Acids and bases
Pure water dissociates into two ions in solution: a proton (H+) and a hydroxide ion (OH–).
H2O <=> H+ + OH–
Pure distilled water contains an equal amount of protons and hydroxide ions. However, if an acid is added to pure distilled water, then the resulting solution will contain more protons and the solution is acidic. Acids are substances that can donate a proton to solutions. Carbonic acid (H2CO3), for example, is formed in blood when CO2 reacts with water in blood plasma. The resulting H2CO3 dissociates in aqueous solution into a proton and bicarbonate (HCO3–).
CO2 + H2O <=> H2CO3 <=> H+ + HCO3–
Bases, on the other hand, accept protons from solution. Hydroxide and bicarbonate, for example, are bases because they are negatively charged ions that can accept protons from solutions. When bases are added to pure distilled water, the resulting solution will contain less protons relative to bases and will be basic.
The acidity of substances is measured on a logarithmic scaled called the pH scale. The pH scale ranges in value from 0 to 14. These values are the negative logarithm of the concentration of protons [H+] in solution. Therefore, each step along the scale represents a 10-fold change in [H+] in solution. The units of concentration are moles per litre (M).
pH = -log10[H+]
Pure distilled water has a pH of 7 and this is considered neutral [neutral = neither acidic nor basic]. Acidic substances have a pH measuring less than 7. Carbonic acid has a pH of ~4.2, for example. Basic substances have a pH measuring greater than 7. Sodium bicarbonate, for example, has a pH of ~8.5.
Most of the solutions inside and outside of cells is kept within a very narrow pH range, generally around neutral pH. For example, blood must be kept between pH 7.35–7.45. Serious harm can result from blood pH falling outside of this range.
Substances that minimize changes in pH are called buffers. Buffers are molecules that can both donate protons to and accept protons from solutions to keep the pH of that solution constant. Buffers DO NOT necessarily keep the pH of solutions neutral! Some buffers keep solutions acidic; some keep solutions basic. Buffers simply prevent large changes in pH from occurring when acids or bases are added to solutions.
Chemistry in the clinic:
Abnormally low blood pH results in a condition known as acidosis. Acidosis results in the denaturation of cellular proteins, disrupting the ability of cells to control solute entry/exit and to make energy.
Similarly, abnormally high blood pH results in alkalosis. Alkalosis decreases the concentration of protons in body fluids as bases accept protons from solution. Alkalosis may also denature proteins and disrupt cellular function.
If left untreated, acidosis and alkalosis disrupt cell, tissue, and organ function, potentially leading to death. You may explore acidosis and alkalosis in more detail if you choose to take Biology 1191.
The primary regulator of blood pH is an enzyme called carbonic anhydrase in red blood cells. The action of carbonic anhydrase maintains blood buffering and is essential to human body function. This topic will be explored further in lectures about blood and the respiratory system.
Summary:
- Water is essential for cell function and life on Earth
- The atoms in a water molecule are held together by polar covalent bonds
- Molecules of water are held together by hydrogen bonds
- The hydrogen bonds of water are responsible for its remarkable properties, including:
- Cohesion
- Ability to moderate temperature
- Expansion upon freezing
- Versatility as a solvent
- Pure distilled water has a pH of 7 and is considered a neutral substance
- Acidic substances donate protons to solution and have a pH less than 7
- Basic substances accept protons from solution and have a pH greater than 7
- Buffers minimize changes to the pH of solution by donating or accepting excess protons
potential hydrogen; a measure of the concentration of protons in solution
denature = to unfold a protein and prevent the protein from performing its cellular function
