pH Measurement in Bioethanol by Using Different Electrodes and according to International Standards

Abstract

The importance of Brazilian bioethanol as biofuel has been increasing significantly. Its origin from renewable sugar cane is able to reduce carbon in the atmosphere and is one of the factors that contribute for reducing global warming. Because of the difficulty in measuring pH in nonaqueous solutions, Inmetro, the Brazilian Metrology Institute, has focused its attention on the needs for accurate and reliable pH measurements, as part of its work in developing and producing certified reference material (CRM) of bioethanol. This paper aims to present the studies of pH measurement in CRM of bioethanol from sugar cane (approximately 99.6 % ethanol content) using two commercial pH electrodes with different internal filling electrolytic solutions. One electrode, containing potassium chloride (KCl), was specified in the American standard, ASTM D6423, and the other one, containing lithium chloride (LiCl), was specified in the Brazilian standard, ABNT NBR 10891, and European standard EN 15490. These studies have shown that there was a significant difference among the pH values (more than 1.0 pH unit) measured by the different electrodes related to its internal filling solution. The results have shown that a harmonization of the standardization on the pH measurement is needed in order to make possible its use and application as analytical parameter.

Unified pH Measurements of Ethanol, Methanol, and Acetonitrile, and Their Mixtures with Water

“…Further influences include: initial purity (including water content) of organic solvent (including bottle to bottle differences), storage conditions and duration, and initial pH of water used in solvent-water mixtures. Further influences may include: differences in electrode construction, including glass composition [ 26 , 48 ] and those associated with interaction between the inner filling solution and the solution under test for combination pH electrodes (electrodes B through F) [ 46 ].…”

“…Similar advantages may be seen in other commercial half-cell electrodes for measurements [ 20 ]. However, differences between commercial electrodes have been noted when measuring solvent-specific pHe in anhydrous and hydrous bio-ethanol according to ASTM D6423-14 [ 26 , 27 ], as well as buffered methanol–water and ethanol–water solutions [ 28 ]. The suitability of various half-cell and combination pH electrodes for measurements of mixtures of water with ethanol, methanol, and acetonitrile are investigated in this work.…”

Reconciling the pHe measurements of bioethanol: pHabs measurements of buffered 50-50 wt% water-ethanol mixtures

“…This method requires the use of a specific electrode, includes detailed measurement constraints, and suggests traceability is achieved through calibration of the specified electrode using aqueous buffer solutions. Reports on measurement of pHe in bioethanol originate with national metrology institutes (NMIs) in Brazil [ [11] , [12] , [13] , [14] , [15] ], the EU [ [16] , [17] , [18] ], and a national research center in the USA [ 19 ]. The significant issues with the ASTM method of measuring pHe have been discussed in Brown et al [ 17 ].…”

“…Issues arising from the use of various glass-electrodes in water-ethanol mixtures has been discussed by Ref. [ 11 ], while differences between ASTM [ 6 ] and various other methods (including EN 15490:2007–10 [ 7 ]) are highlighted in Ref. [ 18 ].…”

“…However, in low water content solvent mixtures, and generally in non-aqueous organic solvents, its applicability is restricted (often to the point of being useless) primarily due to: Poor performance of the commonly used glass electrodes in non-aqueous media. One of the major issues with the use of aqueous buffer calibrated glass electrodes, including those specifically marketed for the measurement of ethanol [ 11 , 14 , 15 , 23 ], is the unknown magnitude of the liquid junction potential (LJP) between the pH electrodes' reference electrode filling solution and the non-aqueous solution under test. Further, glass electrodes behave differently when in contact with aqueous solutions vs. mixed water-ethanol solutions.…”