Factors Affecting pH Sensors

The performance of a pH electrode can be adversely affected by a number of physical and chemical factors. Harsh wastewater characteristics can damage the external gel layer of the electrode; this will shorten the electrode’s measurement span and slow its response. Damage to the gel layer could occur if the electrode remains dry or is abraded by slurries, etched by hydrofluoric acid (below pH 3) or sodium hydroxide (above 12 pH), or dehydrated by low pH (below pH 1) or low water content in the wastestream (40 percent). Temperature, absorption, face velocity and fouling of the probe, buffering of the solution, alkalinity and acidity all affect the pH electrode accuracy and response time.

Temperature
The change in pH to temperature is zero as the pH approaches its isopotential point (the point where levels of potential are equal). This point is generally accepted as being a pH of 7 but, in fact, this is rarely the case. However, using this premise, if a wastewater containing strong electrolytes shows a pH of 8 at 60GC (140GF), it would change to a pH of 9 if the wastewater were at 25GC (77GF). pH changes in the opposite direction of the temperature change.

Temperature compensation is slow because the temperature sensor is embedded in the interior of the electrode. The farther the sensor is from the tip of the electrode, the longer it takes to sense the change in temperature. Generally, the temperature changes in a wastewater stream are relatively slow and of little consequence. However, there are two situations where temperature compensation needs to be seriously considered.The first is when there is a significant and rapid change in the wastewater temperature and the second is in the laboratory.
For example, the manufacture of barium ferrite (used in making rubberized and ceramic magnets) uses hot concentrated hydrochloric acid to dissolve the impurities and divide paired crystals of barium ferrite. The discharge of the 40GC (109GF) wastestream into a small pH control receiving pit dramatically affects the temperature of the wastewater. The second situation is one that occurs more frequently. It is in the laboratory where a pH electrode is moved between solutions of different temperatures. Remember to allow enough time for the temperature compensator to reach the solution temperature.

Absorption Effects
Absorption affects the alkaline buffer solutions and, there- fore, the calibration of the pH electrode. An exposed buffer solution of 10 pH can drop 0.1 pH unit per day through the absorption of carbon dioxide in the air. While the rate is much lower, carbon dioxide can also be absorbed through plastic bottle walls. For this reason, it is a good idea to make up a fresh buffer solution each time you calibrate your pH electrodes. pH 9 buffer solution is preferred in some cases be- cause it is much less affected by CO2 absorption than a pH 10 buffer solution.

Face Velocity, Fouling and Response Time
A slow response time to a change in pH in the process solution means poor control of the process. As the velocity past the face of the electrode increases, the boundary layer thickness decreases and the response time to a pH change is almost instantaneous. Good mixing to create a high flow velocity past the electrode or the use of flow-through pH electrodes will reduce the response time of a pH electrode. Velocities up to 7 feet/second (fps) past the probe will improve the response time and help to keep the electrode clean. Velocities greater than 10 fps can cause excessive signal noise and wear. Please note that when calculating the face velocity, we must take into account the reduction in area due to the electrode being in the stream.
A slime layer or coating of one millimeter on an electrode can severely slow the response time from 10 seconds to 7 minutes. A coating on an electrode may trap the proton [H+] concentration and the pH reading freezes. The thickness of the glass electrode also affects the response time. Some of the more rugged electrodes have response times as high as 100 seconds. Air bubbles clinging to or passing by a pH electrode can cause signal noise or shift. Particulates can also cause a similar shift or noise.
Response of a pH electrode is highly nonlinear and depends on the degree of buffering, the size of the pH change and the direction of change. In general the response is slower as the buffering decreases and the size of the pH change increases. The response time is also slower in the negative direction. For example, the response time may increase up to 100 seconds with a decrease from 1.55 to 1.38 pH and up to 200 seconds from 1.55 to 1.36 pH.

Alkalinity Error
The glass electrode reads low if the concentration of alkali ions is high enough to penetrate the gel layer and create a millivolt potential like a proton. The smaller the size of the alkali ion, the more error it can cause. Thus, at the same concentration lithium will show a greater error than sodium and sodium will give a greater error than potassium. Alkalinity error also increases with a higher pH and a rise in temperature. Care must also be taken with solutions that may be neutral or even slightly acidic if there is a high salt concentration because of the high sodium or potassium concentration.
Alkalinity error can be confusing. If the electrode is removed from the highly alkaline solution, the electrode will typically return to normal condition when immersed in buffer solutions or brought down to a lower temperature.

Acidity Error
pH electrodes will read high if the pH is below 1. You will need to reduce the pH reading to get the true value. This error increases with time. Short immersions in hydrochloric acid will strip away coatings or the old hydrated gel layer, but repeated cleaning will eventually “kill” the electrode.