To determine the concentration of acetic acid in different household vinegars by titrating with a standard sodium hydroxide solution.
Specific Learning Objectives:
By completing this exercise, the user will 1) gain a familiarity with stoichiometry, 2) express solution concentration in moles per liter 3) use a titration curve to determine the equivalence point and the end point of the titration.
For thousands of years people have known that vinegar, lemons and many other foods taste sour. The reason why was discovered hundreds of years later; they are all acids. The first fundamental definition of an acid was a substance that tasted sour and was corrosive to metals. A base, on the other hand, feels slippery and changes litmus paper blue.
In the late 1800's, Arrhenius suggested that acids are compounds that, when dissolved in water, release hydrogen ions (H+) into solution. For example, hydrofluoric acid dissolves in water as follows:
HF → H+ + F-
Likewise, bases were defined as compounds that, when dissolved in water, release hydroxide ions (OH-) into solution. For example, sodium hydroxide dissolves in water as follows:
NaOH → Na+ + OH-
We know that acids release H+ ions into solution and bases release OH- ions into solution, but what happens when you mix an acid and a base together? The H+ ions combine with the OH- ions to neutralize the mixture by forming water.
H+ + OH- → H2O
Neutralization indicates the formation of water and a salt. When hydrochloric acid is mixed with sodium hydroxide the following reaction occurs:
Acid Base Salt Water
HCl + NaOH → NaCl + H2O
The formation of a salt is a byproduct of the dissociation of the H+ and OH- to form water. Now that we know the products of a general acid/ base reaction, how can we express how much acid or base is present?
The pH of a solution is defined as the concentration of hydrogen ions (H+) present in solution.
pH = - log [H+]
We can state the pH of a solution as a way to denote the amount of acid present in solution on a scale that ranges from 0 to 14. This allows for an easy comparison of acid and base strength. Acids have pH< 7, and bases have pH>7, with water being neutral at pH=7. Click here to see a pH chart of common acids and bases
Concentration is a measurement of the amount of substance per volume of solution. For our purposes, concentration will be expressed as molarity, the number of moles of substance per liter of solution. Click here for a review of concentration and molarity.
The concentration of a solution can also be expressed as a percentage.
|% Composition =||mass solute
For our purposes, the mass of solute is the mass of the acid present in the vinegar. This can be determined from a mass to mol conversion. The mass of solution is the mass of the vinegar before the titration. This can also be used as a way to compare acidity among multiple acid solutions.
A titration is an experimental procedure used to determine the volume of a solution needed to react with a given amount of another substance. When titrating vinegar, which is comprised of acetic acid, with sodium hydroxide, the reaction is:
CH3COOH + NaOH → CH3COO- Na+ + H2O
We rely on the stoichiometry, the relationship between the amount of reactants to the amount of product formed, to determine the amount of acid present in vinegar. The equivalence point is the point in the titration in which the acid and base are present in equal quantities. For a review of equivalence point, Click here In this experiment, we will monitor the titration with a pH probe. The region of most rapid pH change will be used to determine the equivalence point. The volume of NaOH added at this point will be used to calculate the acetic acid content of vinegar.
Experimental Data Collection:
To determine the concentration of acetic acid in vinegar, we will perform a titration according to the following procedure.
A burette of standardized 1.0M NaOH solution is attached to a burette stand and the initial volume is recorded. Then, 25 mL of vinegar is added to a graduated cylinder. To obtain the mass, place a 100 mL beaker on a scale, tare, and add the 25 mL of vinegar. Then add a stir bar and set up the apparatus as shown.
|Figure 1. The complete apparatus with burette and pH probe in place.||Figure 2. Close-up to show pH probe is in contact with the vinegar and the burette tip is lower than the lip of the beaker.|
|Figure 3. When reading a burette, read from the bottom of the dark meniscus.||Figure 4. The reagents used; white vinegar and standard 1.0M NaOH.|
Titrate the vinegar by adding volumes of NaOH that change the pH approximately 0.2 units. Continue adding small additions until the equivalence point is reached. Titrate past the equivalence point with slow additions, recording the pH and the burette reading each time.
The pH probe will be interfaced with a computer for data collection. The pH is recorded throughout the entire reaction, resulting in an x-y style graph of pH (on the y-axis) vs. volume of NaOH added (on the x-axis). From this graph, the equivalence point can be determined, which denotes the volume of NaOH needed to react with the acetic acid present in vinegar.
Referring to the balanced equation for the reaction:
CH3COOH + NaOH → CH3COO- Na+ + H2O
We see that each mole of NaOH added reacts with a mole of acetic acid, therefore:
1 mol NaOH = 1 mol CH3COOH
Consequently, if we can determine the number of mols of NaOH added, we can determine how many mols of CH3COOH were present in the vinegar. Using these relationships, we can determine the concentration in molarity (M) of the vinegar.
We also want to express the concentration of acetic acid in vinegar as a percentage. To do this, we refer back to the percent composition equation. To determine the mass of solute (in this case acetic acid, CH3COOH), we must convert the number of mols solute into mass in grams. The molecular mass of the solute can be determined from a periodic table and the conversion is as follows:
|mass substance (g) = mole substance x molecular mass of substance (||g
Click here for more information on calculating molecular mass. Suppose we have 3 mols of sodium chloride (NaCl) in 500 g solution and we want to know the percent composition:
|mass NaCl(g) = 3 mole NaCl x 58.44||g
|% Composition=||175.32g NaCl
|x 100% = 35.06% NaCl|
Using the data from the titration, the concentration and percent composition of vinegar can easily be determined.
Volume vinegar: 25.0 mL
Mass vinegar: 24.6785g
Concentration NaOH: 1.0M
Click here to see the titration curve.
To determine the concentration of acetic acid in vinegar:
1.Click on the titration curve above and determine the equivalence point. *Remember: The equivalence point is usually halfway between the region of steepest slope.
2.Convert this volume of sodium hydroxide, NaOH, added to mols of NaOH added.
3. Using the equation to solve Molarity of acetic acid:
|Molarity CH3COOH =||Mole acetic acid
To determine the percent composition:
1.Perform a mol to mass conversion of the mol CH3COOH present.
2.Plug this value, as well as the mass of vinegar into the percent composition equation.
Jessica, a Florida native and avid skydiver, came to Stetson in 2005. She is a double major in Biology and Chemistry, and graduated in May 2009. She hopes to earn her Ph.D in chemistry and teach at the college level.