Article pubs.acs.org/EF
Electrical Conductivity and pHe Response of Fuel Ethanol Contaminants Jon Luecke and Robert L. McCormick* National Renewable Energy Laboratory, Golden, Colorado 80401, United States S Supporting Information *
ABSTRACT: Electrical conductivity and pHe are used in some parts of the world as fuel specification parameters for denatured fuel ethanol (DFE). Conductivity has been correlated with the presence of corrosive ions such as chloride, and high-conductivity fluids are more likely to cause electrochemical or galvanic corrosion. This study examined how electrical conductivity and pHe of DFE are affected by impurities, including sodium chloride (NaCl), magnesium chloride (MgCl2), hydrochloric acid, magnesium sulfate (MgSO4), sulfuric acid, acetic acid, water, and hydrocarbon denaturant. Conductivity and pHe response data were measured at impurity concentrations permissible by ASTM D4806, the most commonly used specification for DFE. Conductivity was determined to be very responsive to strong acids, NaCl, and MgCl2, which have high solubility and dissociation constants in DFE. Molar conductivity of solutions containing these ions measured 40−60 S cm2/mol. Conductivity was relatively unresponsive to MgSO4 (a salt with low solubility), water (up to 1 wt %), hydrocarbon denaturant, and acetic acid, with molar conductivities measuring 99.5% ethanol supplied by Sigma-Aldrich. Chloride and sulfate ion concentrations were below method detection limits for this material. Acid content was 3.3 mg/L (as acetic acid). The very low conductivity of 0.02 μS/cm also attests to the purity of this material. The chemicals that were used are listed as follows: Sigma-Aldrich, ethanol, >99.5%, Catalog No. 459844; Sigma-Aldrich, sodium chloride, >99.999%, Catalog No. 38979; Sigma-Aldrich, magnesium chloride, >98%, Catalog No. M8266; Sigma-Aldrich, magnesium sulfate, >99.5%, Catalog No. M7506; Sigma-Aldrich, acetic acid, >99.7%, Catalog No. 695092; Sigma-Aldrich, heptane, >99.5%, Catalog No. 51745; JT Baker, sulfuric acid, ACS grade, Catalog No. 9681-00; EMD, hydrochloric acid, ACS grade, Catalog No. HX0603-4. Conductivity. Conductivity was measured using a modified European Standard EN 15938 (2010−12) “Automotive fuels Ethanol blending component and ethanol (E85) automotive fuel Determination of electrical conductivity.” The method repeatability and reproducibility are shown as follows (x is the average value of the compared results): repeatability = 0.0342 + 0.0129 reproducibility = 0.0685 + 0.1191x For example, for a measured value of 2 μS/cm the method repeatability (meaning that 95% of repeat measurements at the same laboratory are expected to fall within this range) is ±0.06 μS/cm. A Thermo Scientific Orion 3-Star conductivity meter with a stainless steel two-electrode cell with nominal cell constant of 0.1 cm−1 was used. Measurements were taken while stirring to eliminate thermal gradients at 25 °C. The meter was calibrated with a National Institute of Standards and Technology (NIST) traceable conductivity calibration kit. The cell constant (k) was verified each time conductivity measurements were taken against a 100 μS/cm sodium chloride (NaCl) conductivity calibration solution. The cell constant did not deviate more than 0.001 cm−1 from k = 0.096 cm−1. The modification to the method relates to container material, because ethanol-rinsed high-density polyethylene (HDPE) containers were used in some experiments instead of the standard specified glass containers. The first set of experiments, described in Figure 1, was performed using glass containers. All subsequent experiments were carried out in ethanol-rinsed HDPE containers. Acid Strength. pHe was measured using the ASTM D6423-08 method designed for DFE and ethanol fuel blends. The method was modified to use a Metrohm 780 pH meter with Metrohm EtOHTrode electrode (Part No. 6.0269.100) with 3 M KCl filling solution for taking measurements. A further modification was the use of HDPE sample containers rather than borosilicate glass containers. The method was calibrated using standard water-based pH 4 and 7 solutions; slope adjustment was used each time measurements were taken. Measurements were taken at room temperature (21.8 °C) while stirring 30 s after the electrode was immersed in 50 mL of the solution. The ASTM repeatability and reproducibility for this method are 0.29 pHe units and 0.52 pHe units, respectively. Sulfate and Chloride. Sulfate and chloride ion concentrations were measured using ASTM D7319-11. A Metrohm 820 ion chromatograph with conductivity detector was used for this analysis.
