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Các lỗi điện cực pH thường gặp (phần 2)

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Getting the optimal performance from your pH electrodes is important. Several factors can influence an electrode’s functionality, including drifting readings, slow response times, and environmental influences. Find out how to determine what may be affecting your electrode—and what you can do about it.

DRIFTING READINGS

Drifting readings can occur if you are using a gel-filled electrode under the following conditions: when measuring a relatively large volume of sample, measuring samples at slightly colder temperatures, or testing fairly clean water. For reference, read the following article: http://www.coleparmer.com/techinfo/techinfo.asp?htmlfile=pHPureWater.htm

Here are some reasons you could be experiencing "drift" and tips to overcome it:

    • Gel-filled electrodes leak solution at a much slower rate than liquid-filled electrodes and will take longer to stabilize. This slower response can be perceived as "drift". Use a liquid-filled or refillable electrode instead or allow longer time for reading to stabilize.
    • When measuring a 500-mL sample (especially with a gel-filled electrode), your readings will take longer to reach equilibrium ("drift") than if you used a smaller volume of sample, say 100 or 200 mL. Use a smaller sample size.
    • When measuring samples that are fairly clean, CO2 can cause the pH to change as the sample is mixed and allowed to sit and react with the air. Again this appears as drift, but it is actually the CO2 changing the pH of your sample.
    • Generally, plastic electrodes are not as good as glass electrodes. Electrodes with built-in temperature sensors aren't as good or don’t respond as fast as separate pH and temperature electrodes. Use glass refillable electrodes or electrodes without ATC.
    • If you are using plasticware (beakers), and especially if you are using a magnetic stir plate and stir bar, you may experience a drift or change in the reading due to static charge that was affecting the reference potential of the electrode. Use a glass vessel, especially if you use a magnetic stirrer.
    • If the body and gel inside of your electrode is at room temperature (22ºC or about 72ºF) and you insert your electrode into a water sample that is 10ºC (about 50ºF), your electrode will drift until it reaches the same temperature as your sample. Allow the temperature of your electrode and sample to equilibrate before taking a reading. Also, always record the temperature in addition to your pH reading.

SLOW RESPONSE TIMES

Consider the following possible sources for slow response times:

    • Plastic gel-filled (non-refillable) electrodes are often preferred for their low cost and low maintenance; however they are much slower to respond than more expensive liquid-filled, glass refillable electrodes. Properly functioning new gel-filled electrodes will go from pH 7 to pH 4 in no more than 60 to 90 seconds. A glass refillable electrode may take about half that time.
    • As electrodes age (especially gel-filled electrodes), they respond more slowly. Single-junction electrodes used with incompatible samples may yield a very slow response after a short time because the electrode may have become irreversibly clogged. Use a double-junction electrode to prevent similar clogging in the future.
    • Consider your response time during standardization with pH calibration standard buffers. In some circumstances, it is normal if your electrode is quick to respond during calibration, but slower to respond in your sample.
    • Electrodes that are dirty or have dried out will give a sluggish response. If rinsing with clean water isn't enough, warm, soapy water works well for most organics, while low concentrations of acids work well for most inorganic material. After cleaning, a period of soaking to rehydrate the glass sensing bulb may be necessary—warm pH 4 buffer works well and is easy to confirm when hydration is complete as the pH reading will be constant.
    • Using plastic beakers during calibration or measurement. The odds of encountering a slow response seem to increase when using a magnetic stirrer to mix solutions. The spinning/friction of the stir bar in a plastic beaker often causes the pH electrode to perform oddly. Turn off or slow down your stirrer and switch to a glass beaker. Removing and reinserting the electrode from the solution may also help.

ENVIRONMENTAL FACTORS

Static Electricity Interference
Using a plastic beaker while stirring with a magnetic stir bar can create a static charge, thus offsetting the reference potential and resulting in erratic pH readings. Switch to a glass vessel or stop the stirring and remove the electrode from the stirrer. Static charge can also occur in industrial applications with plastic pipes.

Electromagnetic Noise
Wiring between the electrode and the controller may cause eletromagnetic noise. Fix the problem by:

    • If the signal is not preamplified, the distance between the electrode and the controller must be kept to an absolute minimum, preferably less than 20 feet.
    • Under no circumstances should the cable for an unamplified signal be spliced.
    • All wiring connections must be kept clean and dry.

    • The electrode wires and any other low-voltage wiring must be physically isolated in a separate conduit at least six inches from any AC power lines. Failure to do so will result in unstable, and inaccurate readings.

    Reference source: http://www.coleparmer.com/TechLibraryArticle/1207