pH is a measure of the acidity or basicity of an aqueous solution. It is a dimensionless measure on a scale typically ranging from 0 to 14 that indicates the relative ionic activity of hydrogen ions (H+) in the solution. A pH of 7 is considered neutral at 25°C with an equal concentration of hydrogen (H+) and hydroxide (OH–) ions.
The pH scale is logarithmic, meaning each whole pH value below 7 (the neutral point) is ten times more acidic than the next higher value, and each whole pH value above 7 is ten times more basic (or less acidic) than the next lower value. For example, a solution with a pH of 3 is ten times more acidic than one with a pH of 4 and one hundred times more acidic than one with a pH of 5.
pH is measured on a logarithmic scale with values indicating the ratio of hydrogen and hydroxide ions.
Why Measure pH?
The pH of aquatic ecosystems influences the health and stability of these environments. It plays a significant role in the biological and chemical processes that occur in natural waters (lakes, rivers, streams, coastal, and ocean). All aquatic species have a pH range in which they can live and an optimal range that maximizes their growth and reproductive success rates. Deviations cause physiological stress that can lead to increased disease and mortality rates.
Impacts on aquatic biota stem largely from the effect that changing pH has on the solubility and availability of nutrients and chemicals in water. The relationship of ammonia to pH is one relevant example. In its unionized form, ammonia (NH3) is toxic to fish. Ammonia that bonds with a hydrogen ion (H+) forms the non-toxic nutrient ammonium (NH4+). While temperature also plays a role, more unionized ammonia is likely to be present at high pH when there is less hydrogen ion activity.
Changes in pH similarly affect the solubility of some salts, including some containing heavy metals that may be toxic to aquatic organisms and humans. Lower pH (increased acidity) can increase the solubility of metals such as copper, lead, zinc, and cadmium. This is because the relatively high availability of hydrogen ions at lower pH causes reactions with the basic anions from the salt compounds, thereby leaving more free metal cations to dissolve in the water. Furthermore, changes in pH can influence other secondary reactions that impact the mobility, bioavailability, and toxicity of such metals once dissolved in the water. 
Long-term pH monitoring is helpful to track conditions and detect any acute or gradual changes. Pollution, such as acid mine drainage, chemical spills, industrial discharges, or wastewater effluents, can potentially alter water pH levels. Acid rain with a pH under 5 caused by the release of sulfur dioxide (SO2) and nitrogen oxides (NOX) into the atmosphere from burning fossil fuels can contribute to the acidification of water bodies.
Even in the absence of human-related causes, pH levels can change due to natural processes. Unpolluted precipitation is slightly acidic with a pH of about 5.6 and can lower the pH of receiving waters. However, the rocks and soils surrounding a water body play a role. For example, runoff from carbonate-rich areas (like limestone) tends to be more alkaline with higher buffer capacity and resulting pH.
Carbon dioxide (CO2) is also a contributor to lowering pH by reacting with water to form carbonic acid. Processes such as photosynthesis, respiration, and decomposition influence CO2 concentrations, as does diffusion from the atmosphere. Increased atmospheric CO2, in particular, is associated with ocean acidification.
How Is pH Measured?
Measuring pH in natural waters is performed using a sensor that measures the potential (voltage) difference between a pH-sensitive electrode and a reference electrode. This type of sensor is generally known as an ion-selective electrode (ISE), where the ion–in the case of pH–is the hydrogen ion (H+).
The electrodes of a pH sensor are constructed of a silver/silver chloride wire or other similar conductive material suspended in a neutral pH solution within a glass bulb. When placed in water, they generate a potential between the bulb and a reference electrode depending on the hydrogen ion concentration in the water.
The reference is filled with an electrolyte of known potential, such as KCl, that does not change with pH. The electrolyte comes in contact directly with the water but is protected from rapid dilution by either a micro-porous junction or by being formed into a gel.
A typical glass bulb pH sensor has a pair of electrodes and measures the voltage between them when placed into a solution.
The potential difference measured between electrodes is then used to calculate the pH of the water sample based on the Nernst equation, which relates voltage to the hydrogen ion concentration in the water.
A pH sensor typically gives a raw output in mV and an equivalent value on the pH scale (0-14) based on its calibration in reference buffers. Temperature is another variable in the Nernst equation, so sensors must include either an internal thermistor or external temperature correction.
