• The Chemistry of Life

    The Chemistry of Life

  • 1

pH

baking soda and lemon juiceThe pH of solutions is an important chemical property. Oxygen is much more electronegative than hydrogen. Both shared electrons in an oxygen-to-hydrogen bond tend to spend more time with the oxygen atom than with hydrogen. Although the vast majority of water molecules remain intact in liquid water, at any given moment a few individual hydrogen atoms succumb to the pressure of the electronegative oxygen and lose their hold on both shared electrons. When this happens, the covalent bond is broken and a hydrogen ion (positive charge because it lost its electron to oxygen, shorthand = H+) is released. The remaining part of the original water molecule is called a hydroxide ion (negative charge because it kept an extra electron, shorthand = OH-). Although present at very low levels in solution, H+ and OH- ions can have enormous effects on the properties of a solution, especially when they are not in balance. In pure water, every hydroxide ion that forms creates a hydrogen ion, so there are equal numbers of anions and cations. If a solute is added to water, however, this balance can change.

Hydrochloric acid moleculeHydrochloric acid (HCl) is a molecular compound that dissociates (separates) easily because chlorine is so strongly electronegative. When put into water, the covalent bonds of HCl break to form H+ and Cl- ions, which increase the concentration of H+ ions in the solution. This creates an acidic solution because there are more H+ ions than OH- ions present. Compounds that add H+ ions to solutions are called acids. In contrast, there are also substances like ammonia (NH3) that are bases because they cause the H+ ion concentration to decrease, resulting in a basic solution.

Because water molecule splitting is extremely rare, the number of hydrogen and hydroxide ions in a solution is so tiny that we use a special logarithm-based formula to measure the number of hydrogen ions present, giving us more manageable numbers in the pH scale. The pH scale ranges from 0 to 14 and represents the hydrogen ion (H+) concentration in a solution. The pH of pure water is 7, which represents 1 x 10-7 hydrogen ions for every one intact water molecule. This is acid warning signonly one H+ ion for every 10 million H2O molecules!

In the pH scale, as H+ ion concentration increases, pH values decrease. This means that a low pH value represents a high H+ ion concentration (acidic solution) and a high pH value represents a low H+ concentration (basic solution). Finally, for each whole number change in the pH scale, the actual H+ ion concentration changes ten-fold. For example, a pH change from 7 to 8 represents a drop in hydrogen ion concentration from 1 in 10 million to 1 in 100 million.

 

pH Tester

In this activity, you will determine the pH of some common household and food items.

 

 

Buffers

bottle of marine aquarium bufferWhy do we care about the pH of a solution? Most of the body’s cells only function within a very narrow range near neutral pH. Enzymes that help us grow and reproduce, break down the food we eat, and assist in other vital functions each work within a specific, narrow pH range. To help maintain this pH, buffers are present in nearly all living solutions.

A buffer is any substance that minimizes change in the pH of a solution. Most buffers consist of a combination of a weak acid and the weak base, where the base is the anion remaining after the weak acid dissociates (separates) to release H+ ions. This may sound confusing, but it is actually a fairly simple back and forth reaction where the buffer acts as a “friend” to H+ ions when necessary, but also to OH- ions when necessary, maintaining a constant balance in the pH value.

Imagine a family with three children. If the oldest child and the youngest child tend to fight a lot, the middle child often acts as a “buffer” between the two fighting children. When we say buffer in this situation, we mean that the middle child will play older kid games at times when the oldest child needs attention and play simpler, younger kid games at times when the youngest child needs attention. By shifting to meet the needs of each child, the middle child buffers the situation, resulting in less angry children. This may not be optimal for the middle child buffer, but it makes the parents happier!

Carbonic acidIn solutions, a chemical buffer acts similarly. For example, carbonic acid (H2CO3) is a weak acid. When it is put into solution, a small amount of carbonic acid dissociates into H+ ions and the remaining bicarbonate anion (HCO3-). This increases H+ ion concentration and lowers pH values (toward acidic). The bicarbonate ion is considered a weak base because if there are a lot of H+ ions in solution, it will re-associate (chemically bind) with the excess H+ ions to re-form the weak acid, which reduces H+ ion concentration, bringing pH values up (back toward basic).

Buffers maintain the pH of a solution by adjusting the direction of their chemical reactions (dissociating or re-associating) in response to increases or decreases in H+ ion concentration that can be caused by other substances entering or exiting the solution. If you add a strong acid like hydrochloric acid (HCl) to a buffered solution, there will suddenly be an excess of H+ ions from dissociation of the HCl. The buffers in the solution will respond by binding these excess H+ ions to re-form the weak acid, using up the excess H+ ions so that the pH can remain around the same value despite the addition of an acid. The presence of carbonic acid/bicarbonate ion in your bloodstream is one of the main ways that your body regulates the pH of your blood to avoid acidosis or alkalosis, both of which are life threatening conditions resulting from changes in your blood pH level.

 

Buffering Solutions

Acids and bases change the pH of solutions. When present, buffers help stabilize pH by binding or releasing hydrogen ions in response to pH changes after addition of an acid or base. In this activity, you will observe the pH changes of two solutions when strong acids and bases are added.

 

Additional Resources

Websites

pH Scale
This short tutorial illustrates the relationship between hydrogen ion concentration and the pH scale.

pH and Buffers
This narrated animation demonstrates what happens when a strong acid or strong base is added to a buffer solution.