An acid-base titration is the determination of the concentration of an acid or base by precisely neutralizing it with a base or acid of known concentration. This lets us quantitatively analyze the concentration of the unknown solution (Figure 2). Acid-base titrations can also be used to quantify the purity of chemicals.
Alkalimetry, or alkimetry, is the specialized analytic use of acid-base titration to determine the concentration of a basic (alkaline) substance. Acidimetry, or acidometry, is the same concept applied to an acidic substance.
The materials used in a titration procedure are:
- white tile (used to see a color change in the solution)
- pH indicator (which one is used depends on the reactants)
- Erlenmeyer or conical flask
- titrant (a standard solution of known concentration; a common one is aqueous sodium carbonate)
- analyte, or titrand (the solution of unknown concentration)
Before you begin the titration you must choose a suitable pH indicator, preferably one that will experience a change in color (known as an "end point") close to the equivalence point of the reaction. The equivalence point of the reaction is the point at which equivalent amounts of the reactants have reacted. Below are some common indicators:
- strong acid-strong base titrations: phenolphthalein indicator
- weak acid-weak base titration: bromthymol blue indicator
- strong acid-weak base titration: methyl orange indicator
- if the base is off the scale (e.g., pH > 13.5) and the acid has pH > 5.5: Alizarine yellow indicator
- if the acid is off the scale (e.g., pH < 0.5) and the base has pH < 8.5: Thymol Blue indicator
The resulting solution at the equivalence point will have a pH dependent on the relative strengths of the acid and base used. The pH of the equivalence point can be estimated using the following rules:
- A strong acid will react with a weak base to form an acidic (pH < 7) solution.
- A strong acid will react with a strong base to form a neutral (pH = 7) solution.
- A weak acid will react with a strong base to form a basic (pH > 7) solution.
When a weak acid reacts with a weak base, the equivalence-point solution will be basic if the base is stronger and acidic if the acid is stronger. If both are of equal strength, then the equivalence pH will be neutral. However, weak acids are not often titrated against weak bases because the color change is brief and therefore very difficult to observe.
The pH of a weak acid solution being titrated with a strong base solution can be found at different points along the way (Figure 1). These points fall into one of four categories: (1) initial pH, (2) pH before the equivalence point, (3) pH at the equivalence point, and (4) pH after the equivalence point.
- The burette should be rinsed with the standard solution, the pipette with the unknown solution, and the conical flask with distilled water.
- A known volume of the solution of unknown concentration should be placed into the conical flask using the pipette, along with a small amount of indicator. The known solution should then be allowed out of the burette into the conical flask. At this stage we want a rough estimate of the amount of known solution it took to neutralize the unknown solution. The solution should be let out of the burette until the indicator changes color, and the value on the burette should be recorded. This is the first titre and is not very precise; it should be excluded from any calculations.
- At least three more titrations should be performed, this time more accurately, taking into account roughly where the end point will occur. The initial and final readings on the burette (prior to starting the titration and at the end point, respectively) should be recorded. (Subtracting the initial volume from the final volume will yield the amount of titrant used to reach the endpoint.)
- The end point is reached when the indicator changes color permanently.