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in conditions it had not worked under before, says Konstantinos Gerasopoulos, a senior scientist at APL who is leading the research.

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Hundreds of thousands...

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internal short circuits that could ignite liquids electrolytes, which can lead to explosions and fires. Engineers at the University of Illinois have developed a polymer-based solid electrolyte that can self-heal after damage. Also...

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Scientists have created an “unkillable” battery
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1. BRIEF CHARACTERISTICS 1.1. PALMITIC ACID, ITS SALTS AND ETHERS 1.1.1. Palmitic acid (Cetyl) 1.1.2. Palmitic acid salts (palmitates) 1.1.3. Palmitic acid esters (p...

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Excellent conductors of electric current are gold, copper, iron, aluminum, and alloys. Along with them, there is a large group of non-metallic substances, the melts and aqueous solutions of which also have the property of conductivity. These are strong bases, acids, and some salts, collectively called “electrolytes.” What is ionic conductivity? Let's find out how electrolyte substances relate to this common phenomenon.

What particles carry charges?

The world around is full of various conductors, as well as insulators. These properties of bodies and substances have been known since ancient times. The Greek mathematician Thales conducted an experiment with amber (in Greek - “electron”). Having rubbed it on silk, the scientist observed the phenomenon of attraction of hair and wool fibers. Later it became known that amber is an insulator. There are no particles in this substance that could carry an electric charge. Metals are good conductors. They contain atoms, positive ions and free, infinitesimal negative particles— electrons. It is they that ensure the transfer of charges when current passes. Strong electrolytes in dry form do not contain free particles. But upon dissolution and melting, the crystal lattice is destroyed, as well as the polarization of the covalent bond.

Water, non-electrolytes and electrolytes. What is dissolution?

By donating or gaining electrons, atoms of metallic and nonmetallic elements become ions. There is a fairly strong connection between them in the crystal lattice. Dissolution or melting of ionic compounds, such as sodium chloride, leads to its destruction. Polar molecules contain neither bound nor free ions; they arise when interacting with water. In the 30s years XIX century M. Faraday discovered that solutions of some substances conduct current. The scientist introduced the following important concepts into science:

• ions (charged particles);
• electrolytes (conductors of the second kind);
• cathode;
• anode.

There are compounds - strong electrolytes, the crystal lattices of which are completely destroyed with the release of ions.

There are insoluble substances and those that are stored in molecular form, for example, sugar, formaldehyde. Such compounds are called non-electrolytes. They are not characterized by the formation of charged particles. Weak electrolytes (carbonic and acetic acid, and a number of other substances) contain few ions.

Electrolytic dissociation theory

In his works, the Swedish scientist S. Arrhenius (1859-1927) relied on Faraday's conclusions. Subsequently, the provisions of his theory were clarified by Russian researchers I. Kablukov and V. Kistyakovsky. They found that when dissolved and melted, not all substances form ions, but only electrolytes. What is dissociation according to S. Arrhenius? This is the destruction of molecules, which leads to the appearance of charged particles in solutions and melts. Basic theoretical principles of S. Arrhenius:

1. Bases, acids and salts are in dissociated form in solutions.
2. Strong electrolytes disintegrate reversibly into ions.
3. Weak ones form few ions.

The indicator of a substance (often expressed as a percentage) is the ratio of the number of molecules that have broken up into ions and the total number of particles in the solution. Electrolytes are strong if the value of this indicator is over 30%, for weak ones - less than 3%.

Properties of electrolytes

The theoretical conclusions of S. Arrhenius were supplemented by later studies of physicochemical processes in solutions and melts carried out by Russian scientists. We received an explanation of the properties of bases and acids. The first include compounds in solutions of which only metal ions can be found among the cations; the anions are OH - particles. Acid molecules break down into negative ions of the acid residue and hydrogen protons (H+). The movement of ions in solution and melt is chaotic. Let's consider the results of an experiment for which you will need to assemble a circuit and include an ordinary incandescent light bulb in it. Let's check the conductivity of solutions of different substances: table salt, acetic acid and sugar (the first two are electrolytes). What is an electrical circuit? This is a current source and conductors connected to each other. When the circuit is closed, the light bulb will burn brighter in a solution of table salt. The movement of ions becomes orderly. Anions are directed to the positive electrode, and cations are directed to the negative electrode.

