A pipette bulb should be used to withdraw and deliver liquids when using a pipette. In this course you will use graduated and volumetric pipettes according to the precision required by the experiment. Graduated pipettes deliver different amounts of liquid by making use of incremental markings inscribed along the pipette. Volumetric pipettes deliver fixed amounts of liquid indicated by a single graduation mark inscribed on them. The volumetric pipette possesses an enlarged portion below the graduation mark in order to reduce the speed of suction and allow a precise volume reading.

Before use, burettes must be rinsed first with distilled water, then with a small portion of the liquid to be measured as follows: add approximately 2 mL of liquid to the burette, hold the burette horizontally and rotate it to allow the liquid to coat the entire inside. Drain the liquid down the tip. When filling the burette using a funnel, make sure the funnel is perfectly clean, the stopcock is closed, and the burette is over a waste beaker. If you see air bubbles trapped inside the tip of the burette, eliminate them by draining a few mL of the liquid out the tip while holding the burette diagonally. Your mentor will demonstrate the correct way to rinse and fill the burette.

a) Determining the mass of distilled water that fills the TC-ware until the bottom of the meniscus coincides with the graduation mark; or collecting the amount of water delivered by the TD-ware in a tared beaker. Repeat the procedure at least three times to assure reproducibility.
b) Measure the exact temperature of the water with a 0.1°C precision thermometer before each measurement.
c) Calculate the calibrated volume using the density of water at the calibration temperature (Knovel Critical Tables http://www.knovel.com/knovel2/crc_handbook.jsp)

1. Record the name of the manufacturer. Fill up the information as in the example below.

2. Using Excel compute the average, variance, and standard deviation for each piece of glassware. Compute the same statistics for the mass determined with the Mettler balance
3. Find the uncertainty given by the glassware manufacturer and by the Mettler Company
4. Compare the values of the computed statistic with the uncertainty provided by the manufacturers. Comment on the statistics given by the manufacturer.
5. Discuss the precision of the calibrated volume for each piece of glassware and for the Mettler balance
6. Report your conclusions

1. Tare two watch glasses and weigh on each a small amount of sodium hydroxide, NaOH and potassium hydrogen phthalate, KHP, a monoprotic acid. Record the masses to the nearest 1 mg. Leave the systems exposed to air for 25 minutes.
2. While you are waiting for the watch glass systems, take a 400 mL beaker of distilled water and measure the pH of the water. Is this the value you expect? Boil the distilled water uncovered with a stir bar for 10 minutes and let it cool to room temperature covered with a watch glass. Measure the pH again. Is there a difference in the reading obtained? If so, discuss why.
3. After time has elapsed, weigh the watch glasses and record your observations. Based on the observations you made, decide which one of the two substances can be weighed directly and dissolved in a well-measured volume of water to prepare a solution of exact concentration.
4. Prepare a 10-1M solution of the substance you decided on in step 3.
5. Calculate the volume needed from this concentrated solution to prepare 250 mL of a 10-2M solution. Use a volumetric pipette to withdraw the aliquot calculated from the concentrated solution. Is it OK to use a graduated pipette to take the aliquot? Set this solution aside to determine its exact concentration later.

PART 3. Standardization of Sodium Hydroxide Solution

After preparing a solution with the substance that changed while exposed to air, it is necessary to perform a standardization (find the exact concentration of the solution) using a procedure called TITRATION. The solution should be kept minimally exposed to air after standardization. The TITRATION procedure entails reacting a volume of a solution whose concentration is exactly known or a mass of solid with known molecular weight with a volume of a solution which concentration is unknown. The volume of sodium hydroxide solution used in a manual titration is the volume needed to reach the endpoint of the titration (where the indicator changes color), while the one obtained from the potentiometric titration is referred to as the equivalence point of the titration. At the equivalence point the number of moles of titrant used is equal to the number of moles of the titrated compound.

1. Add about 450 mL of distilled water to about 150 mL of NaOH (aq) solution provided (note the approximate concentration of the NaOH(aq) from the bottle). Cover the bottle and homogenize. Neither of these volumes needs to be exact since the solution will be standardized. Keep the dilute solution covered as much as possible.
2. Weigh two samples of about 0.2-0.3 g of KHP (Potassium Hydrogen Phthalate, record exactly the amount of KHP weighed) and quantitatively transfer them into two separate, clean 250 mL Erlenmeyer flasks. Dissolve the samples in about 100 mL distilled water (you may need to slightly heat for complete dissolution, but make sure the solutions cool to room temperature before titrating) and add 2-3 drops of phenolphthalein indicator.
3. Rinse a clean burette with distilled water and then with small portions of the dilute solution prepared in step 1. Discard the rinse solutions in the liquid waste container and clamp the burette to the burette clamp. Fill the burette with the dilute solution prepared in step 1.
4. Knowing the exact amount of KHP and its molar mass (204.22 g/mol), can you estimate the approximate volume of dilute sodium hydroxide solution needed to reach the endpoint of the titration?
5. Titrate the KHP solution with the diluted NaOH solution from the burette, swirling continuously, until the faint pink color lasts about 20 seconds. Each group member should perform at least one manual titration.
6. Calculate the concentration of the solution from the volume used in each titration. If the values of the volume used for each titration fall within the experimental error, use the average value as the concentration of the solution. If the values are scattered showing low precision, repeat the titration one more time.

Time(min)	Mass NaOH (g)	Mass KHP (g)
0
25

Standardization of NaOH

Manual Titration

	Trial 1	Trial 2	Trial 3
Weight of KHP used (g)
Initial burette reading (mL)
Final burette reading (mL)
Volume NaOH used (mL)

Discussion

Discuss with your group whether the endpoint volume of the equivalence point should be used to calculate the concentration of the titrated sodium hydroxide solution. What is the exact concentration of the sodium hydroxide solution?

Comment on the pH range at which phenolphthalein changes color. Is phenolphthalein a good indicator for standardizing NaOH solutions against KHP? Explain.

1Skoog D.A., West D.M., Analytical Chemistry, 2ndEd, pp. 89-91, Holt, Rinehart and Winston (1965).