The Spectrum of H+: Exploring the pH Scale and Its Diverse Applications

The pH scale is simultaneously one of the most ubiquitous ideas in chemistry yet one of the least explained. This article takes a deep dive into the pH scale: how it came to be, how it works, and how it applies to life in fields like medicine and agriculture.

Though the idea of compounds being “acidic” existed in simple forms as early as the era of the ancient Greeks, it was not until the 1700s that scientific interest in understanding the nature of acids and bases truly began. Researchers began their inquiry by experimenting to determine what substances, if any, formed the underlying properties of acids. Initially, it was thought that oxygen could be responsible; however, not all acids contain oxygen. As later researchers eventually discovered, the only element all acids have in common is hydrogen—specifically, positively charged hydrogen ions (H+), which are released in one way or another when acids are added to water. Bases were originally understood as compounds that neutralized acids, but the definition was narrowed down later on: bases are substances that release an abundance of hydroxide ions (OH-) when added to water.

With a rudimentary understanding of acids and bases established, all that remained was to develop a system to smoothly categorize them. This was accomplished in 1909 by the Danish chemist Søren Peder Lauritz Sørenson. Sørenson designed a simple logarithmic formula that quantified the concentration of hydrogen ions in a solution as follows:

pH =  -log10[H+]

“[H+]” is replaced by the number of hydrogen ions in the solution, represented in molar form.

This equation gave way to the pH scale, the fundamental measurement used scientifically to determine how acidic or alkaline (basic) a substance is. The scale is centered at seven, representing a completely neutral pH which is the pH value of pure water. Solutions with a higher concentration of hydrogen ions than pure water (acids) result in a lower output value from the pH formula; therefore, any pH value below seven is acidic. pH values above seven are alkaline because their lower concentration of hydrogen ions results in a higher output value from the formula. The pH scale varies between zero and fourteen.

Pure water, the substance which is neutral on the pH scale, in molecular form is a structure with two hydrogen atoms and one oxygen atom—H2O. These two basic elements each carry their own charges, with the oxygen atom having a greater electronegative charge than the hydrogen atoms. Because of the difference in charges, electrons are drawn more towards the oxygen atom, creating the polarity in water. The polarity of water facilitates the dissociation of water into two ions: H+ (in the form of H3O+) and OH-. At any given moment, about two water molecules in every billion molecules will lose an electron to another water molecule, a reversible change, which produces the charged ions H3O+ and OH-. A room temperature sample of water contains approximately 1 × 10-7 hydrogen ions per Mole that have been “liberated” from their original water molecule form, delineating why water has a pH of seven.

Since the separated ions in water are easily able to reunite, water’s electrical and chemical properties are very neutral. However, the addition of other substances immediately changes such neutrality. Operational definitions for acids and bases rely on what happens when a substance is added to water. If a substance reacts with the water molecules in such a way that the concentration of hydrogen ions in the solution increases, it is deemed an acid. If the substance reacts with the water molecules in a way that captures hydrogen ions, thus increasing the concentration of hydroxide ions in the solution, it is called a base. 

When carbon dioxide diffuses into water, some of the carbon dioxide molecules will react with the water molecules to form carbonic acid in a reversible reaction– CO2 + H2O ⇌ H2CO3.

However, since the carbonic acid exists in a solution full of polar water molecules, it is quicker to dissociate into ions instead. In particular, a hydrogen ion and a bicarbonate ion are released as indicated in the reaction H2CO3 ⇌ HCO3- + H+. This reaction is also reversible so that the hydrogen ions can be reclaimed, but the overall effect of adding carbon dioxide to water increases the hydrogen ion concentration of the solution, which is why carbonic acid is considered an acid. 

On the other end of the scale, when ammonia is added to water, ammonium hydroxide is formed. The chemical equation for the reaction is NH3 + H2O ⇌ NH4OH. Similar to the reaction with carbon dioxide and water, the product in this reaction, ammonium hydroxide, dissociates into ions when placed in water as expressed through the equation NH4OH ⇌ NH+ + OH-. When ammonium hydroxide dissociates, it releases a hydroxide ion. Hydroxide ions very readily combine with any hydrogen ions present to form water, reducing the hydrogen ion concentration of the solution, which is why ammonia is considered a base.

Carbon dioxide, carbonic acid, and ammonia are examples of weak acids and bases. On the chemical level, acids and bases go through very similar processes when added to water. The way that they react with water results either in a dissociation full of hydrogen ions or a dissociation full of hydroxide ions. Hydrogen ions by themselves are the basis for acidification, and hydroxide ions “soak up” loose hydrogen ions to make their solutions more basic. On the other hand, buffers also play an important role in the fundamentals of pH. 

