For optimal pH measurements, the correct probe must first be selected. The most important sample criteria to be considered are:
- chemical composition
- process pressure
- pH range
- container size (length and width restrictions)
The choice becomes particularly significant for non-aqueous, low conductivity, protein-rich and viscous samples where general purpose glass probes are subject to various sources of error.
The response time and accuracy of a pH probe is dependent on a number of factors. Measurements at extreme pH values and temperatures, or in low conductivity samples may take longer than those of aqueous solutions at room temperature with a neutral pH and high conductivity. The significance of the different types of samples is explained below by taking the different probe characteristics as a starting point.
Different kinds of junction
The opening that the reference part of a pH probe uses to maintain contact with the sample can have several different forms. These forms have evolved through time because of the different demands put on the probes when measuring diverse samples. The “standard” junction is the simplest one and is made from a ceramic material. It consists of a porous piece of ceramic which is pushed through the glass shaft of the probe. This porous nature allows the electrolyte to slowly flow out of the probe, but stops it from streaming out freely.
This kind of junction is very suitable for standard measurements in aqueous solutions. The METTLER TOLEDO InPro 325x series is an example of a pH probe with a ceramic junction. Even though this is probably the most widely used junction because of its simplicity of use with aqueous solutions, it has one main drawback: Because of the porous structure of the junction it is relatively easy for particles in a sample to block the junction, especially if the sample is viscous or if it is a suspension.
You also have to be careful with some aqueous samples such as those with a high protein concentration, as proteins may precipitate within the ceramic junction if they come in contact with the reference electrolyte, which is often KCl. This reaction will cause the porous structure to become filled with protein debris, blocking the junction and rendering the pH probe useless. Measurements are not possible if the electrolyte cannot flow freely since the reference potential will no longer be stable.
The same problem can also be caused if the inner electrolyte reacts with the sample solution being measured and the two meet in the junction. This reaction can create a precipitate which may block the junction, for example if KCl electrolyte saturated with AgCl is used with samples containing sulfides, the silver and sulfides react to form Ag2S which then blocks the ceramic junction. Factory-filled, prepressurized liquid / gel electrolyte pH probes are suited to a wide scope of applications in the biotechnology, pharmaceutical and chemical process industries. This design ensures the best possible measurement performance under the most diverse operating conditions.
PTFE annular diaphragm
An annular PTFE diaphragm, instead of a ceramic junction, increases the surface exposed to the media to prevent clogging of the diaphragm. Highly contaminated process conditions makes pH measurement and control a complicated issue. An annular PTFE reference diaphragm (e.g. as in METTLER TOLEDO‘s InPro 480x series) is designed for service in tough environments. It resists fouling from hydrocarbon contaminants and sulfides, ensuring high accuracy and fast response throughout its long life. For process media containing particles and aggressive chemicals, the optional flat glass membrane pH probe is the optimal solution.
The third type of junction is the open junction. This means the reference electrolyte is completely open to the environment and is in direct contact with the sample solution. This is only possible with a solid polymer reference electrolyte.
The great advantage of this junction type is clearly the fact that it is unlikely to clog. Open junctions can easily cope with very dirty samples and constantly provide good measurements. However, the solid polymer reference electrolyte which is used for this open junction has a slower reaction time and low electrolyte flow. This necessitates that the samples measured have a high enough ion concentration for stable measurements to be possible. Nevertheless, these probes are suitable for most samples and are very robust.
Dual-membrane without junction
The cell membrane chlor-alkali process is very tough on conventional pH probes. It exposes them to high temperatures, and clogging and poisoning from a variety of compounds. This is particularly true in the anode side of the process' electrolysis cell. Chlorine diffuses through the probe’s diaphragm and attacks the reference system. This results in incorrect pH measurement and shorter sensor lifetime.
Reliable pH measurement can be achieved with sensors such as the InPro 4850 i from METTLER TOLEDO. This is a dual-membrane pH probe that has been designed specifically to provide long-term accurate measurement in chlor-alkali applications. The main difference in measuring technology between dual-membrane pH probes and conventional pH probes is the presence of a sodium-reference (pNa) system. Such probes feature a sodium-sensitive glass membrane which is charged by the sodium ions in the process medium. The sodium concentration in the brine is used as the reference. The pNa reference system is hermetically sealed: there is no diaphragm; therefore, no oxidants can enter the probe to attack the reference system. These probes also feature a high-alkali-resistant pH membrane glass for pH measurement. It is the amalgamation of pH measurement and pNa reference that is the reason that this kind of probe is highly suited to chlor-alkali processes.
More to come.