pH values ​​of solutions. Hydrogen indicator: concept and norm. Reasons to measure pH

The value of the pH value plays an important role for many chemical and biological transformations that take place both in laboratories and in production, as well as in living organisms and the environment. The number of hydrogen ions not only affects the result of any reaction, but also the possibility of its occurrence. To maintain a given pH value, buffer solutions are used. Their task is to maintain this level when diluting solutions or adding acids and alkalis to them.

The pH value of water is one of the indicators of the quality of water for various purposes. In nature, the development of plants and the aggressiveness of the environment on metal and concrete structures depend on it. It should be remembered that the pH value changes the toxicity of pollutants for organisms living in rivers, lakes, and ponds.

pH value

This parameter characterizes the content of H + ions in solutions. It is designated pH. Mathematically, the hydrogen index is equal to the reciprocal of the concentration Η + (С Н+, mol/l): pΗ = −logC Н+. The number of H+ ions in water is determined by the dissociation of H 2 O molecules, which occurs according to the expression: H 2 O<->H + + OH - .

Despite the fact that water is not usually classified as an electrolyte, it is a low-dissociating substance. For it, we can write the dissociation constant: K d = (C H+ ·C OH-)/C H2O. At t=22 °C its value is 1.8ˑ10 -16.

This figure is so small that the H + and OH - ions in water could be neglected. But in solution chemistry, the pH value is used to create a pH scale. Let's consider its meanings.

pH scale

It can be used to quantitatively express the acidity of a solution.

The hydrogen index of the medium is easy to calculate. You just need to know the concentration of hydrogen cations and use the formula: C n+ = 10 n, where n is the pH value with the opposite sign. For example, the concentration of H + in a solution is C H + = 10 -5 mol/l. That is, n = -5, and pH = 5.

pH values ​​of some media and solutions

Everything in the human environment has its own specific pH values. This helps various body systems cope with their tasks more easily. As you know, for pure neutral water the pH value is 7. However, human skin has a slightly acidic reaction. Their pH=5.5. This fact partly influences the appearance of dry skin when it comes into frequent contact with water. Below are pH values ​​for some substances.

Types of solutions

Aqueous solutions, as indicated above, can have a neutral, acidic or alkaline reaction. The fact that the acidity of a solution is due to the presence of H+ ions, and the alkalinity is due to OH- ions, does not mean that they do not contain others. In acidic environments, an excess of hydrogen ions can be found, and in alkaline environments, an excess of hydroxide ions.

In neutral solutions, the hydrogen index is 7. This means that the concentration of H + cations in them is 10 -7 mol/l, but at the same time the content of hydroxide anions is also 10 -7 mol/l. In other words, in neutral solutions there is no excess of H+ or OH- ions.

Ionic product of water

Why can pH take values ​​from 1 to 14? To answer this question, it is worth returning to the expression of the dissociation constant. Having transformed it, we can write K d ·C H2O =C H+ ·C OH- . The value of Kd is known, and water can be easily calculated. Considering water as a solution of H 2 O in H 2 O, you can find out its molar concentration by making the proportion: 18 g H 2 O - 1 mol, 1000 g H 2 O - x mol. Hence x=1000/18=55.6 mol/l. This constant value is denoted by Kw and is called the ionic product of water.

This value allowed us to conclude that pΗ + pOΗ = 14, which is the answer to the question posed above.

Acidic environment

All strong acids in water dissociate irreversibly. Thus, hydrochloric acid completely decomposes into cations Η + and chloride anions Cl -: ΗCl = Η + + Cl -. If 1ˑ10 -2 mol ΗCl was added to water with a volume of 1 liter, then the concentration of Η + ions will also be equal to 1 . 10 -2 mol. That is, for such a solution the pH value is 2.

Weak acids dissociate reversibly, that is, as in the case of water, some of the oppositely charged ions recombine into acid molecules. For example, carbonic acid breaks down into the following ions: H 2 CO 3<->Η + + ΗCO 3 - . Not only do not all molecules dissociate, but those that disintegrate again form a single whole. Therefore, to find the pH value of acids, the dissociation constant is used.

