CS526-L Digital ISFET pH Probe

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1. What are the main components of the CS526-L?

   An ISFET chip pH sensor has three main components:

   • ISFET chip—The ISFET chip is made of durable plastic and contains three electrodes: the source, the drain, and the gate. The gate is the ISFET chip’s only electrode that comes in contact with the sample solution.
   • Reference electrode—Inside the reference electrode’s membrane, there is a silver wire element submersed in an electrolyte solution. The reference electrode, as a whole, is submersed in the sample solution.
   • Reference junction—The reference junction is located at the end of the reference electrode by the sample solution. It interacts with both the reference electrode’s electrolyte solution and the sample solution to complete the electrical circuit.
2. When is cleaning or calibration needed for a pH sensor?

   Cleaning and/or calibration may be required when the measurements are scattered, drifting occurs, or there is physical evidence of fouling. Measurements for pH must be monitored regularly to check for scattering. However, just because the results are scattered does not necessarily indicate the need for an adjustment. For example, there may be a change in the water source that causes the scattering. As a sensor ages, however, the scattering of the measured values tends to increase.

   To check the performance of a pH sensor, use it to measure a buffer solution in the correct range. If the value returned is within the specified range, the sensor does not need to be calibrated.

3. How is a pH sensor calibrated?

   The recommended calibration method listed in a specific pH sensor’s instruction manual should be followed to guarantee the best results. Calibration must be performed correctly to ensure accurate and repeatable measurements. Before performing calibration, the pH sensor should be cleaned.

   Calibration is commonly done using a known-value pH solution called a buffer. The buffer solution is formulated to resist pH changes caused by external contaminants. However, the pH of the buffer solution changes as the temperature changes. To compensate for this, manufacturers list the pH of the buffer solution at various temperatures on the buffer solution’s bottle so that the correct value for calibration is selected.

   The most common calibration method is a two-point calibration using two buffer solutions. Each buffer solution has known and accurate pH values at different temperatures. The buffers used should be based on the normal measurement range that the pH sensor operates in for the application. One buffer solution should have a 7.0 pH. The second buffer solution should have a pH that is near the expected pH value of the sample solution. 

4. What are the advantages of using an ISFET chip pH sensor instead of a glass bulb pH sensor?

   • The ISFET chip is housed in a non-glass body that is more durable than glass. Because an ISFET chip pH sensor is more rugged, it can be deployed in the field for longer durations without requiring maintenance.
   • Because an ISFET chip pH sensor does not use fragile glass in its construction, this type of sensor provides an alternative in applications where the potential for broken glass could be problematic to personnel, organisms, or processes.
   • Because an ISFET chip pH sensor does not have a measuring electrode with a glass membrane, the pH sensor can be stored dry. Also, the need to refill the buffered solution in the measuring electrode is eliminated.
   • ISFET chips have increased sensor stability at low temperatures, so the calibration frequency is reduced.
   • The response rate of ISFET chip pH sensors can be ten times faster than glass bulb pH sensors, thus providing faster measurements and enabling better process control.
   • An ISFET chip pH sensor is smaller than a glass bulb pH sensor because an ISFET chip can be made smaller than a glass membrane.
   • ISFET chip pH sensors are less susceptible to aging than glass bulb pH sensors.
   • Because the ISFET chip pH sensor does not have a pH sensitive glass membrane, it is more resistant to attack from highly acidic or highly alkaline sample solutions. ISFET chip pH sensors reduce the acid and alkaline errors common in extreme pH applications without the need for special glass. 
5. What is a multipoint calibration, and when is it done on a pH sensor?

   In the event that both alkaline and acidic sample solutions are measured using a single pH sensor, a multipoint calibration is done using three buffer solutions. As in the two-point calibration, the first buffer solution has a 7.0 pH. The second buffer solution should be near in pH value to either the acidic or alkaline sample solution, and the third buffer solution should be near in pH value to the other.

6. In the CS526-L, do the electrodes function the same way?

   The source and the drain are two of the three electrodes contained within the ISFET chip, and they behave in much the same way. The third electrode in the ISFET chip, the gate, has an electrical field that influences the current that flows between the source and the drain. The electrical potential in the ISFET pH sensor is measured between the reference electrode and the source.

7. What can cause ion interference in the CS526-L?

   In an ISFET chip pH sensor, the chemical coating on the gate electrode can vary depending on the model and manufacturer of the pH sensor. Depending on which chemical coating is used and what elements are present in the sample solution, there may be some ion interference.

8. What is PEEK, and why is it used in the CS526-L?

   PolyEtherEtherKetone (PEEK) is a plastic material that has very good thermal stability and chemical resistance properties. This material was chosen for use in the manufacture of the CS526-L because of its natural resistance to organic acids (acetic, carbonic, citric, tartaric, etc.) and its hydrolysis resistance to fresh and saltwater.

9. How does a reference electrode in a pH sensor become contaminated?

   A reference electrode can become contaminated when poisoning ions such as lead, iron, chrome, cyanide, or sulfide enter the reference electrode and react either with the silver wire or with the electrolyte solution.

   The contamination may not become apparent until the silver-chloride coating is completely dissolved and the electrical potential from the reference electrode has changed greatly. If this occurs, the reference electrode must be replaced.

10. Why is silver used for the wire in the reference electrode of a pH sensor?

    Silver is the best electrical conductor of all the metals because it has the lowest electrical resistance. The silver wire, coated in silver chloride, is relatively insensitive to changes in temperature.