White Paper. Thermometric Titration of Acid Number According to ASTM D8045

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1 Thermometric Titration of Acid Number According to ASTM D8045 Acid number (AN) determination, a common analysis in the oil industry, is used to monitor oil degradation and acidity. It helps prevent machine breakdowns and pipeline leakage due to corrosive acidic oil. Although standard potentiometric acid-base titrations work well for the determination of AN, they can be time consuming measurements. The ph electrode needs to be precisely calibrated and conditioned, and the titration endpoint is not always easily found. One approach to avoid such pitfalls is to measure the temperature change that occurs during a catalyzed titration. This catalyzed thermometric titration, described in the ASTM D8045 standard (1), is faster, needs less sensor maintenance, and is as precise and reliable as the standard determination of AN, according to the ASTM D664 standard. Contents 1 Thermometric Acid Number Titration: Principle 2 Blank and Titer Measurement 3 Sample Measurement 4 Thermometric vs. Potentiometric Titration of AN 5 Solution 6 Conclusion 7 References

2 1 Thermometric Acid Number Titration: Principle Every chemical reaction is accompanied by a change in energy. During endothermic reactions, energy is absorbed and a temperature drop is observed. The opposite is true for exothermic reactions where energy is released. It is possible to detect the endpoint of a titration by monitoring the temperature evolution (see Figure 1). In the course of an exothermic titration, the temperature increases until the endpoint is reached. After that, the temperature stabilizes with the surroundings and a temperature drop is observed. The opposite happens for endothermic titrations (right hand side of Figure 1). Figure 1: Schematic view of an exothermic titration (left) and of an endothermic titration (right). A temperature decrease is observed during the course of the reaction. Once the endpoint has been reached, the temperature is stabilized. The detection of the endpoint is made by calculating the second derivative of the curve, which shows changes in the slope of the titration curve and happens at the endpoint (see Figure 2). Figure 2: The accepted formal process for how to investigate OOS results. 2

3 The only requirements of such a thermometric titration are: a chemical reaction with a large energy change, a precise and fast thermometer, and a titrator capable of performing a segmented evaluation of the titration curve. The energy change of the acid-base titration used to determine the acid number in oils is too small to be detected directly. Therefore, paraformaldehyde (PFA), a thermometric indicator, must be added. Analogously to the way that color indicator changes color at the equivalence point of an acid-base titration, a thermometric indicator induces a change of temperature at the endpoint. Once all the acid has been neutralized by the titrant, KOH in 2-propanol, paraformaldehyde is decomposed by the excess KOH in a strong endothermic reaction. A temperature drop can thus be observed at the end of the titration. An example of a real AN titration in oil is shown in Figure 3 (blue line), together with the second derivative of the curve (red line). The small increase of temperature at the beginning corresponds to the neutralization of all acids by the addition of KOH in 2-propanol. After the endpoint has been reached, paraformaldehyde reacts with the excess of KOH and a sudden temperature drop is observed. In order to detect the endpoint accurately, the thermometer must be very precise, with a minimum resolution of 10-3 C, and very fast, with a response time < 0.3 s. Figure 3: Thermometric AN titration curve of an oil. Blue line: titration curve. Red line: second derivative of the curve. 2 Blank and Titer Measurement Prior to the sample measurements, it is necessary to measure the blank value of the solvent (mixture of xylenes:2- propanol 75:25) and the titer of the titrant. The blank value is generally very small and is difficult to be directly measured. It is therefore measured indirectly using a stable sample with a known acid number. The consumption of at least three samples of increasing sample size is measured and a linear regression of the consumption vs. the sample mass is performed (see Figure 4). The blank value corresponds to the y-intercept of the linear regression, since it represents the consumption of titrant for a sample size of 0.0 g. According to ASTM D8045, the titrant consumption for each measurement must be 0.15 ml and the R2 of the linear regression must be 0.98 or greater. The blank value must be lower than 0.1 ml, which is usually the case with high purity 2-propanol. When the blank value is found within the limits, it is not necessary to apply any correction to the sample measurements. When the blank value is higher than 0.1 ml, a fresh solvent mixture must be prepared and the blank must be measured once again. 3

4 Once the blank of the solvent is known, the titer of 0.1 M KOH in 2-propanol can be determined in the same way as samples, by direct measurement, using Benzoic acid as standard substance. However, as the concentration of the titrant is directly proportional to the slope of the linear regression, it is possible to obtain the titer value for free, if the measurements are performed with a solution of benzoic acid of known concentration. Only an additional calculation is necessary to obtain the titer from the value of the slope. Figure 4: Blank and titer determination principle. A linear regression is performed on the 4 measurement results. The y-intercept corresponds to the blank and the titer is the slope of the linear regression. The linear regression is calculated as follows. The slope, given in ml/g of the linear equation for the best fit line is first calculated: mmmm * vvvvvvvvvvvv (mmmm vvvvvvvvvvvv) slope = 2 mmmm 2 ( mmmm) Where, m is the mass of the sample vol is the consumed titrant volume n is the number of samples The blank, which corresponds to the y-intercept of the linear regression, is calculated with the relation: Blank= vvvvvvvvvvvv mmmm slope* The titer is then calculated as follows: Cst Titer= slope*c*m 4