In addition, the pH of the solution can be used to indirectly estimate the strength of the acid: the greater it is, the lower the pH value.

Alkaline environment

When bases are dissolved in water, they begin to dissociate with the appearance of hydroxide anions. They interact with H+ ions that are present in neutral clean water. This leads to a decrease in their concentration, that is, to an increase in pH.

For example: NaOΗ = Na + + OΗ - ; Η + + OΗ - = Η 2 O.

In a sodium hydroxide solution with a concentration of 1ˑ10 -2 mol/l, 1ˑ10 -2 mol/l hydroxide anions appear. The concentration of Η + cations in such a solution will be equal to 1ˑ10 -12 mol/l, and pΗ has a value of 12.

In all base solutions, the number of H + cations is always less than 1ˑ10 -7 mol/l, and the hydrogen index is greater than 7.

Determination of pH by indicators

One of the easiest ways to approximately determine the pH of a solution is to use universal indicator strips. By comparing their color, which appears after dipping into the working solution, with the indicator scale, you can estimate the concentration of Η + ions. A universal indicator is a mixture of several substances that changes its color sequentially from red to purple (like a rainbow) with decreasing acidity.

The main disadvantages of this method are the impossibility of determining the hydrogen index in colored or turbid solutions, as well as only an approximate estimate of the concentration of H + ions in the solution.

To determine the pH of the medium even more roughly, various indicators are used. The most commonly used are litmus, methyl orange, phenolphthalein and others. By changing their color, one can only find out whether the composition being studied is acidic, alkaline or neutral.

pH measurement with devices

A much more accurate value of the concentration of H + ions, and therefore pH of the solution, can be found using a pH meter. This method of analysis is called potentiometric. It is based on measuring the electrode potential and determining the relationship between its value and the concentration of the component in the solution under study. The electrode potential arises due to the electrochemical process occurring at the metal-solution interface.

To carry out measurements, a galvanic cell is made up of two half-cells with electrodes, the potential of one of which is known in advance. Then the EMF is measured. Most often, the determination of the hydrogen index in aqueous solutions is carried out using silver chloride and glass electrodes. The first of them is the reference electrode. The value of the second potential depends on the concentration of Η + ions in the solution.

Also, the pH value in laboratories is determined colorimetrically. This method is based on the ability of two-color indicators to change their color or color intensity, depending on the content of hydrogen cations. The color that appears in the solution is compared with a standard scale, which is compiled on the basis of data on solutions with a known pH value.

Reasons to measure pH

They are as follows:

1. For the production of products with specified properties. During the production process, deviations from the technological pH value can cause disturbances leading to changes in product characteristics. Such indicators may be taste or appearance.

2. To reduce costs. In some industries, product yield directly or indirectly depends on the pH of the reaction medium. Accordingly, the higher the yield of the reaction product, the lower its cost.

3. For occupational safety or environmental reasons. Since many compounds exhibit their harmful properties only at a certain pH, it is very important to control its value.

4. To ensure product compliance with standards. In Many regulatory documents that standardize the quality of goods, products, medicines, etc., have a list of indicators that they must comply with. One of them is pH. Thus, its definition contributes to some extent to the protection of the population from harmful substances.

5. To protect equipment. Most manufacturing equipment that comes into contact with chemicals is susceptible to corrosion. The rate of its development very much depends on pH values. In other words, it is important to protect production equipment from unnecessary damage.

6. For research purposes. The pH level is important for studying various biochemical processes. It is also measured for medical purposes to confirm a particular diagnosis.

Mathematical definition of pH

To calculate the pH of a solution, data on the molar concentration of the H + or OH - anion cations is required. If they are known, then you can immediately use one of the formulas:

• pΗ = - log[Η + ].
• pOΗ = -log.
• pΗ + pOΗ = 14.

The concentration of a particular ion in mol/l in an electrolyte solution can be easily determined by the equation:

C m ion = C m ˑαˑ⋅n, where:

C m ion and C m are the molar concentrations of the ion and electrolyte, respectively (mol/l).

α is the degree of dissociation.

n is the number of ions of the type in question, which is formed during the decay of only one electrolyte molecule.

