Those who know vinegar know acidity is of prime importance in measuring the completion and strength of vinegar. Acetic acid is what distinguishes vinegar and global standards of taste and safety specify minimum acidity levels for vinegar. The more widely recognized measurement of the strength of an acid (or base) is the pH scale. Many people know pH is important for controlling microbes and a maximum level of 4 (under 3.7 is better) is required for any application where rogue microbes could cause issues in food or sauce preservation. Please note you should consult local and FDA regulations as well as acidified food guidelines before settling on a pH and water activity for any preserved food.
The most easily measured value is pH. Everything from $10 meters to professional meters like my own that are hundreds of dollars can be used to give an instant pH reading. Acidity measurements are more technical and careful. The typical method is titration using a strong base like sodium hydroxide (NaOH) to determine the percent acid in vinegar. Percent acid is defined as the number of grams of acetic acid per 100 mL of vinegar. So the 5% vinegar you buy in the store has 5 g of acetic acid per 100 mL (or 50g per L). Vinegar makers occasionally use the term ‘grain’ which is just the acidity multiplied by 10 so 5% acidity is 50 grain.
Many people default to pH to see how done their vinegar is given the difficulty of measuring acidity which requires specialized chemicals and lab equipment like a burrette and ring stand. There are also basic chemistry calculations required that, while not difficult, can be daunting for some. For measuring the basic progress of vinegar and its microbial safety, pH is acceptable. However, for using homemade vinegar for canning and for selling vinegar commercially, pH can be deceptive and even dangerous.
First, canning requires a minimum recommended acidity (typically 5%) because the dilution of the vinegar in recipes still requires a minimum level of acid. Remember pH is a logarithmic scale. An acid pH of 3 is 10 times more acidic than that of a pH of 4.Vinegar only measured by pH risks being too low in acidity and can be diluted too much which is dangerous for canning where preventing botulism and other bugs is paramount. As will be explained later, pH cannot replace acidity because the pH can vary widely for different types of vinegar of the same acidity.
To understand their difference, let’s look at how they are calculated. Warning – chemistry ahead.
First pH. An acid is a chemical compound that contains a positive charged hydrogen ion (H+) combined with a negative charged so-called “conjugate base”. In water, both parts dissociate and the H+ concentration is what defines pH. The H+ ion, combines loosely with water to make an ion called Hydronium H30+ whose concentration is often used in lieu of H+ in equations and pH calculations.
The formula for acetic acid is CH3COOH and CH3COO– is the conjugate base.
For acetic acid, dissociation in H2O yields
CH3COOH + H2O → CH3COO– + H30+
Now all of the reactants (left side) and products (right side) have equilibrium concentrations in the solution. The concentration of a chemical, in terms of moles/liter, is designated with square brackets. So the concentration of acetic acid is [CH3COOH]. At standard temperature (25 C) and pressure (1 atm) the equilibrium constants for acid dissociation (acid dissociation constant) Ka determines the relative concentrations in equations like the below:
Ka = [CH3COO–][H30+]/[CH3COOH]
Ka is usually calculated to neglect the water in the reactants. The pH is the negative logarithm (base 10) of the H30+ concentration, pH=-log10 [H30+]. You often see Ka shown as pKa where pKa =-log10Ka. For acetic acid, Ka and pKa are 1.76 x 10-5 and 4.75 respectively at standard temperature and pressure.
So for example, let’s take 5% acetic acid like the standard grade sold in retail stores. 50 g per liter of acetic acid where the molar mass of acetic acid is 60 g means these vinegars are 0.83M (M stands for molar or moles/liter). Given Ka and the fact that 1 mole of CH3COO– is generated per mole of H30+ in the reaction we can see that the concentration [H30+] is 3.8 x 10-3 M and pH should be 2.4.
On the other hand, when you mesaure acidity you are titrating vinegar with a base until you find out what volume of base makes all the acetic acid disappear. The H30+ concentration or acid dissociation constant has little relevance except in how fast the titration occurs.
So why aren’t they interchangeable in some nifty formula? Here is the deal: take two different acids with the same acidity in g/100 mL. So 5% vinegar and 5% hydrochloric acid (HCl). First, their pH levels are different because 1) the molar mass of each acid is different so their molar concentrations vary at the same acidity and 2) their acid dissociation constants vary so different amounts of [H30+] come out in equilibrium.
But there are still complications even if we have the same acid, as in different types of vinegar. I know you are saying, “OK acidity and pH aren’t the same and pH varies for different acids but a pH of 2.4 is equivalent to a 5% acidity acetic acid, right?”
Well, not quite. Even though white distilled vinegar approaches this pH level at 5%, no vinegar gets that low. The main reason is most natural vinegars have many other compounds in the vinegar, including organic acids and other exotic compounds. Some even have small amounts of bases (for example many fruits) and these help increase the ‘buffering capacity’ of the vinegar. A buffer is a mixture of an acid and its conjugate base in proportions that resist pH changes with added acid or base. Finally, there is a reaction called esterification where the acetic acid reacts with leftover ethyl alcohol in the vinegar to form flavor chemicals called esters. The main one is ethyl acetate and this is present in all vinegars. The small levels of other organic acids like formic acid and tartaric acid (in grapes) also form their own esters. These reactions consume acetic acid. This isn’t a bad thing since the amount is usually not large and the development of esters in the aging process helps make vinegar less sharp.
So what you end up seeing are pH levels that are wildly different for vinegars of the same acidity. White distilled vinegar of 5% can range from a pH of 2.5 to 2.7 on average. Pineapple vinegar ranges from 2.8 to 2.9. Red and white wine vinegar can be low, 2.6 to 2.8 but this is helped by the other acids like tartaric acid from grapes. The highest is apple cider vinegar which is typically 3.3 to 3.5 at 5%. It is also one of the chemically more complex vinegars.
So the bottom line is pH and (titratable) acidity both have importance but are not interchangeable or even predictable across vinegars. If you are making the same vinegar from roughly the same raw material over and over, there may be a relation that can be worked out but it will be hard to generalize. So if you are doing canning with home vinegar, make sure you measure the acidity yourself or send it to a local wine lab or university food lab for measurement. Definitely if you want to sell your vinegar you have a legal requirement to make sure the acidity exceeds 4%.