Acids, Bases, and Buffers 1: Monoprotic and Polyprotic Acids Overview Getting Started Techniques Procedure FAQ Full Lab Manual Introduction & Goals Chemistry & Background Key Questions Prelab Problems Safety Procedure In Your Write-up Experiments Index ChemLab Home		Titration of Acetic Acid Your titration of acetic acid will be done with a pH meter and a large volume of titrant, to observe the entire titration curve. Use a beaker large enough to contain the volume and to hold the pH meter electrode. Arrange the experiment so that the stir bar, electrode, buret, and your hand all fit comfortably. Record the initial pH and initial volume reading. Then you can begin to add titrant. As you titrate, you will record pH vs. volume of base added. A plot of this data is the experimental titration curve for the acid. In the buffer regions, the pH does not change rapidly, with the addition of base. Titrant can be added in larger aliquots between pH measurements. You can add titrant in a stream in these regions. Near the equivalence points, the pH changes rapidly with the addition of base. Add titrant in smaller portions in these regions, so you measure the titration curve as accurately as you can. Plot your data by hand or with a graphing calculator as you measure it. This will help you to anticipate the equivalence point and assure you of the quality of your data, as you are taking it. If your buret nears the 50 mL mark, record the volume and refill it with titrant. Remember to take a new reading after you refill and before you resume titrating. Making and testing a buffer Acetic acid and solid sodium acetate will be available for you to use in making a buffer. Use a top loading balance to weigh the solid. Prelab Problem 3. Describe how to make 100 mL of buffer solution from 10.0 mL of 1.5 M acetic acid and an equal number of moles of sodium acetate. Sodium acetate will be supplied as solid sodium acetate trihydrate (MW = 136.080), so calculate the number of grams needed. What will be the pH of your buffer? How would you make 100 mL of a buffer of pH 5, starting from 10.0 mL of 1.5 M acetic acid and a supply of solid sodium acetate trihydrate? Equation 6 for acetic acid and the pKa values in Table 1 will be useful here. You can also use the molecular mass calculator in the periodic table applet, elsewhere on this website. Add the solid sodium acetate to the vinegar in a beaker and mix thoroughly. Transfer this solution to a graduated cylinder, with rinsing, and dilute to a final volume of 100 mL. Transfer the buffer from the graduated cylinder to a clean, dry beaker and mix well. Measure the pH of your buffer, when it is thouroughly mixed. What do you expect the pH value to be? Divide your buffer solution into two equal portions. Titrate one half with acid to a low pH limit. Titrate the other half with base to the adjacent equivalence point. Your goal is to determine the buffer capacity in moles H+ or OH- required to change the pH by one unit for one liter of buffer solution. Titration of Phosphoric Acid Titrate 100 mL of phosphoric acid with standardized NaOH solution, a 50 mL buret, and a magnetic stirrer. Remember to refill the buret, if you are close to the 50.0 mL mark.
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