Buffers are pairs of weak acids or weak bases and their conjugates, or the acids and bases that “match” them in such a way that they exist in equilibrium. When other acids or bases are added to a solution, the existence of acids and bases in the buffer can neutralize them. This stabilizing property extends until either the acid or base in the buffer is depleted; upon reaching depletion, the pH is subject to sharper increases and decreases because it is less resistant to change. 

Additionally, the application of acids and bases is global. The way acids and bases interact has a myriad of impacts on day-to-day lives. 

Medicine– The Delicate Balance of Our Blood

Like any aqueous solution or solution where the solvent is water, blood has a known pH in a very narrow range between 7.35 and 7.45. Various buffering systems exist in the body to maintain this ideal range. The cardiopulmonary systems, for example, help manage the acidity of blood by increasing or decreasing the rate at which carbon dioxide is expelled. Carbon dioxide is a byproduct of cellular respiration that circulates constantly in the bloodstream, which has a slight acidifying effect in water-based blood. Carbon dioxide is a crucial part of the body’s buffer system; however, when substances make blood too acidic, the body can compensate by expelling more carbon dioxide than usual. Carbon dioxide is naturally expelled through breathing, so an increase in respiration rate combined with an increased heart rate to bring as much carbon dioxide to the lungs as possible can help free up the bases present in bodies to address the increased presence of acid.

Disorders of blood pH can yield serious consequences, and when the body’s natural buffering systems fail, medical experts must harness the properties of acids and bases to address the imbalance.  Failing to do so can have severe effects on the body, including coma and death.

In the medical field, disorders of the blood pH are sorted into four different categories: respiratory alkalosis, respiratory acidosis, metabolic alkalosis, and metabolic acidsosis. The respiratory version occurs when a person, for any reason, breathes too much or too little. Breathing too much results in a great deal of carbon dioxide being expelled. If there is not an excess of acid present in the body, an increased rate of breathing can tip the blood’s balance towards being too basic. Breathing too little, on the other hand, has the opposite effect; not enough carbon dioxide is expelled, so the blood becomes more acidic.  

While respiratory alkalosis and acidosis can certainly cause problems or be a sign of more serious issues, medical professionals are typically more concerned with metabolic acidosis and alkalosis, conditions where the body’s balance is thrown off. Substances enter the body and acidify or alkalize the blood. These sorts of changes are often more drastic than changes caused by respiratory alkalosis and acidosis, which lead to more dangerous side effects. Causes for metabolic acidosis and alkalosis can be external such as overdosing on certain medications like antacids or laxatives or internal such as medical conditions like diabetes or dehydration. In order to appropriately treat metabolic pH imbalances, doctors must be able to determine the root cause and address it appropriately, though adding weak acids or bases to the blood intravenously can help stabilize the situation in the interim.

Agriculture and Horticulture — pH-Sensitive Plants

On the other hand, plants are just as susceptible to pH changes as humans are. Plants require very particular environmental conditions to grow, and the presence of too much acid or base in their surroundings can throw off their ideal balance. High soil acidity, in particular, tends to be a more common issue, and it can have a devastating effect on agriculture if left unchecked.

Most plants thrive in slightly acidic conditions, in soil with a pH range of five to six. If the soil becomes more acidic than that, however, vital nutrients like nitrates, potassium, and magnesium are leached away as they react with the prevalent hydrogen ions. The acidic conditions also make metals like aluminum, iron, and zinc more available, which are toxic to many plants. High soil acidity can be a byproduct of how land is managed for agricultural use. Too much of certain fertilizers, a lack of natural plant and animal wastes, or an overabundance of decaying plant material can have an acidifying effect on the soil. Soil acidity can also simply result from the geology of the surrounding land and local water conditions. In either case, farmers who want their crops to thrive must remedy the imbalance by neutralizing the acid with some sort of base. Lime, a strong base containing calcium oxides and hydroxides derived from rocks like limestone, is the traditional remedy.

While a plant’s ability to thrive is the primary application of pH, the way some plants look is also impacted by pH. Certain species of hydrangea, for example, will change the color of their flowers according to the pH of their soil: a natural litmus test! In standard, slightly acidic conditions, their blooms will be blue. In slightly alkaline conditions, on the other hand, their flowers will be pink or even red. Neutral pH can produce purple flowers or cause a mosaic of pink and blue. Gardeners interested in a particular shade of hydrangea can alter the pH of the soil with lime to alkalize the soil or sulfur which acidifies the soil.

Medicine and agriculture are just two of many fields that use the properties of acids and bases.  Though it is easy to relate the pH scale to a chemistry lab, acids and bases are all around. Understanding how they work is handy in more ways than one might expect.

*This article was originally published on the Stem Explorers (STEMx) website www.stemexplorers.net and submitted by the author to Broncology.

References

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