pH and acidity – their difference and importance in vinegar

Reginald SmithAll About Vinegar, Making Vinegar24 Comments

Burette and Ring Stand (courtesy: Wikimedia)

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 CH3COOis 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%.

24 Comments on “pH and acidity – their difference and importance in vinegar”

  1. Evan

    Thank you so much! I am living in China at the moment and I bought a bottle of white vinegar concentrate. Had no idea what 30g/100ml meant. This was the perfect explaination!

    1. Reginald Smith

      Thanks! Be careful when handling 30% vinegar though. Wear gloves since if it gets in your eyes or a cut it will BURN!

  2. Trang

    Thank you so much! But i don’t know that can we drink pineapple vinegar. and how much pH of that product? Thanks!

    1. Reginald Smith

      Is it our pineapple vinegar? That is 5% acidity with a pH of about 2.7. I wouldn’t drink more than 1 tablespoon at a time.

  3. Emma

    Thank you for such an interesting article! We did a food chemistry practical at university, and one of our questions is to analyze the acetic acid content of filtered apple cider vinegar with no preservative, apple cider vinegar with preservative, apple cider vinegar unfiltered and balsamic vineger. How can the presence of a preservative affect the acetic acid content? And how does it affect pH? Thank you so much!

    1. Reginald Smith

      Hi Emma, the presence of preservatives does not affect the acidity (acetic acid content determined by titration). It can affect the pH since the pH depends on many factors besides just the acid concentration. If the preservative–often sulfites or for wine vinegar, sorbic acid from potassium sorbate–create a small buffering effect, you can see the pH differ slightly, usually lower. However, the differences should not be huge. Unfiltered vinegar often has a lower pH since compounds which can create buffers are removed in filtering. Balsamic vinegar can have a higher acidity, around 6%, than apple cider which is typically 5%. I hope this helps.

    1. Reginald Smith

      I would use 5%. It provides the right acidity and is not too strong or too weak to provide preservative actions.

  4. David

    I hated organic chemistry ,physical chemistry, even basic. Never got it. Still don’t understand the periodic table.

    But at 67 I totally understood your “chemistry ahead” explanation.

    Something must have stuck in there from 1974!!!!

    Thanks. Well done!!

  5. Liz

    Hey, thanks so much for the article. I skimmed thru the chem part though…

    I make vinegars at home, all kinds, but mostly the base is from kombucha that has been secondary fermented and has gone past my preferred level of sweetness. I then add a piece of the mother to it and wait. It’s amazing, and I have all kinds of flavors.

    My question for you is how do I dilute them, to give them away to friends? Can I do this with filtered water, and without equipment? I’m sure I have a ph garden tester somewhere…
    I have lots of vinegars and flavors, but don’t need them all a d want to share.

    Also, does the mother lose strength over time? My lastest batch is taking a while to ferment.

    Thanks so much!

    1. Reginald Smith

      You can dilute them with tap water, it won’t affect subsequent fermentation. I would recommend not diluting the raw vinegar and mother you give away. Keeping it at full strength makes their own fermentations go faster and prevents mold growth.

      If you are using pH as a guide for dilution I would not hand out any raw vinegar with a pH higher than 3.5.

      Mothers don’t typically lose strength over time. A long fermenting batch may mean not enough nutrients (doubtful with kombucha but add a few ounces of sweet tea or wine to kickstart it) or just a slower fermenting strain of bacteria. Also, check the pH. Sometimes the smell can be deceiving and less pungent than typical for the acidity.

  6. Winfred Tan

    Hi.

    If the pH Level is 3.33 and the titratable acidity is 9%, does that mean the vinegar is extremely acidic?

    1. Reginald Smith

      9% acidity is very acidic. The most acidic wine vinegars are typically only 7% max. On the other hand, 3.33 is a rather high pH for that much acidity. What type of vinegar are you making? I would expect a pH of 2.4 to 2.7. For use, dilute with 2 parts vinegar, 1 part water to get about 6% acidity.

  7. Sal

    Hello, I purchased a drum of 12% vinegar, can I dilute it to be used in regular household applications such as cleaning or in sauces (to consume etc.) ? Woud I dilute with regular drinking water or distilled water?

    Thank you,
    Sal.

    1. Reginald Smith

      Yes, if you want to dilute it to regular strength vinegar you can just add regular drinking water (no need for distilled) to get it down to 5% which is standard acidity (1.2 parts water to 1 part vinegar just to give you margin for error). If you want to make it easy, 6% is fine too and you can dilute 1 part vinegar with 1 part water. Then it can be used in household applications or sauces, etc.

  8. John

    Mr. Smith, how much vinegar would need to be added to one liter of water at a pH level of 14 to reduce the pH to a level of 9, please?

    1. Reginald Smith

      This is a harder question that it seems. You first need to specify the strength of the vinegar (i.e. 5%) and the type (like white distilled). White distilled is the easiest since it is close to pure acetic acid. Review the equations in the post and you can derive that pH (which is log_10 [H30+]) can be solved by

      pH = -(1/2)*log_10 (Ka*[CH3COOH])

      The level of [H30+] already in the water is 10^(-14). To be honest this is small so just neglect it.

      The concentration of acetic acid in moles in the water + vinegar is calculated by (vinegar acidity*volume in L vinegar added*10)/(1L water + volume of vinegar added in L)

      So solve [CH3COOH] in the equation above assuming a pH of 9 and then use this to calculate the volume of vinegar you need to add.

  9. Luanne Steele

    Hi, I’m from Canada originally and my mum used pickling vinegar 7% acidity to can with. I now live in Kentucky and found a vinegar with 6%, which is better than plain white vinegar; but would refer the 7%. Is there any way to bring the 6% I can buy up to the 7% acidity I want? Thanks so much! Luanne

    1. Reginald Smith

      Hi, the easiest thing would be to add concentrated vinegar to raise the acidity.You can buy the German 25% acidity vinegar Essig Essenz on Amazon. One 13 oz bottle should be able to get you 2 gallons of 7% vinegar. Mix 120 ounces (15 cups) of 6% vinegar with 6.5 oz of Essig Essenz to get 1 gallon 7% vinegar.

      CAUTION: Use gloves and safety goggles when handling concentrated vinegar like Essig Essenz, it will burn hard if it touches your eyes or skin.

  10. Fatima

    Hi, I made a honey vinegar in a factory and I measured its acidity. Unfortunately, its acidity is 1.33 % and I know its too low. I need to increase the acidity until 5.5%. Also, it is really important for me to make organic honey vinegar. Please help me to find a way for making it.

    1. Reginald Smith

      Hi, I would need a lot more information to determine the issue. If you are fermenting it slow and it has not been fermenting for a long time, it could just be slow progress in the fermentation. As far as organic, you need to use organic honey as a starter and approved sanitizing chemicals per your local organic authority.

    1. Reginald Smith

      I am sorry, I am not pickling expert but I know you should use at least 5% vinegar. I would recommend a fermentation blog like phickle.com.

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