Med-Hycos - les Données
Base de connaissance
WHAT IS pH ?
(Text and Image from HORIBA Company)
pH determines the acidity or alkalinity of a solution. Actually, it is determined by the concentration of hydrogen ions, the percentage of hydrogen ions contained in the solution. Let's take water as an example. As you know, the formula for water is H2O. Most of the molecules in water are in that extremely stable form we know as H2O. However, a very tiny percentage of those molecules have broken up into hydrogen ions (H+) and hydroxide ions, (OH-), as illustrated in the figure. Actually, this balance of hydrogen ions and hydroxide ions determines the pH of the water.
If the temperature does not vary, the following relationship between the densities of hydrogen ions (H+) and hydroxide ions (OH-) is found with any solution:
So far, we cover the basic principle of pH. Have
you added to your knowledge of pH? By the way, subsequent studies showed
that the electromotive force of the battery Sorensen used to calculate
pH was found to have a relationship with not only with the concentration
of hydrogen ions, but also with activity of the hydrogen ions. The march
of progress in the understanding of thermodynamics and actual measurement
of pH played an important role in this study.
call the degree of this restriction f. If we multiply the degree of restriction f by the number of balls in the box, the result will correspond to the number of balls that have freedom of moment at a given instant.
Next, apply this example to hydrogen ions within a solution, where the balls are hydrogen ions (H+), the number of balls is hydrogen-ion concentration ([H+]) and the number of balls that can move about freely is the activity of hydrogen ions . And "moving about freely" means that an ion can "exert its particular characteristics." We use f as the activity coefficient.
This leads us to the following formula:
As already mentioned, activity cannot actually
be measured directly. Thus, when measuring pH in actual practice, we measure
pH by defining a solution of known pH as a standard solution (in this case,
a solution whose pH is very unlikely to vary) and comparing it with pH
of the target solution.
The methods for measuring pH fall roughly into the following three categories. A brief description of each method follows:
* Various errors include;
- Error due to high salt concentration in the test liquid
- Error due to the temperature of the test liquid
- Error due to organic substances in the test liquid
However, this method is not appropriate for daily use because of the effort and expense involved, with the inconvenience of handling hydrogen gas and great influence of highly oxidizing or reducing substances in the test solution.
The quinhydron-electrode method of involves
immersing the tip of a polished antimony rod into a test solution, also
immersing a reference electrode, and measuring pH from the difference in
potential between them. This method was once widely used because the apparatus
is sturdy and easy to handle. However, its application is now quite limited
because results vary depending on the degree of polish of the electrode,
and reproducibility is low.
In the glass-electrode method, the known pH of a reference solution is determined by using two electrodes, a glass electrode and a reference electrode, and measuring the voltage (difference in potential) generated between the two electrodes. The difference in pH between solutions inside and outside the thin glass membrane creates electromotive force in proportion to this difference in pH. This thin membrane is called the electrode membrane. Normally, when the temperature of the solution is 30°C, if the pH inside is different from that of outside by 1, it will create approximately 60 mV of electromotive force.
The liquid inside the glass electrode usually has a pH of 7. Thus, if one measures the electromotive force generated at the electrode membrane, the pH of the test solution can be found by calculation. A second electrode is necessary when measuring the electromotive force generated at the electrode membrane of a glass electrode. This other electrode, paired with the glass electrode, is called the reference electrode. The reference electrode must have extremely stable potential. Therefore, it is provided with a pinhole or a ceramic material at the liquid junction.
A glass-electrode pH meter consists of a detector, indicator and reference solution. A brief description of each part follows:
Fourth, too large a difference in potential (asymmetric difference in potential) must not be generated between the solutions inside and outside the electrode when the electrode is immersed in a solution of identical pH to that of the solution inside of the electrode. Another requirement is that the glass membrane be resistant to shock and chemical
reactions.Generally, silver chloride is used as the material for the internal electrode. Potassium chloride solution maintained at pH 7 is usually used as the internal solution.
