Anal. Chem. 1995, 67,4239-4246
Amperometric Measurement of Moisture in Transformer Oil Using Karl Fischer Reagents Sam A. Margolis Analytical Chemistry Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899
Moisture was measured in two oil Reference Materials (RMs), transformer oil (RM 8506) and mineral oil (RM 8507), by the volumetric and coulometric Karl Fischer methods. A variety of analysis conditions were used, including the solvent composition of the titration vessel. The maximumamount of moisture was obtained when the chloroform content of the titration vessel was above 65% after the solvent had been titratedto dryness. The highest moisture titers were measured by the volumetric method using either the Hydranal or pyridine-basedKarl Fischer reagents or by the coulometric method using Hydranal AG-H reagent. Other coulometric reagents measured lower amounts of moisture even after the addition of organic solvents. Although the Hydranal AG-H reagent measured moisture levels comparable to the volumetric reagents, it was necessary to correct the measurements for the reduced titration of the standard when using this reagent. The effect of the dissolution of the oil in the titration vessel solution was examined. When the oil is not completely dissolved in the titration vessel solution, it is capable of binding or sequestering a portion of the water so that the moisture is unavailable to react with the Karl Fischer reagent. Transformer oil is a natural mineral oil distilled from crude sources and refined to maintain stable thermal and electrical characteristics for many years. It is composed predominantly of naphthenic, p a r m c , and aromatic hydrocarbons as well as small amounts of a wide variety of other organic substances. The dielectric breakdown voltage of transformer oil is significantly reduced when the water content rises above 50% saturation. The solubility of water in oil is temperature dependent and varies from about 20 yg/g at 0 "C to 800 yg/g at 100 "C. Typically, the oil used as transformer insulation is relatively dry and has a water content between 5 and 50 yg/g. Specificationsfor the content of water in oil used in new power transformers are no greater than 10 yglg.1 In order to accurately assess the moisture status of transformer oil, it is necessary to accurately and precisely measure moisture content from 5 to 50 yglg. At these moisture levels, the members of the American Society for Testing and Materials (ASTM) D-27 committee observed systematic biases which were dependent on both the type of apparatus and the nature of the solvent system employed in the measurement (personal communication). The studies described herein were done at the request of this ASTM committee and were designed to provide reference materials for evaluating these biases and defining their sources. (1) Griffin, P.; Bruce, C.; Christie, J. Minutes of the 55th Annual International Conference of Doble Clients. Doble Engineering Co., Watertown, MA, 1988. This article not subject to US. Copyright. Published 1995 Am. Chem. Soc.
The measurement of moisture in transformer oils is routinely made by titration with Karl Fischer reagent using either a volumetric or more often a coulometric method.2 The end point of the titration in both cases is defined as that point at which the rate of titration falls below a preset value. This end point is dependent upon several factors including the following: the nature of the solvent, the solubility of the moisture-containing material in the solvent, the presence of materials other than water that react with the Karl Fischer reagent, the occurrence of chemical side reactions (such as esterification) that produce water, and the accessibility of the moisture for reaction with the Karl Fisher reagent.3 The accurate measurement of moisture requires the complete solubilization of the sample in the titration solvent. Particular care must be taken to avoid trapping moisture in oil micelles or emulsions. Chloroform, trichloroethane, toluene, and alcohols such as 1-pentanol and 1-octanol are the solvents of choice for achieving the maximum solubility of oils for moisture titration by the Karl Fischer reagent because they increase the solubility of the oil and are compatible with the titrant. The volumetric method of moisture measurement permits the use of a wide variety of solvent types and concentrations to facilitate the complete solubilization of the analyte. The coulometric method, which is more sensitive, lacks flexibility in the selection of solvent types and concentrations. The titration curve of a typical Karl Fischer titration (see Figure l), which represents the accumulation of titrant as a function of time, is composed of a linear baseline slope (Figure 1, segment A of the titration curve), the initial segment of the titration curve after the addition of the sample where the titrant is added at a constant rate (segment B), a nonlinear portion where the rate of titration returns to the original linear baseline slope (segment C) , and the final baseline slope (segment D). The measurement of large amounts of water is relatively unambiguous since the small bias resulting from the use of a fixed end point can be neglected for routine measurements. For the accurate measurement of trace levels of moisture, the nonlinear segment of the titration curve accounts for a significant amount of the titrated moisture. Thus, the use of a fixed end point (a predetermined time period, usually 3-30 s, during which no additional titrant is added to the titration vessel) is not advisable because that point may occur at several places along the slope of the nonlinear segment of the titration curve (Figure 1, segment C). Furthermore, if the method is standardizedwith a material containing a relatively large amount of water as compared to that present in the oil sample, the error in the estimation of the end point of the curvilinear terminal part of the titration curve may account for a much greater error in the (2) Gedemer, T.; Frey, R Am. Lob. 1975,8 (March), 47-53. (3) MacLeod, S. Anal. Chem. 1991,63, 559A-566A
Analytical Chemistty, Vol. 67, No. 23, December 1, 1995 4239
10
-
8h
E
:
v
,
2
6 -
0
2
: 4:
2 -
0
5
10
15
20
Time (min) Figure I. Typical Karl Fischer volumetric titration curve of moisture in an oil sample: (A) initial baseline, (B) linear portion of the titration curve, (C) nonlinear portion of the titration curve, and (D) final baseline; (1) slope of the initial baseline, (2) slope of the final baseline, and (3) distance measured for calculation of the titrated moisture.
