7. Lab- pH Meter

  • Electrometric pH Measurement:

    • The principle involves determining the activity of hydrogen ions ( ext{H}^+) through a potentiometric measurement.

    • Utilizes a glass pH indicating electrode joined to a reference electrode.

    • When immersed in a solution, the reference electrode contacts the sample, completing an electrical circuit with the indicating electrode.

Indicator Electrode

  • Composition:

    • The pH electrode is made of silver wire coated with ext{AgCl}, immersed in a solution of 0.1 mmol/L ext{HCl}, and has a glass membrane tip.

  • Functionality:

    • The glass membrane is selectively sensitive to ext{H}^+ ions. This sensitivity is due to the presence of lithium, cesium, lanthanum, barium, or aluminum oxides in silicate.

    • The movement of ext{H}^+ ions near the tip creates a potential difference between the internal solution and the test solution, which is quantified as pH.

  • Combination Electrodes:

    • These may include a built-in reference electrode, such as ext{Ag/AgCl} or calomel ( ext{Hg/Hg}2 ext{Cl}2), immersed in saturated ext{KCl} solution.

    • The glass membrane dissolves slowly over time, allowing for long-term use but can be affected by high concentrations of other cations like sodium.

Reference Electrode

  • Calomel Electrode:

    • Made of mercurous chloride in contact with metallic mercury and a potassium chloride solution.

    • Generates a stable voltage as long as the temperature and electrolyte concentration remain constant.

    • Contains a filling hole for potassium chloride solution and a tiny opening to maintain electrical contact between the electrodes.

  • Performance Considerations:

    • The calomel electrode is slow to stabilize after temperature changes and unstable above 80°C.

    • Alternatives include ext{Ag/AgCl} electrodes, which can function at higher temperatures (up to 275°C) and are more compact.

    • For chloride-sensitive measurements, a mercury sulfate and potassium sulfate reference electrode may be preferred.

Procedure for the Accumet AR10 pH Meter

Instrument Setup

  1. Set the mode to pH, % Slope to 100%, and temperature to the standard buffer solution's temperature.

  2. Utilize the pH electrode and Automatic Temperature Compensation (ATC) probe for each measurement. If the probe cannot be used, immerse it in deionized water at the same temperature as the samples.

  3. Remove the pH electrode from the storage solution. Open the fill hole and add saturated KCl if needed (below 1/4 inch).

  4. Rinse probes with deionized water and blot dry with kimwipe before each use.

  5. Immerse the pH electrode in a buffer solution when not in use to avoid sluggish responses from storage in deionized water.

Calibration Procedure

  1. Standardize the pH electrode before measuring samples, using pH 7.0 buffer (yellow) and either pH 4.0 (pink) or pH 10.0 (blue) buffers.

    • Samples with pH above 7.00 can be standardized with 4.0 and 7.0 if accuracy is verified.

  2. Immerse the electrode in the 7.00 buffer solution and adjust the standardize knob until the display shows the buffer value at the current temperature.

  3. Immerse in the second buffer solution and adjust the % slope knob until the display indicates the correct value.

  4. Verify accuracy with control samples: use low ionic strength 0.0001 N ext{HCl} (pH 3.95-4.25) and a certified pH buffer solution (pH 4.61-4.65).

    • Troubleshoot if results are not within specified limits before proceeding to analyze samples.

Analysis Procedure

  1. Rinse each container (sample, standard, control) three times with deionized water and then with a small amount of the respective solution.

  2. Pour the sample into the rinsed container and insert both the pH electrode and ATC probe, ensuring that all sensitive parts are immersed.

  3. Swirl the sample until uniformly mixed and wait for the meter to stabilize (approximately 5 minutes). Record the pH reading.

Instrument Shutdown

  1. Thoroughly rinse probes with deionized water and dry with kimwipe.

  2. Close the fill hole of the pH electrode and store it in saturated potassium chloride solution.

  3. Place the ATC probe in deionized water.

  4. Set the pH meter mode to standby.

Procedure for the Accumet AB15 pH Meter

Standardization Steps

  1. Press the mode key until the digital display shows pH mode (toggle modes: pH, mV, and Rel mV).

  2. Press the setup key twice and then enter to clear any existing standardization.

  3. Follow the same protocol for electrode preparation as previously described:

    • Rinse the electrode and immerse it in pH 4 buffer solution.

  4. Access the Standardization mode by pressing 'std'. The selected buffer group is briefly displayed.

  5. Wait for stabilization before pressing 'std' again to initiate standardization.

  6. Repeat with the pH 10 buffer solution. The meter displays the electrode's performance and gives a GOOD ELECTRODE message if within 90-102% range, otherwise an ELECTRODE ERROR message appears, requiring instructor notification.