The pH Scale

What you’ll learn to do: Demonstrate familiarity with the pH scale

Most people are familiar with the words acid and acidic—whether it’s because of acid rain or acidic foods like lemon juice. However, fewer people are aware of acid’s opposite: base (also called alkaline). Basic substances include things like baking soda, soap, and bleach. Distilled water is a neutral substance. The pH scale, which measures from 0 to 14, provides an indication of just how acidic or basic a substance is.

Most parts of our body (excluding things like stomach acid) measure around 7.2 and 7.6 on the pH scale (a 7 is neutral on the scale). If foreign strong substances dramatically change this pH, our bodies can no longer function properly.

In this outcome, we’ll learn about acids and bases, and what impact they can have on living systems.

Learning Outcomes

  • Identify the characteristics of acids
  • Identify the characteristics of bases
  • Define buffers and discuss the role they play in human biology

The pH scale ranges from 0 to 14. The pH of a solution is a measure of its acidity or alkalinity (base). You have probably used litmus paper, paper that has been treated with a natural water-soluble dye so it can be used as a pH indicator, to test how much acid or base (alkalinity) exists in a solution. You might have even used some to make sure the water in an outdoor swimming pool is properly treated.

The pH scale measures from 0 to 14, with 0 being acidic, 7 being neutral, and 14 being basic. Along the scale, example substances are given for each pH scale unit. Gastric acid measures as a 1. Lemon juice measures as a 2. Orange juice measures as a 3. Tomato juice measures as a 4. Black coffee measures as a 5. Urine measures as a 6. Distilled water measures as a 7 (a neutral pH). Seawater measures as an 8. Baking soda measures as a 9. Milk of magnesia measures as a 10. An ammonia solution measures as an 11. Soapy water measures as a 12. Bleach measures as a 13.

Figure 1. The pH scale measures the amount of hydrogen ions (H+) in a substance. (credit: modification of work by Edward Stevens)

This pH test measures the amount of hydrogen ions that exists in a given solution. High concentrations of hydrogen ions yield a low pH (acidic substances), whereas low levels of hydrogen ions result in a high pH (basic substances). The overall concentration of hydrogen ions is inversely related to its pH and can be measured on the pH scale (Figure 1). Therefore, the more hydrogen ions present, the lower the pH; conversely, the fewer hydrogen ions, the higher the pH. A change of one unit on the pH scale represents a change in the concentration of hydrogen ions by a factor of 10, a change in two units represents a change in the concentration of hydrogen ions by a factor of 100. Thus, small changes in pH represent large changes in the concentrations of hydrogen ions. Pure water is neutral. It is neither acidic nor basic, and has a pH of 7.0. 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. The blood in your veins is slightly alkaline (pH = 7.4). The environment in your stomach is highly acidic (pH = 1 to 2). Orange juice is mildly acidic (pH = approximately 3.5), whereas baking soda is basic (pH = 9.0).

Acids are substances that provide hydrogen ions (H+) and lower pH, whereas bases provide hydroxide ions (OH) and raise pH. The stronger the acid, the more readily it donates H+. For example, hydrochloric acid and lemon juice are very acidic and readily give up H+ when added to water. Conversely, bases are those substances that readily donate OH. The OH ions combine with H+ to produce water, which raises a substance’s pH. Sodium hydroxide and many household cleaners are very alkaline and give up OH rapidly when placed in water, thereby raising the pH.

Buffers

Most cells in our bodies operate within a very narrow window of the pH scale, typically ranging only from 7.2 to 7.6. If the pH of the body is outside of this range, the respiratory system malfunctions, as do other organs in the body. Cells no longer function properly, and proteins will break down. Deviation outside of the pH range can induce coma or even cause death.

So how is it that we can ingest or inhale acidic or basic substances and not die? Buffers are the key. Buffers readily absorb excess H+ or OH, keeping the pH of the body carefully maintained in the aforementioned narrow range. Carbon dioxide is part of a prominent buffer system in the human body; it keeps the pH within the proper range. This buffer system involves carbonic acid (H2CO3) and bicarbonate (HCO3) anion. If too much H+ enters the body, bicarbonate will combine with the H+ to create carbonic acid and limit the decrease in pH.

Likewise, if too much OH is introduced into the system, carbonic acid will rapidly dissociate into bicarbonate and H+ ions. The H+ ions can combine with the OH ions, limiting the increase in pH. While carbonic acid is an important product in this reaction, its presence is fleeting because the carbonic acid is released from the body as carbon dioxide gas each time we breathe. Without this buffer system, the pH in our bodies would fluctuate too much and we would fail to survive.

In Summary: Buffers, pH, Acids, and Bases

The pH of a solution is a measure of the concentration of hydrogen ions in the solution. A solution with a high number of hydrogen ions is acidic and has a low pH value. A solution with a high number of hydroxide ions is basic and has a high pH value. The pH scale ranges from 0 to 14, with a pH of 7 being neutral. Buffers are solutions that moderate pH changes when an acid or base is added to the buffer system. Buffers are important in biological systems because of their ability to maintain constant pH conditions.

Practice Question

Using a pH meter, you find the pH of an unknown solution to be 8.0. How would you describe this solution?

  1. weakly acidic
  2. strongly acidic
  3. weakly basic
  4. strongly basic

The pH of lemon juice is about 2.0, whereas tomato juice’s pH is about 4.0. Approximately how much of an increase in hydrogen ion concentration is there between tomato juice and lemon juice?

  1. 2 times
  2. 10 times
  3. 100 times
  4. 1000 times

Check Your Understanding

Answer the question(s) below to see how well you understand the topics covered in the previous section. This short quiz does not count toward your grade in the class, and you can retake it an unlimited number of times.

Use this quiz to check your understanding and decide whether to (1) study the previous section further or (2) move on to the next section.