measurement of the moisture in the oil sample than the standard. Since the objective of these studies was to measure accurately and precisely trace levels of moisture in oils, we have modifled the standard end point volumetric titration procedure to include the measurement of the moisture detected by the nonlinear part of the titration curve. This type of measurement could not be assessed on the commercial coulometric instruments. We report herein a study of the properties of the titration of moisture in oils as a function of oil solubility using both coulometric and volumetric methods of Karl Fischer moisture titration. A variety of solvent systems and instruments were used to assess the contribution of these parameters to the measurement of the moisture in the oils. These methods were calibrated with a watersaturated octanol (WSO) solution, and the moisture was measured in transformer oils containing different levels of moisture. EXPERIMENTAL SECTION The transformer oil (Exxon Univolt N61) and the mineral oil (Exxon Coray 22) are hydrotreated, light naphthenic distillates which were purchased from Tilley Chemical Co. (Baltimore, MD). The Univolt oil contains 0.3%antioxidant by mass. RM 8506 and RM 8507 were prepared at the National Institute of Standards and Technology (NIST) by distributing 10 mL aliquots of the Univolt and Coray oils, respectively, into dried ampules under dry argon. The pyridine-based Karl Fisher reagents were purchased from Fisher Scientific (Pittsburgh, PA). The Hydranal reagents for both the coulometric and volumetric procedures were purchased from Crescent Chemical Co. Inc. (Hauppauge, NY), and the Ericsen (Watermark) reagents were purchased from GFS Chemicals Inc. (Powell, OH). Some studies were done on the ampuled material and others were performed on bulk oil samples containing varying amounts of moisture. The values reported in a given table were performed on the same bulk sample. Volumetric moisture measurements were made on Metrohm Instruments Models E 547 and 633 (Brinkmann Instruments 4240
Analytical Chemistry, Vol. 67, No. 23, December 1, 1995
Inc., Westbury, NY.),and the titration curves were recorded on a strip chart recorder. The coulometric measurements were made with a Metrohm Model 684 coulometric titrator fitted with either a membrane or a membraneless cell (Brinkmann Instruments Inc.) or with an Aquapal I11 coulometric titrator (CSC Co., Inc., Fairfax, VA). The moisture titrators were calibrated with WSO at laboratory temperature (20-22 "C) which contains 2.3 mol of water/L of organic phase (0.0501 mg of water/mg of organic phase)! All moisture measurements were performed in a closed titration vessel, and the samples were introduced through a septum. Water-saturated butanol (WSB), WSO, and oxalic acid were used to assess the accuracy and linearity of the response of each of the instruments and to assess any bias that might be due to the composition of the standards. The instruments were calibrated with 10 ,uL of WSO. Each oil sample (0.5-5 mL, depending on the moisture content) was weighed and titrated. Standards were routinely run (unless otherwise noted) after every two to four oil samples to ensure that the response to a standard water sample was constant and to detect any systematic bias introduced by the increasing amounts of oil present in the solvent. Volumetric Measurements. Several solvent systems were employed. The primary solvent was methanol. Varying amounts of chloroform, l,l,l-trichloroethane,or toluene were added to the methanol to increase the solubility of the oil in the titration vessel solvent. The titrants used were Hydranal Composite 2 (2 mg of water titrated/mL of reagent) or diluted pyridine-based Karl Fischer reagent (approximately 2 mg of water titrated/mL of reagent). The titration vessel solvent (30-80 mL) was titrated in a closed chamber to a constant rate of addition using a 1mL buret calibrated to flpL. Each titration consisted of the addition of a weighed amount of oil sample (1-4 g) or standard (10 pL or 8.2 mg of WSO) after a constant rate of titrant addition was maintained for 10 min (Figure 1). The titration of the moisture proceeded until the rate of solvent addition returned to the initial constant rate for 10 min. The 10 min time period was necessary to permit the graphic estimation of the initial and final rates of titration. This background rate of titration is always present regardless of the nature of the sample. The progress of each titration was recorded on a strip chart recorder. Lines were drawn through the initial and final slopes of the recording and were extended into the region of the sample titration Figure 1,lines 1and 2). The consumption of Karl Fischer reagent was determined by measuring the distance between the extended lines, 1 and 2, at the point where the titration of the sample was approximately 50% complete, thus graphically subtracting out the background moisture consumption. The usable chart width was 250 mm, and at least one-third of the chart was used for each measurement. The moisture content of each sample was calculated by determining the chart distance traversed (Figure 1, line 3) during the titration of a standard of known water content (WSO) and using this to calculate the amount of water represented by the chart distance traversed during the oil sample titration. This technique is referred to in the succeeding text as the "graphic method" of moisture measurement. Coulometric Measurements. In selected experiments, chloroform, l,l,l-trichloroethane,or toluene was added to the anodal solution to decrease the polarity of the solvent and increase the solubility of the oil. Four different anodal solutions were used in this study. The Hydranal AD solution was formulated to be used (4) Leo, A; Hansch. C. J. Org. Chem. 1971,36, 1539-1544.
