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pH electrodes & Meters

By Terry Coleman,2014-01-20 02:44
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pH electrodes & Meters

    pH Electrodes & Meters

    By Peter Henderson, pHentron, 2007

    pH Electrode

The standard glass pH electrode is a combination of 2 separate electrodes;

    a) Glass electrode b) Reference electrode (often silver/silver

    Glass membranes (made of a special chloride)

    glass) are permeable to hydrogen ions,

    thus; When a reference

    electrode is in use, there

    should be a minute flow

    (but so small, it can not be

    measured!) of electrolyte

    out of the electrode. Thus Many manufacturers coat the glass the cap on the refilling

    membrane with a gel layer to opening should be improve its permeability thus the removed and the electrode membrane should not be wiped with must not be immersed tissues, etc for cleaning it should deeper into the test only be flushed with the relevant solution or distilled water. solution than the level of the electrolyte in the

    Always follow the manufacturer’s recommendations. probe.

Combined Glass pH electrode a combination of both above

    When in use, a voltage is produced (across the 2 wires) which is proportional to the

    pH of the solution being measured.

    Because of the high impedance of the glass membrane, the “volt” meter used to

    12measure this potential must have extremely high internal impedance (ie > 10 Ω, a 7typical digital multimeter is only 10).

     o+This potential is expressed by the Nernst Equation: E = E + RT ln [H]

     nF oAt 25C this becomes E (in mV) = constant 59.16 x pH.

    Thus pH 6 = 59.16 mV, pH 7 = 0.0, pH 8 = -59.16, pH 9 = -118.32, etc

    The voltage from a pH electrode will vary depending on its age & condition.

pH meters are calibrated in pH units (some can also display mV), they have an adjustment for temperature

    variations and can also vary the calibration (ie slope & offset).

    Cleaning pH Electrodes

Wash electrode after every measurement with distilled water, a hard stream of water from a wash bottle is good

    for stubborn dirt. NEVER WIPE THE MEMBRANE WITH TISSUES, etc as they will damage/remove the

    membrane coating (membrane coating rejuvenation solutions are available commercially).

Dirty glass membranes usually give a sluggish response & reduced slope:

    95 100% (ie -56.2 -59.2 mV/pH) good condition

    85 90% (-50.3 -53.2 mV/pH) electrode needs cleaning

    < 85% (less negative than -50.3 mV/pH) electrode needs conditioning or replacing

    > 100% (<-60 mV/pH) check the buffers, they are probably old or contaminated

Blocked junction (often coloured black) is usually caused by sulphide contamination of a Ag/AgCl reference,

    soak in a thiourea/HCl cleaning solution (available commercially). Contamination of the junction or

contamination of the reference electrolyte, usually affects the offset of measurements. The electrode should be

    replaced if the offset exceeds ?30 mV.

Contamination of the electrodes reference electrolyte can also give a large offset drain the electrolyte, flush &

    then re-fill with the manufactures recommended electrolyte (normally 3M KCl but this solution may also be

    saturated with AgCl).

Proteins (such as milk, meat, blood, etc) are bad for electrodes, use commercial pepsin/HCl electrode cleaner.

Oils & greases try to remove with ethanol (usually not effective), my next choice is iso-propanol. If you have

    to use longer chain hydrocarbons, I then remove the hydrocarbon by washing with ethanol or iso-propanol &

    then give the electrode a long soak in its storage solution.

Damage to the electrodes lead or dirt in its plug (or the meters socket) will lower the impedance & can cause

    drifting of results this is more obvious with low & high pH readings.

     Calibrating pH Meter

Without proper calibration, readings are often wrong by 0.5 1 pH unit (it gets worse, the further from pH 7).

Always follow the manufactures recommendation. 2 point calibration is the most common (never trust 1 point

    calibration, some good quality pH meters can also do 3 point calibrations).

The following procedure works for meters with manual adjustable slope & offset;

    1. Turn meter on & let stabilise for 20 minutes.

    2. Let buffers thermally stabilise to room temperature.

    3. If the meter (or electrode) does not have temperature sensing, measure the temperature of the buffers &

    adjust the temperature compensation on the meter.

    4. Measure the pH of buffer 7, adjust the offset so the meter reads the correct pH for the buffer 7 at that

    particular temperature.

    5. Select the second buffer, either pH 4 or 10 (sometimes pH 9.2) according to the pH of your samples (ie

    acidic or alkaline).

    6. Measure the pH of the second buffer & adjust the meter reading using the slope adjustment.

A good method to check the accuracy of calibration & to check the condition of the electrode, is to measure the

    third buffer that was not used in step 5. The result should be less than 0.1 pH unit from the true buffer value.

If slope or offset are accidentally altered, the meter should be re-calibrated. If the temperature of your samples

    ochanges adjust the temperature compensation. Note: slope changes by 1 mV/pH per 5C (about 0.05 pH units

    at about pH 4).

    pH Electrode Storage

All manufactures have their own recommendation & storage solution there are many variations to the standard

    glass membrane pH electrode, so follow their recommendations.

NEVER STORE ELECTRODES IN DISTILLED OR DI WATER this will strip ions out of the electrode &

    shorten its life.

NEVER STORE ELECTRODES DRY.

The manufacturer’s storage solution is normally the same solution as used in the reference electrode, for standard

    glass pH electrodes this is normally 3 M KCl (or 3 M KCl saturated with AgCl, for a Ag/AgCl reference

    electrode).

Short term storage, stand the electrode in the storage solution or pH 7 buffer. If the electrode is giving a sluggish

    response try storing it over night in pH 4 buffer or pH 10 buffer (the opposite value of the pH you normally

    measure).

Over night & for longer storage, replace the cap on the refill opening.

Long term storage place a few drops of storage solution in the storage cap (a plastic cap that fits over the glass

    membrane supplied with new electrodes). I also like storing the electrode vertically, so the internal electrolytes

    stay in the membrane end.

     Common Mistakes With pH Measurements

Open the cap on the refill opening, before making any measurements or calibrating (failure to open the cap will

    cause results to drift & often blockage of the junction).

Before making any measurements, check there is sufficient filling solution to cover the wire in the reference part

    of the electrode. The height of this liquid must also be above the height of the sample being measured, to give a

    very slight flow of this filling solution out of the electrode (this also helps to prevent contamination of the filling

    solution.

Calibrate the meter at least daily (follow the manufacturers recommendations).

Some meters will need to be re-calibrated if they have been turned off (particularly older ones). Also re-calibrate

    if the electrode is changed.

Contamination of the electrode can occur with some solutions, especially if it contains fats, oils or proteins

    follow the manufacturers’ recommendations. Also, solutions containing sulphides, hydrofluoric acid or high

    concentrations of halide can cause problems.

Always wash the electrode with distilled water after every measurement.

Never store the electrode in distilled or D.I. water (see earlier articles).

Stir the sample when making measurements.

Most poor measurements are due to problems with the electrode. Other causes of poor measurements are

    calibration errors due to bad techniques, not compensating for temperatures, then contaminated buffers. The least

    likely fault is an electrical problem with the pH meter.

    Peter Henderson

    ? pHentron 2007

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