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Self-Instructional on Water, pH, and Buffers
Photo taken during IUS field trip to Belize 2001.
Water - the Fluid of Life
Water is indispensable to life as we know it. The special chemistry of water makes it a major component of all life on earth. Three quarters of the surface of the earth is covered by water and we humans are composed of approximately 66% water. Water has many unusual physical properties that result from its unique chemical structure. Before talking about these properties, it is first necessary to define some terms.
Physical properties of matter include such things as
Oxygen is an electronegative element and hydrogen is an electropositive element. Since water is composed of one oxygen atom and two hydrogens, water molecules tend to develop slight negative and positive charges at different ends of the molecule. This charge differentiation makes water a polar molecule. The slight charges of the polar molecule can interact with other polar molecules or ions.
The two hydrogens on one end of the water molecule have partial positive charges (denoted by the symbol d +). The oxygen at the other end of the molecule has a partial negative charge (denoted by the symbol d -). Because of these partial positive and negative charges, water molecules can form hydrogen bonds with other water molecules. One of the partially positive hydrogens will form a weak bond with the partially negative oxygen of another water molecule. See Figure 1 for an illustration of how this works.
The ability of water molecules to hydrogen bond with other water molecules gives water special physical properties. The hydrogen bonds between water molecules allow water to form a highly ordered lattice structure in the solid state. When water freezes a crystal lattice structure forms. This lattice structure is what gives snowflakes and ice crystals their beautiful design. The polar, ordered nature of water makes it an excellent solvent. It is often referred to as the universal solvent, although not everything is soluble in water obviously!
A solvent is an agent that dissolves, or solubilizes, a substance called a solute. If something is soluble in water it dissolves in it. However, solubilizing a substance does not cause it to vanish. As you certainly know, when sugar dissolves in water the water tastes sweet, indicating the presence of the sugar. When the polar water molecules orient themselves around the partially charged polar or fully charged ionic molecules of the solute, the solute has been solubilized in the water.
Examples of polar and ionic compounds that you may know about include table sugar and salt. Table sugar (sucrose C12H24O12) is a polar compound. When sugar and water are mixed together, the sugar dissolves in the water. This is the result of polar water molecules surrounding each polar sugar molecule.
When table salt (sodium chloride NaCl) is mixed with water and dissolves, the sodium and chlorine atoms actually separate in the water, floating freely in the solute as ions. The partial positive charges of the hydrogens on the polar water molecules surround each negatively charged chlorine ion, while the partial negative charges of the oxygen on the polar water molecules surround each positively charged sodium ion. When water molecules orient themselves around charged or polar molecules, spheres of hydration, or hydration shells, are formed. This is diagrammatically illustrated in Figure 2 below.
The following table compares some of the physical characteristics of water with two other polar compounds that look identical to pure water.
| Compound | Melting Pt. | Boiling Pt. | Heat of Vaporization |
|---|---|---|---|
| Water | 0º Celsius | 100º Celsius | 540 Cal/g |
| Methanol | -98º Celsius | 65º Celsius | 263 Cal/g |
| Ethanol | -117º Celsius | 78º Celsius | 204 Cal/g |
Note the wide range in values between these three seemingly similar liquid compounds. All are clear colored, polar solvents, and all contain oxygen and hydrogen. Yet, the table above shows that these substances have extremely different properties. Imagine if lakes were composed of either methanol or ethanol. What might happen in the heat of the summer? What would happen in the winter?
If you guessed that the lakes would evaporate very quickly in the summer and fail to freeze in the winter you would be correct! As you may imagine, this would have a devastating effect on pond life. The formation of ice is very important to many aquatic organisms as a form of protection against extreme cold and rapid temperature fluctuations.
Water has some additional physical properties that also make it essential to life.
Practice Round One
1. List several of the physical properties of water and speculate what would happen to living organisms if these properties were to change.
2. Define the following terms and concepts: hydrogen bonds, polar molecules, electronegative element, electropositive element, solvent, solute, and solubilization.
3. Discuss the contribution of hydrogen bonding to water's unique properties.
pH and Water
In addition to all of its other properties, water can act as either a weak acid or a weak base. How can water act as both? First we must define acids and bases. For our purposes here we will use the simplest definition:
An acid is a substance that can donate hydrogen ions (H+) to the surrounding aqueous environment.
A base is a substance that can absorb or accept hydrogen ions from the surrounding aqueous environment.
An acid will increase the hydrogen ion concentration of an aqueous solution in which it is solubilized. A base will decrease the hydrogen ion concentration of an aqueous solution in which it is solubilized. Acids and bases must be dissolved in aqueous solution in order to dissociate and become active as hydrogen donors or acceptors.
Before we go on we should better define the hydrogen ion (H+). Remember that the element hydrogen is composed of one electron and one proton. Hydrogen normally exists in nature as a gas composed of two hydrogen atoms covalently bonded together. A hydrogen ion is one hydrogen atom that has lost its electron, and therefore is merely a single proton. Knowing what you know about atoms and what makes them stable or unstable, do you suppose that a single proton by itself would be very stable?
Of course it wouldn't! Hydrogen ions can only exist when their positive charges can be neutralized by opposite charges. For example, a hydrogen ion can be present in water when the negative ends of the polar water molecules can surround the positively charged hydrogen ion in a sphere of hydration.
Ionization or separation of water into hydrogen ions (H+) and hydroxide ions (OH-) occurs to a very small extent in water at all times. Water acts as a hydrogen donor (an acid) when a molecule of water separates and releases a hydrogen ion. Water acts as a hydrogen acceptor (a base) when one of the hydroxide ions accepts a hydrogen ion to form a complete molecule of water. The following equation represents the ionization of water: H2O --> H+ + OH- Ionization of water causes the neutral water molecules to separate into charged ions.
