Quantitative Analysis of Acids and Bases

Quantitative analysis of acid-base solutions is the process of determining concentrations of H⁺ or OH^– ions. Gravimetric analysis uses the mass of a precipitate formed by a reaction to determine the starting concentration of an analyte, or the solution being measured. A solution of unknown concentration is combined with another substance that will cause the formation of a precipitate, a solid that falls out of solution. The mass of that precipitate can then be determined experimentally, and the molar quantities of one or more analyte ions present in the initial solution can be determined.

For example, start with a solution of NaOH. Adding a measured amount (mass) of magnesium sulfate, MgSO₄, to the solution will immediately precipitate Mg(OH)₂, which has a very low solubility relative to the starting materials. The mass of precipitate Mg(OH)₂, an insoluble hydroxide, can then be used to determine the starting concentration of OH^–.

Acid-base titration is a quantitative method that relies on measuring the volume of a solution of a known concentration necessary to neutralize a given volume of acid or base. Often, acid-base titration involves an indicator, a chemical that undergoes a color change near or at the equivalence point in a titration. In the case of a titration, use an indicator that changes color at the equivalence point, the pH at which all of acid or base molecules in an acidic or basic solution have been neutralized.

Steps in performing titration analysis:

1. Obtain a measured amount of the analyte, which is the solution being analyzed (titrated) to determine its concentration, or dissolve a known amount of solute, add a few drops of indicator, and add water to a measured amount. Either of these techniques will result in a solution of known volume but unknown concentration of the analyte, the species being analyzed (often H⁺ or OH^–).

2. Create a solution of known concentration, called the titrant, that is used to neutralize a solution of unknown concentration (the analyte) in order to determine its concentration.

3. Pour some of the titrant into a buret, a type of graduated tube with a stopcock at the end that allows fine control of the release of the titrant. Note the amount of solution in the buret.

4. Slowly add titrant to the analyte solution until the color change occurs.

5. Note the volume of titrant used, and use this datum to calculate the molarity of the analyte solution.