Acid and Base Concentrations

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’.

• 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