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MODULE 2. TITRATION

What would happen if an acid and a base were mixed up?

Let's think about it this way.  What would happen if a hydrogen ion (H+) from an acid reacted with a hydroxide ion (OH-) from a base?  That's right.  Water would be produced. 

If the spectator ions (other than H+ and OH- ions) from the acid and the base are put back into the equation, it would read, for example,

showing that an acid reacts with a base to yield water plus an ionic compound called a salt.  This acid-base reaction is called a neutralization reaction

What is titration?

Using the neutralization reaction, you can determine the concentration of an acid or a base solution.  As shown in the previous equation, one hydrogen ion from an acid reacts with exactly one hydroxide ion from a base to produce a water molecule.  If adding 100 hydroxide ions to an acid solution  makes it a neutral solution (only water and salt), there must be 100 hydrogen ions in the original acid solution and you can calculate the concentration of the acid solution.

To titrate an unknown acid/base solution, take a certain amount of the unknown solution and add a standard reagent of the known concentration carefully until the neutralization reaction is completed. This point where the number (or mole) of hydrogen ions and hydroxide ions are equal is defined as the equivalence point. To determine the equivalence point, scientists use an indicator or a pH meter.  With the data of volume of the standard reagent used, the concentration of the unknown solution can be calculated.  This whole process is called "titration."

Titration procedure

First, you need to choose volume of an unknown acid or base solution and put it in an erlenmeyer flask

Second, fill a buret with a standard reagent of known concentration and read the initial volume of the solution.  Of course, if you put an acid in the erlenmeyer flask, you need to put a base in the buret and vice versa.  A buret is a good apparatus for the determination of an equivalence point in acid-base titration because you can accurately read the volume of solution used. 

Third, add a couple of drops of an indicator in the flask for titration.  An indicator is a soluble dye that changes its color noticeably over a fairly short range of pH. Different indicators show color changes at different pH values and it is important to determine an indicator to be used according to the expected equivalence point.

If a pH meter is available, put a pH electrode in the flask.

 

First, you need to choose volume of an unknown acid or base solution and put it in an erlenmeyer flask. 

Second, fill a buret with a standard reagent of known concentration and read the initial volume of the solution.  Of course, if you put an acid in the erlenmeyer flask, you need to put a base in the buret and vice versa.  A buret is a good apparatus for the determination of an equivalence point in acid-base titration because you can accurately read the volume of solution used. 

Third, add a couple of drops of an indicator in the flask for titration.  An indicator is a soluble dye that changes its color noticeably over a fairly short range of pH. Different indicators show color changes at different pH values and it is important to determine an indicator to be used according to the expected equivalence point.

If a pH meter is available, put a pH electrode in the flask. 

Fourth, slightly open the cork of the buret and add the standard reagent into the unknown solution.  Around the expected equivalence point of the titration, you need to drop the solution very slowly and mix the solutions very well because, around the equivalence point, just one drop of solution from the buret can make a radical pH change in the mixed solution.  If the color of the solution in the erlenmeyer flask changes, record the volume of the solution in the buret and add a few drops of the solution to make sure the the equivalence point you found is correct.

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For example, if you choose 10.0 mL hydrochloric acid solution as a concentration-unknown solution and it takes 10.0 mL of 0.100 M sodium hydroxide solution to titrate, the concentration of the hydrochloric acid is 0.100 M.

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Finally, using the data from your acid-base titration, you can calculate the concentration of the unknown solution. The equation for this procedure is as follows. (Here, M means molarity (concentration) of solution , and V means volume of solution.)

 

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