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Measurement of pH by ECE company.

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.
When the hydrogen ions outnumber the hydroxide ions, the solution is acidic. If the reverse is true, then the solution is alkaline. 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:
(Kw is called the ion product of water or dissociation constant of water.) In pure water or neutral solution, the following formula holds true, because
If you know the value of either [H+] or [OH-], you can find the value of the other. Thus, we measure only [H+] and use it as the standard for pH. In this way, pH is determined by hydrogen-ion concentration. So, pH is defined by the following formula
The concentration of hydrogen ions in any solution we are likely to encounter will range from 1 mol to 0.000001 mol per liter of solution. However, solutions with extremely low hydrogen-ion concentration could conceivably rack up a pretty long parade of zeros after the decimal point. Danish biochemist S.P.L. Sorensen was the first to use the pH system we know today, which defines inverse numbers of hydrogen-ion concentration shown in common logarithm as pH. That is to say, in the case of a neutral solution:
This means, for example, that a hydrogen-ion concentration of a solution with a pH of 4 is , meaning it contains 0.0001 mol of hydrogen ions in a solution of 1 liter. In the same way, a solution with a ph of 5 contains   of hydrogen ions, while the solution with a pH of 6 contains  of hydrogen ions. You will notice that if you compare solutions with pH of 4 and pH of 6, the difference in pH is only 2, but the concentration of hydrogen ions with a pH of 6 is 100 times as high as with a pH of 4.

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.
Imagine a box of known size that contains one steel ball. If you move the box back and forth or side to side, the ball rolls around freely within the box. Then, suppose you have two balls in an identical box. These two balls will sometimes collide, which places certain limits on possible directions of movement of the balls. But movement is not severely restricted with only two balls. However, as the number of balls increases, the movement of the balls becomes more and more limited. Suppose we 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.

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:
  • (A) Indicator methods
This category basically includes two methods: One involves comparing the standard color corresponding to a known pH with the color of an  indicator immersed in the test liquid using buffer solution. The other method involves preparing pH test paper which is soaked in the indicator, then immersing the paper in the test liquid and comparing its color with the standard color. This method is simple, but prone to error. A high degree of accuracy cannot be expected.
        * 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
 
  • (B) Metal-electrode methods(including the hydrogen-electrode method, quinhydron-electrode method and antimony-electrode method)
A hydrogen electrode is made by adding platinum black to platinum wire or a platinum plate. It is immersed in the test solution and an electric charge is applied to the solution and platinum black with hydrogen gas.
The hydrogen-electrode method is a standard among the various methods for measuring pH. The values derived using other methods become trustworthy only when they match those measured using hydrogen electrode method.

This means

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-.
 
  • (C) Glass-electrode methods
This method is most widely used for pH measurement because the balancing time of electrical potential is short, it has high reproducibility, it is rarely affected by oxidizing and reducing agents, and it can measure pH of various solutions. This method is used not only in industry but in a wide variety of fields.

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.
In other words, a glass electrode is devised to generate accurate electromotive force due to the difference in pH. And a reference electrode is devised not to cause electromotive force due to a difference in pH. A glass-electrode pH meter consists of a detector, indicator and reference solution. A brief description of each part follows:
  1. The detector consists of a glass electrode, a reference electrode, and a temperature-compensation electrode. There is also a composite electrode, in which the glass electrode and reference electrode are integrated into one unit, and the integrated electrode, into which all three of the above-mentioned electrodes are integrated into a single unit
  2. A glass electrode consists of an electrode membrane that responds to pH, a highly isolating base material to support the unit, solution inside the glass electrode, an internal electrode, a lead wire, and a glass electrode terminal. The most critical item in this system is the electrode membrane. First, the membrane glass must generate a potential that accurately corresponds to the pH of the solution. Second, even though it must be accurately sensitive to acidity and alkalinity, it must not be damaged by them. Third, the electric resistance of the membrane itself must not be too large.

  3. 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.
  4. In 1906, Cremer blew the tip of a glass tube into a bubble and measured the difference in potential between two kinds of solutions (0.6% NaCl + diluted H2SO4 and O.6% NaCl + diluted NaOH). This is considered the birth of the glass electrode. In 1909, Habert and Klemensiewicz measured the difference in potential between a silver chloride electrode and a mercurous chloride electrode, and found that they could obtain a titration curve similar to that of a hydrogen electrode. They called this a glass electrode. So, the glass electrode took its first step toward becoming a practical pH electrode. However, early glass electrodes had large electrical resistance and very thin glass membranes. Therefore, they were very fragile and difficult to handle. Later, with the introduction of glass containing lithium, which is chemically strong and has low electric resistance and with development of technology for fabricating electronic parts and insulation materials, the glass electrode made rapid progress after the Second World War. Now it is widely used as the standard for measuring pH.


  5. 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.

  6. A reference electrode is used in combination with a glass electrode so that the difference in potential generated between it and the glass electrode can be measured. It also indicates a known potential irrespective of the pH of the solution.

  7. 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.

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.