What Is a Digital pH Meter?
A digital pH meter is an instrument used to measure the pH of a solution.
It is expressed as a number between 0 and 14, with a value of 7 indicating neutrality, a value less than 7 indicating acidity, and a value greater than 7 indicating alkalinity.
A digital pH meter can be used to read the hydrogen ion concentration index of a solution as a digital value. pH is called “pH” or “pH (pHH),” but JIS Z8802:2011 states that it should be read as “pH or pHH.
Uses of Digital pH Meters
Digital pH meters are used for environmental measurement and chemical analysis. In environmental measurements, digital pH meters are used for water quality control of drinking water and wastewater, and in chemical analysis, digital pH meters are used for quality control in laboratories and on industrial production lines.
In chemical analysis, pH is one of the indicators that affect sample pretreatment and analysis conditions. pH also affects the progress of chemical reactions, making it an important control item at manufacturing sites.
Principle of Digital pH Meters
The measurement method most commonly used in digital pH meters is the glass electrode method. The glass electrode method uses two electrodes (a glass electrode and a reference electrode). When a liquid with a different pH is present in the two areas separated by the glass films of the electrodes, an electromotive force corresponding to the difference is generated, and the pH is calculated by measuring the potentials of the glass and reference electrodes in the main unit.
Note that the pH meter must be calibrated before use. pH standard solutions are generally calibrated with pH 7 as the zero point and pH 4 and pH 9 as the three-point calibration, but the pH standard solution used for calibration may be changed depending on the pH of the sample. It should also be noted that the slope of the calibration curve is affected by temperature.
To measure pH accurately, the temperature of the pH standard solution and the sample must be matched as much as possible, the electrode must be properly maintained, and the electrode’s drop must be immersed in the sample. pH measurement itself is a simple task, but it is important to keep in mind the key points during measurement and to establish a maintenance method. It is important to follow the procedure for the measurement and maintenance of the electrode.
Other pH measurement methods besides the glass electrode method include the use of indicator such as litmus paper, hydrogen electrode, quinhydrone electrode, and antimony electrode methods.
Other Information on Digital pH Meter
1. Calibration of Digital pH Meter
Calibrating a pH meter means correctly adjusting the zero and span points (sensitivity) of the pH meter using a pH standard solution. Since there are specific operations depending on the digital pH meter used, the instructions provided with the meter should be given priority when performing the actual calibration.
There are three types of calibration: one-point calibration using one type of pH standard solution, two-point calibration using two types, and three-point calibration using three types, but two-point calibration is often used to calibrate pH meters. One-point calibration is a simplified calibration. For more rigorous calibration, three-point calibration is used.
The actual value of “electromotive force per pH” at a glass electrode is affected by contamination of the glass film and “alkaline error” and “acid error” (errors in strong alkaline and strong acid conditions, respectively). Therefore, the actual value is generally lower than the theoretical value (59.16mV at 25°C). The “pH7 EMF” also deviates from the theoretical value (0mV).
By performing calibration prior to measurement and periodically, highly accurate pH measurement can be achieved.
2. Calibration Solution for Digital pH Meter
The types, quality, and composition of pH standard solutions are as follows.
Oxalate pH Standard Solution
0.05mol/kg potassium trihydrogen dioxalate solution, pH 1.68 at 25°C
Phthalate pH Standard Solution
0.05mol/kg potassium hydrogen phthalate in aqueous solution, pH 4.01 at 25°C
Borate pH Standard Solution
0.01mol/kg sodium tetraborate aqueous solution, pH 9.18 at 25°C
Carbonate pH sStandard Solution
0.025mol/kg sodium bicarbonate and 0.025mol/kg sodium carbonate solution at pH 10.02
Neutral Phosphate Standard Solution
0.025mol/kg potassium hydrogen phosphate, 0.025mol/kg disodium hydrogen phosphate solution, pH 6.86 at 25°C