a,
in the membraneless the Hydranal AG solution was formulated to be used in both the membrane cell and the membraneless cell, the Ericsen solution was formulated to be used in the membraneless cell, and the Hydranal AGH solution was formulated to be used in the membrane cell for the titration of moisture in hydrocarbon materials. In every case, the anodal solution (100 mL) was titrated in a closed system to a constant drift which was different for each solvent system. An extraction period of 1.5 min (i.e., a delay between sample addition and the start of titration) was used to ensure that all the moisture was extracted from the oil sample. Longer extraction periods did not modify the results of the titration process. The instrumentmeter gave the total water content in micrograms of water. Collaborative Study. This study consisted of 14 laboratories who routinely measured moisture in transformer oils. Each laboratory analyzed in duplicate two samples of each reference material (RM 8506 and RM 8507) using the method that was used for routine measurements in their laboratory. The samples were randomly selected from the entire lot and randomly distributed to each laboratory. RESULTS The accuracy of the measurement of the moisture content of oils has not been clearly established although many methods used by the electric power industry are capable of making precise measurement^.^-^ A collaborative study among 14 analytical laboratories, which was coordinated by NIST during 1991 with the assistance of the electric power industry and the ASTM D-27 Committee, demonstrated that moisture in oil could be measured by coulometric methods with the Karl Fischer reagent with good precision. The results obtained for the two NIST Reference Materials RM 8506 and RM 8507 were 21 f 3 and 47 f 2 mg/kg, respectively, where the 1-SD uncertainty represents material heterogeneity and intra- and extralaboratory measurement effects. These results were independent of the instruments and the reagents that the laboratories used. We obtained similar values using the coulometric method of measurement and unmodified commercial reagents. However, in our laboratory volumetric measurements with the Karl Fischer reagent in the presence of added nonpolar solvents such as chloroform were greater than those obtained in the collaborative study. Differences of '50% were observed between our volumetric results and the results of the collaborative study (compare the results in Table 1 to those of the collaborative study). We have addressed this methoddependent disparity in moisture content by comparing the results obtained in our laboratory by both the volumetric and coulometric Karl Fischer methods of moisture measurement. Measurements by the Volumetric Karl Fischer Method. Preliminary studies using volumetric titration with the classical pyridine-based Karl Fischer reagent indicated that when the oil was completely dissolved in a titration solvent composed of ~~
~~~~
(5) ASTM D 1533-88 Standard Test Methods for Water in Insulating Liquids (Karl Fischer Reaction Method). Annu. Book ASTM Stand. 1993,10.03, 189. (6) ASTM D 1744-92 Standard Test Methods for Water in Liquid Petroleum Products by Karl Fischer Reagent. Annu. Book ASTM Sfand. 1994,5.01, 592. (7) ASTM D 4377-93a Standard Test Methods for Water in Cmde Oils by Potentiometric Karl Fischer Titration. Annu. Book ASTMSfand. 1994,5.02, 854. (8) ASTM D 492889 Standard Test Methods for Water in Crude Oils by Coulometric Karl Fischer Titration. Annu. Book ASTM Stand. 1993,5.03, 216.
Table I.Effect of Chloroform in the Titration Vessel Solution on the Titration of Moisture in RM 8506 and RM 8507 by the Volumetric Karl Flscher Method Using a Pyridine-BasedKarl Fischer Reagent
chloroform
apparent water contentO (ug/g f SD)*
(%)
RM 8506
nc
RM 8507
n
40 55 65
23.3 f 2.5 31.0 f 1.9 39.7 f 3.3
4 4
62.6 5 4.2 78.3 Ik 4.5 76.8 f 4.8
4 8 16
16
a The WSO titrated at a constant value throughout the analyses. SD, one standard deviation. The uncertainty represents the internal heterogeneity and the intralaboratorymeasurementeffects. n,number of measurements.
