some of the water by suspended oil. The measurement of water in pure hydrocarbons and mixtures of hydrocarbons (e.g., transformer and mineral oils) that are liquid at room temperature is a complex process. Previous studies indicated that the highest water concentration in a given hydrocarbon mixture (transformer oil) was consistently obtained by the use of the volumetric Karl Fischer method.1 Lower values for the water concentration were generally obtained when the coulometric technique was used. This discrepancy between the results obtained by the two methods appeared to be a function of the solubility of the hydrocarbon mixtures (oils) in the Karl Fischer reagent that was used in the titration vessel. The titration of water with Karl Fischer reagents is also subject to several types of systematic errors. The sources of bias for the volumetric method are the accurate estimation of the endpoint of the titration and the use of an accurate standard.1 These biases can be surmounted by using the graphical method of endpoint estimation and by using water-saturated octanol (WSO) as a standard.1 The sources of bias for the coulometric method include the accuracy of the water standard, the nonadjustable instrumental * Tel.: (301) 975-3137. Fax: (301) 977-0685. E-mail: [email protected]. (1) Margolis, S. Anal. Chem. 1995, 67, 4239-4246.

4264 Analytical Chemistry, Vol. 70, No. 20, October 15, 1998

bias, the operator adjustable instrumental parameters, the solvent composition, the cell design, and the sample composition.2 The contribution of the sample composition to the error in the measurement of water in hydrocarbons by the Karl Fischer method has not been fully evaluated. This was particularly evident in the results of an interlaboratory collaborative study sponsored by NIST on transformer oil [Reference Material (RM) 8506] and machine oil (RM 8507), in which the amount of water obtained coulometrically was approximately 50% lower than the corresponding value obtained by the volumetric titration method. In the case of machine oil (RM 8507) using the Coulomat AG-H solution with a diaphragm cell, water values similar to those obtained by the volumetric method were observed. Even after the instrumental and solvent parameters were optimized, significant differences still existed between methods. Furthermore, in previous studies there was the indication that the titration pattern was a function of the oil composition.2 In the current study, the instrument with the lowest instrumental systematic bias2 has been used, and the adjustable parameters have been optimized. WSO was used as the water standard, and the recommended solvents were used for the coulometric measurements. The objective of this study was to evaluate the relationship between the general chemical composition of hydrocarbons and hydrocarbon mixtures and the water titrated by the coulometric and volumetric Karl Fischer methods. EXPERIMENTAL SECTION Hexane and methanol were purchased from J. T. Baker (Phillipsburg, NJ); xylene, petroleum ether, kerosene, and chloroform were obtained from Mallinckrodt (Phillipsburg, NJ); Diala A transformer oil was produced by Shell Chemical Co. (Houston, TX); and Univolt 60 transformer oil, RM 8506 (Univolt N61), and RM 8507 (Coray 22) machine oil were prepared by Exxon Co. (Baytown, TX). Heavy and light paraffin (mineral) oils and the pyridine-based Karl Fischer reagent were purchased from Fisher Scientific Co. (Pittsburgh, PA). Some of the physical properties of these hydrocarbons and oils are summarized in Table 1. Hydranal reagents were purchased from Crescent Chemical Co. Inc. (Hauppauge, NY). In all of the Hydranal reagents, the base or buffer is imidazole instead of pyridine, and these will be referred to generically as imidazole-based Karl Fischer reagents. Volumetric measurements were made on Metrohm Instruments (2) Margolis, S. Anal. Chem. 1997, 69, 4864-4871. S0003-2700(98)00149-8 Not subject to U.S. Copyright. Publ. 1998 Am. Chem. Soc.

