Water content in jet fuels is detected by thermometric titration (TMT), and the optimal detected system is 2,2-dimethoxypropane as titrant, cyclohexane and isopropanol as titration solvents, and methanesulfonic acid as catalyst in this method. The amounts of oil, concentration and delivery rate of titrant, volumes, and the reliability and accuracy of thermometric titration were emphasized. The results show that the accuracy, validity, and reliability of TMT are excellent by different indicated spiked water contents. The obtained results between TMT and Karl Fischer titration have been proven to be in accord. But, the duration of titration merely spends 3–5 min in the whole process, greatly shortening the detected time. Therefore, rapid and accurate determination of trace water in a jet fuel can be realized by TMT.
Water in jet fuels can result in fuel system icing, microbial contamination, and corrosion, which was considered as an important quality of fuel performance. Under a low-temperature environment, dissolved water content of jet fuels is reduced and free water in fuels generates small ice crystals that block the oil line and decrease the normal fuel supply, so as to threaten flight safety [
At present, several field and laboratory methods are used to determine water in oil products. Field method is mainly visual observation method (VOM), which is conducted for the qualitative determination of water. However, as is characteristic of poor accuracy and repeatability, VOM cannot perform accurate quantitative determination of water in oil products [
In the present investigation, the water of jet fuel was subjected to determination by TMT. As the same time, results by TMT are compared with by Karl Fischer, and accuracy and repeatability of results by TMT are further testified.
TMT is an analytical method used to determine the content of substances, which is measured on the basis of different values of enthalpy of reaction [
Water content in jet fuel is detected with DMP as titrant. DMP has undergone an endothermic reaction with water under the catalytic action of acids, and its enthalpy is +27.6 kJ/mol. The reaction equation of DMP and H2O is exhibited as (
DMP (98%), isopropyl alcohol (A.R.), cyclohexane (A.R.), and methanesulfonic acid (MSA; 99%) were purchased from the Shanghai China National Medicines Corporation Ltd. Number 3 jet fuel was purchased from the Tianjin branch of Sinopec. The obtained reactants were dehydrated through a molecular sieve and placed in a dryer.
A self-developed automatic TMT device was equipped with titration software, high-precision temperature-sensing probe, volumetric dispenser, and magnetic stirrer. The titration software has the unique capability of processing large number of data points at the rate of 8 measurements every second which is critical for reliable detection of the endpoint; the probe can measure the temperature to 0.01°C and have a response time of less than 0.3 s; volumetric dispenser can consistently deliver desired volume of titration solvent. Electronic scales from German Sartorius Corporation are capable of weighing to ±0.1 mg. 10
Preparation of the titrant: 12.5 mL DMP was accurately weighed, added into 100 mL volumetric flask, and dissolved into a constant volume with cyclohexane; the DMP titrant with a concentration of approximately 1.0 mol/L was obtained. Then, 0.1 mol/L, 0.5 mol/L, 1.5 mol/L, and 2.0 mol/L DMP titrants were prepared according to the above method. Preparation of the titration solvent: the titration solvent was obtained by mixing isopropyl alcohol and cyclohexane with a volume ratio of 2 : 3. Preparation of the standard aqueous solution: 1
TMT: the appropriate amount of jet fuel was accurately weighed and added to the thermal insulation reaction flask; then, 20 mL titration solvent and 200 Karl Fischer titration: the water content in jet fuel was measured by Karl Fischer method of ASTM D6304.
The water contents in jet fuel are relatively small, so the mass of fuel, concentration, and delivery rate of titrants will affect the accuracy of the determination results in thermometric titrations. As Table
Results of different mass.
Mass (g) | 2.0 | 5.0 | 10.0 | 15.0 | 20.0 | 30.0 |
Recovery rate (%) | 97.2 | 99.3 | 100.8 | 102.1 | 104.8 | 105.5 |
The results of titrant consumption under different concentrations of titrants are listed in Table
Results of different concentrations of titrant.
Concentration (mol·L−1) | 0.1 | 0.5 | 1.0 | 1.5 | 2.0 |
Recovery rate (%) | 108.2 | 101.1 | 99.5 | 98.3 | 92.2 |
As showed in Table
Results of different delivery rates of titrant.
Delivery rate (mL·min−1) | 0.5 | 0.8 | 1.0 | 1.5 | 2.0 | 3.0 |
Recovery rate (%) | 105.8 | 101.5 | 99.3 | 96.5 | 92.8 | 89.5 |
The prepared DMP titrant was calibrated by five groups of standard solutions. Linear fitting of the data points was conducted, and titrant concentration was calculated according to the slope. The determination results were displayed in Table
Corresponding relationship between spiked water content and DMP titer.
