Titration Method

PROCEDURAL VIDEOS TO WATCH

• • Performing a volumetric titration (Note that he is titrating a solution, so he needs a pipet to begin with a known volume. We won't need pipets since we'll be titrating accurately weighed and dissolved solid samples.)

  • Using an analytical balance (The video says to use weigh paper for masses less than 1.5 g. I prefer weigh boats in almost all cases, but this is a personal preference. In either case the key is not to spill.)

CHEMICALS PROVIDED

• • ~0.05 M NaOH stock solution (MSDS)

  • Potassium hydrogen phthalate (KHC8H4O4 or KHP, MW = 204.221 g/mol), dried for 2 hr at 110°C (MSDS)

  • KHP unknown mixture, dried for 2 hr at 110°C (MSDS)

  • Phenolphthalein indicator (MSDS)

SPECIAL EQUIPMENT

• • Class A buret

  • Ring stand and buret clamp

  • Small plastic bottle for delivering titrant

  • Stir plate

PROCEDURE

Notes on Titrations 

• • 1. Obtaining accurate data is critical for the success of this experiment, and part of your grade will be determined by the quality of your own data. A good titration is one where the solution is clear, then the addition of one more drop (or a fraction of a drop) of titrant turns the solution to the desired endpoint color. 

    2. You’ll be performing a minimum of eight good titrations – four gravimetric (two standardizations and two unknowns) and four volumetric (two standardizations and two unknowns).

    3. For each volumetric titration, be sure to record both the initial and final volume readings on the buret (to 2 decimal places).

    4. For each gravimetric titration, use a top-loading balance (not analytical) to record both the initial and final masses of the plastic bottle containing the titrant. Be sure not to spill any drops. Also, if any titrant splatters onto the side of the titration flask, be sure to wash it into the reaction mixture with a separate squirt bottle of RO water.

A.  Standardization of NaOH Solution

• • 1. Use an analytical balance to accurately weigh approximately 0.2 g of KHP into a weighing boat. Record the exact mass.

    2. Use a funnel and squeeze bottle of RO water to quantitatively transfer the KHP into a 250-mL Erlenmeyer flask.

    3. Add approximately 50-mL of ultrapure water and swirl gently to dissolve the solid.

    4. Add 2-3 drops of phenolphthalein indicator to the solution.

    5. Gently drop a magnetic stir bar into the solution, place the flask on a magnetic stirrer, and begin stirring gently. (Be careful not to splatter solution on the walls of the flask.)

    6. Titrate until the solution turns a faint shade of pink. Remember you want a single drop of titrant to turn the solution from colorless to pink.

    7. Repeat steps 1-6 for total of four good titrations (2 volumetric and 2 gravimetric). Flasks can be re-used, but make sure they've been well-rinsed between trials. (Flasks do not need to be dry.)

B.  Determination of KHP in Unknown 

• • 1. Repeat the steps in part B, but use approximately 0.5 g of unknown solid in place of the KHP. Again record the exact mass of solid and be sure to quantitatively transfer the solid to the flask. 

C.  Data Entry

• • 1. On the class spreadsheet enter the data for each of your titrations. (Notice there are four tabs at the bottom of the where you can switch between sheets for the different types of titration.) 

D.  Waste Disposal

• • 1. All titrated solutions can be poured down the drain.

    2. Collect all unused base solution and pour it into the container labeled “Acid/Base Waste”.

    3. Dump any extra KHP or unknown solid down the drain.

    4. Return the unknown sample vial to the plastic bin in the hood labeled "Used Sample Containers".