Kohlrausch’s law: Λ m = Λo m − kc1/2
(1)
Kohlrausch also showed that the limiting molar conductivity is a summation of the limiting molar conductivities of the cations and anions in solution. This is known as the law of independent migration of ions (eq 2) where v is the number of cations and anions per formula unit of electrolyte. Its practical implementation is the understanding that a conductivity measurement is an additive sum of the contributions of each ion in solution. law of independent migration of ions: Λo m = v+Λ+ + v−Λ−
EXPERIMENTAL SECTION
(2)
The objective of this research is to further understand the significance of conductivity and its relationship to the concentrations of specific ions, acetic acid, and pHe relevant to DFE and potentially to ethanol fuel blends. The study used a 5223
dx.doi.org/10.1021/ef5013038 | Energy Fuels 2014, 28, 5222−5228
Energy & Fuels
Article
Water. Water content was measured using Karl Fisher titration as described in ASTM D6304-07. A Metrohm 831 KF Coulometer was used for the analysis and samples were measured by mass. Acidity. Total acidity as acetic acid was measured using ASTM D7795-12 test method A, for potentiometric titration. This method includes nitrogen sparging of the sample before and during measurement to reduce the possibility of dissolved carbon dioxide in solutions. The titrant used was 0.01 N potassium hydroxide in isopropanol.
DFE specification limit for inorganic chloride is 10 ppm by mass. MgSO4 and H2SO4 were chosen as the two sulfate dopant forms. Sodium sulfate was found to be insoluble in ethanol. The specification limit for total sulfate is 4 ppm by mass. Water was doped using 18.3 MΩ water, at the 1% by volume specification limit for ASTM D4806. Acetic acid is representative of weak acids that can exist in DFE. The specification limit for total acidity is 56 mg as acetic acid per liter. Pure ethanol, ethanol denatured with 3.1% by volume heptane, and ethanol denatured with 3.1% by volume natural gasoline were used as controls. The results are shown in Figure 1. The conductivity measurement shows a large response for
■
RESULTS AND DISCUSSION Container Contamination. Inconsistencies in early experimentation led to the finding that the use of different types of glassware severely affected conductivity and pHe results in prepared standards. Experiments were performed to determine how different types of glass and rinses affected pure ethanol conductivity. The results are shown in Table 2. Table 2. Container Testing Using Conductivity Measurements of Pure Ethanol container type
rinsing method
ethanol conductivity (μS/cm)
borosilicate glass vial borosilicate glass vial amber soda-lime amber soda-lime TraceClean amber soda-lime HDPE HDPE
none ethanol none ethanol DI water none ethanol
0.291 0.261 2.494 0.272 0.134 0.061 0.021
The lowest conductivity measurements for pure ethanol were found using HDPE containers. Ethanol rinsing reduced the conductivity for all container types, but all types of glass containers exhibited more interference than HDPE containers. Given that the Brazilian standard for ethanol conductivity is 0.5 μS/cm,13 the fact that the glass containers can cause pure ethanol to exhibit a conductivity of 0.1−0.3 μS/cm, or even higher, is very significant. Standard solutions prepared in brand new amber soda-lime glass containers produced considerable conductivity errors and pHe errors. For example, a solution of 4 ppm by mass magnesium sulfate (MgSO4) in pure ethanol prepared in an ethanol-rinsed HDPE container resulted in a conductivity of 0.055 μS/cm and a pHe of 6.2. Preparing this solution in a brand new amber soda-lime container increased conductivity by 5000% to 2.75 μS/cm and pHe by 129% to 8.0. The data suggest that contamination is likely caused by residue left on the inside of the glass containers from the manufacturing and cleaning processes. The solution may also be leaching cations from the glass into solution, causing a persistent lower magnitude conductivity measurement error. Some common cations that leach are silicon, sodium, potassium, calcium, and magnesium.17 Current conductivity and pHe measurement methods may be improved by requiring that HDPE materials be used instead of glass for sample and measuring vessels. This issue is particularly important when working with low-conductivity conditions. Single Contaminant Conductivity Response in Denatured Ethanol. Pure ethanol denatured with 3.1% by volume heptane was used for experiments with added electrolytes. All samples were prepared in borosilicate glass vials (before we recognized that these produce a significant background conductivity response). NaCl, magnesium chloride (MgCl2), and HCl were chosen as the chloride ion dopant forms. The
Figure 1. Solution conductivity of various contaminants in heptanedenatured ethanol (borosilicate glass vials). Error bars are EN15938 method repeatability.
