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Titration is the slow addition of one solution of a known concentration (called a titrant) to a known volume of another solution of unknown concentration until the reaction reaches neutralization, which is often indicated by a color change. The solution called the titrant must satisfy the necessary requirements to be a primary or secondary standard. In a broad sense, titration is a technique to determine the concentration of an unknown solution.
For acid base titrations, a pH indicator or pH meter is used in order to determine whether neutralization has been reached and titration is complete. The information obtained from the process of titration can then be inserted into the equation, \(M_iV_i=M_fV_f\), to determine the concentration of the unknown solution. \(M_i\) and \(M_f\) are the initial and final molarities, and \(V_i\) and \(V_f\) are the initial and final volumes.
For example, when a color indicator is being used:
To clear confusion, the endpoint and equivalence point are not necessarily equal, but they do represent the same idea. An endpoint is indicated by some form of indicator at the end of a titration. An equivalence point is when the moles of a standard solution (titrant) equal the moles of a solution of unknown concentration (analyte).
The use of an indicator is key in performing a successful titration reaction. The purpose of the indicator is to show when enough standard solution has been added to fully react with the unknown concentration. However, an indicator should only be added when necessary and is dependent upon the solution that is being titrated. Therefore, indicators must only be added to the solution of unknown concentration when no visible reaction will occur. Depending on the solution being titrated, the choice of indicator can become key for the success of the titration.
The following types of titrations are categorized based on chemical reactions.
Titration of acid/base reactions involve the process of neutralization in order to determine an unknown concentration. Acid-Base titrations can be made up of both strong and weak acids or bases. However, in order to determine the unknown concentration of an acid or base, you must add the opposite so that neutralization can be reached. Therefore, an acid of unknown concentration will be titrated using a basic standard solution and a base of unknown concentration will be titrated using an acidic standard solution. Examples of acid-base titrations include include:
Acid-Base titrations often require the use of some kind of indicator depending on the strength of acid or base that is being titrated. In some cases a weak base or weak acid is used or a ph meter which reads the pH of the solution being titrated. Once the pH of the titrated solution equals seven, either indicated by a change in color or on a pH meter one can determine that titrations is complete.
Another type of titration is the Redox, or Oxidizing-Reducing Titration, which is used to determine the oxidizing or reducing agent in a solution. When performing redox titrations, either the reducing or oxidizing agent will be used as the titrant against the other agent. The purpose of this titration is to determine the transfer of electrons from one substance to the other, similar to that of a redox reaction to determine the reductant or oxidant. The end point of such titrations can be determined by either a color changing indicator or potentiometer.
Combination reaction titrations inclued two different types of titrations:
The purpose of back titrating is to return to the endpoint after it was passed. Back titrating should only be used when made necessary. It is often used when the solution being titrated is either too weak or too slow to give a reaction. It is also used if too much titrant was added, and the solution turned too dark. This means the experiment must be done over. The way to back titrate is to add an excess volume of another reactant of known concentration.
A back titration, or reverse titration, is most useful when the endpoint of a normal titration is difficult to identify.
The graphs of titration curves effectively show the relationship between the pH of the solution of unknown concentration as the standard solution is added to it in order to reach neutralization.
The pH of the final solution of titration changes as a result of the concentration of the standard solution. Ideally, if the titration has been done precisely and accurately, the final solution of the titration process should be neutralized and have a pH of 7.0. However, this is not always the case. The pH of the final solution often fluctuates depending upon the concentration of the unknown solution and the standard solution that is being added. Therefore, the effects that titration has on pH can best be defined by a generalized trend exhibited by the equivalence points on a titration curve. For more information of pH and pOH click here.
\[ M_1V_1 = M_2V_2 \]
Ideally when performing titration reactions the molarity multiplied by the volume of solution one should equal the molarity multiplied by the volume of solution two. Assume solution one is the standard solution, titrant, and solution two is the solution of unknown concentration, analyte. The volume of the titrant solution can be determined by subtracting the final burette readings from the initial.
An example of the equation for Acid-base titrations:
If done correctly, the final solution after titration should be neutralized and contain equal moles of hydroxide and hydrogen ions. So the moles of acid should equal the moles of base:
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