The Basic Process

• The standard solution is the opposite of what the solution of unknown concentration is. If one is an acid, the other is a base. This means the titrant "drags" the solution of unknown concentration toward a neutral pH, neither acid nor base.
  
  The standard solution is initially added quickly. When a dye added to the solution of unknown concentration (the "titer") starts changing color, it indicates the solution of unknown concentration is getting closer to a pH of 7.0 (neutral), that is to say, it's being neutralized. The standard solution is then added more and more slowly. When neutralization is about to be achieved, the titrant is added drop by drop. This makes the experiment highly accurate in measuring the titer's concentration.
  
  

Indicators

• Acid-base titration indicators ideally change color when the titer passes a pH of 7.0 (neutral). Generally, used indicators don't hit 7.0 on the nose, but they do get close. For example, litmus is a good indicator for an acid-base titration, because it changes color at a pH of around 6.5---close enough, as will be explained below. Since indicators react with the solution being measured, they should be used in moderation---only a few drops, if possible.
  
  

Equivalence Point

• The "equivalence point" is when the titer has just been fully neutralized. Another few drops of titrant would change the pH significantly. This is because the concentration of hydronium (H3O+) and hydroxide ions (OH-) at the neutral pH of 7.0 are both so low---in technical term, 10^-7 molals (moles per liter) for both. Normal acids and bases have far higher concentrations of hydrogens or hydroxides than this, and a drop or two of acid or base would increase the former or latter by orders of magnitude. This is enormously helpful for indicator use, because an indicator like litmus will change color only a drop or so away from the equilibrium point.
  
  

Titration Curve

• If pH meter readings are taken with every addition of titrant, the titer's pH can be graphed against the volume or mass of titrant added. By the concentration argument above, the curve will be very steep at the equilibrium point, making its determination fairly easy graphically.
  
  

Reactions

• Some neutralization reactions that can be done in the lab as titration experiments are:
  
  NaOH+HCl'NaCl+H2O
  2HCl+Ba(OH)2'BaCl2+2H2O
  HCl+NH3'NH4Cl
  
  Elements are swapped between molecules in the first two equations, and are therefore called "displacement reactions." Neutralization reactions need not be displacement reactions or produce water, as shown by the last equation, which a "combination reaction."