pH meter

pH electrode

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Undergraduate Instrumental Analysis Amazon

Undergraduate Instrumental Analysis

by James W. Robinson

pH meter

To ensure your electrode works correctly you may want to measure its parameters.

example correct pH electrode parameters
isopotential point± 15 mV original value
slope55 - 61 mV/pH unit
glass membrane resistance20-100 MΩ
electrolyte leak rate0.2 to 1.5 mL/24 hours

Note that values presented can be wrong in case of specific electrode (for example electrolyte leak rate in electrodes with sleeve junction can be much faster). Consult owners manual if in doubt.

To test isopotential point and slope, switch your pH meter to display results in mV, not as pH units. If not possible, you have to use other pH meter or laboratory voltmeter able to work with pH electrodes.

Isopotential point of most general use electrodes is set at pH=7.00. To measure it put the electrode into pH 7 buffer and measure the electrode potential. Remember to take the measurements in the temperature electrode was originally calibrated in.

To check the slope move electrode to the pH 4 buffer. When potential stabilizes, read the value. The difference between previous reading and current reading should be in the 166-184 mV range.

Measurements of the electrode resistance and electrolyte leak rate are more difficult. Glass membrane resistance measurements require electrode test stand. To check electrolyte leak rate fill the electrode wil filling solution. Use a waterproof pen to mark the initiall fill level. Suspend the electrode in a beaker of pH 7 or pH 4 buffer so that the lower electrode plug is level with the water. Wait for 24 hours. Refill the electrode using a serological pipet, noting the volume of filling solution required. Amount of solution added divided by 24 is a flow rate.