Published on Web 09/03/1998

Table 1. Boiling Point, Distillation Range, and Viscosity of the Materials Tested boiling range (°C)

distillation range (°C)

octanol hexane petroleum ether xylene kerosene

98 69 35-60 137-140 270a

155-365

Univolt 60 RM 8506 Shell Diala A

Transformer Oils 380a 288-479 380a 290-479 372a 283-499

sample

RM 8507

Engine Oil (Mineral Oil) 423a 294-561

Parafin Oils (Mineral Oil) light (Drakeol 13) 470a 381-547 heavy (Drakeol 35) 498a 431-550 a

viscosity (m2/s at 40 °C)

8 8 9.6 22 24-26 66-71

The boiling point at which 50% of the sample is volatile.

models E 547 and 633 (Brinkmann Instruments Inc., Westbury, NY), and the titration curves were recorded on a strip chart recorder. Coulometric measurements were made with Metrohm model 684 and 737 coulometric titrators fitted with either a membrane or a membraneless cell (Brinkmann Instruments Inc.) or with an Aquapal III coulometric titrator (CSC Co. Inc., Fairfax, VA). With each instrument, the titrations were performed using the solvent system recommended by the manufacturer. Instruments were calibrated, the samples were analyzed, and the water content was calculated as previously described.1 Conditions for titration with the Metrohm 737 coulometric instrument were, unless otherwise stated, start drift 14 µg of water/min, stop drift 6 µg of water/min, and delay 30 s. All coulometric measurements were corrected for the deviation of the standard from the theoretical water content of WSO of 39.2 g/L of solution.4 Titration Procedure. The coulometric titration process basically consists of the addition of the oil to the anode compartment, the reaction of the water with the iodine-containing Karl Fischer reagent, and the electrochemical regeneration of iodine to a preset potential of the indicator electrode. The pyridine- and imidazole (Hydranal Composite 2)-based Karl Fischer reagents were used for the volumetric titrations, and the vessel solvent was a mixture of chloroform and methanol. The residual water in these cosolvents was titrated with Karl Fischer reagent, and the chloroform content of this mixture was reported in vol %. The Hydranal Coulomat AG-H solution (imidazole-buffered reagent), designed for use with hydrocarbons, was used in the diaphragmless cell, and the Hydranal Coulomat A solution (containing approximately 40 vol % chloroform), which was also designed to be used with hydrocarbons, was used in the anode compartment of the diaphragm cell. Hydranal Coulomat CG solution was used in the cathode compartment. Several experiments were done with Coulomat AD anode solution supplemented with 25 vol % chloroform. In general, the titration protocol consisted of the following sampling sequence: two samples of WSO (10 µL each), three to five oil samples (0.5-3 mL, depending on the water content), and (3) Cedergren, A.; Lundstrom, M. Anal. Chem. 1997, 69, 4051-4055. (4) Leo, A.; Hansch, C. J. Org. Chem. 1971, 36, 1539-1544.