Spiked water content (%) | 0.01 | 0.02 | 0.03 | 0.04 | 0.05 |
DMP titer (mL) | 0.947 | 1.013 | 1.073 | 1.127 | 1.173 |
Linear fitting of the standard water content and DMP titer was conducted. As demonstrated in Figure
Calibration curve of the DMP.
Jet fuel with mass of 10.015 g was accurately weighed. Then, spiked water with different mass fractions was added to the jet fuel and mixed with 20 mL titrant and 200
Results of jet fuels with different standard water contents.
Spiked water content (%) | 0.01 | 0.02 | 0.04 | 0.08 | 0.16 | 0.20 |
Spiked recovery rate (%) | 102.4 | 101.5 | 98.6 | 99.0 | 101.5 | 98.2 |
Relative standard deviation (%) | 2.8 | 3.1 | 3.5 | 1.8 | 1.6 | 1.5 |
Linear fitting of the standard water content and DMP titer was established. Figure
Linear regression equation of determining the spiked water contents.
Under room temperature (20°C) and humidity (80%) conditions, jet fuels with mass of 10 g were accurately weighed. Calibrated DMP was used to determine the water contents in jet fuels. The curve of temperature titration was shown in Figure
The curves of temperature titration.
The selection of the endpoint of titration was automatically carried out with no interference with the manual. A second derivative was rapidly formulated to confirm the automatic selection of endpoint by titration software. TMT and Karl Fischer method were used to determine the water content in jet fuels under different conditions. As displayed in Table
Results of the water content (
Factors | TMT | Karl Fischer | ||||
---|---|---|---|---|---|---|
Temperature (°C) | Humidity (%) |
|
|
|
Mean value | |
0 | 60 | 30 | 36 | 39 | 35 | 31 |
6 | 65 | 35 | 46 | 45 | 42 | 38 |
9 | 71 | 52 | 45 | 53 | 50 | 51 |
16 | 77 | 47 | 62 | 62 | 57 | 55 |
20 | 80 | 57 | 54 | 63 | 58 | 60 |
24 | 83 | 59 | 68 | 68 | 65 | 63 |
31 | 85 | 70 | 75 | 83 | 76 | 69 |
On the basis of the influence of the laws of the titration conditions, the reaction principle between DMP and H2O was analyzed during the determination process. The DMP titrant contained H3C–OH groups, which would react with water by absorbing heat and realizing water determination. The reaction mechanism between DMP and H2O was shown in Figure
Proposed mechanism between DMP and H2O.
The structure of DMP titrant contains two methoxy groups connected to one carbon atom forming large sterically hindered and strong chemical activity. This structure easily undergoes nucleophilic substitution with water. Under catalyst-free conditions, SN2 nucleophilic substitution occurs in the reaction. Particularly, H2O induces backside attack of the leaving groups as nucleophilic reagents. Fracturing of old bonds and formation of new bonds simultaneously proceed. However, the stronger the nucleophilicity of the reagent in the SN2 reaction is, the higher the reaction velocity is. The nucleophilicity of H2O is smaller than that of methoxyl. Therefore, the reaction slowly occurs between DMP and H2O without catalyst, so slowly so that the reaction cannot be noticed. Herein, we focus on acid as catalyst to accelerate the reaction and convert the reaction process from SN2 substitution into SN1 substitution. Protonation effect occurs in DMP because of the bond between the proton and methoxyl. As a result, methyl alcohol molecules were separated from DMP and formed carbocations. After a methoxyl was lost, steric hindrance was reduced, so it would be easily attacked by the nucleophilic reagent. As the nucleophilic reagent, H2O was bonded with carbocations, then the second methyl alcohol molecule was separated from a center carbon atom, and formation of protonated acetone occurred. As heat was absorbed in this process, the protons were finally removed to form acetone, and the removed protons continued the catalytic reaction.
The determination results on 0.01%–0.2% (
The authors declare that they have no conflicts of interest.
This work was subsidized the fund from the National Natural Science Foundation of China (Grant 51575525), the Natural Science Foundation of Jiangsu Province (Grants BK20151157, BK20150166, and BK20161188), Natural Science foundation of the Anhui Higher Education Institutions of China (Grant KJ2017A400), and the project was supported by the Tribology Science Fund of State Key Laboratory of Tribology (Grant SKLTKF14B10).