# pH, Buffers, Acids, and Bases

## Acids release H+ while bases release OH-; consequently, buffers maintain a system's required pH if exposed to these solutions.

#### Key Points

• Alkaline substances have a pH above 7.0, while acidic substances have a pH below 7.0.

• Since the body requires near-neutral pH for survival, buffers absorb excess H+ ions or OH- ions to maintain the desired pH.

• The negative of the log (base 10) of the concentration of H+ ions is used to calculate pH; pH = - log10 (concentration of H+).

#### Terms

• A solution used to stabilize the pH of a liquid.

• Having a pH greater than 7.

• Having a pH less than 7, or being sour, or having the strength to neutralize alkalis, or turning a litmus paper red.

#### Figures

1. ##### The pH Scale

The pH scale measures the concentration of hydrogen ions (H+) in a solution.

2. ##### Buffers in the Body

This diagram shows the body’s buffering of blood pH levels. The blue arrows show the process of raising pH as more CO2 is made. The purple arrows indicate the reverse process: the lowering of pH as more bicarbonate is created.

### pH, Acids, and Bases

The pH of a solution indicates its acidity or alkalinity. The pH scale is an inverse logarithm and ranges from 0 to 14 (Figure 1). Anything below 7.0 (ranging from 0.0 to 6.9) is acidic, and anything above 7.0 (from 7.1 to 14.0) is alkaline. Extremes in pH in either direction from 7.0 are usually considered inhospitable to life. The pH inside cells (6.8) and the pH in the blood (7.4) are both very close to neutral. However, the environment in the stomach is highly acidic, with a pH of 1 to 2.

Non-neutral pH readings result from dissolving acids or bases in water. Using the negative logarithm to generate positive integers, high concentrations of hydrogen ions yield a low pH number, whereas low levels of hydrogen ions result in a high pH. An acid is a substance that increases the concentration of hydrogen ions (H+) in a solution, usually by having one of its hydrogen atoms dissociate. A base provides either hydroxide ions (OH–) or other negatively-charged ions that combine with hydrogen ions, reducing their concentration in the solution and thereby raising the pH. In cases where the base releases hydroxide ions, these ions bind to free hydrogen ions, generating new water molecules.

The stronger the acid, the more readily it donates H+. For example, hydrochloric acid (HCl) completely dissociates into hydrogen and chloride ions and is highly acidic, whereas the acids in tomato juice or vinegar do not completely dissociate and are considered weak acids. Conversely, strong bases are those substances that readily donate OH– or take up hydrogen ions. Sodium hydroxide (NaOH) and many household cleaners are highly alkaline and give up OH– rapidly when placed in water, thereby raising the pH. An example of a weak basic solution is seawater, which has a pH near 8.0, close enough to neutral pH that marine organisms adapted to this saline environment are able to thrive in it.

Hydrogen ions are spontaneously generated in pure water by the dissociation (ionization) of a small percentage of water molecules into equal numbers of hydrogen (H+) ions and hydroxide (OH-) ions. While the hydroxide ions are kept in solution by their hydrogen bonding with other water molecules, the hydrogen ions, consisting of naked protons, are immediately attracted to un-ionized water molecules, forming hydronium ions (H30+). By convention, scientists refer to hydrogen ions and their concentration as if they were free in this state in liquid water.

The concentration of hydrogen ions dissociating from pure water is 1 × 10-7 moles H+ ions per liter of water. Moles (mol) are a way to express the amount of a substance (which can be atoms, molecules, ions, etc), with one mole being equal to 6.02 x 1023 particles of the substance. Therefore, 1 mole of water is equal to 6.02 x 1023 water molecules. The pH is calculated as the negative of the base 10 logarithm of this concentration. The log10 of 1 × 10-7 is -7.0, and the negative of this number (indicated by the “p” of “pH”) yields a pH of 7.0, which is also known as neutral pH. The pH inside human cells and blood are examples of two areas of the body where near-neutral pH is maintained.

### Buffers

So how can organisms whose bodies require a near-neutral pH ingest acidic and basic substances (a human drinking orange juice, for example) and survive? Buffers are the key. Buffers readily absorb excess H+ or OH, keeping the pH of the body carefully maintained in the narrow range required for survival. Maintaining a constant blood pH is critical to a person’s well-being. The buffer maintaining the pH of human blood involves carbonic acid (H2CO3), bicarbonate ion (HCO3), and carbon dioxide (CO2). When bicarbonate ions combine with free hydrogen ions and become carbonic acid, hydrogen ions are removed, moderating pH changes. Similarly, excess carbonic acid can be converted to carbon dioxide gas and exhaled through the lungs (Figure 2). This prevents too many free hydrogen ions from building up in the blood and dangerously reducing the blood’s pH. Likewise, if too much OH is introduced into the system, carbonic acid will combine with it to create bicarbonate, lowering the pH. Without this buffer system, the body’s pH would fluctuate enough to put survival in jeopardy.

