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