Acids and bases are classified according to their ability to donate or accept hydrogen ions. A strong acid can donate hydrogen ions easily when dissolved in aqueous solution. A strong base can accept hydrogen ions easily.
Now that we have defined acids and bases chemically, let's find out what effects they have on biological systems.
As you already know, charged particles, or ions, are highly reactive. This basically means that ions are looking for other molecules to react with so they may attain a more stable electron configuration. When charged particles react with biomolecules such as protein and nucleic acid, they can have devastating effects upon the organism. Since acids and bases can dissociate easily into charged hydrogen ions and hydroxide ions, they have the potential to be very destructive to living tissues. Hydrochloric acid, a strong acid, can be highly destructive to organic matter when in its concentrated form. The same is true for sodium hydroxide, a strong base. Few people realize that strong bases are just as dangerous as strong acids.
Most oven cleaners (the foamy substances that eat encrusted food on your oven walls) contain concentrated sodium hydroxide base. Many drain openers are often concentrated acids. Drain openers operate by breaking down organic materials like hair that may clog the drain. Knowing what strong acids and bases due to the organic materials in dirty ovens and clogged drains should give you a hint about their potential for tissue destruction. However, even weak and dilute acids and bases can have a devastating effect when not compensated for inside living cells and tissues.
Although we have not yet discussed the role of the major biological molecules I think most of us know that our cells and tissues stay alive by performing many complex biochemical reactions. Biological life revolves around a class of molecules called proteins. Proteins are very sensitive to even a slight increase or decrease in the hydrogen ion concentration (pH) of the surrounding solution. So how can we be exposed to and ingest acidic and basic compounds all the time and yet not increase and decrease the hydrogen ion concentration in our blood, tissues, and cells?
Buffers!
A buffer is a substance that can accept hydrogen ions when there are too many present or release hydrogen ions when there are too few present. In this way buffers maintain the pH at a relatively constant level.
pH is a measure of how many hydrogen ions are present in a substance. pH actually represents the number of hydrogen ions per unit volume of a liquid, or the concentration of hydrogen ions. Here's an example of how concentration works: one package of Kool-Aid makes two quarts. If you place one cup of sugar in that two quarts you have a specific concentration of sugar in your Kool-Aid. If you place two cups of sugar in that two quarts you would have increased the concentration of sugar 2-fold, or 2X.
Buffers are essential to all organisms because most biochemical reactions can only occur at neutral pH's of about seven. A substance with a neutral pH has just the right amount of hydrogen ions present for biochemical reactions to occur. Buffers maintain pH by absorbing or releasing hydrogen ions when necessary to maintain a specific hydrogen ion concentration. In our laboratory we perform many extractions of biologically active proteins which we later use in an in vitro reaction. We use buffering agents in all of our liquids so that we can maintain a near neutral pH, similar to the pH found in most biological systems. As a matter of fact, scientists almost universally refer to these liquids used in their laboratories as buffers even though it is technically incorrect because most of them contain components in addition to the buffering compound. Buffers are indispensable in all aqueous solutions in which biochemical reactions are taking place, whether in a living organism or in the laboratory.
Is water very susceptible to a pH change? Yes! Add only one drop of concentrated hydrochloric acid to a liter (approximately a quart) of pure water and the pH will be reduced from seven to approximately one. A pH of seven is neutral, and a pH of one is very acidic. What really is the difference between a solution with a pH of seven and a solution with a pH of one?
As we have noted before the pH represents the concentration of hydrogen ions in a specific volume of a substance. pH is defined as the negative log of the hydrogen ion concentration as represented by this equation: -log [H+]. The brackets indicate concentration of H+.
Right now you might be thinking: negative log of what?! Relax, it's pretty simple.
The log of a number q in base 10 is equal to the exponent of q when written in exponential notation. For example, the log of 100 is equal to 2, because 100 written in exponential notation is 1 x 102. When figuring pH, we use the negative log. It is the same as taking the log and then multiplying it by negative one.
But wait, pH's are not negative numbers! That is because the pH represents a small number from 0.1 (1 x 10-1) to 0.00000000000001 (1 x 10-14). These numbers correspond to a pH of one and a pH of fourteen. As you can see, the log of 1 x 10-14 is -14, but the negative log of 1 x 10-14 is 14. The definition for pH was fabricated to simplify the representation of hydrogen ion concentration. pH values represent a kind of shorthand value for acidic and basic solutions.
Note that the higher the concentration of hydrogen ions, the smaller the exponent, and therefore the smaller the pH value. For this reason, acids will have small pH values, such as a pH of one for a strong acid. Bases will have large pH values because their hydrogen ion concentration is smaller. A solution with a hydrogen ion concentration of 1 x 10-1 has a relatively large number of hydrogen ions. This solution is a strong acid of pH one. A solution with a hydrogen ion concentration of 1 x 10-14 has a very small number of hydrogen ions. This solution is a strong base with pH fourteen.
Practice Round Two
1. Explain what acids and bases do in aqueous solution.
2. Write and label the equation which represents the ionization of water.
3. Define pH.
4. If a solution has a pH of 3, what number represents the actual hydrogen ion concentration? If a solution has a pH of 10, what number represents the actual hydrogen ion concentration?
5. What do buffers accomplish in solution?
Post Test on Water, pH, and Buffers
1. Describe how water solubilizes an ionic compound such as KCl (potassium chloride).
2. What are we referring to when we say 'sphere of hydration?'
3. Why are buffering compounds included in many solutions used in biochemical experiments?
4. Explain the pH scale and its relationship to hydrogen ion concentration.
© copyright by Dr. Gretchen Kirchner 1996, 2001
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