Glass/pH electrodes : Performance Check and Cleaning

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Introduction

This protocol covers all combination electrodes and electrode pairs consisting of a glass/pH indicator and a Ag/AgCl reference with 3M KCl salt-bridge isolution.

Instrumentation and reagents

• All pH meters and all titrators that can be operated independently as pH meters may be used
• Standard pH buffered solutions, minimum of two must be used

Procedure

• Microprocessor-based pH meters and titrators
  1. Calibrate the electrode as per meter/titrator instructions.
  2. Measure the potentials (mV) of two or more buffer solutions.
• Analog pH meter
  1. Zero the meter first by shorting the electrode inputs.
  2. Measure the potentials (mV) of two or more buffer solutions, and compare them with those listed in Table 1.

Observations

• Potentials of standard buffered solutions are listed in Table 1. Deviations of greater than ±15 mV signify the need of thorough cleaning of the electrodes.

  Table 1 : pH and mV relationship

• Electrode calibration : The values of slope, asymmetry voltage, Uas, or asymmetry pH, pHas, reflect the working conditions of a glass/pH electrode. Cleaning becomes necessary whenever one of the conditions below is observed:
  • Slope falls outside of 1.025 and 0.975
  • Uas is greater than ±15 mV
  • pHas falls outside of 6.75 and 7.25

Cleaning and Conditioning

• Reagents
  1. Alcohol : methyl, ethyl, or isopropyl.
  2. Ammonium hydroxide, NH₄OH, concentrated.
  3. Mixed acid cleaner : mix concentrated H₂SO₄ and HNO₃ in 2:1 ratio.

• Procedure
  1. Gently wipe the glass membrane (bulb portion) with alcohol.
  2. Submerge the lower 1-cm portion in the acid cleaner for a period as listed in Table 2 below.
  3. Remove electrode and rinse thoroughly with deionized water.
  4. For combination electrodes only: Soak the lower 1-cm portion in concentrated NH₄OH for 15 minutes (longer if the slope falls below 0.950). Remove electrode and rinse thoroughly with deionized water.
  5. Store all glass/pH electrodes in the electrode filling solution (e.g., 3M, 4M and sat’d. KCl , sat’d KNO₃ for at least 10 minutes before use.

Table 2 : Acid cleaning periods

Appendix

The relationship between temperature and theorectical slope, i.e., mV/pH, of glass/pH electrode is described by the equation :

U_M = 0.198357 T + 54.20 mV,

where: U_M is the theorectical slope, or mv/pH; T is temperature, in °C.

Table 3 : Effect of electrolytes on pH buffer potentials*

* Measured with a glass combination pH electrode at 25°C.
** SCE = stanard calomel electrode = Hg/Hg₂Cl₂ - sat’d KCl