Determination of Lower Acidity of Jet Fuel by Catalytic Thermometric Titration Using Paraformaldehyde as a Thermometric EndPoint Indicator the Regime of during

The effect of concentrations of titrant, delivery rates, stirring rates, and oil mass on catalytic thermometric titration for the determination of the lower acidity of jet fuel were investigated, using KOH in isopropanol and paraformaldehyde as titrant and a catalytic thermometric indicator respectively. The results show that paraformaldehyde used as a catalytic indicator exhibits strongly endothermic effects to reflect end point significantly. When the oil mass is from 10g to 30g, the titration concentration is 0.01mol/L and the delivery rate is 1.0mL/min with moderate stirring, the tested acid numbers have good reproducibility and accuracy. The linear coefficient R 2 of the fitting curve is 0.995. Using benzoic acid as a standard acid with concentration of 0.0105mg KOH/g to verify the accuracy of catalytic thermometric titration, the verified acid number is 0.0115mg KOH/g and basically consistent with the actual acid number, indicating that catalytic thermometric titration has good agreement with standard potentiometric titration methods and can be used for determination of acid number of jet fuels. It can accurately determine the acid number of jet fuel as low as 0.015mg KOH/g or even lower at optimized test conditions. The procedure is fast, easy to use, accurate, and highly reproducible to measure lower acidity in jet fuel. It is very suitable for the routine process and quality control of many types of oils.


PPS.MS.ID.000564. 3(3).2020
range of indication reaction is unlimited because all of reactions are accompanied with temperature changes, the magnitude of which can be adjusted by changing reagents' concentrations [8][9][10][11]. The basic principle of this method is catalytic initiation of an exothermic or endothermic reaction with an excess of titrant, as a consequence, the end-point can be indicated by obvious temperature changes of the solution [8][9][10][11][12][13][14][15]. It has been successfully applied for determining acidic substances in aluminum ion concentration of waste water and vegetable oils [16][17][18][19]. However, as so far, there are few reports on application of catalytic thermometric titration on lower acidity of jet fuels.
When small amounts of weak acidic species are titrated in nonaqueous solution with a titrant of strong alkali, the heat produced from the neutralization reactions may be quite small and easy to be confused by solvent evaporation and the mixing heat of the titrant with sample solution [4][5][6][7]. If the special thermometric indicator is added to sample solution, excess hydroxide ions would react quickly with them in endothermic or exothermic reactions, the end-point can be easily determined by temperature increase or decrease of the solution [20,21]. However, practical experience has demonstrated that the endpoint in thermometric titration showed excessive rounding, with consequent loss of precision and accuracy for some oils with lower acidity, such as aviation oil, hydraulic oil and fuel oils. Many studies have shown that titration error or the sharpness of the endpoint can be related to the concentration and delivery rate of titrant, volumes, and types of thermometric indicator [22][23][24][25][26][27]. In our previous studies [9,10], the trace water in jet fuels, as well as the acidity of several coloured oils, could be accurately and rapidly determined by catalytic thermometric titration using the mixture of acetone and chloroform as the endpoint indicator. In this paper, we report herein our results on the determination of the lower acidity of jet fuels with catalytic thermometric titration employing paraformaldehyde as the endpoint indicator, which exhibits strongly endothermic effects to reflect end point significantly and can determinate much more lower acidity than ASTM thermometric titration, and compared with potentiometric titration, the accuracy and repeatability of the thermometric titration is further investigated.

Materials
Paraformaldehyde, potassium hydroxide and isopropanol were of analytical reagent quality. A 0.10mol/L potassium hydroxide isopropanol solution was prepared and standardized with potassium hydrogen phthalate by the usual procedures. The jet fuel NO.3 was purchased from Sinopec Tianjin Shihua. Standard acid solution was prepared from approximately 37.8mg benzoic acid dissolved with 250mL isooctane, and acid value of standard acid is 0.10mg KOH/g.

Apparatus
In thermometric titration, two motor-driven micrometer syringes were employed to add the titrant to samples at a constant delivery rate, and a magnetic stirrer was provided to dissolve oil solutions, the temperature changes were detected by locating the thermistor in one arm of a Wheatstone bridge and were recorded with a strip chart recorder. At the end of each titration, sample and titration data were automatically sent for identification and computation, the acidity of oil samples was obtained from autocalculation. In potentiometric titration, a pH probe containing both a glass and a reference electrode in the same body was employed.

Methods
In visual titration, the amount of oil was chosen in accordance

Results and Discussion
Effect of concentrations of titrant on thermometric titration As listed in Table 1 to obtain more precise acid number.   As Table 2

Effect of stirring rates on thermometric titration
As Figure 3

Effect of oil mass on thermometric titration
As Table 3 and Figure 4 show, when the oil mass is 5.0g, the temperature change of thermometric titration curve is obvious and the end-point of titration is easily judged. However, due to the small oil mass and the lower acidity, the titration system is relatively vulnerable to other factors, and the repeatability of titration results is not stable. When the oil mass is between 10.0g and 30.0g, the thermometric titration curves are smooth with obvious titration end-point, and the second-derivative plots are presented along with the solution temperature plot, which have sharp peak and obvious titration end-point. As Table 3 exhibits, the results of acid number are basically the same with reasonable RSD value. As Figure 5 shows, the coefficient of correlation (R2) for the linear calibration

Thermometric titration in standard acid samples
In order to testify the accuracy of the method of catalytic thermometric titrimetry, different mass of benzoic standard acid samples was investigated, and the results are listed in Table 4. The coefficient of correlation (R2) for the linear calibration curve was found to be 0.9924, which was shown in Figure 6. We can see clearly that the acid numbers of three standard acid samples were very similar to real acid number (0.10mg KOH/g) with low RSD. These observations confirm that thermometric titrimetry is an accurate and highly reproducible method, which was easy to determine acid number of colored or additive-containing petroleum oils in titrations. Comparison of thermometric and potentiometry titration in jet fuel NO.3    Table 5, from which we can see clearly that the acid numbers of jet fuel NO.3 obtained from catalytic thermometric titrimetry are basically consistent with potentiometry and visual titrimetry with less error.

Conclusion
The catalytic thermometric titration is a fast, easy to use, accurate, and highly reproducible method to determine lower acidity in jet fuels, using paraformaldehyde as the end-point indicator. Paraformaldehyde used as the end-point indicator can exhibit strongly endothermic base catalyzed reaction in thermometric titration, which has good agreement with standard acid samples. It can accurately determine the acid number of jet fuels as low as 0.015mg KOH/g or even lower at optimized test conditions. The procedure is fast, easy to use, accurate, and highly reproducible to measure lower acidity in jet fuel. It is very suitable for the routine process and quality control of many types of oils.