strong acids (HCl, H2SO4) and inorganic chloride (NaCl, MgCl2) at allowable concentrations in DFE. Inorganic sulfate (MgSO4), water, and acetic acid show almost no additional conductivity response above the control samples. Conductivity response factors are usually presented in the form of molar conductivity, the solution conductivity divided by the molar concentration of the electrolyte. The molar conductivity at very low concentrations should be very close to the limiting molar conductivity for fully solvated, strong electrolytes. Apparent molar conductivity results are presented in Table 3, and calculations are shown in Supporting Information. In line with the law of independent migration of ions, the conductivity of the control heptane-denatured pure ethanol sample was subtracted from each of the doped solution’s conductivity measurements before dividing by the molarity of each solution to obtain molar conductivity. The strong molar conductivity responses seen from NaCl, HCl, MgCl2, and H2SO4 are 30−60 S cm2/mol. The relatively weak molar conductivity response seen from MgSO4 and acetic acid is between 0.02 and 0.7 S cm2/mol. Water has the weakest 5224
dx.doi.org/10.1021/ef5013038 | Energy Fuels 2014, 28, 5222−5228
Energy & Fuels
Article
by mass sulfate, the maximum allowable level for DFE, yielding a solution conductivity of 0.05 μS/cm in pure ethanol. These results are more accurate than the data presented in Table 3 because they were prepared in ethanol-rinsed HDPE containers, reducing the error introduced by the containers. Each contaminant was doped in five solvent environments: (1) nondenatured pure ethanol; (2) 4% by volume natural gasoline denatured ethanol; (3) 4% by volume natural gasoline denatured ethanol containing 50 mg/L acidity as acetic acid; (4) 4% by volume natural gasoline denatured ethanol containing 1% by volume water; (5) 4% by volume natural gasoline denatured ethanol containing both 50 mg/L acidity as acetic acid and 1% by volume water. Solution conductivity results are shown in Figure 2, and detailed results, including pHe and molar conductivity data, are
Table 3. Electrical Conductivity Response in Denatured Ethanol sample pure ethanol denatured ethanol (3.1% heptane) denatured ethanol (3.1% gasoline) 10 ppm Cl− (NaCl) 5 ppm Cl− (NaCl) 1 ppm Cl− (NaCl) 0.25 ppm Cl− (NaCl) 7 ppm Cl− (HCl) 3.5 ppm Cl− (HCl) 0.5 ppm Cl− (HCl) 10 ppm Cl− (MgCl2) 5 ppm Cl− (MgCl2) 1 ppm Cl− (MgCl2) 3 ppm SO42− (H2SO4) 1.5 ppm SO42− (H2SO4) 0.75 ppm SO42− (H2SO4) 4 ppm SO42− (MgSO4) 1 ppm SO42− (MgSO4) 1.0% H2O 0.5% H2O 0.1% H2O 56 mg/L acidity (as CH3COOH) 28 mg/L acidity (as CH3COOH) 5.6 mg/L acidity (as CH3COOH)
solution conductivity (μS/cm)
molar conductivitya (S cm2/(mol of electrolyte))
0.219 0.207 0.174 9.032 4.996 1.123 0.366 11.590 5.610 0.794 4.750 3.050 0.792 1.794 0.778
40 43 41 29 73 69 53 41 51 53 64 46
0.392
30
0.229
0.68
0.207
0.06
0.516 0.348 0.280 0.426
0.0006 0.0005 0.001 0.02
0.353
0.03
0.362
0.17
a
Molar conductivity calculated by subtracting baseline 3.1% heptane denatured conductivity result from solution conductivity before dividing by molarity. Measurements made in borosilicate glass. Figure 2. Conductivity of contaminants in various solvent conditions (ethanol-rinsed HDPE containers). Water added at 1 vol %, acidity is acetic acid at 50 mg/L. Error bars are EN15938 method repeatability.