methanol and chloroform which was titrated to dryness, greater amounts of water were detected in the oil samples than when the same titration was performed in the absence of chloroform. More moisture was measured as the chloroform content was increased up to a sample-dependent limiting value (Table 1). Because of the moisture content of the chloroform and the necessity of titrating the vessel solvent to dryness (a requirement of the graphic estimation of moisture content), it was not possible to increase the chloroform concentration above 65-70%. The use of chloroform in the titration solvent or working solution is the recommended procedure for the volumetric titration of moisture in transformer oil, ASTM D 1533-7g4because it permits the oil to form a homogeneous transparent solution with the titration vessel solvent. When the oil sample does not form a homogeneous solution, one cannot be confident that all the moisture is accessible to the titrant. Commercial titration vessel solvents (other than the solvent Hydranal AGH) used in coulometric methods without adding additional chloroform frequently are not completely miscible with the oil samples. To evaluate whether the discrepancy between the results obtained by the coulometric vs the volumetric methods was due to the solubility of the oil in the vessel solvent, the volumetric method was examined by using methanol or various mixtures of methanol and chloroform in the titration vessel and titrating to a constant rate with the Karl Fischer reagent. The standard (WSO) and then two or three oil samples were alternately titrated through the transition point from a homogeneous solution to a multiphasic solution. A typical titration sequence is illustrated in Figure 2. The moisture detected in the WSO was constant throughout the entire experiment. In these studies (Tables 2-4 and 6-9) we used oil samples that were slightly wetter than those used in the studies summarized in Table 1. Using the Hydranal Composite 2 reagent, the measurable moisture content of RM 8507 (Table 2) when the vessel solution was homogeneous was 93.1 i 6.3 pg/g, 102%greater than that measured after the solution became multiphasic (45.6 i 1.6 pg/ g) and 52%greater than the maximum amount measured when the vessel solution was pure methanol (61.2 i 7.6 pg/g). The maximum moisture content of RM 8507 titrated with the pyridine reagent was essentially the same as that measured with the pyridine reagent at similar chloroform concentrations (100.5 f 5.6 pglg). No phase change was observed with the pyridine reagent, but there was a breakdown of the potential across the electrode after 14 mL of oil (seven samples) was added to the titration vessel. Table 3 summarizes a similar experiment in which samples of RM 8506 were titrated with Hydranal Composite 2, Analytical Chemistry, Vol. 67, No. 23, December 1, 1995
4241
100 90
Table 4. Effect of Organic Solvents on the Coulometric Titration of Water in Oils
-
t
apparent water contenta (uglg)
70
I
1
organic solvent
3
5
7
9
11
13
15
17
Figure 2. Sequential titration of RM 8506 oil samples and WSO with Karl Fischer reagent, The clear bars represent the WSO samples (10 pL); response is in millimeters of chart distance. The solid bars represent the transformer oil samples (2-3 9); response is in micrograms per gram of water. The titration vessel solvent, 30 mL of chloroform-methanol 4:l (v/v), was titrated to dryness with Hydranal Composite 2 reagent, giving a final chloroform concentration of 62% before the addition of samples. Table 2. Effect of Chloroform on the Measurement of Moisture in RM 8507 by the Volumetric Karl Fischer Method Using the Hydranal Reagent.
methanol 66%chloroform/34%methanol
RM 8507
RM 8506
Membrane Cell, Hydranal AD Solvent System none 42.4 & 6.2 34.1 ZIZ 4.1 chloroform 25 51.0 2Z 2.7 24.1 ZIZ 1.7 Membraneless Cell, Hydranal AD Solvent System 35.3 f 3.8 30.0 f 5.9 none chloroform 17 47.4 2Z 3.7 chloroform 25 65.4 f 4.0 31.1 f 3.2 Membraneless Cell, Ericsen Anodal Solvent System 49.0 f 3.7 31.4 f 3.3 none 50b 53.0 1.9 32.6 f 3.6 chloroform 31.2 ZIZ 3.6 chloroform 58b 65.1 k 2.5 33.3 f 0.9 trichloroethane 14 52.3 f 3.7 trichloroethane 20 56.5 2Z 3.8 33.5 2Z 1.6 toluene 12.5 55.3 f 5.9 31.4 f 2.0
*
Sample No.
solvent
conc (%)
water (ug/g f SD)b
nc
soln state
61.2 f 7.6 93.1 2Z 6.3 45.6 f 1.6
5 5
cloudy clear cloudy
3
The WSO titrated at a constant value throughout the analyses. Oil sample size was 2 mL. SD, one standard deviation.The uncertainty (I
represents the internal heterogeneity and the intralaboratory measurement effects. n, number of measurements. Table 3. Effect of Chloroform on the Measurement of Moisture in RM 8506 by the Volumetric Karl Fischer Method Using the Hydranal Reagent'
solvent
@g/g f SD)b
water n
soh state
methanol 62%chloroform/38%methanol
28.7 f 1.5 58.1 f 7.5 30.1 f 3.1
7 5 6
cloudy clear cloudy
The WSO titrated at a constant value throughout the analyses, and the first two samples on the first line were titrated before any WSO was titrated. SD, one standard deviation. The uncertainty represents the internal heterogeneity and the intralaboratory measurement effects.
using a mixture of methanol and chloroform 1:4.5 (v/v) which was 70%chloroform after titration of the moisture in the solvent mixture. When RM 8506 was completely miscible in the titration vessel solution, the amount of moisture measured was twice that measured after the solution had become multiphasic or cloudy or when the moisture was titrated in pure methanol (Table 3). It is important to note that the amount of moisture detected at the transition point from a homogeneous solution to a multiphasic system, the point at which the vessel solution became cloudy, was about 10%or less than that measured in a homogeneous solution for both types of oil. This would suggest that the majority of the moisture in the oil sample added at the transition point was sequestered in one of the phases of the multiphasic system, 4242 Analytical Chemistry, Vol. 67, No. 23, December 1, 1995
a Mean f SD of the population; each value represents the mean of three to six measurements. Includes the chloroform in the Ericsen solvent system (38%).
