Pseudo halide ions situation. When reduced sulfur or cyanide is present, with porous or open aperture junctions these pseudo halide ions migrate into the reference electrode, contaminate the reference electrolyte and combine with the elemental silver wire to form silver sulfide. Excessive sulfide ions may form a highly water insoluble silver sulfide coating on the wire. Using double junction reference electrodes (Figure 4) with KNO3 in the first reference chamber will help overcome the sulfide and cyanide ion problems. A potassium nitrate solution prevents the sulfide or cyanide ions from getting into KCl reference chamber.
Solid state sensor. It’s possible to avoid glass issues by opting for a sensor based on the ion selective field effect transistor (ISFET), which was developed in the early 1970s. Such sensors particularly suit food and pharmaceutical processes. However, ISFET technology poses its own set of challenges. The sensor’s measuring surface is reduced to approximately a 2-mm-dia. circle. Processes with high suspended solids can create problems with this limited measuring surface area. Installing the sensor at a 45° angle to the process flow can mitigate this situation.
Signal transmission. Most sensors provide analog mV signals; such signals are susceptible to galvanic interference, which we’ll discuss shortly. It’s possible to avoid such problems by choosing a sensor that digitizes the signal before sending it to the analyzer. Figure 5 shows such a sensor that uses our Memosens technology. These sensors also feature an inductive coupling of the pH sensor and cable, allowing the data and power transmission to travel bidirectionally.
The process environment
Proper sensor selection is the key to long probe life and good performance. Combination pH probes are expected to respond very quickly, almost 90% of a pH step change in 10 seconds. This requires a constant free-flowing and pure reference electrolyte and a hydrated measuring glass.
In choosing a sensor, you must consider process dynamics, conditions and surroundings. Factors to watch out for include:
- coating of the glass membrane
- liquid junction plugging
- fluid velocity
- temperature and pressure of the measured fluid
- abrasion, drying and aging of the glass membrane
- galvanic isolation
- calibration and storage
- hypoionic processes (low ionic strength)
Let’s look at several of these.
Coating. As the sensor remains in contact with process fluid, that fluid starts depositing onto the measuring glass surface. Periodic cleaning of the glass membrane with soap and water will remove such deposits. Automated cleaning and calibration systems are available and may be useful in explosion-proof locations or ones posing health hazards.
Aging of the pH sensors. A new probe when calibrated will have 0 mV offset at pH 7.0, and a 59.3 mV/pH slope. As the probe ages, both the offset and slope will change. The altered slope may translate into sluggish response to changes in pH. This will call for frequent calibration. Figure 6 explains the aging process of pH sensors and how it impacts the accuracy of pH measurement.
Galvanic interference — the ground loop. A static buildup in the process fluid may produce an electrical potential that affects the mV output of the sensor. This mismatch in potential starts at the pH electrode’s measuring tip and travels through the cable into the pH transmitter’s input circuitry.
A ground loop can cause the following problems:
- an offset from the actual pH reading
- a drift of the actual pH reading. This could be an increasing or decreasing drift and can start and stop at any time
- saturation of the signal to a high or low level, or to a level anywhere in between.
You can confirm galvanic interference by placing the sensor in a plastic or glass beaker of process sample and then grounding the solution to the process piping. If the pH changes by more than ±0.1 pH, a ground loop is present.