How to Select a pH Sensor?
Project duration and the associated maintenance requirements and costs are significant factors to consider when selecting a pH sensor. Since the reference electrolyte is consumed over time, pH sensors often require more frequent maintenance and calibration than some other sensor types. Consider the costs associated with maintenance visits and parts during the lifetime of the application.
Also, consider the performance of the sensor in the water body that it will measure. Some sensors may have a minimum conductivity in which they can measure accurately. Relatively pure waters may not have sufficient ionic strength for good signal quality. In such conditions, “flowing-junction” reference electrodes–characterized by a liquid electrolyte that easily flows through the membrane to the water–tend to perform well.
In addition to internal characteristics, external factors, such as sensor fouling, should be considered as well. A high-fouling environment may interfere with sensor readings even if there is sufficient electrolyte for measurement. Anti-fouling features such as a wiper mechanism can help in such cases.
What to Consider When Preparing a pH Sensor?
Understanding the calibration and maintenance procedures of a pH sensor is critical to the collection of high-quality field data. Study the routines for periodic maintenance, and practice the procedure of exchanging the sensor electrolyte if relevant and reasonable to do so.
For calibration, sensors are placed into standards called pH buffers, named such because they can maintain a fairly constant pH value, even if exposed to extraneous ions. Common values are pH 4, 7, and 10, though others may be found.
A typical calibration should be either a 2-point or 3-point calibration. Prior to beginning, consider the expected pH in the body of water to be measured. If, for example, it is known to always be on the acidic side (that is, pH less than 7), a 2-point calibration with pH 4 and 7 buffers is likely sufficient. However, if the pH is unknown or is expected to fluctuate both above and below the neutral point, then a 3-point calibration is preferred.
Since pH sensors give a mV signal, pay close attention to the signal strength during calibration. An ideal neutral value per the Nernst equation is 0 mV in pH 7 buffer, and approximately +175 to +180 mV in pH 4 buffer or -175 to -180 mV in pH 10 buffer.
Actual values will almost always vary from ideal to some degree, but these values set the slope of the calibration equation. Therefore, they should remain fairly consistent between calibrations and with similar magnitude on the order of 170-180 mV between the 4-7 and 7-10 intervals.
If the sensor demonstrates a significantly lower response than 180 mV in successive buffers, or no response at all, then consider factors such as age and possible contamination of the buffers, cleanliness of the sensor, blockage of the reference junction, damage to the glass bulb, and conditions in which the probe was stored.
It is important to store the pH sensor in a moist environment so that it does not dry out. However, it should not be left submerged in deionized (DI) water for extended periods, as DI water can leach ions from within the sensor and similarly cause damage.
How to Deploy a pH Measurement System?
Both shoreside and open water deployments can be used for field installation of pH sensors. In a typical stream or river monitoring application, the sensor and logging system can be established using an existing structure such as a bridge, natural elements such as rocks and trees, or by placing mounting posts in cement or driving into solid soils.
pH sensors can be deployed from a mast-mounted system in rivers and streams.
The sensor should be well protected in a deployment pipe that shields against damage to the glass bulb from any debris loads that the water may carry. However, accessibility to the sensor is also necessary for periodic maintenance.
The same principles apply in a buoy-based deployment, except that the system will be placed in open water with a single-point or multi-point mooring system to keep the buoy in place. Near-surface measurement is achieved with a perforated deployment pipe for secure mounting of the pH sensor, or the sensor can be mounted onto a line suspended from the buoy for deeper measurement.
pH sensors can be mounted near the surface in a pipe or on the frame of a buoy or can be suspended at greater depths.
The buoy hull houses a data logger for recording and transmission of the pH measurements, with a solar-charged battery for continuous operation.
Conclusion
pH has a complex role in water chemistry as it influences the solubility, reactivity, and potential toxicity of a wide range of minerals, nutrients, gases, metals, and other elements commonly found in water. Therefore, maintaining the health and stability of aquatic ecosystems and drinking water supplies depends in part on understanding pH dynamics.
Real-time monitoring systems can provide baseline readings and alert to any sudden or gradual changes in pH. This helps water resource managers to determine if changes are caused by pollution or natural events, thus empowering informed decision-making and timely action when remedial action is required.
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