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As was previously reported by the news department at www.chapelboro.com, on July 8th, 10 people and 3 pets who reside at Pine Gate Apartments in Chapel Hill received chemical burns when they came in contact with an alkali-based paint removal product called Stripper Cream. As I was following the incident, I was reminded that popular culture teaches us to be afraid of acids, which is certainly appropriate, but generally overlooks the fact that alkali burns can be far worse. So let me explain why the high end of the pH range (strong bases) can be far more hazardous than the low end (strong acids).

To understand what happened at Pine Gate, we need to review the science of solutions, as well as some details of acid and base chemistry. As you probably know, water is a great solvent. Often when materials are dissolved in water, they are completely disassembled into their constituent atoms. For example, table salt (NaCl) contains sodium and chlorine atoms which are bonded together in a crystal structure. If you immerse table salt in water, the crystal structure is ripped apart into individual sodium and chlorine atoms that now float around in the water on their own. In the process of being torn asunder by the water, the sodium and chlorine atoms acquire an electric charge. The sodium atoms end up with one fewer negatively-charged electrons than positively-charged protons, leaving them with a net positive charge of +1. The chlorine atoms end up with one too many electrons, so they acquire a net negative charge of -1. These dissolved, charged atoms are now referred to as ions. Notation for ions uses brackets and superscripted plus and minus signs. Thus, the notation used for the sodium and chloride ions are [Na+] and [Cl]. I have reviewed the science in some detail here because the concept of an ion having one too many or one too few electrons to maintain a neutral electric charge is key to understanding why acids and bases are hazardous.

Water, H2O, is such a good solvent that, to a degree, it dissolves itself. In pure water, the vast, vast majority of the water exists as intact water molecules. But a small number of the water molecules are, in effect, dissolved by the intact water molecules and thus dissociated into a hydrogen ion with a positive charge [H+] and a hydroxyl ion [OH] with a negative charge. In neutral water, which has a pH of 7, there are an equal number of [H+] and [OH] ions. (1)  Acids and bases are created when you dissolve something in water that throws off the balance of the [H+] and [OH] ions. If you end up with more [H+] than [OH], you have an acid. If you end up with the reverse, you have a base. Please note that “base”, “caustic”, and “alkali” are all synonyms.

Let’s walk through the chemistry of making two familiar acids and bases, hydrochloric acid and sodium hydroxide solution. In its normal state, hydrogen chloride (HCl) is a gas and is electrically neutral. When you dissolve HCl in water, it is disassembled into [H+] and [Cl] ions. This results in there being more [H+] than [OH] in the water, so the solution is now acidic.   Acids are named based on the source of the [H+] ions, thus this solution is called hydrochloric acid. Sodium hydroxide (NaOH) in its normal state is a solid crystalline salt with a neutral electric charge. When you dissolve it in water you create [Na+] and [OH] ions, so now you have a base. For some reason lost to history, bases don’t have a sensible naming system like acids. Therefore, solutions of NaOH are often referred to as “caustic”, but so are other bases as well, which can result in confusion.

To understand why acids are highly chemically reactive and therefore dangerous, let’s anthropomorphize a bit. Since the [H+] ions in an acid are short one electron from being neutral, they really, really “want” to steal an electron from some other molecule or atom. So if an acid is put into contact with a substance from which it can appropriate electrons, it will do so. If it is a strong acid, pH<4, this reaction can be rather dramatic. In bases, the [OH] ions have an extra electron that they really, really “want” to get rid of. Therefore, if a strong base, pH >10, contacts a material to which it can donate electrons, it will do so enthusiastically.

The explanation above was a bit longer and a bit more technical than I had intended, so here is the brutally short summation. Acids are chemically reactive because they are trying to extract electrons from other substances and bases are reactive because they want to donate electrons to other substances. As I will discuss below, the reason that bases (alkalis) are more dangerous to people than acids stems from the fact that electron-donating chemical reactions cause more damage to you skin cells that electron-accepting reactions.

Your skin is primarily comprised of proteins and fats (triglycerides). Proteins are long chain polymers of amino acids which wrap around themselves into specific and intricate three-dimensional configurations. When the configuration of a protein is disrupted by heat or a chemical reaction, this is referred to as denaturation. Denaturing a protein totally disrupts is shape and function. The graphic below can help you to visualize the process.

Protein naturation

 

The reason that scrambled eggs turn from liquid to solid is that the heat from the stove is denaturing the proteins.

Proteins can be chemically denatured through both electron-donating and electron-accepting reactions. Therefore, both acids and bases can denature the proteins of your skin cells. Given that this process is akin to turning your skin cells into scrambled eggs, it’s easy to understand why getting a chemical burn in this manner is excruciatingly painful and also can result in significant scarring. If the burn is caused by an acid, then the denatured proteins remain in place and effectively form a protective barrier against deeper skin damage. However, if the burn is from a base then you’ve got an additional problem, a big one.

Bases decompose fats (triglycerides) through a chemical reaction called saponification. Below is a graphic of the chemical reaction which occurs.

saponification

As you can see saponification shreds the fat (triglyceride) molecule into four separate pieces, which transforms them from solid to liquid. Saponification is the first step in the process used to render animal and vegetable fats into soaps and glues. As the fats are turned to liquids they flow away from the site of the burn which exposes new skin to further injury. This s allows a chemical burn from an alkali to burrow deeply into the skin in a process known rather gruesomely as liquefaction necrosis and explains why chemical burns from a base are often far worse than from an acid.

The Stripper Cream used at Pine Gate Apartments has a pH of 13.5 which makes it extremely hazardous. I am frankly quite surprised that it is permissible to use a product such as this in a public environment like an apartment complex. Given that it is permissible, the residents of Pine Gate Apartments should have been given far better information on the hazards present to themselves and their pets. Better yet, a less hazardous paint removing approach should have been considered. I hope that it at least the new paint applied after stripping was completed looks nice.

Listen to Jeff’s conversation about this column with WCHL’s Aaron Keck.

 

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  1. The pH of a solution is a measure of the concentration of [H+] ions and is a logarithmic scale that runs from 1 to 14. Low numbers correspond to strong acids and high numbers to strong bases.   As such, a solution with a pH of 6 has an [H+] concentration 10 times higher than a solution with a pH of 7 and so on. Therefore, an acid with a pH of 1 has an [H+] concentration one million times higher than neutral water. Further, it is important to understand that the concentrations of [H+] and [OH] work like a see-saw; when one goes up the other goes down.   Therefore, a solution with a pH of 14 has a million times more [OH] ions than neutral water and a million times fewer [H+].