A small number of charged particles are involved in this process in acetic acid. Sugar is not an electrolyte and does not conduct current. There will be an insulating layer between the electrodes in this solution; the light bulb will not light.

Chemical interactions between electrolytes

When draining solutions, you can observe how the electrolytes behave. What are the ionic equations for such reactions? Let's look at the example of the chemical interaction between and sodium nitrate:

2NaNO 3 + BaCl 2 + = 2NaCl + Ba(NO 3) 2.

We write the formulas of electrolytes in ionic form:

2Na + + 2NO 3- + Ba 2+ + 2Cl - = 2Na + + 2Cl - + Ba 2+ + 2NO 3- .

The substances taken for the reaction are strong electrolytes. In this case, the composition of the ions does not change. Chemical interaction between is possible in three cases:

1. If one of the products is an insoluble substance.

Molecular equation: Na 2 SO 4 + BaCl 2 = BaSO 4 + 2NaCl.

Let us write the composition of electrolytes in the form of ions:

2Na + + SO 4 2- + Ba 2+ + 2Cl - = BaSO 4 (white precipitate) + 2Na + 2Cl - .

2. One of the substances formed is gas.

3. Among the reaction products there is a weak electrolyte.

Water is one of the weakest electrolytes

Chemically pure does not conduct electric current. But it contains a small amount of charged particles. These are H + protons and OH - anions. A negligible number of water molecules undergo dissociation. There is a quantity - the ionic product of water, which is constant at a temperature of 25 ° C. It allows you to find out the concentrations of H + and OH -. Hydrogen ions predominate in solutions of acids, hydroxide anions are more abundant in alkalis. In neutrals, the amount of H + and OH - is the same. The solution environment is also characterized by pH value(pH). The higher it is, the more hydroxide ions are present. The medium is neutral with a pH range close to 6-7. In the presence of H + and OH - ions, indicator substances change their color: litmus, phenolphthalein, methyl orange and others.

The properties of solutions and melts of electrolytes are widely used in industry, technology, agriculture and medicine. The scientific basis lies in the work of a number of outstanding scientists who explained the behavior of the particles that make up salts, acids and bases. Various ion exchange reactions occur in their solutions. They are used in many production processes, in electrochemistry, and electroplating. Processes in living beings also occur between ions in solutions. Many non-metals and metals, toxic in the form of atoms and molecules, are irreplaceable in the form of charged particles (sodium, potassium, magnesium, chlorine, phosphorus and others).

Electrolytes are substances whose solutions or melts conduct electric current. Electrolytes include acids, bases and salts. Substances that do not conduct electric current in a dissolved or molten state are called nonelectrolytes. These include many organic substances, such as sugars, etc. The ability of electrolyte solutions to conduct electric current is explained by the fact that when dissolved, electrolyte molecules disintegrate into electrically positively and negatively charged particles - ions. The amount of charge on an ion is numerically equal to the valence of the atom or group of atoms that form the ion. Ions differ from atoms and molecules not only by the presence electric charges, but also other properties, for example, ions have no odor, color, or other properties of chlorine molecules. Positively charged ions are called cations, negatively charged ions are called anions. Cations form hydrogen H +, metals: K +, Na +, Ca 2+, Fe 3+ and some groups of atoms, for example the ammonium group NH + 4; Anions form atoms and groups of atoms that are acidic residues, for example Cl -, NO - 3, SO 2- 4, CO 2- 3.

The disintegration of electrolyte molecules into ions is called electrolytic dissociation, or ionization, and is a reversible process, i.e., an equilibrium state can occur in a solution in which as many electrolyte molecules disintegrate into ions, so many of them are formed again from ions. The dissociation of electrolytes into ions can be represented by the general equation: , where KmAn is an undissociated molecule, K z+ 1 is a cation carrying z 1 positive charges, And z- 2 is an anion having z 2 negative charges, m and n are the number of cations and anions , formed during the dissociation of one electrolyte molecule. For example, .

The number of positive and negative ions in a solution may be different, but the total charge of the cations is always equal to the total charge of the anions, so the solution as a whole is electrically neutral.

