Acid and Base Concentrations

Source: Smaa Koraym at Johns Hopkins University, MD, USA

1. Preparation of ReagentsExpand

   Here, we show the laboratory preparation for 10 students working in pairs, with some excess. Please adjust quantities as needed.

   • Prepare 25 mL of 50% w/w NaOH solution. Weigh 25 g of NaOH pellets and add them to a 125-mL Erlenmeyer flask.
   • Measure 25 mL of deionized water and add it to the flask.
   • Add a magnetic stir bar and mix the solution on a stir plate until the solution appears homogeneous. After mixing, cover the opening of the flask with paraffin film. Note: The solution produces heat, so allow it to cool before handling.
   • Once cooled, use a powder funnel to pour the solution into a polyethylene bottle labeled ‘50% w/w NaOH’. Store the solution in a corrosive storage cabinet until it is needed.
   • Prepare 25 mL of 0.5 M phosphoric acid. Use a 1-mL volumetric pipette to measure 0.85 mL of stock 14.8 M phosphoric acid and add it to a 25-mL volumetric flask.
   • Fill the flask with deionized water to the volume marking. Add a magnetic stir bar and mix the solution until it is homogeneous.
   • Use a powder funnel to transfer the solution to a polyethylene bottle labeled ‘0.5 M H₃PO₄’. Store the solution in a corrosive storage cabinet until it is needed.
   • Prepare 5 mL of 1% phenolphthalein. Use a volumetric pipette to transfer 2.5 mL of ethanol into an Erlenmeyer flask.
   • Weigh 0.05 g of phenolphthalein and add it to the ethanol. Rinse the weigh boat with ethanol to transfer residual phenolphthalein.
   • Add a stir bar and mix the solution until the phenolphthalein dissolves and the solution appears homogeneous.
   • Add another 2.5 mL of deionized water to the flask and stir. Once the solution is well-mixed, transfer it to a labeled dropper bottle.
   • Prepare dry potassium hydrogen phthalate (KHP). Weigh approximately 20 g of stock KHP into a glass container with a lid.
   • Place the uncovered glass container in an oven at 125 °F to dry the KHP overnight.
   • When the KHP is dry, secure the lid and set it by the balance to cool.
   • Make sure there is a clean and dry spatula, several weigh boats, and a 150-mL beaker labeled, ‘Excess’.
2. Preparation of LaboratoryExpand
   • Set out the following glassware and equipment at each lab station (we suggest that students work in pairs):

     1    50-mL glass burette

     1    60-mL plastic burette with 2-valve system

     3    250-mL Erlenmeyer flasks

     1    10-mL graduated cylinder

     1    50-mL graduated cylinder

     1    100-mL beaker

     1    250-mL beaker

     1    400-mL beaker

     1    Glass stirring rod

     1    Ring stand

     1    Medium 2-prong clamp

     1    Burette clamp

     1    1-mL pipette

     15    1-mL pipette tips

     1    500-mL polyethylene bottle with cap

     1    Drop counter

     1    pH sensor probe

     1    Stirplate

     1    Stir bar

     1    Data acquisition system

     1    Flash drive

Source: Smaa Koraym at Johns Hopkins University, MD, USA

1. Preparation of ReagentsExpand

   Here, we show the laboratory preparation for 10 students working in pairs, with some excess. Please adjust quantities as needed.

   • Prepare 25 mL of 50% w/w NaOH solution. Weigh 25 g of NaOH pellets and add them to a 125-mL Erlenmeyer flask.
   • Measure 25 mL of deionized water and add it to the flask.
   • Add a magnetic stir bar and mix the solution on a stir plate until the solution appears homogeneous. After mixing, cover the opening of the flask with paraffin film. Note: The solution produces heat, so allow it to cool before handling.
   • Once cooled, use a powder funnel to pour the solution into a polyethylene bottle labeled ‘50% w/w NaOH’. Store the solution in a corrosive storage cabinet until it is needed.
   • Prepare 25 mL of 0.5 M phosphoric acid. Use a 1-mL volumetric pipette to measure 0.85 mL of stock 14.8 M phosphoric acid and add it to a 25-mL volumetric flask.
   • Fill the flask with deionized water to the volume marking. Add a magnetic stir bar and mix the solution until it is homogeneous.
   • Use a powder funnel to transfer the solution to a polyethylene bottle labeled ‘0.5 M H₃PO₄’. Store the solution in a corrosive storage cabinet until it is needed.
   • Prepare 5 mL of 1% phenolphthalein. Use a volumetric pipette to transfer 2.5 mL of ethanol into an Erlenmeyer flask.
   • Weigh 0.05 g of phenolphthalein and add it to the ethanol. Rinse the weigh boat with ethanol to transfer residual phenolphthalein.
   • Add a stir bar and mix the solution until the phenolphthalein dissolves and the solution appears homogeneous.
   • Add another 2.5 mL of deionized water to the flask and stir. Once the solution is well-mixed, transfer it to a labeled dropper bottle.
   • Prepare dry potassium hydrogen phthalate (KHP). Weigh approximately 20 g of stock KHP into a glass container with a lid.
   • Place the uncovered glass container in an oven at 125 °F to dry the KHP overnight.
   • When the KHP is dry, secure the lid and set it by the balance to cool.
   • Make sure there is a clean and dry spatula, several weigh boats, and a 150-mL beaker labeled, ‘Excess’.
2. Preparation of LaboratoryExpand
   • Set out the following glassware and equipment at each lab station (we suggest that students work in pairs):

