How to Choose the Best pH Sensor: Laboratory Style Sensors

How do you know which laboratory pH sensor is the best pH sensor for your application? You want a sensor that will provide the best measurement accuracy with the maximum lifetime, but the number of different laboratory pH sensors out there can be overwhelming. Before making a laboratory pH sensor purchase it is crucial to have an understanding of your application and the conditions the sensor will endure while in use. Based on this knowledge you can make a decision on what features are required. Here are some features you may want to consider:

Body Material

The first step to choosing the best pH sensor for an application is to decide on the body material. Some things to think about when deciding on a body material are:

• Temperature
• Chemicals in solution
• Handling

Temperature

As a rule of thumb, polymer body sensors, including epoxy, plastic, and Ultem®, can be used in applications in low to medium temperature ranges. In higher temperature ranges (above 100°C), glass is a better choice.

Chemical Compatibility

For polymer bodied electrodes you always want to check the compatibility between the sensor body material and the chemicals that may be present in samples. Generally, polymer body probes are never used in solutions containing inorganic substances. Additionally, when sample solutions contain highly corrosive constituents, you want to use a glass bodied electrode. This is because glass is a very inert material, which means that it resists reactions with chemicals that might degrade other substances (like polymers).
In applications where the sensor will be handled with a heavy hand or experience any type of impact, the best pH sensor will have a polymer body, as polymer bodies can absorb more shock than glass and resist breakage.

Single or Double Junction

If your application involves testing samples that may contain proteins, heavy metals (Ni, Cd, Cu, Cr, Ag), sulfides, cyanides or iodides, or any other material that will react with silver or chloride ions, a Double Junction electrode will provide added protection to prevent reference fouling. This extends the working life of the sensor. Single junction electrodes can be used in more general applications.

Sealed or Refillable Electrodes

Sealed pH sensors require lower maintenance and are more cost effective. However, sealed designs, which contain gel reference solutions, will eventually deplete or become contaminated and need to be replaced. Refillable pH sensors have a resealable hole near the top of the body that allows users to refill the liquid internal solutions as they deplete. This extends the lifetime of the sensor and ensures that measurements are accurate throughout the sensor’s lifetime.

Some refillable lab pH sensors will also offer a flushable junction, which is specially designed to respond quickly and accurately to rapid temperature changes. Refillable lab pH sensors with a flushable junction are generally referred to as research grade.

Temperature Compensation

A pH measurement is temperature dependent. Based on the temperature and the pH of a sample there may be an error factor. If you are looking for highly accurate pH measurement in applications with extreme temperatures, then the best pH sensor for your application will include a temperature integration. Often times laboratory pH sensors are offered with integrated temperature sensors for this purpose. When selecting a probe with Automatic Temperature Compensation (ATC), you must also consider whether your pH meter will accept a temperature element and whether the temperature element from the pH sensor matches that of the meter.

Connector

Last but not least, always make sure the connector on the pH sensor matches up with that of the pH meter. The most common connector for a pH sensor is a BNC connector, but there are many other different connectors out there. Also, if your pH sensor has an integrated temperature sensor, you want to be sure that the connector for the temperature element is also compatible with the pH meter you will be using.