Remember:

A neutralization reaction is a reaction between an acid and a base that produces salt and water;

By pure water, chemists understand chemically pure water that does not contain any impurities or dissolved salts, i.e. distilled water.

Acidity of the environment

For various chemical, industrial and biological processes, a very important characteristic is the acidity of solutions, which characterizes the content of acids or alkalis in solutions. Since acids and alkalis are electrolytes, the content of H+ or OH - ions is used to characterize the acidity of the medium.

In pure water and in any solution, along with particles of dissolved substances, H+ and OH - ions are also present. This occurs due to the dissociation of the water itself. And although we consider water to be a non-electrolyte, it can nevertheless dissociate: H 2 O ^ H+ + OH - . But this process occurs to a very small extent: in 1 liter of water only 1 ion breaks down into ions. 10 -7 mol molecules.

In acid solutions, as a result of their dissociation, additional H+ ions appear. In such solutions there are significantly more H+ ions than OH - ions formed due to slight dissociation of water, therefore these solutions are called acidic (Fig. 11.1, left). It is commonly said that such solutions have an acidic environment. The more H+ ions contained in the solution, the more acidic the medium.

In alkali solutions, as a result of dissociation, on the contrary, OH - ions predominate, and H + cations are almost absent due to insignificant dissociation of water. The environment of such solutions is alkaline (Fig. 11.1, right). The higher the concentration of OH - ions, the more alkaline the solution environment is.

In a solution of table salt, the number of H+ and OH ions is the same and equal to 1. 10 -7 mol in 1 liter of solution. Such a medium is called neutral (Fig. 11.1, center). In fact, this means that the solution contains neither acid nor alkali. A neutral environment is characteristic of solutions of some salts (formed by alkali and strong acid) and many organic substances. Pure water also has a neutral environment.

pH value

If we compare the taste of kefir and lemon juice, we can safely say that lemon juice is much more acidic, i.e. the acidity of these solutions is different. You already know that pure water also contains H+ ions, but the sour taste of the water is not felt. This is due to the too low concentration of H+ ions. Often it is not enough to say that a medium is acidic or alkaline, but it is necessary to characterize it quantitatively.

The acidity of the environment is quantitatively characterized by the hydrogen indicator pH (pronounced “p-ash”), associated with the concentration

Hydrogen ions. The pH value corresponds to a certain content of Hydrogen cations in 1 liter of solution. Pure water and neutral solutions contain 1 liter in 1 liter. 10 7 mol of H+ ions, and the pH value is 7. In acid solutions, the concentration of H+ cations is greater than in pure water, and in alkaline solutions it is less. In accordance with this, the value of the pH value also changes: in an acidic environment it ranges from 0 to 7, and in an alkaline environment it ranges from 7 to 14. The use of the pH value was first proposed by the Danish chemist Peder Sørensen.

You may have noticed that the pH value is related to the concentration of H+ ions. Determining pH is directly related to calculating the logarithm of a number, which you will study in 11th grade math classes. But the relationship between the content of ions in the solution and the pH value can be traced according to the following scheme:

The pH value of aqueous solutions of most substances and natural solutions is in the range from 1 to 13 (Fig. 11.2).

Rice. 11.2. pH value of various natural and artificial solutions

Søren Peder Laurits Sørensen

Danish physical chemist and biochemist, President of the Royal Danish Society. Graduated from the University of Copenhagen. At the age of 31 he became a professor at the Danish Polytechnic Institute. He headed the prestigious physicochemical laboratory at the Carlsberg brewery in Copenhagen, where he made his main scientific discoveries. His main scientific activity was devoted to the theory of solutions: he introduced the concept of pH value and studied the dependence of enzyme activity on the acidity of solutions. For his scientific achievements, Sørensen was included in the list of “100 outstanding chemists of the 20th century,” but in the history of science he remained primarily as the scientist who introduced the concepts of “pH” and “pH-metry.”

Determination of medium acidity

To determine the acidity of a solution in laboratories, a universal indicator is most often used (Fig. 11.3). By its color, you can determine not only the presence of acid or alkali, but also the pH value of the solution with an accuracy of 0.5. To more accurately measure pH, there are special devices - pH meters (Fig. 11.4). They allow you to determine the pH of a solution with an accuracy of 0.001-0.01.

Using indicators or pH meters, you can monitor how chemical reactions are progressing. For example, if chloride acid is added to a solution of sodium hydroxide, a neutralization reaction will occur:

Rice. 11.3. A universal indicator determines the approximate pH value

Rice. 11.4. To measure the pH of solutions, special devices are used - pH meters: a - laboratory (stationary); b - portable

In this case, solutions of reagents and reaction products are colorless. If a pH meter electrode is placed in the initial alkali solution, then the complete neutralization of the alkali by the acid can be judged by the pH value of the resulting solution.

Application of pH index

Determining the acidity of solutions is of great practical importance in many areas of science, industry and other areas of human life.