Strong electrolytes almost completely dissociate into ions at any concentration in solution. These include strong acids (see), strong bases and almost all salts (see). Weak electrolytes, which include weak acids and bases and some salts, such as sublimate HgCl 2, dissociate only partially; the degree of their dissociation, i.e., the proportion of molecules disintegrated into ions, increases with decreasing solution concentration.

A measure of the ability of electrolytes to disintegrate into ions in solutions can be the electrolytic dissociation constant (ionization constant), equal to
where the concentrations of the corresponding particles in the solution are shown in square brackets.

When a direct electric current is passed through an electrolyte solution, cations move to a negatively charged electrode - the cathode, anions move to a positive electrode - the anode, where they give up their charges, turning into electrically neutral atoms or molecules (cations receive electrons from the cathode, and anions give up electrons at the anode) . Since the process of adding electrons to a substance is reduction, and the process of giving up electrons by a substance is oxidation, when an electric current is passed through an electrolyte solution, reduction of cations occurs at the cathode, and oxidation of anions occurs at the anode. This redox process is called electrolysis.

Electrolytes are an indispensable component of fluids and dense tissues of organisms. In physiological and biochemical processes, an important role is played by inorganic ions such as H +, Na +, K +, Ca 2+, Mg 2+, OH -, Cl -, HCO - 3, H 2 PO - 4, SO 2- 4 (see Mineral Metabolism). H + and OH - ions are found in very small concentrations in the human body, but their role in life processes is enormous (see Acid-base balance). The concentration of Na + and Cl - ions significantly exceeds that of all other inorganic ions combined. See also Buffer solutions, Ion exchangers.

Electrolytes are substances whose solutions or melts conduct electric current. Typical electrolytes are salts, acids and bases.

According to Arrhenius' theory of electrolytic dissociation, electrolyte molecules in solutions spontaneously disintegrate into positively and negatively charged particles - ions. Positively charged ions are called cations, negatively charged ions are called anions. The amount of charge on an ion is determined by the valence (see) of the atom or group of atoms that form the given ion. Cations are usually formed by metal atoms, for example K+, Na+, Ca2+, Mg3+, Fe3+, and some groups of other atoms (for example, the ammonium group NH 4); Anions, as a rule, are formed by atoms and groups of atoms that are acidic residues, for example Cl-, J-, Br-, S2-, NO 3 -, CO 3, SO 4, PO 4. Each molecule is electrically neutral, therefore the number of elementary positive charges of cations is equal to the number of elementary negative charges of anions formed during the dissociation of the molecule. The presence of ions explains the ability of electrolyte solutions to conduct electric current. Therefore, electrolyte solutions are called ionic conductors, or conductors of the second kind.

The dissociation of electrolyte molecules into ions can be represented by the following general equation:

where is a non-dissociated molecule, is a cation carrying n1 positive charges, is an anion having n2 negative charges, p and q are the number of cations and anions included in the electrolyte molecule. For example, the dissociation of sulfuric acid and ammonium hydroxide is expressed by the equations:

The number of ions contained in a solution is usually measured in gram ions per 1 liter of solution. Gram ion is the mass of ions of a given type, expressed in grams and numerically equal to the formula weight of the ion. The formula weight is found by summing the atomic weights of the atoms forming a given ion. So, for example, the formula weight of SO 4 ions is equal to: 32.06 + 4-16.00 = 96.06.

Electrolytes are divided into low molecular weight, high molecular weight (polyelectrolytes) and colloidal. Examples of low molecular weight electrolytes, or simply electrolytes, are ordinary low molecular weight acids, bases and salts, which in turn are usually divided into weak and strong electrolytes. Weak electrolytes do not completely dissociate into ions, as a result of which a dynamic equilibrium is established in the solution between the ions and undissociated electrolyte molecules (equation 1). Weak electrolytes include weak acids, weak bases and some salts, such as sublimate HgCl 2. The dissociation process can be quantitatively characterized by the degree of electrolytic dissociation (degree of ionization) α, the isotonic coefficient i and the electrolytic dissociation constant (ionization constant) K. The degree of electrolytic dissociation α is the fraction of electrolyte molecules that disintegrates into ions in a given solution. The value of a, measured in fractions of a unit or in %, depends on the nature of the electrolyte and solvent: it decreases with increasing solution concentration and usually changes slightly (increases or decreases) with increasing temperature; it also decreases when a stronger electrolyte is introduced into the solution of a given electrolyte, forming nones of the same name (for example, the degree of electrolytic dissociation of acetic acid CH 3 COOH decreases when hydrochloric acid HCl or sodium acetate CH 3 COONa is added to its solution).

