“The control of the pH in a system can help prevent problems of corrosion and/or scaling and can save huge amounts of money and wastewater for certain applications such as cooling towers.”
pH Control
pH - Definition
Measuring pH
pH is a measure of acidity or alkalinity. A pH reading less than seven is considered acidic, while a pH reading greater than seven is considered alkaline. pH 7 is considered neutral because it is the pH of pure water at 25 °C. pH is formally dependent upon the activity of hydrogen ions (H+). Because pH is dependent on activity, a property which cannot be measured easily or predicted theoretically, it is difficult to determine an accurate value for the pH of a solution. The pH reading of a solution is usually obtained by comparing unknown solutions to those of known pH, and there are several ways of doing this.
Naming Conventions
The name, pH, has been purported to come from a variety of places including: pondus hydrogenii (Latin), potentiel hydrogène (French), and potential of hydrogen (English). However pH is actually shorthand for its mathematical expression: in chemistry a small p is used in place of writing - log10 and the H should more correctly be H+, representing the concentration of hydrogen ions.
Where 'a' denotes the activity of the hydrogen ions
In simpler terms, the more activity of the H+/Hydrogen ions we have in a solution, the more acidic it becomes. Since pH is by definition logarithmic, an increase of one in pH means a tenfold decrease of H+ activity. Here is an example:
The pH in water is raised from neutral (pH 7) to pH 8.
Since
we can see that the original activity is:
Raising the pH to 8 results in:
or, one tenth of the original value
Activity and concentration
In solutions that contain other ions, activity and concentration are not the same. The activity is an effective concentration of hydrogen ions, rather than the true concentration; it accounts for the fact that other ions surrounding the hydrogen ions will shield them and affect their ability to participate in chemical reactions. So despite the fact that there is a certain real concentration of hydrogen ions, maybe only half of the ions can effectively participate in any process that involves H+. This reduces the activity.
simplified version
Often the above formula is simplified since activity is hard to measure. In the simplified version, the activity is considered approximately equal to the numeric value of the concentration of the H+ ion, measured in moles per litre (also known as molarity). Hence, the formula becomes:
The use of this formula explains why many soft drinks have very low pH readings (e.g. 2.3), but not the obvious aggressiveness of acids; a lot of the hydrogen ions (=concentration) are tied up by other components in the liquid, reducing the acidic activity. Still, if pH is measured based on the high concentration of H+ according to the simplified formula, it results in a very low pH.
Total Alkalinity
Total alkalinity denotes the buffering capabilities of a liquid; in other words, the pH resistance of a liquid in response to external changes.
Definition
Total alkalinity is defined as the total concentration of bases in water expressed as parts per million (ppm) or milligrams per liter (mg/L). These bases are usually bicarbonates (HCO3) and carbonates (CO3). They act as a buffer system that prevents drastic changes in pH.
For example, in waters with low alkalinity, pH might fluctuate from 6 or lower to as high as 10 or above; while in high alkalinity waters, pH might fluctuate from about 7.5 to 8.5. Human blood is an example of high total alkalinity. Even though we might drink huge quantities of acidic drinks (e.g. grapefruit or lemon juice), the body pH is not easily changed, because the blood acts as a buffer system due to its high alkalinity that absorbs the excess hydrogen ions.
pH Control in various applications
pH control is an important function in a whole range of applications. The control of the pH in a system can help prevent problems of corrosion and/or scaling and can save huge amounts of money and wastewater for certain applications such as cooling towers. Controlling the pH in a narrow band also greatly improves the efficacy of Copper/Silver Ionisation equipment.
Measuring pH
The most common method of measuring pH is the electrode method with an electronic pH meter. A typical pH meter consists of a special measuring probe (a glass electrode) connected to an electronic meter that measures and displays the pH reading. It requires measuring the voltage that develops between two electrodes that are immersed in the liquid and then comparing this to a calibration measurement from the same electrodes. The resulting voltage is very low and requires sophisticated microprocessor post-processing.
A schematic of a pH probe is shown in this diagram
- a sensing part of electrode, a bulb made from specific glass
- sometimes electrodes contain small amount of AgCl precipitate inside the glass electrode
- internal solution, usually 0.1M HCl for pH electrodes or 0.1M MeCl for pMe electrodes
- internal electrode, usually silver chloride electrode or calomel electrode
- body of electrode, made from non-conductive glass or plastics.
- reference electrode, usually the same type as 4
- junction with studied solution, usually made from ceramics or capillary with asbestos or quartz fiber.
Peristaltic pumps to control pH
Our pH control equipment makes use of peristaltic pumps for pH adjustments. A peristaltic pump is a positive displacement pump used for pumping a variety of fluids. The fluid is contained within a flexible tube fitted inside a circular pump casing. A rotor with a number of 'rollers', 'shoes' or 'wipers' attached to the external circumference compresses the flexible tube. As the rotor turns, the part of tube under compression closes thus forcing the fluid to be pumped to move through the tube. This process is called peristalsis and is used in many biological systems such as the gastrointestinal tract.
Maintenance of pH control equipment
In order to ensure accurate performance of the measurement, the pH probe needs to be maintained on a regular basis to prevent effects like drift and noise, which is done in conjunction with scheduled maintenance for the relevant application by one of our factory trained technicians.