Image credit: Flir Commercial Systems | Kobold | ABB Measurement

A pH instrument is an electronic device used to measure the pH of a typically-liquid substance.

Basics of pH Measurement

pH is a measure of the acidity or alkalinity of a substance. While the exact abbreviation is disputed, pH is usually taken to stand for "power" or "potential" of hydrogen. Specifically, pH instruments measure the relative quantity of hydrogen ions (H⁺) and hydroxyl ions (OH^-) in a substance. Acidic solutions have a higher concentration of hydrogen ions, while alkaline solutions have higher hydroxyl concentrations.

pH is defined as the negative logarithm of hydrogen ion activity, as shown in the formula below. Hydrogen ion activity (aH) is defined as the product of the molar concentration of hydrogen ions and an activity coefficient; the activity coefficient provides an adjustment for the hydrogen atoms which have interacted with other chemicals within the solution.

The logarithmic characteristics of pH values are shown in the table below: each increment of the pH scale is equal to a tenfold difference in hydrogen ion concentration. Acidic solutions at 25° C have pH values less than 7, while alkaline solutions have values greater than 7. The image at right shows the pH scale labeled with common substances.

Image credit: Cal State Stanislaus | UC Davis

pH Applications

pH measurement is important in diverse applications, including chemistry, human biology, agriculture, water quality, and oceanography. Specific uses include:

• Emissions testing for water and other liquids
• Nutrient testing
• Monitoring of blood and anatomical processes
• Agricultural soil testing
• Detecting changes in potable water sources
• Food and beverage stability testing

pH Measurement Devices

pH measurements are typically conducted using a pH meter. This device uses a glass electrode connected to an electronic meter.

pH meters actually consist of at least two electrodes: a specialized glass electrode as well as a reference electrode. Many meters also include a temperature sensor to provide a temperature signal to the meter. Construction of the main electrode is dependent upon the principle that an electrical potential difference occurs between two interacting metals or liquids with differing electron mobility.

To this end, the electrode is used to measure the potential difference between a known liquid within a thin membrane (potassium chloride within the doped glass) and an unknown liquid. This voltage difference can be translated into a pH measurement for the unknown liquid because the pH of the potassium chloride is standardized at 7.0 (neutral). The nominal "electrode" is actually a silver chloride tip submerged in the potassium chloride electrolyte.

The reference electrode is used to complete the circuit and provide a known reference potential for the meter; it consists of a mercuric chloride electrode within a potassium chloride solution. Technically, a pH meter measures several sets of potential differences — between the electrodes and their electrolytic solutions; and between the unknown liquid and the electrolytic solutions — but only the latter measurement is useful in determining pH.

A standard glass electrode and a reference electrode are typically configured to provide a 0 mV output signal at 7.0 pH at a temperature of 25° C. It becomes clear that, by using the two electrodes in conjunction with a temperature sensor, the meter can output an accurate pH signal by compensating for varying temperatures within the unknown liquid using this calibrated signal.

The image below illustrates a pH meter circuit using a glass (left) and a reference electrode.

Image credit: SeaFriends

Errors

A substance's pH may be affected by a number of outside factors, all of which in turn affect a pH meter's measurement capabilities. Also, conditions like reference poisoning do not affect the substance's pH, but rather result in an inaccurate potential difference and output. Some common factors and details are listed below.

• pH extremes - Strongly acidic or basic substances have been known to cause measurement errors. Acids with pH values of 1 or less can cause acid errors and even corrode electrodes; for this reason, acid conductivity tests are used in place of pH measurement for more accurate output. Conversely, conductivity tests are also used on strongly alkaline solutions, as they can quickly destroy electrodes. At a pH of 10 or greater, where sodium ion concentrations are much higher than those of hydrogen ions, sodium error — in which the electrode begins mistakenly responding to the sodium ions instead of the hydrogen ones — may occur. Sodium error results in low pH measurements and can be overcome by specially-designed high-pH electrodes.
• Temperature - Temperature causes the dissociation of weak acid and alkaline molecules, causing an alteration in a solution's pH. As described above, modern pH meters are temperature compensated for values above and below the nominal process temperature of 25° C. Through process monitoring of a solution's temperature as well as utilizing a user-provided solution temperature coefficient, pH meters can accurately recalculate pH for all but the most extreme temperatures.
• Reference poisoning - Errors within the reference electrode cause inaccurate measurements. Problems with the junction between the reference electrode and the solution being measured can cause an open circuit (if the junction is plugged) or reference potential changes (if the junctions around the electrolytes degrade). Use of a double-junction electrode (in the case of the latter error) or complete electrode replacement are solutions to reference poisoning.

Combination Instruments

Instruments which measure pH are often configurable to measure other characteristics of a substance, most often oxygen reduction potential (ORP). These instruments can be used with interchangeable probes for each parameter to be measured. In addition to ORP, other typical measurement capabilities of pH meters include:

• Temperature (as mentioned above)
• Conductivity
• Dissolved oxygen
• Dissolved carbon dioxide

The image below shows a typical water analysis meter, an example of a pH instrument which measures several other parameters. This particular meter has the potential to measure pH, temperature, conductivity, and total dissolved solids (TDS), some simultaneously.

Image credit: OMEGA Engineering

Device Maintenance

pH electrodes require periodic cleaning and maintenance to avoid the errors listed above. Devices should be periodically conditioned using the parameters below.

• Periodic checking of liquid electrolyte levels
• Storage of electrode in specialized storage solution
• Regular rinsing of glass with pure water or cleaning formula
• Preventing reference electrode junction dryout

For more about cleaning of pH electrodes, please reference the pH Electrodes Selection Guide.

Standards

pH instruments may be produced, tested, and maintained based on a variety of published standards. Example standards include:

• ASTM E70 - Standard test method for pH of an aqueous solution using a glass electrode
• BS 3145 - Specification for laboratory pH meters
• ANSI IT4.36 - Photographic processing solutions: pH calibration and measurement

References

Emerson Process Management - The Theory of pH Management