pH, Buffers, Acids, and Bases


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The lower case p upper case H scale, which ranges from zero to 14, sits next to a bar with the colors of the rainbow. The p H of common substances are given. These include gastric acid with a p H around one, lemon juice with a p H around two, orange juice with a p H around three, tomato juice with a p H around four, black coffee with a p H around five, urine with a p H around six, distilled water with a p H around seven, sea water with a p H around eight, baking soda with a p H around nine, milk of magnesia with a p H around ten, ammonia solution with a p H around 11, soapy water with a p H around 12, and bleach with a p H around 13.
The pH scale measures hydrogen ions’ (H+) concentration in a solution. (credit: modification of work by Edward Stevens)

OpenStax Biology 2e

The pH of a solution indicates its acidity or basicity.

You may have used litmus or pH paper, filter paper treated with a natural water-soluble dye for use as a pH indicator, tests how much acid (acidity) or base (basicity) exists in a solution. You might have even used some to test whether the water in a swimming pool is properly treated. In both cases, the pH test measures hydrogen ions’ concentration in a given solution.

Hydrogen ions spontaneously generate 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, immediately attract to un-ionized water molecules, forming hydronium ions (H3O+). Still, by convention, scientists refer to hydrogen ions and their concentration as if they were free in this state in liquid water.

– What is any chemical compound that, when dissolved in water, gives a solution with a pH of less than 7.0?

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.). One mole represents the atomic weight of a substance, expressed in grams, which equals the amount of the substance containing as many units as there are atoms in 12 grams of 12C. Mathematically, one mole is equal to 6.02 × 1023 particles of the substance. Therefore, 1 mole of water is equal to 6.02 × 1023 water molecules. We calculate the pH 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 a neutral pH. The pH inside of human cells and blood are examples of two body areas where near-neutral pH is maintained.

– What is a measure of the concentration of a chemical species, in particular of a solute in a solution, in terms of amount of substance per unit volume of solution?

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 hydrogen ions’ (H+) concentration 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.

– What is a scientific instrument that measures the hydrogen-ion activity in water-based solutions, indicating its acidity or basicity expressed as pH?

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 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 we place them in water, thereby raising the pH. An example of a weak basic solution is seawater, which has a pH near 8.0 This is close enough to a neutral pH that marine organisms have adapted in order to live and thrive in a saline environment.

– What refers to the tendency of an acid, symbolised by the chemical formula HA, to dissociate into a proton, H+, and an anion, A−.?

The pH scale is, as we previously mentioned, an inverse logarithm and ranges from 0 to 14. 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 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. As a result, how do stomach cells survive in such an acidic environment? How do they homeostatically maintain the near neutral pH inside them? The answer is that they cannot do it and are constantly dying. The stomach constantly produces new cells to replace dead ones, which stomach acids digest. Scientists estimate that the human body completely replaces the stomach lining every seven to ten days.

– What is the main constituent of gastric acid that is produced by parietal cells in the gastric glands in the stomach?

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 body’s pH 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, it removes hydrogen ions and moderates pH changes. Similarly, excess carbonic acid can convert to carbon dioxide gas which we exhale through the lungs. 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 enters 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.

– What is an aqueous solution consisting of a mixture of a weak acid and its conjugate base, or vice versa, and its pH changes very little when a small amount of strong acid or base is added to it?

An upper case H subscript 2 baseline upper case O molecule can combine with an upper case C upper case O subscript 2 baseline molecule to form H subscript 2 baseline C O subscript 3 baseline, or carbonic acid. A proton may dissociate from H subscript 2 baseline C O subscript 3 baseline, forming bicarbonate, or H C O subscript 3 baseline superscript negative, in the process. The reaction is reversible so that if acid is added protons combined with bicarbonate to form carbonic acid.
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.

Source: OpenStax Biology 2e

Other examples of buffers are antacids that some people use 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 who suffer “heartburn” after eating. Water’s unique properties that contribute to this capacity to balance pH—as well as water’s other characteristics—are essential to sustaining life on Earth.

– What is a substance that neutralizes stomach acidity and is used to relieve heartburn, indigestion or an upset stomach?


Clark, M., Douglas, M., Choi, J. Biology 2e. Houston, Texas: OpenStax. Access for free at: