Is Acetic Acid A Strong Acid or Weak Acid?

Acids vary from adding a tangy flavor to our food to burning through anything they touch. Knowing the difference is important since the latter can be dangerous to handle. You’d likely find acetic acid in products around the house, but should you be handling it with more care?

Acetic acid is a weak acid as it has a low acid dissociation constant, meaning it is relatively harmless and is safe to handle. Acetic acid is found in several household items, such as vinegar and wood glue, meaning that, to an extent, skin contact is not harmful either.

For the curious, that answer may have raised even more questions than when you started, such as how they calculated an acid dissociation constant or where else they use acetic acid. Wonder no more, as we’ll explore all of these questions!

Measuring Acidic Strength

Chemists developed the acidic strength metric to ensure safety around potentially harmful substances.

Understanding how they measure it involves rather advanced chemistry, but this will help you grasp the difference between a strong acid and a weaker one.

Acid strength refers to acid’s tendency to dissociate into a proton, H+, and an anion, A- (source).

This measure is therefore called the acid’s dissociation constant, represented as Ka. More potent acids have larger Ka and, conversely, smaller logarithmic constants — where pKa= -logKa — than weaker acids (source).

For comparison, one of the strongest measurable acids has a Ka value of 3.2×109, whereas acetic acid, or ethanoic acid, has a value of 1.8×10-5 (source).

Acid Strength in Layman’s Terms

For non-chemists, a simplified explanation would be that a strong acid remains strong when diluted.

Think of diluting different juice concentrates — some brands require you to mix one part juice to three parts water, while others require one part juice to eight parts water.

The stronger the concentrate, the more water you can mix it with while still maintaining its flavor integrity. The opposite is true of weaker concentrates; when someone mixes them with too much water, they just taste watery.

Similarly, the presence of protons and anions allows for a scientific measure of the reaction’s equilibrium after diluting the acid. 

Even after dilution, strong acids remain volatile and pose a serious health risk if not handled with care. For many of the stronger acids, they require storage in a diluted state to limit the risk of catastrophic reactions should there be an accident.

What Purpose Do Weak Acids Serve?

Armed with that knowledge, you may wonder what purpose weak acids serve. In a world where we often prefer bigger, better, and faster, acids operate slightly differently.

On the one hand, strong acids dominate reactions and call for a high level of care. A small mistake could set off a reaction that creates toxic fumes, intense heat, or explosions, making weaker acids easier to manage.

Taking a closer look at types of acids, we note that stronger acids are typically man-made, essentially combining molecules that rarely occur outside of man’s interference.

Weaker acids are mostly naturally occurring substances, and we can find these readily in items such as food or household solutions.

This is an important distinction to make — strong acids are almost exclusively used in laboratories where their reactions are monitored and documented. You can purchase weaker acids in supermarkets, and they have more day-to-day uses.

Make no mistake, though, in a high enough concentration, even weak acids can be lethal. At a certain point of concentration, acetic acid becomes corrosive and can burn your skin.

The Chemical Makeup of Acetic Acid

Acetic acid is a colorless liquid organic compound with the chemical formula CH3COOH, which we can also write as CH3CO2H, C2H4O2, or HC2H3O2 (source).

Another name for it is ethanoic acid, but acetic acid is the most commonly used, though ethanoic acid is the chemical name given to the compound by IUPAC.

As you can see by looking at the elements in its makeup, acetic acid is a simple compound, consisting of just carbon, hydrogen, and oxygen molecules.

It is interesting to see the occurrence of pure oxygen in acetic acid, represented by O2, as we discussed in “Is Oxygen O2 or Just O: Understanding the Molecular Structure of Oxygen.”

It is also easy to see why the scientific community refers to acetic acid as an organic compound. We can find all three elements that make up the acid in abundance in our bodies, the food we eat, and the world around us.

A Quick Comparison to Other Acids

Taking a quick look around the house, you would probably also find citric acid (H3C6H5O7), lactic acid (C3H6O3), ascorbic acid (C6H8O6), and sulfuric acid (H2SO4).

You don’t need a microscope to see that these four are extremely closely related to acetic acid since they share many of the same elements, just varying slightly in how much of each they contain (source).

Citric acids occur naturally in citrus fruits, though its uses in household cleaning products and as a flavoring agent mean two million tonnes are manufactured each year. It is also a weak organic acid.

Lactic acid is found in dairy products. It is ten times more acidic than acetic acid, though still relatively weak.

Ascorbic acid is more commonly known as vitamin C. As many already know, it occurs naturally in food sources, though it is also manufactured as a dietary supplement.

Sulfuric acid is the strongest acid of the lot, dehydrating compounds at high enough concentration levels. Individuals use it most commonly in household drain cleaners, where it puts its corrosive nature to good use in unblocking drains.

It is intriguing to see how a simple tweak in molecular structure can make a completely different compound, even affecting the level of acidity.

The Uses of Acetic Acid

Understanding what happens to acids on a molecular level is one thing, it’s also interesting to look at the various places we use it to further understand its properties. Acetic acid is extremely versatile, as these uses illustrate.

Vinegar

Since vinegar is essentially diluted acetic acid, you may think that vinegar is simply the common name for acetic acid.

Though this an understandable assumption to make, it is technically incorrect since vinegar is mostly water, and concentrated acetic acid is something different.