The degree of dissociation of weak electrolytes can be determined by Ostwald's dilution law: α = √ (K d / C m).

Examples of problem solving

1. It is required to calculate the pH of a 0.001 N NaOH solution.

Solution: Since sodium hydroxide is a strong electrolyte, its dissociation in an aqueous solution is irreversible. It goes according to the equation: NaOΗ → Na + OΗ.

Let's use the formula C m ion = C m ˑαˑn. Let us take the degree of dissociation equal to 1. When one NaOH molecule is destroyed, one OH- ion is formed, which means n=1. According to the conditions of the problem, cm is known and equal to 0.001 or 10 -3. Hence C OH− =10 -3 ˑ1ˑ1=10 -3 .

The concentration of H+ ions can be determined from the relation K w = C H+ ·C OΗ- = 10 -14. Transforming the formula, we obtain С Н+ =К w /С ОХ- =10 -14 /10 -3 =10 -11 . Next we can calculate the hydrogen index: pH = -lg10 -11 =11.

Answer: pH=11.

2. It is required to calculate [H + ] and [OH-], if pH = 4.3 in a given solution.

Solution: The easiest way is to first find the concentration of hydrogen cations: [H + ] = 10 -рΗ =10 -4.3 = 5ˑ10 -5 mol/l.

It is convenient to find the concentration of hydroxide anions from the ratio of the ionic product of water: C OΗ- = K w / C Η+ =10 -14 /5ˑ10 -5 =2ˑ10 -10 mol/l.

Answer: 5ˑ10 -5 mol/l and 2ˑ10 -10 mol/l.

Story

Equations relating pH and pOH

Display pH value

In pure water at 25 °C, the concentrations of hydrogen ions () and hydroxide ions () are the same and amount to 10 -7 mol/l, this directly follows from the definition of the ionic product of water, which is equal to · and is 10 -14 mol²/l² (at 25 °C).

When the concentrations of both types of ions in a solution are equal, the solution is said to have neutral reaction. When an acid is added to water, the concentration of hydrogen ions increases, and the concentration of hydroxide ions correspondingly decreases; when a base is added, on the contrary, the content of hydroxide ions increases, and the concentration of hydrogen ions decreases. When > they say that a solution is sour, and when > - alkaline.

For convenience of presentation, in order to get rid of the negative exponent, instead of the concentrations of hydrogen ions, their decimal logarithm is used, taken with the opposite sign, which is actually the hydrogen exponent - pH).

pOH

The inverse pH value is somewhat less widespread - an indicator of the basicity of the solution, pOH, equal to the negative decimal logarithm of the concentration of OH ions in the solution:

as in any aqueous solution at 22 °C = 1.0 × 10 − 14, it is obvious that at this temperature:

pH values ​​in solutions of varying acidity

• Contrary to popular belief, pH can vary not only in the range from 0 to 14, but can also go beyond these limits. For example, at a hydrogen ion concentration = 10 -15 mol/l, pH = 15, at a hydroxide ion concentration of 10 mol/l pOH = −1.

Since at 25 °C (standard conditions) = 10 -14, it is clear that at this temperature pH + pOH = 14.

Since in acidic solutions > 10 -7, then the pH of acidic solutions is pH< 7, аналогично pH щелочных растворов pH >7, the pH of neutral solutions is 7. At higher temperatures, the dissociation constant of water increases, and the ionic product of water increases accordingly, so the pH turns out to be neutral< 7 (что соответствует одновременно возросшим концентрациям как H + , так и OH -); при понижении температуры, напротив, нейтральная pH возрастает.

Methods for determining pH value

Several methods are widely used to determine the pH value of solutions. The pH value can be approximately estimated using indicators, measured accurately with a pH meter, or determined analytically by performing acid-base titration.

1. To roughly estimate the concentration of hydrogen ions, acid-base indicators are widely used - organic dye substances, the color of which depends on the pH of the medium. The most well-known indicators include litmus, phenolphthalein, methyl orange (methyl orange) and others. Indicators can exist in two differently colored forms - either acidic or basic. The color change of each indicator occurs in its own acidity range, usually 1-2 units.

