Acids and Bases       

One very important reaction is constantly occurring in any body of water.  At any given time, some of the water is being ionized while some ions are joining back together into water molecules.  When water is ionized, it has been broken down from one molecule into two ions (charged particles).  The reaction can be shown by the equation H₂O = H⁺ + OH^-.  This reaction also happens in reverse when the H⁺ and OH^- ions join back together into water.  

The H⁺ ions can be considered acids while the OH^-  ions can be considered bases.  OH- ions tend to attract oils and fats and become soap (lye soap).  H⁺ ions tend to ionize metals such as Iron, Fe⁺⁺, which becomes rust, Fe₂O₃. 

pH is a measure of the concentration of H⁺ ions in the water, which can also be thought of as the ratio of H⁺ to OH^- ions.  In neutral water, the number of H⁺ ions is equal to the number of OH^- ions.  But if you add hydrochloric acid (HCl) to the water, the hydrochloric acid will ionize and add H⁺ and Cl^- ions to the mixture.  Now there are more H⁺ ions in the water than OH^- ions.  The water is said to be acidic and has a lower pH value. 

Bases, such as sodium hydroxide (NaOH), can release OH^- ions into the water.  As a result, the water becomes more basic and the pH value is higher.  Bases can also work by attaching to the H⁺ ions already in the water, taking them out of solution and lowering the concentration of H⁺ ions in the water.  

Natural systems also have acids and bases.  Carbon dioxide, CO₂, combined with water becomes carbonic acid, which is a weak acid.  Limestone (MgCO₃ and CaCO₃) dissolved in water is a weak base. 

Measuring pH 

How do you think the number of H⁺ ions in water is measured?  The pH scale refers to the number of H⁺ ions in one liter (approximately four cups) of water.  Since molecules and ions are so tiny, it would be impossible to count all of the H⁺ ions in even one teaspoonful of water.  Instead, we count the number of moles of H⁺ ions in the water.  One mole (also known as a gram molecular weight) is equal to 6.023 x 10²³ molecules - billions and billions of molecules.   

Neutral water has a pH of 7, meaning that it has 10^-7 moles (or 0.0000001 moles) of H⁺ ions in every liter of water.  Lemon juice, with a pH of 2, has 10^-2 moles (or 0.01 moles) of H⁺ ions in every liter of water.  That is, there are more H⁺ ions in the lemon juice than in the water, so the lemon juice is more acidic. 

Since the number of moles of H⁺ ions can vary so much in a solution, we use a logarithmic scale to measure the ratio.  A very basic solution is given a pH of 14 while a very acidic solution is given a pH of 1.  The acidic solution does not have 14 times as many H⁺ ions as the basic solution.  Instead, the acidic solution has 100,000,000,000,000 as many H⁺ ions.  Each number on a logarithmic scale, such as a pH scale, is ten times more than the number below it.   

The Basics of pH 

The pH scale ranges from 0 to 14.  Neutral water has a pH of 7 while anything with a pH below 7 is acidic and anything with a pH above 7 is basic.

The pH of water does not change as the temperature changes.

Living organisms are sensitive to pH change and can survive only in a narrow range.  For example, if human blood, which is normally 7.45, becomes less than 6.8 or greater than 8.0 for more than a few seconds, death results.  Too low pH in the blood causes depression in the central nervous system and a too high pH causes a hyperactive state.

pH is can be easily measured with litmus paper or with a pH meter.  The original pH indicators were probably salts extracted by boiling red cabbage and beets. 

The following scale shows the pH of some common substances: