Definition
pH is an abbreviation of “pondus hydrogenii” and was proposed by the Danish scientist S.P.L. Sørensen in 1909. pH provides the degree of activity of an acid or a base in terms of hydrogen ion activity.
pH = -log[H+]
pH scale goes from 0 to 14. pH 0 means a very high acid activity The alkaline substances, range from 8 to 14. With a pH 7 pure water is said to be neutral.
| Substance | pH Value |
| Stomach acid | 1.0 |
| Lemon Juice | 2.4 |
| Vinegar | 2.8 |
| Orange Juice | 3.0 |
| Tomato Juice | 4.0 |
| Black Coffee | 5.0 |
| Detergent | 6.5 |
| Milk | 6.8 |
| Blood | 7.4 |
| Seawater | 8.0 |
| Baking soda | 9.0 |
| Milk of Magnesia | 10.7 |
| Domestic Bleach | 11.0 |
| Caustic Soda | 14.0 |
Ionization
An ion is an atom or molecule which has either gained or lost electron(s). Ion presence in a solution allows electrical energy to be passed through the solution.
Different compounds form ions in different amounts. Molecules that dissociate easily will form strong acids or bases when in aqueous solution.
In an aqueous solution, hydrogen ions combine with water to form the hydronium ion (H3O+). The terms "hydronium ion" and "hydrogen ion" are used interchangeably on pH measurements.
Combined electrodes
Since it is easier to handle one electrode instead of two, combined electrodes (single stem) are very popular. The indicating glass electrode and the reference electrode are simply built into a single physical entity. This helps to ensure that the two electrodes have the same temperature during operation. In these electrodes the inner electrode of the glass electrode is the same type (Ag/AgCl) and has the same dimensions as the reference electrode, and the inner solutions are as identical as possible (saturated with KCl). Glass Electrodes are usually quite fragile. Recent developments have replaced the glass with more durable solid-state sensors.
Temperature Effects
The pH electrode is temperature dependent, and must be compensated for in the pH meter circuitry. The circuitry of the pH meter utilizes the Nernst equation:
E = Ex- 1.98 Tk pH
Where E is sensor potential, Ex is a sensor’s constant and TK is temperature in ºK
Changes in temperature of a solution will vary the millivolt output of the glass pH electrode in accordance with the Nernst equation. Electrode sensitivity versus temperature is a linear function.
Calibration and Buffers
A solution with a precisely known pH is used for calibration,. Such a solution must have certain insensitivity to being lightly contaminated, i.e. it must have a buffering capacity. This is where buffer solution or buffer originates from.
Calibration is required to match the pH meter to the electrodes. pH meters require calibration at specific pH values. One calibration is usually performed near the isopotential point, and a second is typically performed at either pH 4 or pH 10. It is best to select a buffer as close as possible to the actual pH value of the sample to be measured.
There can be small differences between the output of one electrode and another, as well as changes in the output of electrodes over time. Therefore, the system must be periodically calibrated.
The length of time between calibrations depends on process conditions and the accuracy and stability expectations. Calibration will correct for some changes. Cleaning the sensors will also help. However, just as batteries, pH electrode's lifetime is finite, and have to be replaced eventually.
pH measurement system
A pH measurement system is made up of the pH sensor, a temperature sensor, a preamplifier; and an analyzer or transmitter. A pH sensor is just a battery where the positive terminal is the measuring electrode and the negative terminal is the reference electrode. The measuring electrode is sensitive to the hydrogen ion. Reference electrode provides a stable potential against which the measuring electrode can be compared.
The pH electrode has very high internal resistance, making its voltage difficult to measure. The preamplifier takes the high-impedance pH electrode signal and changes it into a low impedance signal, less susceptible to electrical noise, which the analyzer or transmitter can accept. The pH meter is basically a high impedance amplifier that accurately measures the minute electrode voltages and displays the results directly in pH units on either an analog or digital display.