The isotonic coefficient, or Van't Hoff coefficient, i is equal to the ratio of the sum of the number of ions and undissociated molecules of the electrolyte to the number of its molecules taken to prepare the solution. Experimentally, i is determined by measuring osmotic pressure, lowering the freezing point of the solution (see Cryometry) and some others physical properties solutions. The quantities i and α are interrelated by the equation

where n is the number of ions formed during the dissociation of one molecule of a given electrolyte.

The electrolytic dissociation constant K is an equilibrium constant. If the electrolyte dissociates into ions according to equation (1), then

Where, and are the concentrations in solution of cations and anions (in g-ion/l) and undissociated molecules (in mol/l), respectively. Equation (3) is a mathematical expression of the law of mass action as applied to the process of electrolytic dissociation. The higher the K, the better the electrolyte breaks down into ions. For a given electrolyte, K depends on temperature (usually increases with increasing temperature) and, unlike a, does not depend on the concentration of the solution.

If a molecule of a weak electrolyte can dissociate not into two, but into a larger number of ions, then the dissociation occurs in stages (stepwise dissociation). For example, weak carbonic acid H 2 CO 3 in aqueous solutions dissociates in two steps:

In this case, the dissociation constant of the 1st step significantly exceeds that of the 2nd step.

Strong electrolytes, according to the Debye-Hückel theory, in solutions are completely dissociated into ions. Examples of these electrolytes include strong acids, strong bases, and almost all water-soluble salts. Due to the complete dissociation of strong electrolytes, their solutions contain a huge number of ions, the distances between which are such that electrostatic attraction forces appear between oppositely charged ions, due to which each ion is surrounded by ions of opposite charge (ionic atmosphere). The presence of an ionic atmosphere reduces the chemical and physiological activity of ions, their mobility in an electric field and other properties of ions. The electrostatic attraction between oppositely charged ions increases with increasing ionic strength of the solution, equal to half the sum of the products of the concentration C of each ion and the square of its valence Z:

So, for example, the ionic strength of a 0.01 molar solution of MgSO 4 is equal to

Solutions of strong electrolytes, regardless of their nature, with the same ionic strength (not exceeding, however, 0.1) have the same ionic activity. The ionic strength of human blood does not exceed 0.15. To quantitatively describe the properties of solutions of strong electrolytes, a quantity called activity a was introduced, formally replacing the concentration in equations arising from the law of mass action, for example in equation (1). Activity a, which has the dimension of concentration, is related to concentration by the equation

where f is the activity coefficient, showing what proportion of the actual concentration of these ions in the solution is their effective concentration or activity. As the solution concentration decreases, f increases and in very dilute solutions becomes equal to 1; in the latter case a=C.

Low molecular weight electrolytes are an indispensable component of fluids and dense tissues of organisms. Of the ions of low molecular weight electrolytes, the cations H+, Na+, Mg2+, Ca2+ and the anions OH-, Cl-, HCO 3, H 2 PO 4, HPO 4, SO 4 play an important role in physiological and biochemical processes (see Mineral metabolism). H+ and OH- ions in organisms, including the human body, are found in very small concentrations, but their role in life processes is enormous (see Acid-base balance). The concentrations of Na+ and Cl- are significantly higher than the concentrations of all other ions combined.

Living organisms are highly characterized by the so-called antagonism of ions - the ability of ions in solution to mutually reduce the inherent effect of each of them. It has been established, for example, that Na+ ions in the concentration in which they are found in the blood are toxic to many isolated organs of animals. However, the toxicity of Na+ is suppressed when K+ and Ca2+ ions are added to a solution containing them in appropriate concentrations. Thus, K+ and Ca2+ ions are antagonists of Na+ ions. Solutions in which the harmful effects of any ions are eliminated by the action of antagonist ions are called equilibrated solutions. Antagonism of ions was discovered during their action on a wide variety of physiological and biochemical processes.