In Japan, Professor Tatsuzo Okada of Kyoto University launched a study on lithium glass electrodes right after the end of the war. Also, studies on reference electrodes and amplifiers were carried out by people in various fields. Horiba Wireless Research Center (the predecessor of Horiba, Ltd.) introduced and integrated these technologies and developed the first glass-electrode pH meter in Japan in 1950. Moreover, Horiba introduced a two-dimensional processing technique in creating the structure for the glass electrode and succeeded in the development of the "sheet-type composite glass electrode," which enlaces the glass electrode and reference electrode, and is only 1 mm in
As shown in the figure, it consists of a liquid junction, internal solution, replenishment inlet, a tube to support the reference electrode, the internal solution of the reference electrode, an internal electrode and an electrode lead wire. In most cases, a silver chloride electrode or mercurous chloride electrode is used as the internal electrode, and potassium chloride is used as the internal solution.
The liquid junction contacts the test solution and the internal solution. This is roughly classified into four types:
(1) the pinhole type, which has a hole a few dozen microns in diameter, (2) the sleeve type, which has a petticoat facing upward, (3) the ceramic type, which contacts foreign material, and (4) the fiber type. The pinhole liquid junction has the
advantage of very small loss of the internal solution; however, it tends to generate liquid potential. The sleeve liquid junction is easy to clean, but loss of internal solution is higher. The ceramic and fiber liquid junctions exhibit less loss of internal solution, but a problem with adherence of test solution. In light of these advantages and disadvantages, a double-junction type was developed by combining two types of junctions.
Temperature-compensation electrode is needed, because the electromotive force generated at the glass electrode varies depending on the temperature of solution. Temperature compensation means compensating for the variation of electromotive force due to a variation in temperature. What needs to be understood thoroughly here is that a variation of pH values due to temperature has nothing to do with compensation for temperature. Therefore, one must record the temperature of a solution along with the pH value, even if using a pH meter that automatically compensates for temperature. Otherwise, the measured values may become meaningless.
With the composite electrode, the glass electrode and reference electrode are fully integrated into one unit. With the integrated electrode, the glass electrode, reference electrode, and temperature-compensation electrode are all integrated into one unit. This enables pH measurement only by immersing a single electrode into the sample solution. It is easy to use and convenient when cleaning and calibrating with standard solution.
Special versions of composite electrodes include
The combination of a glass electrode and reference electrode can bethought of as a battery with high internal resistance. Thus, you cannotmeasure the difference in potential accurately if you connect it to anordinary potentiometer (voltmeter) as-is. You need an amplifier with highinput impedance. The indicator of the pH meter has such an such amplifierbuilt in, and allows adjustment. A dial for adjusting asymmetricalpotential, a dial for temperature compensation, and a dial for adjustingsensitivity are indispensable for the indicator of a pH meter. The dial foradjusting asymmetrical potential is intended for adjusting readings on thepH meter so that they will correspond to the pH values of the referencesolution when you immerse the electrode in the reference solution,moving to zero point of the amplifier electrically. The dial for temperature compensation is provided for compensating forvariations of electromotive force of the glass electrode per 1 pH due totemperature and for adjusting so that the indicator will indicate the correctpH values irrespective of temperature. However, this adjustment isautomatic with any pH meter that has a temperature-compensationelectrode (automatic temperature compensation). Therefore, such a pHmeter would usually not have a temperature-compensation dial. Instead, itwould have a dial for adjusting sensitivity. This dial is for adjusting the sensitivity of the amplifier so that it willcorrespond to the electromotive force of the glass electrode per 1 pH of ata certain temperature. Its electrical action is not different from that of thetemperature-compensation dial, though its adjustment range is rathernarrow.
A reference solution must always be calibrated for the pH meter before measuring pH. A buffer solution whose pH is unlikely to vary is used as the reference solution. Without a reliable reference solution, erroneous results can be expected.