perhaps as oil micelles. Yet while this phase change and the variation in the moisture measurements in the oils were occurring, the measured moisture content of the WSO remained at the same level throughout the titration both in the absence and in the presence of chloroform. This indicates that the presence of oil and the multiphasic nature of the titration vessel solvent infiuences the amount of moisture measured in the oil but not in the WSO (except when the WSO was added immediately after the transition point). Under these conditions, the measurable moisture content of the WSO was reduced in a manner similar to that when the oil sample was added. Thus, the optimum condition for the volumetric titration of moisture in oil was a titration vessel solution titrated to dryness and composed of W O chloroform, 40%methanol Karl Fischer reagent. Under these conditions, both volumetric reagents yielded similar results. Finally, both RM 8506 and 8507, which are oils of very different composition, gave a similar pattern of response. Measurements by the Coulometric Karl Fischer Method. Because of the discrepancy between the moisture content of the reference oils determined by the volumetric method at NIST (line 3, Table 1) and those reported by the round robin study using the coulometric method (21 & 3 and 47 & 2 mg/kg of oil), it was necessary to evaluate the coulometric method for sources of bias. Table 4 summarizes the effects of cell design and solvent composition on the moisture titer of the two RM oils. The moisture content obtained by coulometric titration of the RM oil samples (Table 4) was not as large as the results obtained by the volumetric method (Table 1) although the amount of detectable moisture again increased with the addition (to the anode solution) of nonpolar solvents, such as chloroform, l,l,l-trichloroethane, or toluene, before the addition of samples. In Table 4, the amount of added organic solvent was limited to that reported in the table because higher amounts resulted in the breakdown of the voltage across the electrode of the coulometric instrument. The direction of the change (increase) in the titratable water, except for RM 8506 in the membrane cell, appeared to be independent of the of the design of the cell and the addition of nonpolar solvents. However, the magnitude of the moisture content of the RM 8507 samples varied with the conditions of the titration, namely, cell
+
Table 5. Composition of Hydranal Anodal Solutions. % composition
reagent methanol imidazole hydriodic acid sulfur dioxide diethanolamine diethanolamine sulfite pentanol N,M-dimethyldodecylamine hydrobromic acid
Hydranal AD
Hydranal AG
Hydranal AG-H
-70 1-5 1-5 -10 -10 12-17
65-70 8-12 3-5
30 10 1 10
1
Data 0 The composition values are taken from the Material Safe Sheets and do not represent the exact composition because of in ustrial confidentiality.
!
type, amount of added organic reagent, and the composition of the anodal solvent. Two types of coulometric cells are used. One type of cell consists of an electrode in a compartment containing a cathodal solution which is separated by a porous membrane from a second compartment containing an anodal solution. The second type of cell lacks a membrane and a single solution is used in the cell. The presence of a membrane forming a twocompartment system influences the moisture titer when the same anodal solution is used. In the absence of a membrane, the addition of chloroform influenced the observed moisture titer of both oil samples more than in the membrane cell (Table 4). In the case of the RM 8506, the addition of chloroform to the anodal solution of the membrane cell decreased the moisture titer. In the membraneless cell, both the Ericsen and the Hydranal AD anodal solutions gave similar results with chloroform and other nonpolar solvents, that is, an increase in the moisture titer of RM 8507 upon the addition of nonpolar solvents as opposed to no change in the moisture content of RM 8506 under the same conditions. In the absence of chloroform, the anodal solution became multiphasic upon the addition of the first oil sample but not upon the addition of WSO. The anodal solution was homogeneous for all of the measurements in Table 4 made in the presence of added organic solvent. In the presence of intermediate concentrations of chloroform, the anodal solution became multiphasic after titration of the moisture in the first few samples. The number of samples titrated before the transition point was reached was dependent upon the amount of chloroform and the type and total amount of oil that was added. As with the volumetric method, we evaluated the relationship between the homogeneity of the anodal solution in the presence of varying amounts of chloroform and the moisture titer of successive oil samples interspersed with WSO standards. (By the time these studies were initiated it was recommended that the Hydranal AD solution be replaced with the newer Hydranal AG and Hydranal AGH solutions. The composition of these reagents is summarized in Table 5.) This type of experiment was essential because the oil never completely dissolved in the AG solution in the absence of added organic solvents when the moisture was measured in the coulometric instruments. Tables 6 and 7 summarize the results of sequentially coulometrically titrating oil samples and WSO standards using the same general protocol as that used for the volumetric titrations. These measurements were performed after those summarized in Table 4,and in the interveningtime the bulk oil samples absorbed some
Table 6. Effect of Chloroform on the Measurement of Moisture in RM 8506 by the Coulometric Karl Fischer Method Uslng the Hydranal AG Reagent
anodal solvent Hydranal AG 40%chloroform/ 60%Hydranal AG 40%chloroform/ 60%Hydranal AG
wateP (ug/g f SDb) n 37.3 f 2.7 35.3 f 2.3 29.9 f 1.1 34.1 f 3.0 28.1 f 1.4
soln state
7 3 5 6 8
cloudy clear cloudy clear cloudy
wso (ug of water/lO Lof WSO f 4Db) 378 f 4.6 377 f 3.9 not done
The moisture content has been corrected for the deviation of the standard from the theoretical water content of WSO (412 pg/10 pL of solution4). * SD, one standard deviation. The uncertainty represents the internal heterogeneity and the intralaboratory measurement effects.