Polyelectrolytes are high molecular weight electrolytes; examples of them are proteins, nucleic acids and many other biopolymers (see Macromolecular compounds), as well as a number of synthetic polymers. As a result of the dissociation of polyelectrolyte macromolecules, low molecular weight ions (counterions), usually of different nature, and a multiply charged macromolecular ion are formed. Some counterions are tightly bound to the macromolecular ion by electrostatic forces; the rest are in solution in a free state.

Examples of colloidal electrolytes include soaps, tannins, and some dyes. Solutions of these substances are characterized by equilibrium:
micelles (colloidal particles) → molecules → ions.

When a solution is diluted, the equilibrium shifts from left to right.

See also Ampholytes.

1. ELECTROLYTES

1.1. Electrolytic dissociation. Degree of dissociation. Electrolyte Power

According to the theory of electrolytic dissociation, salts, acids, and hydroxides, when dissolved in water, completely or partially disintegrate into independent particles - ions.

The process of decomposition of substance molecules into ions under the influence of polar solvent molecules is called electrolytic dissociation. Substances that dissociate into ions in solutions are called electrolytes. As a result, the solution acquires the ability to conduct electric current, because mobile electric charge carriers appear in it. According to this theory, when dissolved in water, electrolytes break up (dissociate) into positively and negatively charged ions. Positively charged ions are called cations; these include, for example, hydrogen and metal ions. Negatively charged ions are called anions; These include ions of acidic residues and hydroxide ions.

To quantitatively characterize the dissociation process, the concept of the degree of dissociation was introduced. The degree of dissociation of an electrolyte (α) is the ratio of the number of its molecules disintegrated into ions in a given solution ( n ), To total number its molecules in solution ( N), or

α = .

The degree of electrolytic dissociation is usually expressed either in fractions of a unit or as a percentage.

Electrolytes with a degree of dissociation greater than 0.3 (30%) are usually called strong, with a degree of dissociation from 0.03 (3%) to 0.3 (30%) - medium, less than 0.03 (3%) - weak electrolytes. So, for a 0.1 M solution CH3COOH α = 0.013 (or 1.3%). Therefore, acetic acid is a weak electrolyte. The degree of dissociation shows what part of the dissolved molecules of a substance has broken up into ions. The degree of electrolytic dissociation of an electrolyte in aqueous solutions depends on the nature of the electrolyte, its concentration and temperature.

By their nature, electrolytes can be divided into two large groups: strong and weak. Strong electrolytes dissociate almost completely (α = 1).

Strong electrolytes include:

1) acids (H 2 SO 4, HCl, HNO 3, HBr, HI, HClO 4, H M nO 4);

2) bases – metal hydroxides of the first group of the main subgroup (alkali) – LiOH, NaOH, KOH, RbOH, CsOH , as well as hydroxides of alkaline earth metals – Ba (OH) 2, Ca (OH) 2, Sr (OH) 2;.

3) salts soluble in water (see solubility table).

Weak electrolytes dissociate into ions to a very small extent; in solutions they are found mainly in an undissociated state (in molecular form). For weak electrolytes, an equilibrium is established between undissociated molecules and ions.

Weak electrolytes include:

1) inorganic acids ( H 2 CO 3, H 2 S, HNO 2, H 2 SO 3, HCN, H 3 PO 4, H 2 SiO 3, HCNS, HClO, etc.);

2) water (H 2 O);

3) ammonium hydroxide ( NH 4 OH);

4) most organic acids

(for example, acetic CH 3 COOH, formic HCOOH);

5) insoluble and slightly soluble salts and hydroxides of some metals (see solubility table).

Process electrolytic dissociation depicted using chemical equations. For example, dissociation of hydrochloric acid (HC l ) is written as follows:

HCl → H + + Cl – .

Bases dissociate to form metal cations and hydroxide ions. For example, the dissociation of KOH

KOH → K + + OH – .

Polybasic acids, as well as bases of polyvalent metals, dissociate stepwise. For example,

H 2 CO 3 H + + HCO 3 – ,

HCO 3 – H + + CO 3 2– .

The first equilibrium - dissociation according to the first step - is characterized by the constant

.

For second stage dissociation:

.

In the case of carbonic acid, the dissociation constants have the following values: K I = 4.3× 10 –7, K II = 5.6 × 10–11. For stepwise dissociation always K I > K II > K III >... , because the energy that must be expended to separate an ion is minimal when it is separated from a neutral molecule.

