The pH of a solution indicates its acidity or alkalinity.
\[\ce{H_2O(I) \leftrightharpoons H^+ (aq) + O^- (aq)} \nonumber\]
litmus or pH paper, filter 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 (acidity) or base (alkalinity) 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 the concentration of hydrogen ions in a given solution.
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+). Still, 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 of human cells and blood are examples of two areas of the body where near-neutral pH is maintained.
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.
The pH scale is, as previously mentioned, an inverse logarithm and ranges from 0 to 14 (Figure \(\PageIndex{7}\)). 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. So how do the cells of the stomach 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. New stomach cells are constantly produced to replace dead ones, which are digested by the stomach acids. It is estimated that the lining of the human stomach is completely replaced every seven to ten days.
Link to Learning
Watch this video for a straightforward explanation of pH and its logarithmic scale.
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, as shown in Figure \(\PageIndex{8}\), excess carbonic acid can be converted to carbon dioxide gas and exhaled 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– 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 on Earth.
Updated May 17, 2018
By Alex Silbajoris
Because water can absorb and transfer heat well, the human body uses it to stabilize temperature. Water has a relatively high heat capacity, meaning it can absorb a lot of heat before its temperature rises. This trait allows the water in every cell of the human body to act as a buffer against sudden temperature changes. Blood, which is made largely of water, moves heat away from the extremities and toward the vital organs when it needs to conserve heat, it flows towards the skin surface to release excess heat when necessary, and it transports muscle heat away as needed. Water also helps expel excess heat from the body as water vapor from the lungs and sweat on the skin.
Water has a relatively high specific heat, or heat capacity, meaning it can absorb a lot of heat before its temperature rises. This trait helps it to stabilize temperature in its surroundings. The water in every cell of the human body act as a buffer against sudden temperature changes. Blood is largely composed of water, and it moves toward vital organs when the body needs to conserve heat, and toward the extremities and skin when the body is in danger of overheating. Water evaporates from the skin and lungs to cool the body.
Burning calories through physical work or exercise generates heat from muscles. Water comprises up to 75 percent of muscle mass. One calorie will heat one gram of water by one degree Celsius - that's ten times more heat absorption than copper. The water in the muscle cells exchanges heat with the water in blood, which carries the heat away. In the brain, the hypothalamus senses the heat increase in the blood, and activates the sweat glands.
Your skin, made wet by its sweat glands, serves as a heat exchanger. Evaporative cooling occurs because the fastest-moving (hotter) water molecules escape as vapor, leaving behind the slower-moving (cooler) molecules. The heat driving the vapor is called the heat of vaporization. This is why a hot beverage or bowl of soup goes cold; the escaping vapor robs the heat. Air flow across the skin increases this effect. That's why, when you're wet with sweat, fans or a breeze help to cool you off more quickly.
The normal human body temperature is 98.6 degrees Fahrenheit, yet people survive when the ambient air temperature is higher. Air temperatures in deserts can reach higher than 120 degrees Fahrenheit, and sunlight adds to the heat absorbed by the body. In these conditions, loose, billowing clothing allowing air flow to help evaporate sweat is the norm. In heat-stressing conditions or in heavy exertion, the body might need as much as 10 liters of water per day to maintain healthy hydration.
The water lost cooling the body through sweat (as well as during other body processes) must be replaced. That's why you often hear "drink plenty of fluids" as advice for anyone working or playing hard. But sweat also excretes electrolytes such as sodium, potassium, chloride and calcium. That's why sport beverages include these among their ingredients.