Table 7. Effect of Chloroform on the Measurement of Moisture in RM 8507 by the Coulometric Karl Flscher Method Uslng the Hydranal AG Reagent
anodal solvent -Hydranal AG 40% chloroform/ 0%Hydranal AG
wateP (ug/g f SDb) 122 92.5 i 3.6 147 f 4.3 136 f 3.3
n 1 6 2
5
soln state
wso b g o f water/lO Lof WSO f !D*)
cloudy cloudy clear cloudy
379 358 f 4.6 380 f 5.7 355 f 4.0
0 The moisture content has been corrected for the deviation of the standard from the theoretical water content of WSO (412 pg/lO p L of solution4). * SD, one standard deviation. The uncertainty represents the internal heterogeneity and the intralaboratory measurement effects.
Table 8. Measurement of Moisture in RM 8506 by the Coulometric Karl Flscher Method Using the Hydranal AG-H Reagent.
anodal solvent Hydranal AG-H 40%chloroform/ 60%Hydranal AGH
wateI.b (ug/g f SDc) n 31.2 f 1.7 35.6 f 1.9 35.0 f 1.4
soh state
8 cloudy 6 clear 4 cloudy
wso (ug of water/lO Lof WSO f tDC) 371 f 1.7 362 f 9
0 The WSO titrated at a constant value throughout the analysis. * The moisture content has been corrected for the deviation of the standard from the theoretical water content of WSO (412 p g / l O p L of solution4). SD, one standard deviation. The uncertainty represents the internal heterogeneity and the intralaboratory measurement effects.
water; thus the data in a specific table are self-consistent, but the moisture content for a given sample may vary from table to table. With the Hydranal AG solution, in both the presence and absence of chloroform there was a decrease in the moisture titer of mineral oil when the anodal solution became multiphasic (Table 6). In the case of the transformer oil, the decrease was observed only in the presence of chloroform (Table 7). In the absence of chloroform, the solution was never clear and the moisture titer remained relatively constant. However, the magnitude and direction of this change for any given oil sample was dependent both on the composition of the anodal solution and the materials that were added to the starting solution such as organic modifiers, external standards, and analytes (Tables 4-9). Effect of Solvent Composition, Organic Modifiers, and Titration Conditions on the Titration of Moisture in Standards Analytical Chemistry, Vol. 67, No. 23, December 1, 1995
4243
Table 9. Comparison of Results of the Titration of Moisture in RM 8507 by the Coulometric and the Volumetric Karl Fischer Methods
wate@
solventa
(ug/g+SDC)
68%chloroform/ 32%methanol
190 f 7.4 118 f 11
Hydranal AG-H
184 f 13
soh
state
n
wso @g of
water/lO pL of WSOfSDc)
7 5
clear cloudy
Coulometric Method 3 clear 157f8.3 13 cloudy
d
365 f 9 339f 9
-
700
I
-Ea
600
$
400
5 6
300
E
a
Volumetric Method
800
/
500
200 100 0
For the volumetric method the solvent refers to the titration vessel solvent; for the coulometric method the solvent refers to the solvent in the anodal chamber. bThe moisture values for the coulometric method have been corrected for the deviation of the standard from the theoretical water content of the WSO (412 u g l l 0 pL of solution*). SD, one standard deviation. The uncertainty represents the internal heterogeneity and the intralaboratory measurement effects. For the volumetric method the value for the WSO standard was constant for the entire experiment. a
0
1
2
3
4
5
6
7
Water (mg)
E
for Moisture Analysis. When the graphic method for estimating the moisture content was used, the moisture content of WSO determined by the volumetric method was independent of the I-Stop, which is the current that determines the end point of the titration. Under the conditions used in these experiments, the only effect of decreasing this value was to increase the duration taken for the completion of the titration of the moisture in WSO. This was observed when the titration vessel solution was either methanol or 85%chloroform/l5% methanol (v/v). When the coulometric method of moisture analysis was used, the absolute amount of moisture titrated in the WSO varied with the instrument and the solvent that was used. This suggested that the calibration and the response of the two instruments used at NIST were different. The theoretical moisture content of WSO is 412 p g l l 0 p L (density of WSO 0.8228 at 25 "C; water content 0.0501 g of water/g of WSO) Using the Brinkmann instrument with the Hydranal AD solution and either a membrane cell or a membraneless cell, the water in 10pL of WSO was 392 f 1.41pg (n = 30). Using the Hydranal AG solution, the value was about 10pg lower. Using the Aquapal I11 instrument with the Hydranal AGH solvent (Tables 7 and 8), the amount of water titrated in 10 pL of WSO varied with the solvent, being noticeably lower particularly in the case of RM 8507 and as the oil content increased. In the same study we also observed that as the oil content increased the instrumental drift increased, suggesting that the water was not completely titrated. When the moisture values for the oil titrated in this solvent were corrected for the incomplete titration of the standard, the moisture content determined by the coulometric method was very similar to that obtained by the volumetric method (Table 9). Thus the inaccuracy of the end point determination and excess drift represent signzcant sources of systematic bias. Commercial coulometric instruments use the end point titration method and are prone to this bias to some degree. This systematic bias is illustrated in Figure 3, where the results of titrating the moisture in oxalic acid, WSO, and WSB are compared. Titration of these standards by the volumetric method using the graphic method of calculation shows that, when the theoretical amount of water over a 5@fold concentration range was plotted against the response, all three materials give the same response 4244
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, 0
IO
