918博天堂

This article describes the definition, principle and various measurement methods of pH acidity meter; the general classification of pH acidity meter. It is mainly for the analysis of the pH acidity meter and sensor principle which are used in the market at present, and the classification of the pH acidity meter is briefly introduced.

Keywords [acidic, basic, neutral, pH acidity meter.

With the improvement of people's living standards, PH acidity meter has gradually entered our lives and is closely related to our life. It is widely used in environmental protection, food, chemical, pharmaceutical, agricultural, aquaculture and other fields.

Definition, principle and measurement method of PH acidity meter

What is PH? PH is Latin "Pondus

Abbreviation for the term hydrogenii (Pondus = pressure, pressure hydrogenium = hydrogen), used to measure the activity of hydrogen ions in a substance. This activity is directly related to the acidity, neutrality and alkalinity of aqueous solutions. Water is chemically neutral. However, there is no ion, even if the chemical pure water is also slightly dissociated: strictly speaking, the hydrogen nucleus does not exist in a free state until it cooperates with water molecules.

H2O+ H2O= + OHˉ

Since the concentration of water total hydrogen ions (H3O) is treated in the same way as the hydrogen ion (H) concentration, the above formula can be simplified into the following common forms:

H2O= + OHˉ

The positive hydrogen ions here are expressed in the chemical as "ion" or "hydrogen core". The hydrated hydrogen nucleus is represented as "hydrated hydrogen ion". Negative hydroxide ions are referred to as "hydroxide ions."

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

Since only a small amount of water is dissociated, the molar concentration of water is actually a constant, and there is an equilibrium constant K to determine the ion product KW of water.

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

In other words, there is 10-7 moles for one liter of pure water at 25 °C.

Ions and 10-7 moles of OH ˉ ions.

In a neutral solution, hydrogen ions

The concentration of the hydroxide ion OH 都是 is 10-7 mol/l.

Such as:

If there is excessive hydrogen ion

, the solution is acidic. Acid is the hydrogen ion in the aqueous solution

Free substance. Similarly, if the hydrogen ions and the OH ˉ ions are free, then the solution is basic. Therefore, the value given is sufficient to indicate the characteristics of the solution, which is acidic and alkaline. In order to avoid the calculation of the negative concentration index of the molar concentration, the biologist Soernsen suggested the value of the inconvenience in 1909. Replace with logarithm and define it as "pH". The mathematically defined pH is the common log negative value of the hydrogen ion concentration. Immediately, therefore, the pH is the negative of the base 10 logarithm of the ion concentration:

acid

neutral

Alkali

← →

PH value

To change the pH of 50 water, 5001 bleach is required from pH 2 to pH 3. However, only 50 l of bleach is required from pH 6 to pH 7.

There are many methods for measuring the pH value, mainly chemical analysis method, test paper method, and potential method. Now we mainly introduce the pH value measured by the potential method.

The electrode used in the potential analysis method is called a primary battery. A primary battery is a system whose function is to convert chemical reaction energy into electrical energy. The voltage of this battery is called electromotive force (EMF). This electromotive force (EMF) consists of two half cells. One of the half-cells is called a measuring electrode, and its potential is related to a specific ion activity such as:

The other half-cell is a reference half-cell, commonly referred to as a reference electrode, which is typically connected to the measurement solution and is connected to a measuring instrument.

For example, an electrode is made of a silver wire inserted in a salt solution containing silver ions, and at the interface between the wire and the solution, a different activity of silver ions in the two phases of the metal and the salt solution is formed. The charging process of ions and the formation of a certain potential difference. Lost electrons of silver ions into the solution. When no external current is applied for reverse charging, that is, there is no current, the process will eventually reach a balance. The voltage present in this equilibrium state is referred to as a half-cell potential or an electrode potential.

Such an electrode (as described above) consisting of a metal and a solution containing the metal ion is referred to as a first type of electrode.

This potential is measured relative to a reference electrode whose potential is independent of the composition of the salt solution. Such a reference electrode having an independent potential is also referred to as a second electrode. For such electrodes, the metal wires are covered with a layer of a slightly soluble salt of such a metal (e.g., Ag/AgCL) and inserted into an electrolyte solution containing such metal salt-limited ions. The half cell potential or the electrode potential at this time depends on the activity of such an anion.

The voltage between the two electrodes follows the Nernst formula:

E=E0+ R·T·1n a Men·F

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 = 273 + 293 Kelvin)

F-Faraday constant (96493 library / equivalent)

N—the valence of the measured ion (silver=1, hydrogen=1)

aMe-ion activity

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

Immerse the electrode at 25 ° C

In the solution with an ion content of 1 mol/l, the half-cell potential or electrode potential referenced for all potential measurements in the electrochemical is formed. The hydrogen electrode as a reference electrode is difficult to implement in practice, so a second type of electrode is used as a reference electrode. The most commonly used one is the silver/silver chloride electrode. The electrode reacts to changes in chloride ion concentration by dissolved AgCl.

The electrode potential of this reference electrode is made constant by a saturated kcl reservoir (eg 3 mol/l kcl). An electrolyte solution in the form of a liquid or gel is connected to the solution to be tested through a separator.

The silver ion content in the film rinsing liquid can be measured by using the above electrode combination-silver electrode and Ag/AgCl reference electrode. It is also possible to replace the silver electrode with a platinum or gold electrode for the measurement of the oxidation-reduction potential. For example: the oxidation phase of a certain metal ion.

The most commonly used PH indicating electrode is a glass electrode. It is a glass tube that is blown into a bubble-like pH-sensitive glass film. The tube is filled with 3 mol/l kcl containing saturated AgCl.

Buffer solution, pH 7. The potential difference reflecting the pH value on both sides of the glass film is controlled by Ag/AgCl conduction system.

Such as the second electrode, derived. PH composite electrode as shown in Figure (1)

This potential difference follows the Nernst formula:

E=E0+ R·T·1n a H3O+n·F

E=59.16mv/25°C per pH

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

As the temperature per 1 °C becomes larger, a potential change of 0.2 mv/per pH is caused. When expressed by pH, the first pH changes by 0.0033 pH per 1 °C.

This means that for measurements between 20 and 30 ° C and around 7 pH, there is no need to compensate for temperature changes; for applications with temperatures >30 ° C or < 20 ° C and pH > 8 pH or 6 pH Compensate for temperature changes.

From the above analysis of the principle of PH measurement, we know that we can display the PH value with a millivolt meter.

Classification of PH acidity meter

According to the needs of production and life, people have scientifically researched and produced many types of acidity meters:

According to the measurement accuracy, it can be divided into 0.2, 0.1, 0.01 or higher precision.

According to the volume of the instrument, there are pen-type (mini), portable, desktop and online continuous monitoring and measurement online.

According to the requirements of use:

Pen type (mini type) and portable PH acidity meter are generally used by the inspection personnel to be used for on-site inspection.