response, with a molar conductivity of around 0.001 S cm2/ mol, at most. Given the low molar conductivity increase, water, acetic acid, and MgSO4, it may be impractical to measure these constituents using conductivity as a tool. Because the solution conductivity for the lowest dopant concentrations was so close to the value of the heptane-denatured ethanol control sample, the molar conductivity calculation in this experiment is more accurate for the highest dopant concentrations. More accurate molar conductivity results were obtained in a subsequent experiment using HDPE containers. Solvent Effects on Conductivity Response. Solvent properties including density, dielectric constant, and other contaminants that do not have a significant conductivity response may change the conductivity response of various ions.16 Experiments were set up to determine solvent effects on conductivity and pHe response to NaCl, MgCl2, HCl, MgSO4, and H2SO4. Pure ethanol and 4% by volume gasoline-denatured ethanol were used as controls. Chloride ion contaminants were doped at 2 ppm by mass chloride, targeting a conductivity response of 2 μS/cm. H2SO4 was doped at 3.5 ppm by mass sulfate, targeting the same conductivity response of 2 μS/cm. MgSO4-doped ethanol could never attain a solution conductivity near the target of 2 μS/cm, so it was doped at 4 ppm
shown in Table 4. The denaturant had no effect on conductivity, and acetic acid had no effect except when present with MgCl2. Water at 1% by volume reduced the solution conductivity of 2 ppm chlorides (from HCl) by 1.4 μS/cm (39%) and 3.5 ppm sulfates (from H2SO4) by approximately 0.75 μS/cm (38%). Water in small quantities within nonaqueous solvents has produced dramatic conductivity effects for strong electrolytes with limiting molar conductivity decreasing at low water levels even though conductivity in pure water is much higher than in the nonaqueous solvent.18 For example, limiting molar conductivity of HCl in ethanol dropped from 88 S cm2/mol at 0.01 wt % water to 49 S cm2/mol at 0.9 wt % water.19 A theoretical analysis led to the proposal that in anhydrous alcohol proton migration occurs via proton jumps along hydrogen-bonded alcohol chains. In the presence of low levels of water the protons become effectively trapped in hydronium ions that are not part of this chain network and are relatively isolated making proton jumps statistically improbable, hence reducing conductivity.20 5225
dx.doi.org/10.1021/ef5013038 | Energy Fuels 2014, 28, 5222−5228
Energy & Fuels
Article
MgCl2, at 2 ppm chloride ion, demonstrated an increase of ∼25% in molar conductivity response when water or acetic acid or a combination of water and acetic acid was added (solution conductivity increase from ∼1.4 to ∼1.8 μS/cm). MgCl2 also forms Mg2+−Cl− ion pairs in ethanol,23 and water may reduce ion pair formation or lead to greater dissociation. There were no solvent effects on the apparent molar conductivity response for NaCl at 2 ppm chloride ion when water or acetic acid was present. As expected, the denaturant had very little impact in any samples because it does not produce ions in solution. Figure 3 compares our results for chloride’s effects on conductivity with results reported by others. Galante-Fox and
