Acid and Base Concentrations
At the end of this lab, students should know...
What are Arrhenius acids and bases?
An Arrhenius acid is an acid that dissociates and produces hydrogen ions when dissolved in water, whereas an Arrhenius base produces hydroxide ions when dissolved in water.
What is pH, and how is it calculated?
pH is a measure of the degree of acidity or alkalinity of a solution. It is calculated as the negative log base 10 of the concentration of hydrogen or hydronium ions...
Source: Smaa Koraym at Johns Hopkins University, MD, USA
- Preparation of Reagents
Here, we show the laboratory preparation for 10 students working in pairs, with some excess. Please adjust quantities as needed.
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Prepare 25 mL of 50% w/w NaOH solution. Weigh 25 g of NaOH pellets and add them to a 125-mL Erlenmeyer flask.
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Measure 25 mL of deionized water and add it to the flask.
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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.
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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.
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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.
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Fill the flask with deionized water to the volume marking. Add a magnetic stir bar and mix the solution until it is homogeneous.
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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.
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Prepare 5 mL of 1% phenolphthalein. Use a volumetric pipette to transfer 2.5 mL of ethanol into an Erlenmeyer flask.
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Weigh 0.05 g of phenolphthalein and add it to the ethanol. Rinse the weigh boat with ethanol to transfer residual phenolphthalein.
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Add a stir bar and mix the solution until the phenolphthalein dissolves and the solution appears homogeneous.
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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.
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Prepare dry potassium hydrogen phthalate (KHP). Weigh approximately 20 g of stock KHP into a glass container with a lid.
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Place the uncovered glass container in an oven at 125 °F to dry the KHP overnight.
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When the KHP is dry, secure the lid and set it by the balance to cool.
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Make sure there is a clean and dry spatula, several weigh boats, and a 150-mL beaker labeled, ‘Excess’.
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- Preparation of Laboratory
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Set out the following glassware and equipment at each lab station (we suggest that students work in pairs):
150-mL glass burette 160-mL plastic burette with 2-valve system 3250-mL Erlenmeyer flasks 110-mL graduated cylinder 150-mL graduated cylinder 1100-mL beaker 1250-mL beaker 1400-mL beaker 1Glass stirring rod 1Ring stand 1Medium 2-prong clamp 1Burette clamp 11-mL pipette 151-mL pipette tips 1500-mL polyethylene bottle with cap 1Drop counter 1pH sensor probe 1Stirplate 1Stir bar 1Data acquisition system 1Flash drive
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View the full transcript and gain access to JoVE Lab Manual videos
Q1: How do you prepare a 50% w/w NaOH solution in the laboratory?
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. Mix on a stir plate with a magnetic stir bar until homogeneous. The solution generates heat, so allow it to cool before transferring to a labeled polyethylene bottle for storage in a corrosive storage cabinet.
Q2: What is the procedure for diluting stock phosphoric acid to 0.5 M concentration?
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. Mix with a magnetic stir bar until homogeneous, then transfer to a labeled polyethylene bottle and store in a corrosive storage cabinet.
Q3: How is phenolphthalein indicator solution prepared for titration experiments?
Transfer 2.5 mL of ethanol into an Erlenmeyer flask and add 0.05 g of phenolphthalein. Stir until dissolved, then rinse the weigh boat with ethanol to transfer residual phenolphthalein. Add 2.5 mL of deionized water and mix until homogeneous. Transfer the solution to a labeled dropper bottle for use.
Q4: Why is potassium hydrogen phthalate dried before use in acid-base titrations?
Potassium hydrogen phthalate (KHP) is dried overnight in an oven at 125 °F to remove moisture that could affect the accuracy of titration calculations. Dry KHP serves as a reliable primary standard for standardizing base solutions. Once dried, the container is sealed and cooled at room temperature before use.
Q5: What glassware and equipment are essential for each student pair in an acid-base titration lab?
Each station requires a 50-mL glass burette, 60-mL plastic burette, three 250-mL Erlenmeyer flasks, graduated cylinders, beakers of various sizes, a glass stirring rod, ring stand with clamps, pipettes, a pH sensor probe, stir plate with magnetic stir bar, and a data acquisition system to record measurements.
Q6: How should corrosive reagents like NaOH and phosphoric acid be stored after preparation?
Both NaOH and phosphoric acid solutions must be transferred to labeled polyethylene bottles and stored in a corrosive storage cabinet. Polyethylene containers resist chemical degradation from strong acids and bases, preventing leaks and contamination. Proper labeling ensures safe identification and prevents accidental misuse.
Q7: What precautions should be taken when preparing concentrated NaOH solutions?
When dissolving NaOH pellets in water, the reaction is highly exothermic and produces significant heat. Cover the flask with paraffin film during mixing and allow the solution to cool completely before handling or transferring. This prevents burns and ensures safe handling of the caustic reagent.