pH and ORP are important parameters of water quality and a wide range of process applications depend upon the ability to measure and control each. In simplest terms, both pH and ORP represent types of ion activity in liquids—pH measures hydrogen ion activity while ORP represents the potential transfer of electrons between chemicals or chemical compounds. Though measuring very different things, both pH and ORP share a close relationship and are often included together while discussing water quality.
Understanding pH
pH stands for "potential of hydrogen" and is a measure of hydrogen ion concentration. A high concentration of hydrogen ions results in acidity while a low concentration results in alkalinity. The concentration of hydrogen ions is represented on a scale from 0 to 14 with 7 being neutral (i.e. an equal number of hydrogen (+) ions and hydroxide (-) ions to cancel each other out). Values below pH 7 are acidic and values above pH 7 are alkaline. The scale is logarithmic, meaning each full point increases or decreases by a factor of 10x. Some extreme substances can score lower than 0 or greater than 14, but most fall within the scale.
pH measurements are important in medicine, biology, chemistry, agriculture, forestry, food science, environmental science, oceanography, civil engineering, chemical engineering, nutrition, water treatment & water purification, and many other applications.
pH Measurement
pH meters generally consist of a glass electrode connected to an electronic meter that measures and displays the pH reading. Meters can range from units that combine all the parts into a simple, handheld unit or more complex, laboratory-quality instruments that provide highly accurate readings and a host of other functions.
The electrode itself is comprised of a measuring electrode, a reference electrode, and, often, a temperature sensor. Electrode components are usually combined into one device called a combination pH electrode. The reference electrode is filled with an electrolyte of a known value which reacts with the medium being tested. That reaction is what determines the potential of hydrogen. Since pH is sensitive to temperature fluctuations, a temperature sensor is included so readings can be properly compensated. Temperature compensation can be achieved manually or automatically. Manual temperature compensation is usually achieved by entering the temperature of the fluid being measured into the instruments menu. Automatic temperature compensation requires input from a temperature sensor and constantly sends a compensated pH signal to the display.
There is a wide range of pH electrode styles to suit particular applications. All electrodes include a junction which separates the electrolyte within the reference electrode from the medium being tested. Double junctions are available for applications which risk contaminating the electrolyte. There are also a number of electrode tip styles which are suitable for particular materials being tested.
Maintenance and Calibration of pH Instruments
Accuracy of pH measurements is dependent upon well maintained and properly calibrated instruments. The main factors that degrade the performance of an electrode are coating, contamination and aging. Necessary maintenance and calibration depends on these factors and, of course, of the accuracy required from measurements.
All glass electrodes have a bulb which must be kept hydrated and a reference junction which must be kept wet to prevent excess leakage of the internal electrolyte solution from the reference junction. It is recommended keeping the electrodes in a storage solution between uses. Special cleaning solutions are also available to ensure electrodes are free from contamination or build-up which can affect performance and shorten the lifespan of the electrode. The electrolyte within some reference electrodes must also be refilled periodically. Other electrodes are gel-filled or "low-maintenance" and do not require refilling.
All pH meters require calibration from time to time. They should be calibrated at a minimum of two points. Most manufacturers offer buffer solution at specific pH levels for calibration. For most applications, pH 7.00 and 4.00 (or 7.01 and 10.01 for alkaline values) are suitable for calibration.
Understanding ORP
ORP is the acronym for "Oxidizing-Reduction Potential" which is the activity of oxidizers and reducers in relation to their concentration. Oxidizers are chemicals and chemical compounds that accept electrons from other substances while reducers donate electrons. ORP is especially important when discussing water quality as it's most notable feature is its ability to measure the oxidation of contaminants.
Chemicals like chlorine, bromine, and ozone are all oxidizers. It is their ability to oxidize - to "steal" electrons from other substances - that makes them good sanitizers because in altering the chemical makeup of unwanted substances, they render those substances inert. Of course, in the process of oxidizing they use up their ability to further oxidize things.
While for most applications ORP measurement is concerned with oxidizers for their sanitizing effect, there has been increasing interest with reducers. Free radicals are atoms and/or molecules with an unpaired electron. Introduced to our bodies as byproducts of biochemical reactions and from the air we breathe and food we eat, free radicals are unstable and desire balance so they try pull electrons (oxidize) from wherever they can. In acquiring balance for itself, the free radical damages other biological molecules in the body. Reducers, also known as antioxidants, donate the electrons sought by free radicals thus minimizing the degenerative effects of free radicals on the body.