Table 4. Detailed Solvent and Contaminant Effects on Conductivity and pHe Results (Water Added at 1 vol %, Acidity Is Acetic Acid at 50 mg/L) sample pure ethanol denatured ethanol (4% gasoline) 2 ppm Cl (NaCl) + water + acetic acid 2 ppm Cl (NaCl) + water 2 ppm Cl (NaCl) + acetic acid 2 ppm Cl (NaCl) denatured 2 ppm Cl (NaCl) nondenatured 2 ppm Cl (MgCl2) + water + acetic acid 2 ppm Cl (MgCl2) + water 2 ppm Cl (MgCl2) + acetic acid 2 ppm Cl (MgCl2) denatured 2 ppm Cl (MgCl2) nondenatured 2 ppm Cl (HCl) + water + acetic acid 2 ppm Cl (HCl) + water 2 ppm Cl (HCl) + acetic acid 2 ppm Cl (HCl) denatured 2 ppm Cl (HCl) nondenatured 4 ppm SO4 (MgSO4) + water + acetic acid 4 ppm SO4 (MgSO4) + water 4 ppm SO4 (MgSO4) + acetic acid 4 ppm SO4 (MgSO4) denatured 4 ppm SO4 (MgSO4) nondenatured 3.5 ppm SO4 (H2SO4) + water + acetic acid 3.5 ppm SO4 (H2SO4) + water 3.5 ppm SO4 (H2SO4) + acetic acid 3.5 ppm SO4 (H2SO4) denatured 3.5 ppm SO4 (H2SO4) nondenatured
molar conductivitya (S cm2/mol)
conductivity (μS/cm)
pHe
0.051 0.037
5.8 5.9
2.12
5.3
47
2.06 2.08
6.2 5.0
45 46
2.05 2.02
6.0 5.9
45 45
1.75
4.0
77
1.75 1.72
5.4 3.2
77 76
1.36 1.40
5.2 5.1
59 61
2.21
2.7
49
2.19 3.60 3.64 3.64
2.7 1.7 1.7 1.7
48 80 81 81
0.135
5.3
3.0
0.060 0.111
6.7 5.1
0.70 2.3
0.055
6.2
0.55
0.054
5.9
0.52
1.24
3.2
37
1.23
3.2
36
1.95
2.1
58
1.98
2.2
59
1.92
2.1
57
Figure 3. Conductivity as a function of chloride content for samples from this study and refs 7 and 14 (CI = corrosion inhibitor additive).
co-workers prepared E85 samples with NaCl concentrations of 2.5−10 ppm chloride ion.14 As Figure 3 shows, the effect of the chloride ion concentration was almost identical to the results reported here. A second study reports chloride content and conductivity for commercial DFE and E85 samples.7 Chloride ion content never exceeds 2 ppm for these samples, and agreement with our results is less exact. For example, a DFE sample with a conductivity of 2.7 μS/cm had chloride content less than 0.1 ppm and pHe greater than 7. As shown above, for the contaminants examined here only the chloride salts examined or strong acid could produce such high conductivity. Therefore, other contaminants may be able to produce high conductivity; alternatively, the result is affected by high background conductivity from the sample container. The limiting molar conductivity of ions is significantly less in pure ethanol than in pure water, as shown in Table 5; however, the magnitude of reduction is different for different ions.18 Our measured molar conductivity data for HCl and NaCl agree reasonably well with others who have measured limiting molar conductivity of those compounds in pure ethanol. As shown in Table 5, the limiting molar conductivity of HCl in pure ethanol decreases further when small amounts of water are added, indicating the molar conductivity change that occurs when a pure ethanol mixture is changed to a pure aqueous mixture is not linear but has a local minimum at some ethanol/water ratio. This is confirmed by published data at higher water levels.24 Water has a much smaller effect on the conductivity of NaCl in ethanol. A recent study of the conductivity of KCl in ethanol− water solutions shows results similar to those for NaCl and further demonstrates that the conductivity at any ethanol level
a
Calculated by subtracting baseline 3.1% heptane denatured conductivity result from solution conductivity before dividing by molarity. Measured in ethanol-rinsed HDPE containers.
MgSO4 showed a very small increase in conductivity response (40 S cm2/mol)and therefore whose concentration might be controlled by a conductivity limitare strong acids, NaCl, and MgCl2. However, for MgCl2 the response per mass of chloride ion in the sample is much lower than for NaCl, at least greater than 1 ppm chloride ion concentration. MgSO4, water, and acetic acid produce much lower molar conductivity (