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Sample (mg)
Figure 3. Titration of the water in WSO, WSB, and oxalic acid standards. (a, top) after methanol (30 mL) was titrated by the volumetric method to a constant rate with Hydranal Composite 2, varying amounts of sample were weighed and titrated. The open squares represent WSO, the solid squares WSB, and the diamonds oxalic acid. (b, bottom) After Hydranal AG reagent was titrated to equilibrium in a Brinkmann instrument using a membraneless cell, varying amounts of sample were added and the water content was recorded. The squares represent WSO, the triangles WSB, and the diamonds oxalic acid. The solid symbols represent the actual results, and the open symbols represent the calculated water content.'
(Figure 3a). When the coulometric method (Figure 3b) was used, the measured moisture of the WSB and the WSO was less than the theoretical amount whereas the oxalic acid showed little or no bias. It is important to note that smaller samples of oxalic acid were not used because the water content was high and smaller samples could not be weighed accurately. We believe that the lack of bias in the volumetric method of moisture determination is attributable to the use of a graphic method of determination of moisture content, particularly at low moisture concentrations, as opposed to the use of the end point method utilized by all coulometric instruments and for many volumetric measurements. Measurements by the Coulometric Karl Fischer Method Using Anodal Solvents Specially Designed for Hydrocarbons. The Hydranal AGH anode solution has been designed to be miscible with nonpolar materials (Table 5). It provides an alternative to the addition of nonpolar solvents to the standard anode solution. The results obtained with this solvent are the first coulometric results to agree with the volumetric results (Table 9). However, the titration vessel solvent of the volumetric instrument accommodated at least seven samples before it became
multiphasic and the measureable water decreased from 190 f 7.4 to 118 f 11 pg/g of oil; the coulometric instrument only accommodated three samples before the moisture titer of the oil samples decreased from 184 f 13 to 157 f 8.3 pg/g of oil. Furthermore, the results of the coulometric measurement had to be corrected for the systematic bias due to incomplete titration of the moisture in the standard. The major component of this bias was probably the result of the increase in the drift of the baseline moisture levels of the instrument which could not be eliminated by increasing either the delay between the introduction of the sample and the initiation of the titration process by 2-5 min. Thus, although the results of the volumetric determination are similar to the corrected coulometric results obtained with the AGH anodal solution, it is difficult to accurately correct for the systematic bias introduced by the constant increase in the drift of the baseline moisture levels. DISCUSSION From the collaborative study it is obvious that the amount of moisture titratable in oils by the coulometric method currently in use by the electric power industry is very reproducible. However, these reproducible results do not represent the total moisture. Both the coulometric and the volumetric Karl Fischer methods are highly dependent upon the nature of the anodal solution (particularly RM 8507, Table 4) and the titration solvent, respectively (Table 1). The maximum amount of moisture was detected by the volumetric method in the presence of chloroform when the oil was completely dissolved in the titration solvent. The amount of titratable material was dependent on the chloroform content and decreased (approximately 40%) when the titration vessel solution changed from a homogeneous system to a heterogeneous system or a suspension. More importantly, even though the titratable material in the oil samples decreased after the transition point was passed and a new equilibrium value was established, the value for the moisture content in the WSO remained the same. Thus, the addition of chloroform and the sequential addition of oil samples did not affect the ability of the Karl Fischer reagent to repeatably titrate all of the water in a known standard. The breakdown voltage for the electrode in the case of each method was reached at the highest chloroform level that was tested. This condition was characterized by an increase in the iodine content of the titration solvent and the inability of the instrument to reach an end point. A second phenomenon was observed in the case of the coulometric method. This consisted of a small increase in the detectable moisture in oil samples introduced immediately after the WSO sample. This effect was not observed when the volumetric method was used, indicating that the solvent composition plays an important role in the availability of titratable material and is consistent with the results obtained with the AGH solvent which contains pentanol. It is clear that the Karl Fischer reagent is capable of titrating the total amount of water in a standard regardless of how much water is measured in the added oil, the amount of chloroform present in the solvent, or the amount of oil that has been added to the titration vessel. It is also clear that when the moisture in oil is titrated by the volumetric method in the presence of chloroform very little moisture is titrated at the transition point when the mixture in the titration vessel changes from a clear solution to a suspension (no moisture in the case of RM 8507 and 4 pg/g in the case of RM 8506). This is less than 10%of the total moisture detected and at least &fold smaller than that