5 Where, Cst is the concentration of the Benzoic acid solution (in mg/g) c is the nominal concentration of KOH in 2-propanol (0.1 mol/l) M is the molar mass of benzoic acid ( g/mol) Finally, the correlation coefficient R2 is calculated with the following formula: mmmm * vvvvvvvvvvvv (mmmm vvvvvvvvvvvv) RRRR 2 = mmmm 2 ( mmmm)2 vvvvvvvvvvvv 2 ( vvvvvvvvvvvv)2 2 All of these calculations can be done directly by an auto-titrator. 3 Blank and Titer Measurement Once the blank and titer are known, the measurement of the sample is straightforward. Simply weigh the appropriate amount of sample, add 30 ml of a mixture of Xylenes:2-Propanol 75:25, stir for 30 s, add 0.5 g paraformaldehyde, and start the titration. The measurement takes less than 2 minutes. It is important to use the amount of sample recommended in Table 1. When the recommended sample weights are used, no blank correction is necessary. Whereas, when the recommended sample weights cannot be used, due to poor solubility or low amount of substance, a blank has to be measured using this particular sample and then a correction must be applied to the measured AN. Expected Acid Number (KOH, mg/g) 0.05 to to to Recommended Sample Mass (g) ± 10 % 10 to Table 1: Recommended sample weights for thermometric AN measurement. Very viscous or solid samples can be liquefied at 120 C to facilitate the sample weighing process. The sample should then be cooled at room temperature before the analysis. After weighing, it is possible to pre-dissolve samples in 10 ml of toluene or xylenes, if they are not readily soluble in the solvent mixture. During the titration, a minimum of 0.15 ml and a maximum of 5 ml of titrant should be used. In any case, a maximum of one burette volume can be titrated; during the refilling of the burette, the sample will equilibrate its temperature with the surroundings, producing an unwanted change of slope in the titration curve and unwanted endpoint recognition. Paraformaldehyde (PFA) can be added by hand prior to every measurement. However, it is a carcinogenic powder and must be handled with care, using gloves, goggles, coat, and protective mask. To avoid a constant exposure to paraformaldehyde, it can be automatically added. In such a case, a suspension of 17 g/l PFA is prepared in the solvent mixture. The solution is constantly stirred on a magnetic stirring plate to ensure a homogenous distribution of PFA and a pump is used to dispense 30 ml of the solution prior to the titration. 5

6 A constant titrant dosing rate of 2 ml/min is advised. Dynamic dosing must be avoided, since this can lead to temperature variations during the titration. After the recognition of the endpoint using the segmented evaluation (second derivative), the acid number, given in mg KOH/g, is calculated as follows: AN= VEQ*c*TITER*M m*z Where, VEQ is the consumed titrant volume at the endpoint (in ml) c is the concentration of the titrant (0.1 mol/l) TITER is the titer of the titrant M is the molar mass of KOH (56.11 g/mol) m is the mass of the sample (in g) z is the number of equivalents (z=1) 4 Thermometric vs. Potentiometric Titration of AN The Canadian Crude Quality Technical Association, CCQTA, performed a comparative study of the thermometric ASTM D8045 standard with the ASTM D664 potentiometric standard on different crude oils (2). Thermometric titration gave similar results as potentiometric titration. With Thermometric titration, thick and sticky crudes are also easy to measure and the sample stays homogeneous during the analysis. Furthermore, the probe is not subject to coating. In addition, the sensor handling is much easier with the thermometric titration as the probe does not require any calibration or special conditioning. Thermometric titration is also much faster than potentiometric titration, as the time consuming conditioning steps can be avoided. Thermometric titration is however limited to crude oils and other oils with little or no additives, like compressor oils, turbine oils or hydraulic oils. For oils containing additives, like motor oils, lubricant oils or heavy marine fuels, the thermometric endpoint is hardly found or remains undetected. In fact, some additives have a tendency to react directly with paraformaldehyde leading to a constant temperature decrease during the course of the titration instead of a sudden drop after the endpoint. For such oils, thermometric titration must be tested for each different sample to determine the appropriateness of the method. 6

7 5 The Solution Thermometric titration is now available on all three new Excellence Titrators (T5, T7, and T9) using the brand new Thermotrode sensor. This precise thermometer, based on NTC technology, has a resolution of 10-4 K, a very stable signal and a response time lower than 0.3 s. It is a maintenance-free sensor that does not require any calibration or systematic conditioning, and it is resistant to most of the organic solvents (see Figure 5). All analyses can be started with just OneClick : all you have to do is just click a button and the titrator will take care of the rest. Thermometric analysis can be automated using the InMotion sample changer: up to 112 samples can be titrated in unattended operation. By automating their analysis, users can reduce timeconsuming, hazardous manual tasks such as cleaning sensors or emptying the beakers whilst increasing their productivity, safety and efficiency. Unique technologies like SmartSample (not compatible with all configurations of InMotion) RFID tagging and LabX software platform with SmartCode functionality, allow the user to transfer all of the main information of the sample or even start the corresponding method quickly and automatically. 6 Conclusion Thermometric acid number determination is a technique to perform analyses on crude oils and oils without interfering additives. It is a titration technique that uses a thermometric indicator to detect the endpoint thermally. Even though the measurement of the blank requires at least three samples to perform a linear regression on the results, this analysis is much faster than standard potentiometric acid number determination according to ASTM D664, but as precise and reliable. Results are generally close to what is obtained by ASTM D664. offers a simple modular solution for thermometric titration of AN according to ASTM D8045, using existing potentiometric titrators. It goes from a standalone instrument with manual sample preparation to a full automated system with automatic solvent addition and automatic weight transfer. The whole system can be controlled by the unique laboratory software, LabX. For more information about thermometric titration, please visit 7 References 1. Acid Number of Crude Oils and Petroleum Products by Catalytic Thermometric Titration. West Conshohocken : s.n., Bd. ASTM Standard D8045. Figure 5: The new Thermotrode, sensor for thermometric titration. 2. Murray, Dave. TAN Thermometric Method Evaluation, CCQTA Project Report. CCQTA

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