     1    50-mL glass burette

     1    60-mL plastic burette with 2-valve system

     3    250-mL Erlenmeyer flasks

     1    10-mL graduated cylinder

     1    50-mL graduated cylinder

     1    100-mL beaker

     1    250-mL beaker

     1    400-mL beaker

     1    Glass stirring rod

     1    Ring stand

     1    Medium 2-prong clamp

     1    Burette clamp

     1    1-mL pipette

     15    1-mL pipette tips

     1    500-mL polyethylene bottle with cap

     1    Drop counter

     1    pH sensor probe

     1    Stirplate

     1    Stir bar

     1    Data acquisition system

     1    Flash drive

Before starting, wear the appropriate personal protective equipment, including a lab coat, gloves, and chemical splash goggles. Be sure to work in a chemical fume hood whenever using high concentrations of acids or bases. First, you'll need to prepare 50%sodium hydroxide by weight.

To prepare 25 milliliters of the final solution, start by weighing 25 grams of sodium hydroxide pellets. Then, pour them into a 125-milliliter Erlenmeyer flask. Using a graduated cylinder, measure 25 milliliters of deionized water, and pour it into the flask.

Add a magnetic stir bar, then set the flask on the stir plate. Turn on the stir setting, and allow the solution to mix until it appears homogeneous. Then, turn the stir setting off, and cover the opening of the flask with paraffin film.

The solution produces heat, so allow it to cool before handling. Once cooled, obtain a polyethylene bottle, and label it 50%by weight sodium hydroxide'Then, place a powder funnel in the bottle, and pour the solution into it. Cap the bottle, and store the solution in the proper corrosives cabinet until the lab session.

Next, we will prepare the 0.5 molar solution of phosphoric acid. For this you will need a stock solution of 14.8-molar phosphoric acid. To prepare 25 milliliters of solution, use a 1-milliliter glass volumetric pipette to measure 0.85 milliliters of stock phosphoric acid.

Add it to a 25-milliliter volumetric flask, and fill the flask with deionized water to the volume marking. Add a stir bar, and set the flask on a stir plate. Stir the solution until it is homogeneous.

Then, turn the stir setting off, and label a 125-milliliter polyethylene bottle as 0.5 molar phosphoric acid'Using a powder funnel, transfer the solution to the bottle and cap it. Store the solution in the proper corrosives cabinet until the lab session. Now, let's prepare the 1%phenolphthalein needed for the experiment.

To prepare 5 milliliters of solution, use a volumetric pipette to transfer 2.5 milliliters of ethanol into an Erlenmeyer flask. Then, weigh 0.05 grams of phenolphthalein, and add it to the ethanol. To transfer residual phenolphthalein, use a pipette to rinse the weigh boat with some of the ethanol.

Then, place a stir bar in the solution, and set the flask on the stir plate. Turn the stir setting on, and mix until the phenolphthalein dissolves and the solution appears homogeneous. Then, stop stirring, and add 2.5 milliliters of deionized water to the solution before turning the stir function back on.

Allow the solution to mix until it appears homogeneous. Then, transfer the solution to a labeled dropper bottle. Lastly, let's prepare the dry potassium hydrogen phthalate, or KHP.

Weigh approximately 20 grams of stock KHP into a glass container with a lid. Place the uncovered glass container in an oven at 125 degrees Fahrenheit to dry the KHP overnight. When the KHP is dry, remove it from the oven, and secure the lid.

Then, set it by the balance to cool to room temperature along with a clean and dry spatula, several weigh boats, and a 150-milliliter beaker labeled Excess'Now, let's get the equipment set up at each lab station. First, each lab group will need glassware, including a 50-milliliter glass burette, a 60-milliliter plastic burette with a two-valve system, three 250-milliliter Erlenmeyer flasks, a 10 and 50-milliliter graduated cylinder, a 100, 250, and 400-milliliter beaker, and a glass stirring rod. The lab groups will also need a ring stand, a medium two-prong clamp, a burette clamp, a 1-milliliter pipette with 10 to 15 tips, and a 500-milliliter polyethylene bottle with a cap.

Finally, the students will need a drop counter, pH sensor probe, stir plate and stir bar, along with a data acquisition system and flash drive.