pH Scale and Indicators

pH pH stands for “the power of hydrogen” and will tell us if a solution is acidic, neutral, or basic. The pH of a substance depends on the concentration of hydrogen ions in the solution (H+)  The pH scale is based on a logarithmic scale meaning that every change of one unit on the scale represents a tenfold effect (multiply or divide by 10). Which means there is a 10x difference between each pH level. e.g. the hydrogen ion concentration for… pH = 1 [H+] = 10-1 = 0.1 pH = 2 [H+] = 10-2 = 0.01 pH = 3 [H+] = 10-3 = 0.001 pH = 14 [H+] = 10-14 = 0.00000000000001

pOH We can also calculate the concentration of hydroxide ions and use the pOH scale. pOH = 1 [OH-] = 10-1 = 0.1 pOH = 2 [OH-] = 10-2 = 0.01 pOH = 3 [OH-] = 10-3 = 0.001 pOH = 14 [OH-] = 10-14 = 0.00000000000001

pH and pOH In every solution the pH and pOH must equal 14. pH + pOH = 14 For example, if…. pH = 3 pOH = 11 (acid) pH = 8 pOH = 6 (base) pH = 7 pOH = 7 (neutral)

Calculating pH To calculate the pH use the formula: pH = - log [H+] e.g. What is the pH of a solution that has a hydrogen ion concentration of 0.00548 M. [H+] = 0.00548 M pH = ? pH = -log [H+] pH = -log (0.00548M) pH = 2.26

Calculating [H+] To calculate the concentration of hydrogen use this formula: [H+] = 10-pH e.g. What is the hydrogen ion concentration of a solution with a pH of 9.46? pH = 9.46 [H+] = ? [H+] = 10-9.46 [H+] = 3.47 x 10-10 = 0.000000000347

pH Scale The pH scale measure how acidic or basic a solution is. It ranges from 0 to 14. A solution with a pH of 7 is neutral. A solution with a pH of less than 7 is acidic. Strong acids have a low pH (close to 0) A solution with a pH greater than 7 is basic (or alkaline). Strong bases have a high pH (close to 14).

pH Scale

pH Scale

Acid-Base Indicators Any substance that changes colour in the presence of an acid or a base They change colour because of the presence of H+ ions or OH- ions.

Common Indicators Indicator Name What is it? How does it work? Red litmus paper A plant extract that is dried onto strips of paper. Turns blue when it is dipped in a basic solution and remains the same colour in a neutral or acidic solution. Blue litmus paper Turns red when it is dipped in an acidic solution and remains the same colour in a neutral or acidic solution Universal indicator A mixture of chemicals that changes colour through a wide range of pH values. Different colours correspond to different pH values.

Common Indicators Indicator Name What is it? How does it work? Phenolphthalein Chemical that changes colour in acids and bases. Turns pink in a base and clear or cloudy in an acid or neutral solution. Red cabbage juice, cranberry juice. Chemicals in these liquids change colour depending on if they are in an acid or a base. Turns purple in neutral, red in acid, and green-yellow in bases.