What is commonly referred to as acidity is the concentration of hydrogen ions in an aqueous solution.
Figure 1. The pH scale by numbers
Figure 2. The percent change in acidity.
What is commonly referred to as "acidity" is the concentration of hydrogen ions (H+) in an aqueous solution. Some common examples are shown in the figure at left. The concentration of hydrogen ions can vary across many orders of magnitude—from 1 to 0.00000000000001 moles per liter—and we express acidity on a logarithmic scale called the pH scale. Because the pH scale is logarithmic (pH = -log[H+]), a change of one pH unit corresponds to a ten-fold change in hydrogen ion concentration (Figure 1).
Many natural processes affect acidity levels in the environment—examples include photosynthesis and respiration—so the acidity may vary by an order of magnitude or more (or in pH units, by 1 or more) as a result of natural biological, physical, and geological processes on a variety of different spatial and temporal scales. Ocean acidification, related to the uptake of CO2 at the ocean surface, causes a relatively slow, long-term increase in the acidity of the ocean, corresponding to a decrease in pH. Since the Industrial Revolution, the global average pH of the surface ocean has decreased by 0.11, which corresponds to approximately a 30% increase in the hydrogen ion concentration.
So why are scientists concerned about such a seemingly small change in pH?
Many organisms are very sensitive to seemingly small changes in pH. For example, in humans, arterial blood pH normally falls within the range 7.35–7.45. A drop of 0.1 pH units in human blood pH can result in rather profound health consequences, including seizures, heart arrhythmia, or even coma (a process called acidosis). Similarly, many marine organisms are very sensitive to either direct or indirect effects of the change in acidity (or H+ concentration) in the marine environment. Fundamental physiological processes such as respiration, calcification (shell/skeleton building), photosynthesis, and reproduction have been shown to respond to the magnitude of changes in CO2 concentrations in seawater, along with the resultant changes in pH and carbonate ion concentrations that are expected over the next century.
Common misconceptions about pH and ocean “acidification”
How scientists describe the percent change in acidity
If you add acid to a solution the concentration of hydrogen ions (acidity) increases and the pH decreases. Frequently people confuse pH with acidity—pH is the scale on which acidity is expressed, but it is not synonymous with acidity. An appropriate way to compare the acidity at two different pH values is to express the relative percentage change of the H+ concentration at the two pH levels, as in Figure 2. For example, we said above that pH decrease of 0.11 corresponds to approximately a 30% increase in acidity, which is an exact change in acidity (H+ concentration) of 28.8% when calculated in this way.
High pH values
pH values above 7 are commonly referred to as “basic” (or “alkaline”). These common terms engender confusion, because a pH value does not directly reflect a quantitative measure of the concentration of bases in the solution, nor do high pH values constitute a measure of alkalinity. What is expressed by pH values >7 is still the acidity of a solution, it’s just that the acidity (H+ concentration) is very, very low (less than 10-7 (or 0.0000001) moles per liter, to be specific). To determine the alkalinity of a solution (which is related to the concentration of bases), a separate, detailed laboratory analysis must be run on the solution, so it is incorrect to characterize the change in hydrogen ion concentration as a decrease in alkalinity.
Calling this phenomenon “ocean acidification” when surface seawater will remain “basic” under future emissions scenarios is alarmist
Just as we describe an increase in temperature from -40°F to -20°F as warming, even though neither the starting nor the ending temperature is “warm,” the term “acidification” describes a direction of change (i.e. increase) in the level of acidity in the global oceans, not an absolute end point. When CO2 is added to seawater, it reacts with water to form carbonic acid (H2CO3); hence acid is being added to seawater, thereby acidifying it. Similarly, in the example about human blood, a drop in pH is referred to as acidosis, even though the point where acidemia begins (7.35) is still above 7.
Natural variability vs. long-term change
Many scientists have observed that natural variability in seawater acidity (and thus pH) is strong and can be much larger on short time scales than the observed and projected changes in acidity due to ocean acidification over the scale of decades to centuries. While this is true, the reason that scientists are concerned about this slow, long-term change is that it constitutes a changing baseline—such that the natural variability in acidity due to photosynthesis, respiration, upwelling, and myriad other processes—will be overlaid on an ever-increasing average concentration of H+ (the atmospheric increase in CO2 at Mauna Loa is a great example of a shifting baseline). Despite the slow and steady nature of this change in the baseline relative to human time scales, on geological time scales, this change is more rapid than any change documented over the last 300 million years, so organisms that have evolved tolerance to a certain range of conditions may encounter increasingly stressful, or even lethal, conditions in the coming decades.
For more information on changes in chemistry and the potential biological impacts due to ocean acidification, see the Ocean Carbon & Biogeochemistry Project’s Ocean Acidification FAQ page.
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.
- Identify the characteristics of acids
- Identify the characteristics of bases
- Define buffers and discuss the role they play in human biology
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.
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.
Using a pH meter, you find the pH of an unknown solution to be 8.0. How would you describe this solution?
- weakly acidic
- strongly acidic
- weakly basic
- strongly basic
Show Answer
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?
- 2 times
- 10 times
- 100 times
- 1000 times
Show Answer
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As we've just learned, chemistry is essential to life: we are all made of compounds and molecules. Think back to the beginning of this chapter, where we briefly discussed the term carbon-based life. In the video below, we'll learn about why carbon is so essential to life:
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Let's look back to our earlier occupation spotlight on the nutritionist. You should now be able see how an understanding of chemistry is essential to this job: a nutritionist needs to understand how the body builds complex molecules from the food a person ingests. A nutritionist also should understand how energy is used and moved about in the body. All this requires a basic understanding of elements, their structures, and how they interact.
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- That's Why Carbon Is A Tramp: Crash Course Biology #1. Authored by: CrashCourse. Project: Crash Course Biology. License: All Rights Reserved. License terms: Standard YouTube License