Preparation and pH Adjustment of Phosphate Buffer

Step 1: Calibration of the pH Meter

  1. Select the pH 4.01 buffer solution.
  2. Immerse the glass electrode of the pH meter into the buffer solution.
  3. Press the Calibrate button on the pH meter.
  4. Confirm that the pH meter is calibrated to pH 4.01.
  5. Empty the flask containing the buffer solution.
  6. Repeat the calibration process using the pH 7.0 buffer solution.
  7. Repeat once more using the pH 9.21 buffer solution.
  8. The pH meter is now fully calibrated. Proceed to the next step.

Step 2: Preparation of Phosphate Solutions

A. Preparation of 100 mL Na₂HPO₄ Solution

  1. Select the Na₂HPO₄ bottle.
  2. Weigh 1.42 g of Na₂HPO₄ using a weighing balance.
  3. Transfer the weighed Na₂HPO₄ into a clean volumetric flask.
  4. Add distilled water to the flask up to the 100 mL mark.
  5. Shake the flask thoroughly to ensure complete dissolution.

B. Preparation of 100 mL KH₂PO₄ Solution

  1. Select the KH₂PO₄ bottle.
  2. Weigh 1.36 g of KH₂PO₄ using a weighing balance.
  3. Transfer the weighed KH₂PO₄ into a clean volumetric flask.
  4. Add distilled water to the flask up to the 100 mL mark.
  5. Shake the flask thoroughly to ensure complete dissolution.
  6. Both phosphate solutions are now prepared. Proceed to the next step.

Step 3: Measurement and Adjustment of pH

  1. Select the prepared Na₂HPO₄ solution.
  2. Select the prepared KH₂PO₄ solution (or the target mixed buffer solution).
  3. Immerse the pH meter glass electrode into the target solution.
  4. Press the Check button on the pH meter and record the pH value.

pH Adjustment

  1. If the measured pH is less than 7.2, add a few drops of NaOH solution to the target solution.
  2. If the measured pH is greater than 7.2, add a few drops of HCl solution to the target solution.
  3. After adjustment, press the Check button again and record the new pH value.
  4. Repeat small additions of NaOH or HCl and recheck until the desired pH of 7.2 is obtained.

Use of calculator for both phosphate and carbonate–bicarbonate buffer systems.

Sample 1:

Acid concentration, [HA]=[H2PO4]=0.10 M
Conjugate base concentration, [A-]=[HPO42-]=0.20M
Acid dissociation constant, Ka=6.2x10-8

It will give pH value=7.51

pH = pKa + log([A−]/[HA])
pH = 7.21 + log(0.20/0.10)
pH = 7.21 + log(2)
pH ≈ 7.21 + 0.301 = 7.51

Sample 2:

Acid concentration, [HA]=[H2CO3]=0.05 M
Conjugate base concentration, [A-]=[HCO3-]=0.15 M
Acid dissociation constant, Ka=4.3x10-7

It will give pH value=6.85

pH = pKa + log([A−]/[HA])
pH = 6.37 + log(0.15/0.05)
pH = 6.37 + log(3)
pH ≈ 6.37 + 0.477 = 6.85