Wood – A High Performance Material for Process pH Analysis

Why would anyone consider a wood reference junction in a pH probe?

Apart from use in furniture, building and some niche applications, wood may be considered an engineering material of years gone by. Wheels for cars are made from aluminum alloy, power boat hulls are fiberglass, sporting goods have shifted largely to composite materials – so why would anyone consider wood for a product having high performance demands? It turns out there are areas where wood provides excellent performance characteristics that challenge those of modern materials. One such area is the reference junction of a pH sensor.

Some distinct advantages of wood

In an industrial application, a pH sensor will generally be placed in process water that contains chemical or physical contaminants such as acids, alkalis, grit, particles or fibres. The junction of the sensor must allow a connection to be maintained between the ions in the process and the ions in the sensor’s electrolyte while not allowing the electrolyte to become contaminated. It is in this regard that wood has some distinct advantages.

Wood prevents contaminants from reaching reference electrode chamber?

There are many types of wood but diffuse porous wood species in particular have beneficial filtering properties. With a fine heterogenous capillary structure with pore sizes in the order of 3 µm, diffuse porous wood can effectively prevent contaminants from reaching the internal reference electrode chamber over long periods of time. Once the internal reference electrolyte becomes contaminated, the pH sensor will no longer read accurately. Accordingly, the junction must block contaminants while still maintaining ion contact with the process.

How a wood junction extends the life of a pH probe

By filtering out contaminants, over time there will be an accumulation within the pore structure of the wood. However, compared to homogeneous synthetic materials, wood has a very complex structure that allows for a relatively high volume of particles to be retained. Synthetic materials by comparison, tend to accumulate material closer to their surface. Wood also exhibits the ability to flush out contaminants by nature of its capillary structure thereby extending the time before it becomes plugged.

Moreover, wood as a natural, heterogeneous material has inherent properties that allow it to perform exceptionally well as a pH sensor reference junction. While it has certain chemical compatibility limitations, where it can be applied, it will increase maintenance intervals over other porous junction alternatives. After all, a tree can draw water and nutrients from soil for decades without plugging!