The dilution level differs between manufacturers, but vinegar should contain no less than 4% acetic acid by volume. Acetic acid is a critical component of vinegar that you can find in all things vinegar-related.

You’d find acetic acid in balsamic vinegar, wine, cider-based vinegar, marinades, mayonnaise, pickled products, cheese, mustard, and ketchup, to name a few.

Photographic Film

Not many people have personally developed film, but you most likely know part of the process is dipping the developing film into liquid baths. This is another moment where acetic acid shines because it stabilizes the reaction that develops film. 

Photography enthusiasts are of the opinion that purer forms of acetic acid, such as glacial acetic acid, which is water-free and 99.5%+ pure acid, deliver better and quicker results.

Other photographers who prefer a more DIY approach know the struggle of getting their hands on something as pure as glacial acetic acid.

They would tell you that you could even get away with using standard white vinegar, relying on the 4%+ acetic acid to do the same job.

Importantly, the acetic acid bath is the second step of the process, with other fluids used in steps one and three to move the film development along to the final product.

As you could imagine, developing film is an incredibly sensitive job; if the film is left in any one of the solutions for too long, it could over- or underdevelop.

They use acetic acid to wash away the developer agent quicker and more thoroughly than water, which is essential for the time-sensitive process.

Not every film photographer uses acetic acid to develop their film, though, since this acid concentration can cause skin issues.

Healthcare professionals recommend that film developers use tongs to move developing film between the acid and a “stop bath” that neutralizes the reaction.

Wood Glue

You may well have a tube of PVA — or polyvinyl acetate for long — sitting in a toolbox at home. This contains acetic acid as well!

First discovered in 1912, polyvinyl acetate goes by a few different names, such as wood glue, white glue, or school glue. They make PVA by the palladium-catalyzed oxidative addition of acetic acid to ethylene.

This chemical bonding on a molecular level is what gives wood glue its holding strength.

It is fascinating to note that, while it undergoes a very specific reaction to become the compound known as wood glue, the chemical formula for polyvinyl acetate — (C4H6O2)n — is still so similar to that of acetic acid — C2H4O2.

It is also interesting to look into how PVA wood glues work since the gluing process is little more than a controlled chemical reaction.

Wood is a porous material, allowing the PVA glue to seep deep into the piece of wood. The glue is water-based, so the majority of the formula evaporates when exposed to air for long enough periods of time.

Evaporation leaves behind a flexible polymer film that forms the bond between the two wood pieces. This film is what you’ll see stuck on your fingertips after working with wood glue.

Manufacturers use the same process to make a bonding agent that extends much further than just carpentry.

Acetic acid also plays a role in creating paper adhesive used in packaging, binding book pages to their spine, drywall primer, cigarette paper, wallpaper, and envelope adhesive.

Fabrics

The textile industry also uses acetic acid, predominantly for dyeing cloth. Getting fabric to be a very specific color requires the various pigments’ exact amounts to make the desired color.

They add acetic acid and some other chemical agents to the batch to transfer the coloring to the fabric during the manufacturing process.

Importantly, they don’t dip garments in acid to color them. Instead, they dye the yarn first before making it into the finished product.

Despite the textile industry’s thoroughness in treating the dyed fabric, someone with incredibly sensitive skin could still become irritated when wearing dyed clothing as the acetic acid leaves a slight acidic residue on the textile.

DIY Household Uses

The list of acetic acid’s uses doesn’t end there. Even when used in some of its alternate forms, such as vinegar, acetic acid still has a wide range of uses around the house as well.

Herbicide

Not every gardener trusts acetic acid as their go-to weed killer, but some swear by it. As an acid, it changes the soil’s pH levels in an attempt to make it inhabitable for weeds. The issue is it sometimes does the same for the plants you’re trying to grow as well!

Multipurpose Cleaner

Vinegar makes a great multipurpose cleaner around the house. Some homeowners even infuse vinegar with other cleaning agents, such as citric acid, to break down greasy fats and bacteria while leaving a fresh, clean smell.

Pickling and Fermentation Substance

Fermenting foods has become popular again, and vinegar plays an important role in this as well. By leaving food in sealed jars with vinegar, a fermentation process begins, which, according to naturalists, unlocks the true potential of the food we eat.

Image by AKuptsova via Pixabay

Medicinal Uses

For the same reasons that vinegar is used as a household cleaner, some also use it to treat mild external infections, such as outer ear infections. It is known to kill bacteria due to its acidic nature, but doctors advise caution!

Final Thoughts

Acetic acid isn’t a strong acid in the scientific sense — its acid dissociation constant value is low. This works in its favor, though; by dissolving in water and being mixed with other chemicals, it has incredibly broad applications.

As with all chemicals, be careful when you handle acetic acid. It is difficult to know how diluted it is, and when in high concentration, it can still cause chemical burns and react with other substances.

Dr. Patrick Capriola

Dr. Patrick Capriola is the founder of strategiesforparents.com. He is an expert in parenting, social-emotional development, academic growth, dropout prevention, educator professional development, and navigating the school system. He earned his Doctorate in Educational Leadership from the University of Florida in 2014. His professional experience includes serving as a classroom teacher, a student behavior specialist, a school administrator, and an educational trainer - providing professional development to school administrators and teachers, helping them learn to meet the academic and social-emotional needs of students. He is focused on growing strategiesforparents.com into a leading source for high-quality research-based content to help parents work through the challenges of raising a family and progressing through the school system.

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