a WSO standard. The second and third steps were repeated either until the voltage across the indicator electrode broke down (i.e., the end point was not detected and the addition of titrant did not stop) or through the transition point until the water titer remained constant for at least three oil samples. Safety Considerations. All of the Karl Fischer reagents contain methanol, sulfur dioxide, and an organic base (imidazole, diethanolamine, or pyridine). Some of them also contain hydriodic acid, chloroform, and/or glycols. These compounds are all toxic to varying degrees with respect to inhalation, ingestion, and/or eye irritation. Several solvents (methanol and pentanol) are flammable. In addition, imidazole is an in vitro mutagen, and chloroform is an animal carcinogen. Proper precautions were used in the laboratory to minimize exposure. The reagents were handled in a chemical hood and disposed of according to OSHA regulations by the NIST Health and Safety Office. The oils used in this study were free from chlorinated biphenyl compounds but were also disposed of by the NIST Health and Safety Office. RESULTS To evaluate the effect of chemical composition on the measurement of water by the Karl Fischer method, a series of hydrocarbons and hydrocarbon mixtures were selected as samples. Their molecular weights spanned the range from an ACS-grade simple linear saturated hydrocarbon (hexane) to a complex mixture of high-molecular-weight (>C25) linear and cyclic hydrocarbons that are extensively saturated (heavy paraffin oil) and contained no additives (Table 1). Three different Karl Fischer methods were selected: (a) the volumetric method, in which the oil is dissolved in a suitable solvent and the water is then titrated amperometrically with Karl Fischer reagent; (b) the coulometric method using a diaphragmless cell consisting of a single compartment and a single Karl Fischer reagent to which the oil is added and the iodine is electrochemically regenerated to a preset potential; and (c) the coulometric method using a diaphragm cell consisting of two separate compartments, the anode and the cathode, separated by a membrane. The results of these measurements are summarized in Tables 2-4. Water in each sample was determined by each method. All of the transformer and machine oils (including RM 8506 and RM 8507) were prepared for interlaboratory collaborative studies and/or as reference materials for the electric power industry and were not saturated with water. All other samples were water saturated to ensure that any variations in the water titer from one analysis to the next were not the result of increased water uptake from the air. In a previous study, we observed that the water content of a given transformer oil varied with the method of analysis, while the water titer in the standard, WSO, was constant and independent of the method of analysis.1 Similarly, in these studies, no variation was observed in the water titer of the WSO as a function of the method or as a function of the amount of oil dissolved or suspended in the titration solvent or the anode solution. Volumetric Karl Fischer Method. The Karl Fischer volumetric method in all cases yielded the highest water concentration in the samples except for 1-octanol, xylene, and RM 8506. In the case of the 1-octanol and xylene, the water values obtained by each method were the same, and in the case of RM 8506, the values obtained by the coulometric method only with a onecompartment cell were statistically indistinguishable from those Analytical Chemistry, Vol. 70, No. 20, October 15, 1998

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Table 2. Effect of Karl Fischer Method and Reagent Composition on the Measurement of Water in Low-Molecular-Weight Hydrocarbonsa method cell type volumetric

samplec octanol xylene hexane petroleum ether

Hydranal Composite 2, 66-70 vol %b 390 (3, 10) 409 (12, 9) 98 (6, 6) 116 (7, 3)d 92 (4, 3) 108 (2, 3)d

coulometric

pyridine, 73-76 vol %b

diaphragmless Coulomat AG-H

diaphragm Coulomat A + CG

389 (2, 20) 382 (12, 9) 95 (7, 6)

396 (3, 20) 389 (13, 7) 75 (4, 8)

390 (6, 7) 389 (10, 12) 70 (7, 9)

87 (7,10)

81 (2, 11)

74 (8, 9)

a The values reported in columns 2-5 represent the water content (µg/g), with the standard deviation and number of measurements given in parentheses. b The vol % of chloroform in the titration vessel solvent after the solvent was titrated to dryness with the Karl Fischer reagent. c The size of each sample varied between 1.5 and 3 g. d The moisture content for the three samples titrated after precipitation of crystals which are probably imidazole.

Table 3. Effect of Karl Fischer Method and Reagent Composition on the Measurement of Water in Kerosenea CHCl3 (vol %)b

titrant pyridine

Composite 2c

75 68 63 48 0 68 59 45 0

single-phase

transition

Volumetric Method 102 (4, 5) 101 (4, 11) 84 (8, 10) 91 (8, 15) 109 (5, 5) 99 (8, 9) 122 (9, 8) n.o.d

bvb bv bv 0 11 260 100

multiphase

65 (3, 3) 75 (8, 5) bv 75 (4, 3) 83 (3, 6) 108 (9, 8)

Coulometric MethodsDiaphragmless Cell 77 (4, 9) 71

54 (2, 4)

Coulometric MethodsDiaphragm Cell 82 (2, 3) 82

81 (1, 6)

a The size of each sample varied between 1.5 and 3 g. The values reported in columns 3-5 represent the water content (µg/g). The numbers in parentheses are the standard deviation and the number of measurements. b “bv” indicates the breakdown of the voltage across the electrode. c This reagent is the imidazole-buffered Hydranal Composite 2 titrant. d “n.o.” indicates that the single phase was not observed.