Other examples of buffers are antacids used to combat excess stomach acid. Many of these over-the-counter medications work in the same way as blood buffers, usually with at least one ion capable of absorbing hydrogen and moderating pH, bringing relief to those that suffer “heartburn” after eating. The unique properties of water that contribute to this capacity to balance pH (as well as water’s other characteristics) are essential to sustaining life.

#### Key Term Glossary

acid
a substance with a pH less than 7, or being sour, or having the strength to neutralize alkalis, or turning a litmus paper red
##### Appears in these related concepts:
acidic
Having a pH less than 7, or being sour, or having the strength to neutralize alkalis, or turning a litmus paper red.
##### Appears in these related concepts:
alkaline
Having a pH greater than 7.
##### Appears in these related concepts:
base
any of a class of generally water-soluble compounds, having bitter taste, that turn red litmus blue, and react with acids to form salts
##### Appears in these related concepts:
basic
Of or pertaining to a base; antonym of acidic
##### Appears in these related concepts:
bicarbonate
In inorganic chemistry, bicarbonate (IUPAC-recommended nomenclature: hydrogen carbonate) is an intermediate form in the deprotonation of carbonic acid.
##### Appears in these related concepts:
blood
A vital liquid flowing in the bodies of many types of animals that usually conveys nutrients and oxygen. In vertebrates, it is colored red by hemoglobin, is conveyed by arteries and veins, is pumped by the heart, and is usually generated in bone marrow.
##### Appears in these related concepts:
buffer
A solution used to stabilize the pH of a liquid.
##### Appears in these related concepts:
carbon
The chemical element (symbol C) with an atomic number of 6.
##### Appears in these related concepts:
carbon dioxide
the normal oxide of carbon, CO2; a colorless, odorless gas formed during respiration and combustion and consumed by plants during photosynthesis
##### Appears in these related concepts:
carbonic acid
A weak unstable acid, H2CO3, known only in solution, and as carbonate salts; it is present in carbonated drinks, and sparkling wine, but decomposes to form carbon dioxide and water.
##### Appears in these related concepts:
cell
The basic unit of a living organism, consisting of a quantity of protoplasm surrounded by a cell membrane, which is able to synthesize proteins and replicate itself.
##### Appears in these related concepts:
concentration
The proportion of a substance in a mixture.
##### Appears in these related concepts:
dissociation
The process by which a compound body breaks up into simpler constituents; said particularly of the action of heat on gaseous or volatile substances.
##### Appears in these related concepts:
hydrogen
The lightest chemical element (symbol H) with an atomic number of 1 and atomic weight of 1.00794.
##### Appears in these related concepts:
hydrogen ion
a proton combined with one or more water molecules; usually written H3O+
##### Appears in these related concepts:
ion
An atom, or group of atoms, bearing an electrical charge, such as the sodium and chlorine atoms in a salt solution.
##### Appears in these related concepts:
lung
A biological organ that extracts oxygen from the air.
##### Appears in these related concepts:
mole
in the International System of Units, the base unit of amount of substance
##### Appears in these related concepts:
molecule
The smallest particle of a specific compound that retains the chemical properties of that compound; two or more atoms held together by chemical bonds.
##### Appears in these related concepts:
organism
a discrete and complete living thing, such as an animal, plant, or fungus
##### Appears in these related concepts:
pH
In chemistry, pH is a measure of the activity of the (solvated) hydrogen ion. p[H], which measures the hydrogen ion concentration is closely related to, and is often written as, pH.
##### Appears in these related concepts:
proton
Positively charged subatomic particle forming part of the nucleus of an atom and determining the atomic number of an element.
##### Appears in these related concepts:
solution
a homogeneous mixture, which may be liquid, gas or solid, formed by dissolving one or more substances
##### Appears in these related concepts:
stomach
an organ in animals that stores food in the process of digestion
##### Appears in these related concepts:
strong base
A strong base is a basic chemical compound that is able to deprotonate very weak acids in an acid-base reaction.
weak acid
A weak acid is an acid that dissociates incompletely. It does not release all of its hydrogens in a solution, donating only a partial amount of its protons to the solution.
work
A measure of energy expended in moving an object; most commonly, force times distance. No work is done if the object does not move.