4.1: Titration of a Diprotic Acid - Experiment

General Chemistry II Lab Manual · LibreTexts

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    SAFETY PRECAUTIONS
    • Chemical splash goggles must be worn at all times in the laboratory.
    • Sodium hydroxide titrant and unknown weak acid solutions are caustic and corrosive. Avoid contact with skin and eyes. Flush affected areas with water immediately.

    EQUIPMENT AND CHEMICALS NEEDED

    Table \(\PageIndex{1}\): Materials Needed
    Equipment Chemicals
    Vernier pH Sensor & LabQuest interface Unknown diprotic weak acid (H2A) solution
    50-mL buret, buret clamp, & ring stand 0.10 M NaOH titrant solution
    Magnetic stirrer, stir bar, & utility clamp pH 4.00 & pH 7.00 buffer solutions
    20.00-mL volumetric pipette & pipette pump/bulb Deionized water
    150-mL beaker, 250-mL beaker, & wash bottle Aqueous Acid-Base Waste container

    EXPERIMENTAL PROCEDURE

    Part A: Set Up the Titration

    VERNIER TECH BOX: DIPROTIC TITRATION SETUP

    1. Sensor Connections & pH Calibration (All Users)

    • Connect the pH Sensor to CH 1 of the LabQuest interface.
    • Tap the red pH readout → Select Calibrate → Tap Calibrate Now.
    • Point 1: Rinse probe, submerge in pH 4.00 Buffer. Enter "4.00" and tap Keep.
    • Point 2: Rinse probe, submerge in pH 7.00 Buffer. Enter "7.00" and tap Keep.
    • Tap OK.

    2. Method A (No Drop Counter): Manual Titration

    • Tap Mode. Select Events with Entry.
    • Set Name: Volume | Units: mL.
    • In this mode, add titrant, wait for pH to stabilize, tap Keep, and manually type in the total buret volume reading.
    • Tip: Take small additions (0.10 mL) around the expected first equivalence point to capture the first inflection step clearly.

    3. Method B (With Drop Counter): Automated Titration

    • Connect the Drop Counter to DIG 1. Set Mode to Drop Counting.
    • CRITICAL STEP: Calibrate Drops
      • Tap red Drop Counting box → Calibrate → Calibrate Now.
      • Place a 10-mL graduated cylinder below the drop tip. Open valve to collect 9–10 mL. Close valve.
      • Enter exact cylinder volume (e.g., 9.40 mL) and tap Keep. Record calibration factor on your data sheet.
    • Open the stopcock valve to maintain a slow, steady drip (1–2 drops/sec). The system will plot the double-inflection curve automatically.
    1. Measure 30 mL of deionized water into a clean 150-mL beaker. Use a 20.00-mL volumetric pipette and bulb/pump to transfer 20.00 mL of the unknown diprotic acid into the beaker.
    2. Place the magnetic stirrer on the base of the ring stand. Carefully place a magnetic stir bar into the beaker and turn on the stirrer to maintain gentle mixing.
    3. Rinse the pH sensor thoroughly with deionized water and place it into the acid solution. Ensure the spinning stir bar does not strike the glass bulb of the pH probe. Clamp the probe securely with a utility clamp.
    4. Obtain approximately 100 mL of 0.10 M \(\mathrm{NaOH}\) solution in a clean 250-mL beaker. Record the exact \(\mathrm{NaOH}\) concentration in your data table.
    5. Rinse and fill the 50-mL buret with \(\mathrm{NaOH}\) solution. Clamp the buret above the reaction beaker.

    Part B: Titrate the Unknown Diprotic Acid

    1. Confirm the initial pH is acidic.
    2. Method A (No Drop Counter)
      1. Record the initial buret volume reading (\(\pm 0.02\text{ mL}\)) and initial pH.
      2. Add \(\mathrm{NaOH}\) titrant in increments of no more than 0.50 mL, recording total buret volume and pH after each addition.
      3. Continue adding \(\mathrm{NaOH}\) titrant until pH reaches at least 11.0. CRITICAL: As you approach each equivalence region, reduce titrant additions to small dropwise increments (0.10–0.20 mL) to capture both inflection points accurately
    3. Method B (With Drop Counter).
      1. Press the green start triangle on the LabQuest. The system will wait until it sees the drops start.
      2. Open the stopcock to start the LabQuest collecting data.
      3. Monitor the data collection making sure that the drop rate is not too fast (about one drop every 1.5-2.0 seconds is best).
      4. Stop the acquisition when the pH reaches at least 11.0.
    DATA SKILL: RESOLUTION CHECK

    Before emptying your beaker: Inspect your titration curve on the screen.

    • Can you observe two distinct inflection steps?
    • The first equivalence point of a diprotic acid is often subtle (a minor rise in slope) compared to the second steep cliff. If base was added too rapidly early on, the first step may be obscured, preventing accurate determination of \(\mathrm{p}K_{\mathrm{a1}}\).
    • Decision Rule: If the first step is unresolvable, repeat the titration using smaller, dropwise additions (0.10 mL) throughout the initial buffer and equivalence region.

    Part C: Repeat the Titration

    1. Perform a second titration trial using a fresh 20.00-mL sample of unknown diprotic acid following Steps 1–7/8.
    2. Rinse the pH sensor thoroughly with deionized water and return it to its storage solution bottle.
    CHEMICAL DISPOSAL
    • Dispose of all titrated solutions and excess chemicals in the designated Aqueous Acid-Base Waste container. Clean all glassware before returning equipment.

    This page titled 4.1: Titration of a Diprotic Acid - Experiment was last modified on Wed, 23 Sep 2026 20:08:53 GMT and is shared under a CC BY 4.0 license and was authored, remixed, and/or curated by Vince Hradil.