detected after reaching the transition point from a clear solution to a heterogeneous solution. This strongly suggests that when the oil forms a suspension, an increased amount of water is sequestered within the oil phase and is inaccessible to the Karl Fischer reagent. Furthermore, the presence of chloroform does not appear to affect the measurable moisture content in heterogenous solutions, since the same moisture values are obtained when the moisture in oil is titrated either in the presence of chloroform after the formation of a heterogeneous solution or in the absence of chloroform. When WSO was titrated in the presence of ,increasing amounts of chloroform, no change in moisture content was observed, thus eliminating the chloroform and the octanol as affecting the moisture titer. The dramatic decrease in moisture titer is observed only at a high concentration of chloroform at the transition point from a homogeneous solution to a heterogeneous solution where maximum levels of moisture are measured prior to reaching the transition point. At lower concentrations of chloroform (below 40%),this is not observed with either the coulometric or the volumetric titration method. When the Hydranal AGH anodal solution is used, the moisture content detected by the coulometric method approaches that of the volumetric method. This solvent is formulated specifically with pentanol in place of methanol for hydrocarbon moisture analysis (Table 5). Using two independent solvents and independent titration systems, similar results were obtained, suggesting that the higher value for the moisture content of the NIST RM oils is the correct value. This conclusion is further supported by the observation that very little moisture (less than 4 pg/g) is detected from a sample introduced immediately before the transition point from a homogeneous to a heterogeneous solution as a result of the sequestering of water in the material suspended in the heterogeneous solution. The next few samples also exhibited lower moisture titers. Thus, it appears that all of the water in the first sample after the transition point and some of the water from the next few samples is redistributed into the suspended oil. Further, when WSO is added immediately after the transition point is reached, the moisture titer of WSO is somewhat decreased. Although the nature of this binding or trapping of moisture in the vessel solution remains unknown, the results are indicative that the additional material measured in the chloroform-containing homogeneous solutions is probably tightly sequestered water. These results are consistent with methods recommended by ScholzG and the ASTM methods which recommend that chloroform be used in the volumetric methods for titrating water in insulating liquids (D1533)2 and petroleum products @1744).3 These methods use methanol/chloroform solutions (1:2 and 1:3, respectively) which are titrated to constant dryness before use. In our experience, after titration of vessel solutions containing chloroform at these concentrationsis completed,the chloroform content of the vessel solution is usually below 60%,which is the minimum chloroform concentration for obtaining reliable results on a variety of oils but not necessarily all oils (Table 1). These considerations apply to both Hydranal and pyridinecontaining Karl Fischer reagentsg (9) (a) Hydranal Manual Eugen SchoLz Reagentsfor Karl Fischer Titration;Reidelde Haen: Dordrecht, The Netherlands, 1988. @) Moisture Measurement by Karl Fischer Titrimetry; GFS Chemicals Inc: Powell, OH, 1991. (c) Scholz E. Karl Fischer Titration Determination of Water, Chemical Laboratory Practice; Springer-Verlag: Berlin, 1984.
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In the case of the coulometric method (ASTM method D4928j) a 3:2 (v/v) mixture of anodal solution/xylene is used. However, it is obvious from our studies with nonpolar organic additives, particularly toluene, that this is probably not adequate for the measurement of the total water content of all oils by coulometric methods (Table 4). Furthermore, the amount of organic additive that is compatible with a given anodal solution varies with the composition of the particular anodal solution. The results reported here demonstrate that the measurement of moisture in oil is highly dependent on the nature and concentration of the nonpolar organic solvent and the solubility of the oil in the vessel or anodal solution. Three major sources of bias in the measurement of moisture in oil are identified: (1) the estimation of the end point of the titration by using a fixed end point, particularly when very low levels of moisture are being measured; (2) the instability of the baseline after titration of the moisture in two or three oil samples especially when using the Hydranal AGH reagent; (3) the incomplete solubilization of the oil sample in the titration vessel solvent, which in the case of the volumetric method, can be circumvented by the addition of adequate amounts of organic solvents but cannot be completely resolved in the case of the coulometric method. Finally, in the case of the measurement of small amounts of moisture, it is essential to use standards such as WSO or water-
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saturated xylene whose moisture content is in the range of the measurement and to use a graphic method of measurement to order to avoid errors in the identification of the end point of the titration. ACKNOWLEDGMENT
The author acknowledges and thanks H. Carl Manger of Baltimore Gas and Electric Co., Paul Griffin of Doble Engineering Co., and Michael Nolte of General Electric Co. for their insights and encouragement and S. B. Schiller for the ANOVA statistical analysis. 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 equipment or material is necessarily the best for the purpose. Received for review June 28, 1995. Accepted September 14, 1995.@ AC9506421 Abstract published in Advance ACS Abstracts, November 1, 1995.