obtained by the volumetric method (Tables 2-4). This occurred when the initial chloroform content was between 68 and 75 vol % in the vessel solution (e.g., kerosene 102 µg vs 101 µg water/mg, respectively). Furthermore, two-phase solutions were not formed with any of the samples when the pyridine-based Karl Fischer reagent was used for the volumetric titration and the initial chloroform content of the titration vessel solvent was 73-76 vol %. The breakdown voltage was reached before multiphase solutions were formed. Similar results were observed when the water in transformer oils was titrated with Hydranal Composite 2. The breakdown of the voltage across the indicator electrode was not observed with the paraffin oils; only the transition to a multiphase state was observed. Measurements with the pyridine Karl Fischer reagent and coulometric measurements were done 4266 Analytical Chemistry, Vol. 70, No. 20, October 15, 1998

at the same time, and the water content of the samples in these measurements was comparable for a given oil. In the case of the Shell and Univolt 60 oils, the volumetric measurements with the imidazole-based Karl Fischer reagents were done 11 months after the other measurements. These oils, which were stored in screwcap bottles, appeared to have absorbed some water from the atmosphere. Thus, the absolute values of these measurements cannot be compared to the rest of the measurements on the same oil sample. All other samples were either water saturated or stored in sealed glass ampules (RMs 8506 and 8507). In the present study, we have surveyed a range of hydrocarbons and hydrocarbon mixtures of different molecular weight distributions (Table 1) to establish at what point this transition occurs and to define more clearly the nature of this event with respect to the hydrocarbon composition. When successive samples of water-saturated kerosene (interspersed with WSO samples at every third or fourth sample) were titrated with the pyridine reagent, this transition to a multiphase system was not observed until the chloroform was less than 50 vol % of the starting titration vessel solution (40 mL) and after 40 mL of kerosene had been added to the titration vessel (Table 3). In the case of the imidazole reagent, using the same amount of titration vessel solution, this transition to a multiphase system was observed at a higher chloroform concentration (59 vol %) after 17 mL of kerosene had been added to the titration vessel. At 68 vol % chloroform, the transition point was observed, but the breakdown voltage was reached immediately thereafter. Once the multiphase system formed, the amount of water measured in the oil decreased in all cases. The water measurement at the transition point did not approach zero, except at chloroform concentrations of 48 vol % for the pyridine reagent and 68 vol % for the imidazole reagent. In both volumetric titration systems, the water content measured when a single phase was present was similar although not identical. In every case (except kerosene) in which a transition was observed to a multiphase system, the water content measured for the multiphase system was lower than that observed for the same samples in the single-phase system (Table 4). It is also striking that, with two exceptions for water-saturated kerosene (Hydranal Composite 2 at 59 vol % and 45 vol % chloroform), the reduction of the amount of water measured in the oil at the transition point was >20 µg of water/g of oil. At 59 vol % chloroform, the high water measurement at the transition point can be attributed to the approach to the point at which the voltage across the indicator electrode breaks down just before the transition to a multiphase system. This was observed for the titration of the water in the sample immediately preceding the complete breakdown of the voltage across the indicator electrode. When methanol was used as a vessel solvent and Hydranal Composite 2 as the titration solvent for the titration of watersaturated kerosene, the water content was the same as that observed in the presence of chloroform. This is the only case in our studies on the measurement of water in oils where this was observed.1 The results obtained for both water-saturated kerosene and water-saturated paraffin oil reveal only a slight difference in the pattern of water titers. In both cases, the highest water titers were obtained at the highest possible concentration of chloroform,

Table 4. Effect of Karl Fischer Method and Reagent Composition on the Measurement of Water in Transformer and Engine Oilsa titrant

CHCl3 (vol %)b

single-phase

transition

multiphase

Univolt 60 (Transformer Oil, Antioxidant ) BHT 0.08%) pyridine Composite 2e

Volumetric Method 25 (2, 10)c 46 (4, 6) 50 (4, 4)

73-76 66-70 58-62

bvd bv 8, 21

29 (6, 5)

Coulometric MethodsDiaphragmless Cell 18 (2, 10)c Coulometric MethodsDiaphragm Cell n.o.f

9 (2, 10)c

RM 8506, Univolt N61 (Transformer Oil, Antioxidant ) BHT 0.3%) pyridine Composite 2

Volumetric Method 40 (3, 16)g 45 (7, 5) 50 (4, 5)

73-76 66-70 58-62

Coulometric MethodsDiaphragmless Cell 46 (7, 3)

bv bv 0, 0 44

Coulometric MethodsDiaphragm Cell n.o.

