PH THEORY What is it pH?What is it pH?/How is pH Measured?/What Equipment is Required to Measure pH?/How is a pH Measurement Device Calibrated?/Why is pH Measured?How is pH Measured?What Equipment is Required to Measure pH?How is a pH Measurement Device Calibrated?Why is pH Measured?

What is it pH? The concentration of the hydrogen ion in a solution is a measure of its acidity or basicity. This concentration is often expressed in terms of the pH of the solution, the negative logarithm of the hydrogen ion concentration. pH = -log[H + ] The pH value is the negative power to which 10 must be raised to give the hydrogen ion concentration. [H + ] = 10 -pH M Water is a weak electrolyte by itself, but it does undergo auto ionization and forms equal amounts of hydronium (H3O + ) and hydroxide ions (OH - ). There are many inconsistencies in terminology found in industry today. One of which professionals in chemical industry know that the hydrogen ion is found in water as the hydronium ion, but they write it as H + or H + (aq). The equilibrium equation can be written as: [H + ][OH - ]=K w =1.0 x o C Where K w is the equilibrium constant for water.

What is it pH? Where K w is the equilibrium constant for water. Similarly, we may define pK w as the negative logarithm of the ion product for water. Now we can write: pH=pK w- pOH As temperature increases the amount of hydronium ions and hydroxide ions that autoionize increases, therefore producing larger concentrations of each ion present in the water. The pH scale is usually presented from 0 to 14 pH. The more acidic a compound is the closer to zero pH it will be, and conversely the more basic a compound is the closer to pH 14 it will be. A solution with the same amount of H + as OH - has a pH of 7. This point is unaffected by temperature changes. The following graph shows the pH error due to temperature.

How is pH Measured? pH is determined electrically by measuring the difference in electrical potential between two electrodes placed in the same solution. For standard pH devices this electrical potential has been determined to be approximately 59.6 mV/pH. This electrical signal is sent to a pH meter where it is amplified and converted to a usable pH value. In some instances a temperature value is also input into the meter to make correction for temperature changes which would introduce measurement error at the measuring electrode. The two electrodes are referred to as the measuring electrode and the reference electrode. The measuring electrode has traditionally been made out of a pH sensitive glass. This is the easiest and least expensive to produce. Other measuring electrodes are the antimony base electrode and the solid state non-glass electrode. All of the measuring electrodes have their weaknesses. The glass electrode ’ s weaknesses are obvious, it is difficult for a piece of glass to hold up in a harsh industrial environment. The antimony electrode is very rugged, but is radioactive. The solid state electrode has many chemical compatibility problems.

How is pH Measured?

As stated before the glass electrode is the most commonly used technique for measuring pH. When the pH sensitive glass comes in contact with an aqueous solution a gel a layer develops on the pH sensitive glass membrane. The pH glass is blown into a bulb and filled with an inner buffer of know value. This is typically a pH 7 buffer. Thee gel layer also forms on the inside of the gel layer also forms on the inside of the glass membrane. The H + ions either diffuses out of the gel layer, or into the gel layer, depending on the pH value of the measured solution. For acidic (pH 7) the H + ions move out of the gel layer. This causes a potential to build on the outer gel layer. Because the internal buffer in the glass bulb is constant the potential between the inner gel layer and the outer gel layer is used to determine the pH value of the measured solution. How is pH Measured?

The reference electrode must have a defined stable potential, which is independent of the measured solution. A reference electrode consists of a reference element, which is immersed, in a defined electrolyte. This electrolyte must be in contact with the measured solution. This contact has historically been achieved through a porous membrane, but recently there have been some advances in reference technology. Some of the new techniques for reference systems include solid polymer references and solid state direct contacting reference system. The most common used reference systems are the mercury/calomel and the silver/silver chloride systems. Due to the hazards associated with mercury the most the silver/silver chloride system is the most widely used system. How is pH Measured?

What Equipment is Required to Measure pH? To measure pH the following is required: a pH sensitive electrode, a reference electrode, a temperature sensing device (optional), interconnecting cable, process housing, pH meter, and a pre-amp (can be located in the pH meter, in the sensor or in a remote location). The pH electrode, the reference electrode and the temperature device are typically manufactured in a combination electrode. The pre-amp is necessary to amplify the signal from the electrode to a signal, which is useable by the electronics of the pH meter. Typically for separation distances between the pH meter and the pH electrode less than 30 feet the pre-amp can be located in the pH meter. For separation distances greater than 30 feet the pre-amp typically must be located in the electrode or a remote location

How is a pH Measurement Device Calibrated? When all of the components of the measuring loop have been installed the pH loop must be calibrated in order obtain accurate pH values. There are two ways to calibrate a pH-measuring device. The two-point calibration is the recommended procedure, but sometimes it may be necessary to perform a one-point standardization. The two-point calibration is used to set the slope of the pH electrode. For new pH electrodes this slope is approximately 59.6mV/pH. If the slope drops below 47mV/pH unit the pH electrode must be replaced. To perform a two-point calibration two pH buffers of know pH value must be used. These buffers are typically made of a stable salt solution and can range anywhere from 1 to 14pH. Typically 4, 7,or 10 are used for calibrations. The pH electrode is placed in the initial buffer and the value is entered into the pH meter. Then the pH electrode is placed in a second buffer, this buffer must be at least two pH units different from the original buffer, and the value is entered into the pH meter. The meter then calculates the pH slope. This slope is usually linear up to pH 13.5.

The second way to calibrate a pH device is to perform a single point standardization. This can be done if you already know the exact slope of the electrode. Typically the slope is not know unless a two-point calibration has be performed. The single point standardization is typically performed to make on pH meter read exactly what a reference pH meter is reading. This is usually done to make a process pH meter match a lab pH meter is discrepancies appear. How is a pH Measurement Device Calibrated?

Ghazi AlMutairi AA Operator Why is pH Measured? pH is measured for many reasons. Here are a few uses for pH: Reaction control Water quality in cooling towers Water quality in wastewater Product quality Acid-Base mixing Most industries that have water in their process will have to measure pH. The following figure will show the pH of some commonly known materials.