ORP readings are measured in mV and range from -2000 mV to 2000 mV. Values above zero indicate an oxidizing substance while values below zero indicate a reducing substance. Values do not indicate the concentration but rather the potential of the substance to oxidize or reduce.
ORP measurement is important for many applications such as drinking water, swimming pools and spas maintenance, cooling tower disinfection, groundwater remediation, bleaching, cyanide destruction, chrome reductions, metal etching, fruit and vegetable disinfection and de-chlorination. ORP is also the only practical method we have to electronically monitor sanitizer effectiveness.
ORP Measurement
An ORP sensor consists of an measuring electrode, a reference electrode, and some sort of display, in much the same fashion as pH measurement. An ORP meter is really a millivolt meter, measuring the voltage across a circuit formed by the reference electrode and the measuring electrode.
Measuring electrodes are made of platinum or gold, which due to their low resistance, will lose electrons to an oxidant or accept electrons from a reductant. Voltage is produced as electrons are added or subtracted. The reference electrode, usually made of silver, is surrounded by an electrolyte solution that produces another tiny voltage. But the voltage produced by the reference electrode is constant and stable, so it forms a reference against which the voltage generated by the measuring electrode may be compared. The more oxidizer or reductant available, the greater the voltage difference between the solutions.
ORP is unaffected by temperature but readings will vary as the pH fluctuates. As the pH goes up, the millivolt reading on an ORP meter will go down. This can greatly affect the effectiveness of sanitizers such as chlorine, which is by far the most commonly used chemical for water sanitation. For example, at a pH 6.0, 96.5% of the Free Available Chlorine in the water is in the active killing form, while at pH 8.5, only 10% is in that form. Bringing the pH down or adding more sanitizer will raise the millivolt reading.
Maintenance and Calibration of ORP Instruments
As with pH meters, regularly cleaning and properly storing ORP electrodes is important in ensuring accurate results. ORP meter, however, do not require calibration.
pH / ORP FAQs
- What if I have a water ionizer?
- You will likely want to measure the pH and ORP. Please make sure you select a meter capable of both measurements and be sure to purchase the required solutions for best results.
- Why is calibration needed?
- Calibration is needed frequently in pH meters to standardize the electrode. Over time the measurements will drift; calibration will correct for this drift so you can accurately measure your pH.
- What Solutions do I need?
- For best results we recommend pH buffer 4, 7, and 10 as most meters will recognize these solutions. We recommend cleaning and storage solution to maximize the life of your electrode. We also offer other calibration solutions to accommodate special applications and our higher end meters which can accept these solutions.
- How often should the meter be calibrated?
- The general rule of thumb is to calibrate your pH meter once per week. For greater accuracy it is recommended to calibrate more frequently. For best accuracy you would calibrate prior to each test.
- How do I calibrate my pH meter?
- Most of our meters are very similar to calibrate and have automatic buffer recognition. Typically you would calibrate with pH7 first, then pH4 and pH10 being sure to rinse to avoid cross contamination between each calibration.
- How should a ph meter be stored?
- For best results and longevity of your electrode we recommend storing your electrode in a pH storage solution. DO NOT USE DEIONIZED OR DISTILLED WATER.
- How long does a pH probe last?
- Most pH probes will last 6 months to 2 years. Proper maintenance of your pH probe is the best way to maximize the life of your pH electrode.
- When do I replace the pH probe?
- When reading reacts unstable or sluggish it may be time to replace your electrode. If you can no longer calibrate the electrode, it is time to replace it.
- Which meter do I need?
- Depending on your application there are many pH meters available. The testers are the least expensive option. These are generally the least accurate and do not have special purpose electrodes available but they are very popular because of their low cost. They are accurate enough for most users. Basic handled meters will offer a larger screen than the testers and will offer greater flexibility with special purpose probes. High-end handheld meters vary but many of them will offer PC connectivity, data logging with date stamp to meet GLP (good lab practice), and very high accuracy. Please contact an Instrumart Engineer to help select the correct meter for you.
If you have any questions regarding pH / ORP meters please don't hesitate to speak with one of our engineers by e-mailing us at sales@instrumart.com or calling 1-800-884-4967.
