Purpose: To determine the amount of acid in an unknown and become familiar with the techniques of titration.
Hypothesis: If one uses a neutralization reaction for a solution, one can determine the concentration of a solution.
Materials: • 50mL burette • 600mL beaker • 500mL Erlenmeyer flask • 250mL Erlenmeyer flasks(3) • Weighing bottle • Ring stand and ring • Balance • Bunsen burner and hose • 1 pint bottle with rubber stopper • Wash bottle • Burette clamp and ring stand • Wire gauze
Chemicals: • 19M NaOH (reduced the amount) • Potassium hydrogen phthalate • Phenolphthalein solution • Unknown acid
1.) Preparation of a Burette for Use
Clean a 50mL burette with soap solution and a burette brush and thoroughly rinse with tap water. Then rinse with at least five 10mL portions of distilled water. The water must run freely from the burette without leaving any drops adhering to the sides. Make sure that the burette does not leak and that the stopcock turns freely.
2.) Reading a Burette
To avoid parallax errors when taking readings, the eye must be on a level with the meniscus. Wrap a strip of paper around the burette and hold the top edges of the strip evenly together. Adjust the strip so that the front and back edges are in line with the lowest part of the meniscus and take the reading by estimating to the nearest tenth of a marked division.
3.) Standardization of Sodium Hydroxide (NaOH) Solution
Weigh from a weighing bottle triplicate samples of between 0.4 and 0.6g each of pure potassium hydrogen phthalate (KHP) into three separate 250mL Erlenmeyer flasks. Do not weigh the flasks. Record the masses and label the three flasks in order to distinguish among them. Add to each sample about 100mL of distilled water and two drops of phenolphthalein indicator solution.
Rinse the previously cleaned burette with at least four 5mL portions of the approximately 0.100M NaOH solution that you have prepared. Discard each portion into the designated receptacle. Completely fill the burette with solution and remove the air from the tip by running out some of the liquid into an empty beaker. Make sure that the lower part of the meniscus is at the zero mark or slightly lower. Allow the buret to stand for at least 30 sec before reading the exact position of the meniscus. Remove any hanging drop from the burette tip by touching it to the side of the beaker used for the washings. Record the initial burette reading on your report sheet.
Slowly add the NaOH solution to one of your flasks of KHP solution while gently swirling the contents of the flask. As the NaOH solution is added, a pink color appears where the drops of the base come in contact with the solution. This coloration disappears with swirling. As the end point is approached, the color disappears more slowly, at which time the NaOH should be added drop by drop. It is most important that the flask be swirled constantly throughout the entire titration. The end point is reached when one drop of NaOH solution turns the entire solution in the flask from colorless to pink. The solution should remain pink when it is swirled. Allow the titrated solution to stand for at least 1 min so the burette will drain properly. Remove any hanging drop from the burette tip by touching it to the side of the flask and wash down the sides of the flask with a stream of water from the wash bottle. Record the burette reading on your report sheet. Repeat this procedure with two other samples. Dispose of the neutralized solutions as instructed.
From the data you obtain in the three titrations, calculate the molarity of the NaOH solution to four significant figures.
B.) Analysis of an unknown acid
1.) Clean another burette
2.) Rinse it with small 5mL portions of unknown acid (3-4 times)
3.) Fill above 0.00mL line and purge tip of bubbles and refill to 0.00mL line