To expand the working range of pH measurements, a so-called universal indicator is used, which is a mixture of several indicators. The universal indicator sequentially changes color from red through yellow, green, blue to violet when moving from an acidic region to an alkaline one. Determining pH by the indicator method is difficult for cloudy or colored solutions.

1. The use of a special device - a pH meter - allows you to measure pH in a wider range and more accurately (up to 0.01 pH units) than using indicators. The ionometric method for determining pH is based on measuring the EMF of a galvanic circuit with a millivoltmeter-ionometer, including a special glass electrode, the potential of which depends on the concentration of H + ions in the surrounding solution. The method is convenient and highly accurate, especially after calibrating the indicator electrode in a selected pH range; it allows you to measure the pH of opaque and colored solutions and is therefore widely used.
2. The analytical volumetric method - acid-base titration - also provides accurate results for determining the acidity of solutions. A solution of known concentration (titrant) is added dropwise to the test solution. When they are mixed, a chemical reaction occurs. The equivalence point - the moment when there is exactly enough titrant to completely complete the reaction - is recorded using an indicator. Next, knowing the concentration and volume of the added titrant solution, the acidity of the solution is calculated.
3. Effect of temperature on pH values

0.001 mol/L HCl at 20 °C has pH=3, at 30 °C pH=3

0.001 mol/L NaOH at 20 °C has pH=11.73, at 30 °C pH=10.83

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Can be determined using acid-base indicators, measured potentiometrically with a pH meter, or calculated using the formula as a value opposite in sign and equal in modulus to the decimal logarithm of the activity of hydrogen ions, expressed in moles per liter:

Accurate pH measurement and control is needed in various fields of chemistry, biology, materials science, technology, medicine and agrochemistry.

Story

In the case of pH, the letter H stands for the concentration of hydrogen ions (H+), or more precisely, the thermodynamic activity of hydronium ions.

Equations relating pH and pOH

Display pH value

In pure water, the concentrations of hydrogen ions () and hydroxide ions () are the same and at 22 °C they are 10 −7 mol/l, this directly follows from the definition of the ionic product of water, which is equal to · and is 10 −14 mol²/l² ( at 25 °C).

When the concentrations of both types of ions in a solution are equal, the solution is said to have neutral reaction. When an acid is added to water, the concentration of hydrogen ions increases (in fact, it is not the concentration of the ions themselves that increases - otherwise the ability of acids to “attach” a hydrogen ion could lead to this - but the concentration of precisely such compounds with a hydrogen ion “attached” to the acid), but the concentration of hydroxide ions decreases accordingly; when a base is added, on the contrary, the content of hydroxide ions increases, and the concentration of hydrogen ions decreases. When > , they say that the solution is acidic, and when > - main.

For convenience of presentation, in order to get rid of the negative exponent, instead of the concentration of hydrogen ions, use its decimal logarithm taken with the opposite sign, which, in fact, is the hydrogen exponent - pH.

pOH

The inverse pH value is somewhat less widespread - an indicator of the basicity of a solution, pOH, equal to the negative decimal logarithm of the concentration of OH ions in the solution -:

as in any aqueous solution at 25 °C [ H + ] [ OH − ] = 1 , 0 ⋅ 10 − 14 (\displaystyle [(\text(H))^(+)][(\text(OH))^(-)]=1(,) 0\cdot 10^(-14)), it is obvious that at this temperature:

pH values ​​in solutions of varying acidity

Since at 25 °C (standard conditions) = 10 −14, it is clear that at this temperature pH + pOH = 14.

Since in acidic solutions > 10 −7, then the pH of acidic solutions< 7, аналогично, у осно́вных растворов pH >7, the pH of neutral solutions is 7. At higher temperatures, the electrolytic dissociation constant of water increases, and the ionic product of water increases accordingly, so the pH turns out to be neutral< 7 (что соответствует одновременно возросшим концентрациям как H + , так и OH −); при понижении температуры, напротив, нейтральная pH возрастает.