19 (4, 4) 43 (6, 5) 18 (2, 12)h

Shell Diala A (Transformer Oil, No Antioxidant) pyridine Composite 2

Volumetric Method 33 (3, 10)c 54 (4, 4)

73-76 66-70

bv 0, 13

26 (3, 4)

Coulometric MethodsDiaphragmless Cell 19 (1, 10)c Coulometric MethodsDiaphragm Cell n.o.

17 (2, 10)c

RM 8507 Coray 22 (Engine Oil, No Antioxidant) pyridine Composite 2

73-76 66-70

Volumetric Method 77 (5, 18)i 73 (5, 6) Coulometric MethodsDiaphragmless Cell 66 (6, 3)

bv 5, 18 64

Coulometric MethodsDiaphragm Cell n.o.

45 (5, 4) 63 (3, 5) 49 (4, 12)h

Drakeol 13 (Light Mineral Oil-Paraffin Oil, Antioxidant ) R-Tocopherol, 15 mg/kg) pyridine

Composite 2

73-76 66 47 methanol 66-70

Volumetric Method 85 (6, 9) 70 (12, 3) n.o. n.o. 73 (0, 2)

bv bv 0 47

59 (7, 6) 51 (6, 6) 43 (4, 7)

Coulometric MethodsDiaphragmless Cell n.o.

47 (5, 9)

Coulometric MethodsDiaphragm Cell n.o.

48 (7, 12)

Drakeol 35 (Heavy Mineral Oil-Paraffin Oil, Antioxidant ) R-Tocopherol, 15 mg/kg) pyridine Composite 2

73-76 66-70

Volumetric Method 76 (6, 9) 83 (0, 2) Coulometric MethodsDiaphragmless Cell n.o.

bv 49

51 (3, 5) 50 (5, 9)

Coulometric MethodsDiaphragm Cell n.o. 34 (2, 9) a The size of each sample varied between 1.5 and 5 g. The values reported in columns 3-5 represent the water content (µg/g). The numbers in parentheses are the standard deviation and the number of measurements. b The vol % of chloroform in the titration vessel solvent after the solvent was titrated to dryness with the Karl Fischer reagent. c These means represent the average of two groups of five samples. All samples were analyzed under single-phase conditions. d “bv” indicates the breakdown of the voltage across the electrode. e This reagent is the imidazole-buffered Hydranal Composite 2 titrant. f “n.o.” indicates that the single-phase condition was not observed with this solvent and oil sample combination. g This mean represents the mean of four groups of four samples analyzed in this manner because the solvent system could accommodate only four samples of 6.5 g each before the voltage across the electrode broke down. h These samples were analyzed using Coulomat AD + 20 vol % chloroform as the anolyte. i This mean represents the mean of three groups of six samples analyzed in this manner because the solvent system could accommodate only six samples of 5 g each before the voltage across the electrode broke down.

