What pH Measures
pH measures how acidic or basic a solution is. The term stands for “potential of hydrogen” or “power of hydrogen.” While commonly known to range from 0 to 14, this applies to most common aqueous solutions.
The scientific basis of pH lies in the concentration of hydrogen ions ([H+]) within a solution. Solutions with a higher concentration of hydrogen ions are acidic, while those with a lower concentration are basic. This relationship is logarithmic, meaning a small change in pH represents a large change in hydrogen ion concentration.
The formula for pH is pH = -log[H+], where [H+] is the molar concentration of hydrogen ions. This logarithmic scale conveniently represents wide ranges of hydrogen ion concentrations. For instance, a pH of 4 is ten times more acidic than a pH of 5, and one hundred times more acidic than a pH of 6.
How Negative pH Arises
The possibility of a negative pH value stems directly from the logarithmic nature of the pH scale. When the concentration of hydrogen ions ([H+]) in a solution exceeds 1 Molar (M), the mathematical calculation of pH = -log[H+] will result in a negative number. For example, if the hydrogen ion concentration is 10 M, the pH would be -log(10), which equals -1.
This phenomenon occurs in extremely concentrated solutions of strong acids. Strong acids are substances that dissociate almost completely in water, releasing a high proportion of their hydrogen ions into the solution. As the concentration of these acids increases significantly, the sheer number of free hydrogen ions can push their molarity above 1 M.
While the standard pH scale is often presented as ranging from 0 to 14, this primarily covers dilute aqueous solutions. Negative pH values, and values greater than 14 for strong bases, are well-established in chemistry. Negative pH simply indicates an exceptionally high concentration of hydrogen ions, exceeding what is typically encountered.
Substances with Extremely Low pH
Extremely low, or even negative, pH values are found in highly concentrated solutions of strong acids. Common examples include concentrated sulfuric acid (H₂SO₄), hydrochloric acid (HCl), and nitric acid (HNO₃). These acids are characterized by their ability to fully dissociate in water, meaning they release nearly all their hydrogen ions when dissolved.
For instance, commercially available concentrated hydrochloric acid can have a pH of approximately -1.1. Similarly, highly concentrated sulfuric acid solutions can also exhibit negative pH values, though their exact pH can vary depending on concentration and other factors. Concentrated nitric acid can also reach significantly low pH levels, with a 10 M solution theoretically having a pH of -1.
While the pH equation provides a theoretical value, the actual behavior of these highly concentrated solutions can be complex due to factors like ion activity and incomplete dissociation at very high concentrations. Nevertheless, these strong acids are capable of pushing hydrogen ion concentration to levels that result in negative pH readings.