The pHe is a measure of acid strength in alcohol fuels and in ethanol. It can be used as predictor of the corrosion potential of an ethanol-based fuel. The determination of the pHe is preferred over the total acidity, because total acidity overestimates the contribution of weak acids (e.g., carbonic acid) and underestimates the contribution of strong acids (e.g., sulfuric acid). Furthermore, the acid strength is an important parameter to determine in order to reduce the risk of failing motors.

The pHe is not to be confused with the pH value. The pH value is only applicable for aqueous solutions, whereas the pHe value is a measure for alcoholic solutions.

The pHe value is highly dependent on the sample itself, but also on the stirring rate and the time the electrode is immersed into solution. Therefore, both the stirring rate and measuring time need to be fixed.

This Application Note describes the determination of the pHe value using the 913 pH Meter and the EtOH Trode according to ASTM D6423, which covers denatured fuel ethanol and ethanol fuel blends.

The method is demonstrated for denatured ethanol fuel.

Temperature has a strong influence on the obtained results. Therefore, sample temperatures are first adjusted to be at the same value in order to be able to compare results.

913 pH Meter equipped with a pH electrode. Example setup for the determination of the pHe value.
Figure 1. 913 pH Meter equipped with a pH electrode. Example setup for the determination of the pHe value.

This application is performed on a 913 pH Meter equipped with an EtOH Trode, a temperature sensor, and an external stirring plate. The EtOH Trode is conditioned and calibrated prior to use.

A defined amount of sample is poured into a 100 mL beaker and placed on an external stirring plate. The EtOH Trode and the temperature sensor are immersed, and the measurement is started immediately. The value after 30 seconds is considered to be the acid strength of the sample.

The analysis demonstrates acceptable and reproducible pHe values. For the tested denatured ethanol fuel, a pHe value of 8.08 (n = 10, SD(rel) = 0.45%) is obtained. An example measurement curve is displayed in Figure 2.

Figure 2. Example curve of a pHe measurement in denatured ethanol fuel.

With a 913 pH Meter it is possible to assess quick and easily the pHe of an ethanol fuel according to ASTM D6423. The used EtOH Trode is specifically mentioned in the standard, and is therefore fully compliant.

By using a 913 pH/DO meter or a 914 pH/Conductometer, it is also possible to assess the dissolved oxygen concentration or the conductivity in parallel, if necessary.

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