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PH meter using the principle
Feb 23, 2018

What is pH? The pH is the Latin abbreviation for "Pondus hydrogenii" (Pondus = pressure, hydrogenium = hydrogen), which measures the activity of hydrogen ions in a substance. This activity is directly related to the acidic, neutral and alkaline aqueous solution. Water is chemically neutral, but it is not free of ions, even if it is chemically pure, with traces of dissociation: Strictly speaking, hydrogen nuclei do not exist in a free state until hydrated.

H2O + H2O = H3O + + OHˉ, since the concentration of hydronium ion (H3O +) is equivalent to the concentration of hydrogen ion (H +), the above formula can be simplified into the following commonly used forms:

H2O = H + + OHˉ

The positive hydrogen ion here, people in chemistry is expressed as "H + ion" or "hydrogen nucleus." Hydronuclear nuclei are indicated as "hydronium ions." Negative hydroxide ions are called "hydroxide ions."

Using the law of mass action, one can find an equilibrium constant for the dissociation of pure water:

K = H3O + × OH ----- H2O

Since only a very small amount of water is dissociated, the molar mass of water is actually a constant, and the equilibrium constant, K, yields the product, KW, of water.

KW = K × H2O KW = H3O + · OH- = 10-7 · 10-7 = 10-14 mol / l (25 ° C)

That is to say there is 10-7 moles H3O + ions and 10-7 moles OHˉ ions for one liter of pure water at 25 ° C.

In neutral solution, the concentration of hydrogen ion H + and hydroxide ion OHˉ is 10-7mol / l. Such as:

If there is an excess of H + ions, the solution is acidic. Acids are substances that make hydrogen ions H + in the aqueous solution free. Similarly, if OH ions are free, then the solution is alkaline. So, given the H + value is sufficient to indicate the nature of the solution, acidic or alkaline, in order to avoid using the molecular concentration of negative index computing, biologist Soernsen (Soernsen) in 1909 recommended this inconvenient to use The value is replaced by a logarithm and is defined as "pH." The mathematical definition of pH is the common logarithm of hydrogen ion concentration. Ie pH = -log [H +].

Therefore, the pH value is the negative logarithm of the base 10 ion concentration:

Changing the pH of 50m3 water requires 500L of bleach from pH2 to pH3. However, only 50 L of bleach is required from pH 6 to pH 7.

There are many ways to measure pH value, mainly chemical analysis method, dipstick method, potential method. Are mainly introduced potentiometric pH measured.

The electrode used for potential analysis is called a galvanic cell. The original battery is a system, its role is to make the chemical reaction energy into electricity. The voltage of this battery is called electromotive force (EMF). This electromotive force (EMF) consists of two half-cells, one of which is called the indicator electrode and whose potential is related to a specific ion activity, such as H +; the other half-cell is a reference half-cell, commonly referred to as a reference Electrode, which is usually the measurement solution is connected, and connected with the measuring instrument.

For example, an electrode is made of a silver wire inserted in a salt solution containing silver ions, forming ions at the interface between the wire and the solution due to the different activities of silver ions in the two phases of the metal and the salt solution The charging process, and the formation of a certain potential difference. Lost electronic silver ions into the solution. This process eventually reaches a balance when no external current is applied for reverse charging, that is, no current flow. The voltage that is present in this state of equilibrium is called the half-cell potential or the electrode potential. This (as described above) electrode consisting of metal and a solution containing the metal ion is referred to as a first type of electrode.

The measurement of this potential is carried out with respect to a reference electrode whose potential is independent of the composition of the salt solution. This reference electrode with its own potential is also referred to as the second electrode. For such electrodes, the metal wire is covered with a layer of a sparingly soluble salt of such metal (eg, Ag / Agcl) and inserted into the electrolyte solution containing the anion of the metal salt. The magnitude of the half-cell potential or the electrode potential at this time depends on the activity of such anions.

The voltage between these two electrodes follows the Nernst equation:


Nernst formula

Where: E-potential

E0 - standard voltage of the electrode

R-Gas Constant (8.31439 Joules / mole and ° C)

T-Kelvin absolute temperature (example: 20 ° C corresponds to (273.15 + 20) 293.15 Kelvin)

F-Faraday's constant (96493 banked / equivalent)

n-valence of the measured ion (silver = 1, hydrogen = 1)

ln (aMe) - The logarithm of ion activity aMe

The standard hydrogen electrode is the reference point for all potential measurements. The standard hydrogen electrode is a platinum wire electroplated (coated) with platinum chloride and surrounded by hydrogen gas (fixed pressure of 1013 hPa).

Immersion of this electrode at 25 ℃ H3O + ion content of 1mol / l solution will be formed in electrochemical all potential measurement reference half-cell potential or electrode potential. Among them, the hydrogen electrode is difficult to be practically used as a reference electrode, so a second type of electrode is used as a reference electrode. The most common of these is silver / silver chloride electrodes. The electrode reacts with changes in chloride ion concentration by dissolving AgCl.

The reference electrode's electrode potential is stabilized by a saturated kcl reservoir (eg, 3 mol / l kcl). Electrolyte solution in liquid or gel form is connected to the solution under test through the membrane.

Silver electrode and Ag / AgCl reference electrode can be used to measure the silver ion content in the film wash solution using the above electrode combination. The silver electrode can also be replaced by a platinum or gold electrode for the redox potential measurement. For example: a metal ion oxidation stage.

The most commonly used pH indicator electrode is a glass electrode. It is an end blown glass for pH-sensitive glass films. Tube filled with saturated AgCl 3mol / l kcl buffer solution, pH 7. The pH difference potential difference existing on both sides of the glass film was measured by a Ag / AgCl conduction system,

Such as the second electrode, export. pH composite electrode and pH solid state electrode,

This potential difference follows the Nernst formula:


Nernst formula

The E0, R, T (298.15K or 25 ℃) and other values into the above formula has been:

E = 59.16 mv / 25 ° C per pH (where ln (H3O +) has been converted to pH)

Where R and F are constants, n is the valence, and each ion has its fixed value. N = 1 for hydrogen ions. The temperature "T" as a variable plays a large role in the Nernst formula. As the temperature rises, the potential value will increase.

For every 1 ° C temperature increase will cause a potential change of 0.2 mv / per pH. The value of pH indicates a change of 0.0033 pH per 1 ° C first 1 pH.

This means that for temperature measurements between 20 ° C and 30 ° C and around 7 pH it is not necessary to compensate for temperature changes and for applications with a temperature> 30 ° C or <20 ° C and a pH value of 8 or 6, Compensation.

Industrial pH meter, is a commonly used industrial equipment, mainly used to accurately measure the pH value of the liquid medium. As well as installation, cleaning, anti-interference and other issues to consider