Ecologists regularly measure the pH of rainwater, rivers and lakes. A sharp increase in the acidity of natural waters may be a consequence of atmospheric pollution or the entry of industrial waste into water bodies (Fig. 11.5). Such changes entail the death of plants, fish and other inhabitants of water bodies.

The hydrogen index is very important for studying and observing processes occurring in living organisms, since numerous chemical reactions take place in cells. In clinical diagnostics, the pH of blood plasma, urine, gastric juice, etc. is determined (Fig. 11.6). Normal blood pH is between 7.35 and 7.45. Even a small change in the pH of human blood causes serious illness, and at pH = 7.1 and below, irreversible changes begin that can lead to death.

For most plants, soil acidity is important, so agronomists conduct soil analyzes in advance, determining their pH (Fig. 11.7). If the acidity is too high for a particular crop, the soil is limed by adding chalk or lime.

In the food industry, acid-base indicators are used to control the quality of food products (Fig. 11.8). For example, the normal pH for milk is 6.8. Deviation from this value indicates either the presence of foreign impurities or its souring.

Rice. 11.5. The influence of the pH level of water in reservoirs on the vital activity of plants in them

The pH value for cosmetics that we use in everyday life is important. The average pH for human skin is 5.5. If the skin comes into contact with products whose acidity differs significantly from this value, this will lead to premature skin aging, damage or inflammation. It was noticed that laundresses who used ordinary laundry soap (pH = 8-10) or washing soda (Na 2 CO 3, pH = 12-13) for a long time for washing, the skin of their hands became very dry and covered with cracks. Therefore, it is very important to use various cosmetics (gels, creams, shampoos, etc.) with a pH close to the natural pH of the skin.

LABORATORY EXPERIMENTS No. 1-3

Equipment: rack with test tubes, pipette.

Reagents: water, chloride acid, NaCl, NaOH solutions, table vinegar, universal indicator (solution or indicator paper), food and cosmetic products (for example, lemon, shampoo, toothpaste, washing powder, carbonated drinks, juices, etc. .).

Safety regulations:

For experiments, use small amounts of reagents;

Be careful not to get reagents on your skin or eyes; If a caustic substance gets in, wash it off with plenty of water.

Determination of Hydrogen ions and hydroxide ions in solutions. Establishing the approximate pH value of water, alkaline and acidic solutions

1. Pour 1-2 ml into five test tubes: into test tube No. 1 - water, No. 2 - chloride acid, No. 3 - sodium chloride solution, No. 4 - sodium hydroxide solution and No. 5 - table vinegar.

2. Add 2-3 drops of a universal indicator solution to each test tube or lower the indicator paper. Determine the pH of solutions by comparing the color of the indicator on a standard scale. Draw conclusions about the presence of Hydrogen cations or hydroxide ions in each test tube. Write dissociation equations for these compounds.

Study of pH of food and cosmetic products

Test samples of food and cosmetic products with a universal indicator. To study dry substances, for example, washing powder, they must be dissolved in a small amount of water (1 spatula of dry substance per 0.5-1 ml of water). Determine the pH of solutions. Draw conclusions about the acidity of the environment in each of the studied products.

Key idea

Control questions

130. The presence of what ions in a solution determines its acidity?

131. What ions are found in excess in acid solutions? in alkaline?

132. What indicator quantitatively describes the acidity of solutions?

133. What is the pH value and the content of H+ ions in solutions: a) neutral; b) weakly acidic; c) slightly alkaline; d) strongly acidic; e) highly alkaline?

Assignments for mastering the material

134. An aqueous solution of a certain substance has an alkaline medium. Which ions are more present in this solution: H+ or OH -?

135. Two test tubes contain solutions of nitrate acid and potassium nitrate. What indicators can be used to determine which test tube contains a salt solution?

136. Three test tubes contain solutions of barium hydroxide, nitrate acid and calcium nitrate. How to recognize these solutions using one reagent?

137. From the list above, write down separately the formulas of substances whose solutions have a medium: a) acidic; b) alkaline; c) neutral. NaCl, HCl, NaOH, HNO 3, H 3 PO 4, H 2 SO 4, Ba(OH) 2, H 2 S, KNO 3.

138. Rain water has pH = 5.6. What does this mean? What substance contained in the air, when dissolved in water, determines the acidity of the environment?

139. What kind of environment (acidic or alkaline): a) in a shampoo solution (pH = 5.5);

b) in the blood of a healthy person (pH = 7.4); c) in human gastric juice (pH = 1.5); d) in saliva (pH = 7.0)?

140. Coal used in thermal power plants contains compounds of Nitrogen and Sulfur. The release of coal combustion products into the atmosphere leads to the formation of so-called acid rain containing small amounts of nitrate or sulfite acids. What pH values ​​are typical for such rainwater: more than 7 or less than 7?