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Table 5. Effect of Water Content on the Volumetric Titration of Water in Coray 22 Oil (RM 8507)

HCl3 (vol %)a

single-phase water, µg/g (SD, N)b

68 66 70

190 (7, 7) 93 (6, 6) 73 (5, 6)

transition water, µg/g

total sample size (g)c

multiphase water, µg/g (SD, N)

50 16, 21, 17 5, 18

1 5 5

119 (11, 5) 36 (2, 3) 45 (5, 4)

a The vol % of chloroform in the titration vessel solvent after the solvent was titrated to dryness. b The size of each sample varied between 1.1 and 5 g. The numbers in parentheses represent the standard deviation of a single measurement (SD) and the number of measurements (N). c This value represents the total oil added for all of the samples (1, 2, or 3) cited in the transition category.

and in the multiphase state the water titer was consistently lower than that measured in the single-phase state. Comparison of the results obtained for the kerosene to those for the paraffin oil yields three primary differences: (a) the water titer at the transition point is lower in the paraffin oil; (b) the appearance of the multiphase system occurs at a higher chloroform concentration; and (c) a larger number of the kerosene samples can be accommodated before the multiphasic state occurs. These trends persist through the entire series of oils assayed. The kerosene appears to represent the type of hydrocarbon mixture whose properties are intermediate between the low-molecular-weight hydrocarbons and the higher molecular weight transformer and paraffin oils. The transformer oils and the engine oil represent a group of oils relatively intermediate in hydrocarbon molecular weight (Table 1). These oils were used in studies to evaluate the measurement of water in dry transformer oils. Titration results obtained with the dry oil samples are similar to those obtained with the water-saturated samples, except that, at the transition point, the titratable water for at least one sample of each oil is very low (between 0 and 8 µg/g). This strongly suggests that the water is redistributed at the transition point for these four dried oils. To understand this process, we evaluated several bottles of RM 8507 that were opened and closed periodically for sampling, thus permitting moist air to enter the bottle. These samples were thus left in contact with a humid atmosphere for extended periods of time and were hydrated to different degrees (Table 5). The chloroform contents of the titration vessel were similar in all three titrations, thus minimizing any role that the solvent could play in this measurement. As the water content of the oil increased, the amount of water titrated at the transition point increased. The total amount of oil necessary to progress beyond this transition point decreased from multiple samples totaling 5 g of oil with a lower water content to a single 1-g sample of oil with a high water content. In all cases cited in Table 5, the measured water in at least one sample at the transition point at each level of water saturation was more than 50% lower than the average amount of water titrated after the multiphasic system had attained an equilibrium state. The basis for the reduction in the water content obtained after the system reached equilibrium is not clear, since the same oil (RM 8507) was used to prepare all of samples discussed in Table 5. It is reasonable to conclude that the primary cause of this variation in water titer at the transition point is the result of the increase in the amount of oil in the 4268 Analytical Chemistry, Vol. 70, No. 20, October 15, 1998

titration vessel solvent rather than the oil composition. This includes the amount of Karl Fischer reagent added during the sequential titrations and the partitioning of chloroform and methanol between solvent phases at the time each sample was titrated. It is possible that trace materials that were present in the oil might have reacted with the Karl Fischer reagent and might be partially responsible for the observed results. However, the amount of water measured at the transition point was very low. This suggests that these additives did not react with the Karl Fischer reagent at the transition point. Coulometric Karl Fischer Measurement. The only hydrocarbon samples that have the same water titer by all of the methods are xylene and 1-octanol (Table 3). These values are very similar to the theoretical value of moisture in xylene4 (389 mg/kg) and 1-octanol, SRM 2890 (39.2 mg/kg). It indicates that the polar and aromatic hydrocarbons are the least responsible for the lower coulometric water titers. Hexane and petroleum ether are similar in that petroleum ether is primarily a mixture of pentanes and hexanes. Both of these samples exhibit similar but not identical water titers by each method. The coulometric titers tend to be lower than the volumetric titers (theoretical value for hexanes, approximately 90 mg/kg), even though all of the samples were saturated with water. Since these do not contain additives, the only conclusion is that the coulometric method does not measure all of the water. Furthermore, the coulometric reagents that were used in this study are the reagents that are recommended by the instrument and the reagent manufacturers for the measurement of water in nonpolar compounds. Kerosene, which contains longer chain hydrocarbons, alkyl benzenes, and naphthalenes, represents a transition type hydrocarbon mixture. The first samples completely dissolved in the coulometric anode solvent (nine samples in the case of the diaphragmless cell and three samples in the case of the diaphragm cell), indicating that the AG-H solvent had a greater capacity than the AD solvent for dissolving the oil sample (Table 3). The later samples cause the anode solution to become cloudy or multiphasic, and, in the case of the diaphragmless cell, the amount of water measured decreased by an additional 22%. This was not observed with the diaphragm cell, where the water content was the same before and after the formation of a multiphasic solution. Regardless of which coulometric method was used to measure the water in kerosene, the results were always more than 17% below the volumetric water titers. In the case of the transformer oils and other oils, the results obtained coulometrically are always more than 17% lower than the volumetric water titers with the lone exception of RM 8506, in which the titers by the volumetric and the coulometric (diaphragmless cell) methods are statistically indistinguishable. The values obtained using the diaphragmless cell are always equal to or higher than those obtained using the diaphragm cell. It is apparent that the coulometric method does not measure water in oil as efficiently as the volumetric method, with the one obvious exception (RM 8506). It is also apparent that a greater amount of water is detected with the diaphragmless cell than with the diaphragm cell, excepting light mineral oil. Finally, it is noteworthy that the presence of pentanol in the Coulomat AG-H solution eliminates the change at the transition point and the reduction of the moisture in the multiphasic state for oils in the class containing

