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Traditionally, pH is defined as the negative of the base 10 logarithm of the activity (read concentration, for simplicity) of hydronium ions in an aqueous solution, that is,
where the activity is a corrected form of the molar concentration. Details aside, pH allows us to represent, on the same scale, concentration values of this ion that can differ by up to 14 orders of magnitude.
In chemistry and biology, the pH scale is used to measure the level of acidity or basicity of an aqueous solution relative to pure water, which is considered neutral. The scale reference point (the neutral point) is defined in terms of the autoprotolysis equilibrium of water, which is given by the following chemical equation:
In a neutral solution, as in the case of pure water, there is an equivalent concentration of hydroxide (OH – ) and hydronium (H 3 O + ) ions , whose value is 10 -7 M, corresponding to a pH of 7.
As can be seen, everything related to pH, from its definition in terms of the concentration of the hydronium ion (which is only present in water), to the definition of the midpoint of the scale or point of neutrality, is established exclusively for water or for aqueous solutions of different solutes.
The pH of vegetable oil
What has been stated in the previous paragraphs means that, if we wanted to talk about the pH of vegetable oil , we would necessarily have to refer to the pH of a solution of vegetable oil in water. However, vegetable oil is insoluble in water, so aqueous solutions of that solute cannot be prepared. The same can be said of mineral oils and any other substance that is not water soluble. As a consequence, vegetable oil has no pH, or rather there is no point in talking about the pH of vegetable oil (or any other oil) since this concept, seen in the traditional way, does not exist in the absence of water.
It could be argued that no solute is absolutely insoluble in a solvent, and that even a minute amount of oil could eventually dissolve in water. If that were the case, the concentration would be so low that it would be almost impossible to distinguish it from the neutral pH of 7.
pH, acid value and free fatty acids in vegetable oil
It is convenient to make a small clarification. The reason many people wonder what the pH of vegetable oil is, even though this doesn’t make chemical sense, is because edible oils, such as vegetable oils, are made up primarily of fatty acid esters called triglycerides and by free fatty acids (that is, they are not esterified).
One of the quality parameters of an edible oil is its acidity index, which is related to the proportion of free fatty acids it contains. The acid value refers to the amount of KOH in milligrams that is required to neutralize all the free fatty acids present in an oil. The higher the acidity index, the worse quality the oil is.
Due to the obvious relationship between pH and acidity of aqueous solutions, it is natural that most people would look for a similar relationship between the “acid number” of vegetable oil and pH, even though they have absolutely nothing. to do with each other.
The “pH” of the oil in other acidity scales
The fact that a substance such as vegetable oil cannot be assigned a level of acidity (read pH) in aqueous solution does not mean that vegetable oil lacks acidity. It simply means that the pH scale is not adequate to measure said acidity, much less to be able to compare the acidity of vegetable oil against that of other chemical substances.
In view of this, acidity scales equivalent to pH have been established but for other solvents such as dimethyl sulfoxide or acetonitrile, to name just two. In these cases, autoprotolysis equilibria similar to those of water and acidity scales similar to that of pH have been proposed, but in terms of the concentration of the ions of said solvents instead of the hydronium ion.
In these cases, instead of a scale that goes from 0 to 14 (which is the pK W value ), the acidity scale is established based on the pK a of the solvent, so each solvent has its own scale associated with it. In these cases, it is possible to find a solvent for which a similar acidity scale has been established, and which is capable of dissolving vegetable oil. Although it would not be called pH nor could it be compared to the values of the pH scale in water, it would have a value for the level of acidity of the vegetable oil.
A universal pH scale
Questions like what is the pH of the oil may seem basic, but chemists and physicochemists have been asking them since what is an acid and what is a base was defined in 1887; and what’s more, the debate is still going on. As recently as 2010, the idea of a unified pH scale that was applicable not only to aqueous solutions and solutions in other solvents, but even to the gaseous state and solid state, systems in which traditional pH is nothing, was published in a recognized scientific journal . Must do.
So the next time someone asks you what the pH of the oil is wanting to sound smart and make you fall into a little trap, ask them which of the pH scales they are referring to.
References
- Himmel, D., Goll, SK, Leito, I., & Krossing, I. (2010). A Unified pH Scale for All Phases. Angewandte Chemie International Edition, 49(38), 6885–6888. https://onlinelibrary.wiley.com/doi/epdf/10.1002/anie.201000252
- Rodríguez, A., Ruiz L., Santoyo, S., Miranda, V. (2016). Determination of the acidity index and total acidity of five mayonnaises . Retrieved from http://www.fcb.uanl.mx/IDCyTA/files/volume1/2/10/146.pdf
- Kardash, E., Tur’yan (2005). Acid Value Determination in Vegetable Oils by Indirect Titration in Aqueous-alcohol Media . Croatica Chemica Acta 78(1):99-103 Retrieved from https://www.researchgate.net/publication/279556165_Acid_Value_Determination_in_Vegetable_Oils_by_Indirect_Titration_in_Aqueous-alcohol_Media Kuselman (1998). pH-Metric Determination of the Acid Value of Vegetable Oils without Titration. JOURNAL OF AOAC INTERNATIONAL VOL. 81, No. 4, 1998.
