pH Sensor: Water Quality Probe

Why measure pH?

pH is a measurement of water’s acidity or alkalinity.  Water with pH less than seven is acidic; water with pH greater than seven is alkaline (basic).  The pH of water is determined by dissolved, pH-active salts.  For instance, dissolving sodium hydroxide in pure water will increase the pH because the hydroxide ion is a pH-active base, and dissolving hydrogen sulfide in pure water will decrease pH because the hydrogen ion is a pH-active acid.   However, dissolving sodium chloride in water will not change the pH because neither sodium nor chloride is a pH-active ion.

The pH of water determines the form of dissolved, pH-active salts.  Suppose, for instance, that an ammonia-based fertilizer flows into a river via rainfall runoff.  If the pH of the river is below nine, most of the ammonia species present will be in the form of ammonia, NH3.  If the river pH is above pH nine, most of the ammonia species present is in the form of ammonium, NH4+.  This is important because ammonia is toxic to biota, where ammonium can be a nutrient for biota.

pH also determines the solubility of metals in the water.  Metals like iron and chromium are more soluble at low pH, and so more available for uptake by fish and other biota.  At high pH, the metals are less soluble, and more likely to be bound into sediment than available for biota.

Changes in long-term pH trends can signal the need for more detailed chemical study of the water and its contamination sources.

How is pH measured?

pH is measured with a pH sensor and reference electrode.  The pH sensor is a small, glass bulb filled with an electrolyte of known pH.  If the pH of the sample water is different from that of the electrolyte, a voltage is created across the glass bulb.  That voltage changes predictably with the pH of the sample water and is measured through the reference electrode.  The reference electrode contains an electrolyte separated from the sample water not by a glass bulb, but by a micro-porous material, often Teflon, which allows the electrolyte to contact the water without being quickly diluted by the water.

There are two types of reference electrodes found in water-quality instruments.  The first employs a small volume of high viscosity, “gelled” electrolyte.  The gelled-electrolyte reference electrodes do not require periodic replacement of the electrolyte.  However, they must be replaced approximately once a year or more often because the user cannot replace the electrolyte.

The second type of reference electrode employs a large volume of low-viscosity (un-gelled) electrolyte.  The resulting “flowing-junction” typically establishes a more stable measurement than do gelled-electrolyte reference electrodes, especially as the gelled electrolyte ages.  The flowing-junction electrolyte require new electrolyte every two months or so.  However, refilling the electrolyte means that you do not have to replace the entire reference electrode (as you do with gelled-electrolyte models).

The reference electrode used for pH measurement can simultaneously facilitate measurements with ORP electrodes and ion-selective electrodes (ISE’s).

pH sensors are ordinarily calibrated with commercially available standards, called pH buffers.  You can choose two- or three-point pH calibration.  Two-point calibrations use a seven (neutral) buffer and a second buffer whose value is near that of the waters you intend to monitor, normally a four buffer (acidic), or a ten buffer (basic).  If you are measuring in waters whose pH might range significantly above and below seven, you can increase your measurement accuracy slightly with a three-point calibration using seven, four, and ten buffers.