RM 8506 and RM 8507 but not for the Drakeol oils, which become multiphasic upon the addition of the first oil sample. DISCUSSION The amount of water measured in the WSO standard was not affected by the transition from a homogeneous to a multiphasic solution in any of the volumetric studies reported herein. The titer of water-saturated xylene is nearly the same for each of the four methods of analysis. The mean titer of water in WSO obtained by the two coulometric methods was between 390 and 396 µg/10 µL of WSO (Table 2), as compared to theoretical amount of 392 µg/10 µL of WSO (SRM 2890). Thus, all instruments can titrate all of the water in two very different samples, xylene and WSO. Consequently, neither the vessel design nor the Karl Fischer reagent(s) are responsible for the method-dependent differences in the amount of water measured in different oils. Thus, the change in the amount of water measured at the transition from a single-phase system to a multiphase system is not a function of the ability of the Karl Fischer reagent to detect and measure all of the water in a sample added to the titration system in either the single-phase or the multiphasic state. Instead, this appears to be the result of the sequestering of the water in the oil in a manner that is not clearly understood and is not accessible to the Karl Fischer titrant. The series of hydrocarbons and hydrocarbon mixtures that have been evaluated increased in boiling range, distillation range, viscosity, and the molecular weight distribution of the hydrocarbons (Table 1). The titration results of these samples (Tables 2-4) indicate that, independent of the water content, the amount of water measured after the formation of a multiphasic system decreases. Furthermore, the water content of these samples measured at the transition point was less than that measured after the transition point was passed. As the degree of water saturation decreased, the amount of water measured at the transition point was smaller, and more sample was required before the constant level of water characteristic of the multiphasic system was achieved (Table 5). The four unsaturated oils show a similar titration pattern progressing from the single-phase to the multiphasic state. Since these four oils are different in composition, the pattern in the titration of water in oils appears to be characteristic of oils in general. The water titers obtained by both volumetric methods appear to be very similar, except for the Shell and the Univolt 60 oils, which absorbed water during the 11 months between the performance of the volumetric assays with each solvent. However, in general, the pyridine reagent, when compared to the imidazole reagent, permits the use of slightly higher concentrations of chloroform and also permits the titration of a greater number of samples before the vessel solution becomes multiphasic. Xylene and WSO were the only materials for which the same values were obtained with all four methods. This is in contrast to the variable results obtained for oils with the Hydranal AG-H solution in the diaphragm cell.1 This observation cannot be attributed to a negative bias in the titration of water in WSO2 and is in agreement with the results reported by Cedergren and Lundstrom,3 who observed no negative bias with a water standard when using a membraneless cell with the Coulomat AG-H type solutions. The explanation for these method-dependent discrepancies must lie either in the interaction between the oil and the reagents