Average (normal) salts, soluble in water, dissociate to form positively charged metal ions and negatively charged ions of the acid residue

Ca(NO 3) 2 → Ca 2+ + 2NO 3 –

Al 2 (SO 4) 3 → 2Al 3+ +3SO 4 2–.

Acid salts (hydrosalts) are electrolytes containing hydrogen in the anion, which can be split off in the form of the hydrogen ion H +. Acid salts are considered as a product obtained from polybasic acids in which not all hydrogen atoms are replaced by a metal. Dissociation of acid salts occurs in stages, for example:

KHCO 3 → K + + HCO 3 – (first stage)

Acids: HCl HBr HI HClO 4 HMnO 4 H 2 SO 4 HNO 3

Bases: hydroxides formed by s-elements 1 and s-elements of group 11, starting with Ca

NaOH KOH Ca(OH) 2 Sr(OH) 2 Ba(OH) 2

Salts - almost everything.

Acids from the point of view of the theory of dissociation, these are electrolytes that dissociate to form a hydrogen cation and an acid residue anion. The presence of hydrogen cations in acid solutions determines their sour taste, the ability to change the color of the indicator, have an irritating and even inflammatory effect.

Acids, depending on their strength, dissociate in different ways.

Strong acids dissociate immediately and irreversibly:

Weak electrolytes dissociate stepwise and reversibly

CH 3 COOH = CH 3 COO - + H +

H 2 CO 3 = H + + HCO 3 -

HCO 3 - = H + + CO 3 2-

The dissociation of weak compounds, as a reversible process, is characterized by the dissociation constant

TO dis. CH3COOH= (CH 3 COO -)*(H +)

Carbonic acid, as a dibasic acid, will be characterized by the presence

TO dis 1 st H 2 CO 3 = (NSO 3 -)*(H +)

TO dis.2st N 2 CO 3 = (CO 3 2-)*(H +)

The dissociation constant, like any constant of a reversible process, is a constant value for each electrolyte (depending on the nature of the substance) and depends on the temperature of the solution. The lower the dissociation constant, the weaker the electrolyte. (K dis. is a constant value and can be found in the reference table)

Grounds – These are electrolytes that dissociate to form a metal cation and a hydroxide anion. Strong bases dissociate immediately and irreversibly:

KOH K + + OH -

Weak electrolytes dissociate stepwise and reversible

Mg(OH) 2 MgOH + + OH -

MgOH + Mg 2+ + OH -

Salts– strong electrolytes, therefore in solution immediately and completely decompose into metal cations and acid residue anions.

Al 2 (SO 4) 3 2Al 3+ + 3SO 4 2-

Na 3 PO 4 3Na + + PO 4 3-

Acid salts first dissociate into a metal cation and an anion of the acid residue

NaHCO 3 Na + + HCO 3 -

And then the acidic residue dissociates as an acid

HCO 3 - H + + CO 3 2-

The concept of pH (ph)

Water is most often used as a solvent. Although water is a weak electrolyte, it dissociates in solution

H 2 O = H + +OH -

Like any reversible process, it is characterized by its dissociation constant

TO dis. = (H +)*(OH -)

It has been experimentally proven that out of 10,000,000 molecules only one breaks up into ions. Therefore, the concentration of water is taken as a constant value and the following expression is obtained

Kdis * (H 2 O) = Kw = (H +) * (OH -) = const = 10 -14 (this value is called the ionic product of water)

Because this value is constant, it is used to calculate the concentration of H + or OH - ions

For example, (OH -) =10 -3 determine (H +) = ?

(H+) = K w= 10 -14 =10 -11

- (OH -) = 10 -1 (H +) = 10 -13 ph =13

- (OH -) = 10 -5 (H +) = 10 -9 ph = 9

- (OH -) = 10 -7 (H +) = 10 -7 ph = 7

- (OH -) =10 -10 (H +) = 10 -4 ph = 4

- (OH -) = 10 -14 (H +) =10 0 =1 ph =1

All subsequent calculations are made similarly to the first. It is inconvenient to use fractional notations for concentrations, so the concept is introduced pH value ( its values ​​are given in the far right column)

(H +)= 10 -6 ph=6, (H +) = 10 -11 ph=11