Quality control of bioethanol

Clive Grantham, group managing director Metrohm UK Ltd explores the issue regarding the testing and analysis of bioethanol fules to ensure that comply with international standards:

London - The EU standard for bioethanol pr EN 15376 lists a number of parameters that the bioethanol must meet. These include the percentage of pure bioethanol, acidity, chloride and sulphur contents as well as water content, which if too high can impair fuel quality and damage vehicle fuel systems.

Bioethanol is mixed with gasoline in various ratios to reduce both the demand for gasoline and environmental pollution. The gasoline-alcohol blend is known in the US as gasohol and in Brazil as Gasolina Tipo C. In the US the mixtures E10 and E85, which contain 10% and 85% bioethanol respectively, are widespread. In Brazil the bioethanol content of most blends lies between 21% and 23%. The ASTM standard E 1064 describes the coulometric Karl Fischer titration for determining the water content. For a water content below 2% the recommended test method is volumetric titration as per ASTM E 203.

The pH value is an important quality criterion for bioethanol. A combined pH glass electrode with ground-joint diaphragm is recommended for measuring the pH in organic solvents. Because of the expected low conductivity of the sample, the electrode should be particularly well shielded in order to suppress electrostatic influences.

Meanwhile, the pH meter used for the measurement should possess the following GLP functions: Three-point calibration with automatic buffer recognition;  Temperature compensation; Calibration data monitoring; Automatic measured value recording; and Result memory with identification

The chloride content in bioethanol fuels is the subject of ASTM D 512. Determinations can be carried out by mercurimetric or argentometric titration or by direct titration with a Cl-sensitive electrode. Argentometric determination (with silver nitrate) is the method of choice because of its environmental advantages and very high precision. A defined volume of the sample is pipetted into the titration cell, treated with 5 mL of 2M nitric acid and then titrated against 0.01M silver nitrate solution.

A proven method for sulphate determination is potentiometric titration with lead nitrate using a Pb-sensitive electrode. The pH of the sample is adjusted to pH 3…5 with perchloric acid and the sulphate is then precipitated out with lead nitrate solution. The Pb-selective electrode detects the first excess of lead ions at the equivalence point. The wide range of potentiometric applications for bioethanol also requires a highly flexible titrator.