in a manner that sequesters the water so that it is unable to react with the Karl Fischer solution, or in the reaction of oil additives with the Karl Fischer reagents. The antioxidants do not appear to be responsible for this effect, since similar patterns of response between the different methods are observed for the Univolt oils, which contain ∼0.05 wt % BHT, the Drakeol oils, which contain ∼15 µg/g R tocopherol, and the Coray and Diala A oils, which contain no antioxidants. Other compounds, such as ketones or sulfur compounds, could, conceivably, increase the water titer by reacting with either the volumetric or coulometric Karl Fischer reagents. However, these types of compounds should react with each reagent to a similar extent. The interaction of the oils with the buffering reagent (imidazole vs pyridine) does not appear to be responsible for these methoddependent discrepancies, since similar results are obtained with both volumetric methods on dried oils stored in sealed ampules (RMs 8506 and 8507) and water-saturated oils (Drakeol oils). The only remaining possible explanation is the distribution and/or solubility of each oil in the Karl Fischer reagent plus the cosolvents. These results indicate that, when the solution in the Karl Fischer titration vessel undergoes the transition from a singlephase system to a multiphase system, the water is nearly always redistributed into the new oil phase and is no longer accessible to the Karl Fisher reagent. The addition of pentanol to the Karl Fischer reagent (Hydranal AG-H) has a variable effect on the coulometrically measured water in the group of oils that were evaluated. In the presence of pentanol, the amount of water measured coulometrically in Univolt N61 and Coray 22 oils using a diaphragmless vessel is equal or close to that observed with the volumetric method. No decrease in water content is observed at or after the transition point. In the case of the Diala A and the Univolt 60 oils, the amount of water measured, when the Hydranal AG-H reagent was used, is lower than that observed by the volumetric method. These results are consistent with the results observed on oils by Cedergren and Lundstrom3 when they compared the titration results using Coulomat AG-H reagent to those using a specially formulated solvent containing 75% chloroform. The effect of the pentanol on the water measured in the oils is consistent with the concept that, under most coulometric conditions, particularly but not exclusively when the system is multiphasic, some of the water is not accessible to titration by the Karl Fischer reagent and that the relatively polar pentanol reduces the amount of water that is sequestered. CONCLUSION These studies were initiated to explain the wide variation in ASTM collaborative studies on the measurement of water in transformer oils. The results in this study confirm the observation that the amount of water measured in an oil sample is dependent upon whether the oil is completely dissolved in the vessel solution of the Karl Fischer apparatus.1,2 This observation has been extended to a wide variety of oils and demonstrates that this is a characteristic property of the oils. Furthermore (except for xylene and 1-octanol), water content measured by the volumetric method is usually higher than that obtained by the coulometric method. The coulometric results obtained using the two-compartment cell are frequently lower than those obtained using a one-compartment (diaphragmless) cell. The miscibility of the oils with a solvent Analytical Chemistry, Vol. 70, No. 20, October 15, 1998

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appears to be not only a function of the presence of a nonpolar cosolvent1 but also a function of the composition of the oil, because as the viscosity and boiling range of the oil increase, reflecting the increase in hydrocarbon molecular weight, the miscibility decreases to zero for the coulometric reagents. In the case of the Hydranal volumetric reagent, with the molecular weight increase of the oil, the breakdown of the voltage across the indicator electrode is no longer observed. It is replaced with a transition to a heterogeneous suspension and a concomitant reduction of approximately 50% in the amount of water measured. ACKNOWLEDGMENT The author acknowledges and thanks Jacob Angelo of Exxon Corp. and Paul Griffin of Doble Engineering Co. for their insights

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and encouragement and David Deuwer for his editorial assistance. Certain commercial equipment, instruments, and materials are identified in this paper to specify adequately the experimental procedure. In no case does such identification imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the equipment or material is necessarily the best for the purpose.

Received for review February 10, 1998. Accepted July 22, 1998. AC980149H