Med-Hycos - the Data
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.
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.
So, there was great scientific progress, although it was found that theoretical calculation of pH based on activity was impossible and that activity could not be measured directly. So, the classical definition of pH (meaning the notion that pH could be determined according to the concentration of hydrogen ions) proposed by Sorensen was subject to slight modification as science progressed. However, such modification did not detract from the advantages of using the pH scale, or from its practical value and biological and chemical meaning.
So, what is activity of hydrogen ions? Let's try to clear that up.
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:
With the widespread need for measuring the pH of various solutions, the problem of getting different measured values from identical samples became prominent. Therefore, it became necessary to establish a clear definition of pH and a standard selection method. Also, it was decided to try to define JIS standards as early as possible for methods for measuring pH. JIS standards for methods for measuring pH were established in March of 1957, after a lot of research and surveys and with the participation of people from a wide variety of fields. In preparation for the planning of the JIS standards, the standards regarding pH in the U.S., England and France were studied. Since pH was used not only in Japan but also abroad, they couldn't just set arbitrary standards, but needed to consider the future international validity of any proposed standards. Compatibility was needed so as not to hinder academic and commercial activities. This concludes our briefing on pH
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.
Note: Quinhydron solution of a certain pH is sometimes used to check whether an ORP meter is operating normally. The principle of the quinhydron electrode is applied in such a case.
Note: The antimony-electrode method is as upper and is now used only in cases where a high degree of accuracy is not required (only for industrial use) and the test solution contains F-.
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.
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 thickness.
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 followings:
As it has a sharp point at the tip of the detector, it can impale solid substances such as meat, processed food, vegetables, fruit, soil, pieces of animal tissue, drugs, and cosmetics to measure pH values.
As the pH-sensitive part and liquid junction are provided on the same surface, this type of electrode can measure pH values on the surface of wet substances such as skin, leather, paper or leaves. Also, it can measure pH if the amount of the sample liquid is very small.
This is an ultrasmall electrode with a long lead wire so that it can be inserted into the digestive tract and reach the stomach and duodenum.
This type is long and narrow so that it can be inserted into a narrow container such as a test tube to measure pH values.