Measuring pH and the Role of Buffers in Biological Systems

What is pH?  pH is a measurement of hydrogen ion (hydronium ion) concentration in a given solution.  Acids are defined as proton donors and bases are defined as proton acceptors. Biological Significance  Enzymes and other biological molecules including macromolecules with hydrogen bonds like structural proteins, DNA, and RNA only function properly at specific pH optimums.  pH is an important homeostatic parameter for all living things – organisms must work to maintain stable pH values through the use of buffering compounds.

Since hydrangeas take up aluminum best at lower pH levels, raising the pH will help to keep the bluing effect of aluminum out of the hydrangea's system. If the soil naturally contains aluminum and is acid (low pH) the color of the hydrangea will automatically tend toward shades of blue and/or purple.

2H 2 O H 3 O + + OH - Water can act as both a proton donor and acceptor for itself. A proton can be transferred from one water molecule to another through a process called disassociation, resulting in the formation of one hyroxide ion (OH-) and one hydronium ion (H 3 O+). H 2 O H + + OH - In the above equilibrium, water acts as both an acid and a base.

However, in pure water at room temperature, the number of water molecules that disassociate is quite small. The disassociation constant of water (K W ) at 25 degrees C is approximately That means that the concentration of H+ is roughly M or moles of H+ per liter of water and the concentration of OH- is also roughly 10-7M or moles of OH- per liter of water.

Rather than expressing the concentration of H+ as some very small number, it is often more convenient to describe in terms of pH, defined as: pH = - log [H+] For example, pure water at 25 degrees C contains equal concentrations of H+ ions and OH- ions, where the concentration of H+ = M. Thus, the pH of the solution is obtained by: pH = - log = 7 The negative log scale is useful for measuring other minute quantities, for example to measure [OH-]: pOH = - log [OH-] pH < 7solution is acidic pH = 7solution is neutral pH > 7solution is basic

 Changes of 1 pH unit indicate a 10 fold change in hydrogen ion concentration.  An acid and a base neutralize each other to create water and a salt. HCl + NaOH  H 2 O + NaCl

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