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Experiment #10
Volumetric Analysis
The Titration of Acids and Bases

This experiment will allow you to gain practical experience in the preparing standard solutions, using a pipette and a buret, and performing standard titrations. You will use this experience to experimentally determine the concentration of acetic acid in an unknown solution. There are three parts to this experiment, they are 1. Preparation a standard solution of oxalic acid (~0.07 M) 2. Preparation a sodium hydroxide solution (~0.1M) which will be standardized using the standard oxalic acid solution. 3. Determination of the concentration of acetic acid in an unknown vinegar solution. (0.1-0.2M)


Standard solutions are solutions with known concentrations, generally to four significant figures. There are two different ways to make a standard solution. We can make a primary or a secondary standard. A primary standard is prepared directly by dissolving a known mass of sample to make a known volume of solution. A secondary standard is prepared by dissolving an approximate amount of sample into a volume of solvent and determining its exact concentration through titration experiments. Primary standards are prepared from compounds that are at least 99.9% pure, have a definite composition, are water soluble, are easily weighed, and do not change composition on contact with air. Oxalic acid dihydrate (H2C2O4•2H2O) fits these criteria and therefore may be used as a primary standard. Sodium hydroxide absorbs water when it comes into contact with air and therefore it is difficult to obtain a pure, dry sample to weigh. For this reason the sodium hydroxide solution will be titrated with the oxalic acid standard to become a secondary standard.

In the first part of this experiment you will prepare a solution of known concentration of oxalic acid. The oxalic acid crystallizes with two water molecules per oxalic acid in the crystalline network. For this reason, we will weigh out an appropriate amount of oxalic acid dehydrate to dissolve in water. The water molecules in the crystal network will become part of the water of solution once it is dissolved. For this reason the molar concentration of oxalic acid dihydrate will be the same as the molar concentration of oxalic acid.

In order to standardize the sodium hydroxide solution you will perform a titration. Sodium hydroxide reacts with oxalic acid according to the reaction below: H2C2O4(aq) + 2 NaOH(aq) ( 2 H2O(l) + Na2C2O4(aq)

You will measure a 25.00 mL aliquot of the oxalic acid solution into a flask and add an indicator. An indicator is a substance that changes color when a solution changes from acidic to basic. The common indicator used for acid base titrations is phenolphthalein. Phenolphthalein is colorless in a solution that is acidic and bright pink in a solution that is basic. In this titration the oxalic acid solution is acidic and therefore phenolphthalein will be colorless. The sodium hydroxide solution will be added drop wise from a buret into the flask containing the oxalic acid and indicator. As the sodium hydroxide is added to the flask it will react with the oxalic acid and be neutralized. At the point where all of the oxalic acid is reacted, the next drop of sodium hydroxide will make the entire solution basic and it will turn pink. At this point you have completed the titration.

In order to determine the concentration of acetic acid in the vinegar solution you will titrate it with the standardized sodium hydroxide solution. The equation for this reaction is HC2H3O2(aq) + NaOH(aq) ( H2O(l) + NaC2H3O2(aq)

In order to get the best precision possible, you should repeat each titration until you get 3 trial that are within 1% of each other.


Safety Notes
➢ Wear safety glasses at all times.

Preparation of Standard Oxalic Acid Solution
Carefully weigh a 100 or 150 mL...
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