Figure 1 Setup for a Typical Titration In a typical titration, the titrant in the buret is added to the solution in the Erlenmeyer flask until the indicator changes color to show that the reaction is complete.
An indicator is added to the solution being titrated. The indicator is a substance that changes color when the reaction is complete. Phenolphthalein has two chemical forms. In acidic conditions, it is in the acid form, which is colorless. The titrant is slowly added to the solution being titrated until the indicator changes color, showing that the reaction is complete.
This stage in the procedure is called the endpoint. In our example, the NaOH solution is slowly added from the buret until the mixture in the Erlenmeyer flask changes from colorless to red.
These react with the phenolphthalein molecules, changing them from the acid form to the base form. Because the base form is red, the solution turns red, telling us that the reaction is complete or just slightly beyond complete. The volume of titrant added from the buret is measured. For our example, let's assume that Once titrated, you could dilute it precisely to the concentration you want.
Some other reagents you might want standard solutions of react with air; these you might also titrate if they have been waiting a long time so you know what the current concentration is.
Titrations might seem a little old-fashioned. Actually, the number of automated titration machines available try a google search! One reason might be that titrations can be good for studying newly discovered molecules, for instance to measure the molecular weight and other properties that we will study more later. Traditionally, you take a known mass or volume of the unknown solution and put it in a flask with the indicator.
Then you add the standard solution in a buret, which is a special tube for adding solution slowly and measuring the volume added at the end. These days, it might be easier to use a plastic squeeze bottle instead of a buret. You put the standard solution in the squeeze bottle, get the mass of the bottle, do the titration, and then mass the bottle again.
The analyte is a solution of unknown concentration. As you slowly add the titrant, you can monitor for signs that a reaction is taking place.
Water is necessary to create the solutions in titrations. Additionally, if you add water to a solution, you change the concentration of the solution. You must incorporate these changes into your calculations. Adding water to a titrant or analyte will change the concentration of that solution. Each solution has a molarity, which is equal to the number of moles of a solvent per liter of solution.
When you add water to a solution, the number of moles of the solvent stays the same while the volume increases. Typically, the dilution factor remains constant for each dilution, resulting in an exponential decrease in concentration. For example, a ten-fold serial dilution could result in the following concentrations: 1 M, 0.
As is evidenced in this example, the concentration is reduced by a factor of ten in each step. Serial dilutions are used to accurately create extremely diluted solutions, as well as solutions for experiments that require a concentration curve with an exponential or logarithmic scale. Serial dilutions are widely used in experimental sciences, including biochemistry, pharmacology, microbiology, and physics.
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