Methods for determining pH value

Several methods are widely used to determine the pH value of solutions. The pH value can be approximately estimated using indicators, measured accurately with a pH meter, or determined analytically by performing an acid-base titration.

1. To roughly estimate the concentration of hydrogen ions, acid-base indicators are widely used - organic dye substances, the color of which depends on the pH of the environment. The most well-known indicators include litmus, phenolphthalein, methyl orange (methyl orange) and others. Indicators can exist in two differently colored forms - either acidic or basic. The color change of each indicator occurs in its own acidity range, usually 1-2 units.
2. To expand the working range of pH measurements, a so-called universal indicator is used, which is a mixture of several indicators. The universal indicator sequentially changes color from red through yellow, green, blue to violet when moving from the acidic to the basic region. Determining pH by the indicator method is difficult for cloudy or colored solutions.
3. The use of a special device - a pH meter - allows you to measure pH in a wider range and more accurately (up to 0.01 pH units) than using indicators. The ionometric method for determining pH is based on measuring the EMF of a galvanic circuit with a millivoltmeter-ionometer, including a special glass electrode, the potential of which depends on the concentration of H + ions in the surrounding solution. The method is convenient and highly accurate, especially after calibrating the indicator electrode in a selected pH range; it allows you to measure the pH of opaque and colored solutions and is therefore widely used.
4. The analytical volumetric method - acid-base titration - also provides accurate results for determining the acidity of solutions. A solution of known concentration (titrant) is added dropwise to the test solution. When they are mixed, a chemical reaction occurs. The equivalence point - the moment when there is exactly enough titrant to completely complete the reaction - is recorded using an indicator. Next, knowing the concentration and volume of the added titrant solution, the acidity of the solution is calculated.
5. Effect of temperature on pH values

pH (from the English power Hidrogen - “Activity/strength of hydrogen”) is an indicator that is used to determine the ratio of acid (base) in any solution. The term is inextricably linked with the concept of acid-base balance (ABC).

Although in scientific terminology the concept of pH environment is applied to literally any solution, in modern popular science this term is used mainly to designate the ratio of acid and alkali in the human body.

What pH indicator can indicate a normal normal environment? It is recognized that at a pH of 7.0, the medium can be called “neutral” - the activity of positively charged ions and negatively charged ions in such a medium is the same. However, an acid-base balance is almost never formed in the human body - three slices of lemon are enough to tip the indicator towards increased acidity.

The human body is constantly fighting for the balance of the acid-base environment, trying to restore this balance through internal mechanisms if it is disturbed, since the absence of ASR can lead to undermining the functioning of the immune system.

The role of acid-base balance in the body's functioning

There are three states of the acid-base environment in the body: a balanced state of the environment, increased acidity (acidosis) and increased alkali content in the body (alkalosis).

Increased acidity leads to poor absorption of minerals by the body: magnesium, potassium - all of them are simply excreted from the body without having time to be absorbed. Increased acidity has a bad effect on the functioning of many organs, especially the gastrointestinal tract, kidneys and cardiovascular system. The main complications that can be caused by acidosis are:

Bone fragility (as a result of inability to absorb calcium);
- weight gain;
- kidney dysfunction;
- allergic reactions;
- decreased immunity;
- general weakness.

Most often, a person feels increased acidity in the intestines and stomach - a slight burning sensation or even heartburn that occurs after drinking a large amount of alcohol or eating lemon indicates increased acidity.

The opposite of high acidity is called alkalosis - an increased level of alkali in the body. In fact, it is not entirely easy to cause truly elevated levels of alkali in the body - most often this condition occurs after long-term use of drugs containing alkali. In addition, this condition does not contribute to the development of such serious problems as acidosis. Possible complications as a result of alkalosis:

Skin problems;
- smell from the mouth;
- problems with the intestines;
- possible occurrence of allergic reactions to some products.

The key to a balanced acid-base environment is a harmonious diet and aversion to alcohol (among those who like to “drink on Fridays,” the acid-base balance is approximately 1.5%-2.0% higher than those who don’t drink). To restore ASR, enzymes, vitamin complexes and mineral complexes are most often used, especially calcium in large dosages.