141. Does the pH of a solution of a strong acid depend on its concentration? Justify your answer.

142. A solution of phenolphthalein was added to a solution containing 1 mol of potassium hydroxide. Will the color of this solution change if chloride acid is added to it in the amount of substance: a) 0.5 mol; b) 1 mol;

c) 1.5 mol?

143. Three unlabeled test tubes contain colorless solutions of sodium sulfate, sodium hydroxide and sulfate acid. The pH value was measured for all solutions: in the first test tube - 2.3, in the second - 12.6, in the third - 6.9. Which test tube contains which substance?

144. The student bought distilled water at the pharmacy. The pH meter showed that the pH value of this water was 6.0. The student then boiled this water for a long time, filled the container to the top with hot water and closed the lid. When the water cooled to room temperature, the pH meter detected a value of 7.0. After this, the student passed air through the water with a straw, and the pH meter again showed 6.0. How can the results of these pH measurements be explained?

145. Why do you think two bottles of vinegar from the same manufacturer may contain solutions with slightly different pH values?

This is textbook material

HYDROGEN VALUE (PH). One of the most important properties of aqueous solutions is their acidity (or alkalinity), which is determined by the concentration of H + and OH – ions ( cm. ELECTROLYTIC DISSOCIATION. ELECTROLYTES). The concentrations of these ions in aqueous solutions are related by a simple relationship = TO w ; (square brackets usually indicate concentration in units of mol/l). The quantity Kw is called the ionic product of water and is constant at a given temperature. So, at 0 o C it is equal to 0.11 H 10 –14, at 20 o C – 0.69 H 10 –14, and at 100 o C – 55.0 H 10 –14. The most commonly used meaning is K w at 25 o C, which is equal to 1.00H 10 –14. In absolutely pure water, which does not even contain dissolved gases, the concentrations of H + and OH – ions are equal (the solution is neutral). In other cases, these concentrations do not coincide: in acidic solutions, H + ions predominate, in alkaline solutions, OH – ions predominate. But their product in any aqueous solution is constant. Therefore, if you increase the concentration of one of these ions, the concentration of the other ion will decrease by the same amount. So, in a weak acid solution, in which = 10 –5 mol/l, = 10 –9 mol/l, and their product is still equal to 10 –14. Similarly, in an alkaline solution at = 3.7H 10 –3 mol/l = 10 –14 /3.7H 10 –3 = 2.7H 10 –11 mol/l.

From the above it follows that the acidity of a solution can be unambiguously expressed by indicating the concentration of only hydrogen ions in it. For example, in pure water = 10 –7 mol/l. In practice, it is inconvenient to operate with such numbers. In addition, the concentrations of H + ions in solutions can differ by hundreds of trillions of times - from approximately 10–15 mol/l (strong alkali solutions) to 10 mol/l (concentrated hydrochloric acid), which cannot be depicted on any graph. Therefore, it has long been agreed that for the concentration of hydrogen ions in a solution, only the exponent of 10, taken with the opposite sign, should be indicated; To do this, the concentration should be expressed as a power of 10x, without a multiplier, for example, 3.7H 10 –3 = 10 –2.43. (For more accurate calculations, especially in concentrated solutions, their activities are used instead of the concentration of ions.) This exponent is called the hydrogen exponent, and abbreviated pH - from the designation of hydrogen and the German word Potenz - mathematical degree. Thus, by definition, pH = –log[H + ]; this value can vary within small limits – only from –1 to 15 (and more often – from 0 to 14). In this case, a change in the concentration of H + ions by 10 times corresponds to a change in pH by one unit. The pH designation was introduced into scientific use in 1909 by the Danish physical chemist and biochemist S.P.L. Sørensen, who at that time was studying the processes occurring during the fermentation of beer malt and their dependence on the acidity of the medium.

At room temperature in neutral solutions pH = 7, in acidic solutions pH< 7, а в щелочных рН >7. The approximate pH value of an aqueous solution can be determined using indicators. For example, methyl orange at pH< 3,1 имеет красный цвет, а при рН >4.4 – yellow; litmus at pH< 6,1 красный, а при рН >8 – blue, etc. More accurately (up to hundredths of a fraction) the pH value can be determined using special devices - pH meters. Such devices measure the electrical potential of a special electrode immersed in a solution; this potential depends on the concentration of hydrogen ions in the solution and can be measured with high accuracy.

It is interesting to compare the pH values ​​of solutions of various acids, bases, salts (at a concentration of 0.1 mol/l), as well as some mixtures and natural objects. For poorly soluble compounds marked with an asterisk, the pH of saturated solutions is given.

The table allows us to make a number of interesting observations. pH values, for example, immediately indicate the relative strength of acids and bases. A strong change in the neutral environment as a result of the hydrolysis of salts formed by weak acids and bases, as well as during the dissociation of acidic salts, is also clearly visible.

Natural water always has an acidic reaction (pH< 7) из-за того, что в ней растворен углекислый газ; при его реакции с водой образуется кислота: СО 2 + Н 2 О « Н + + НСО 3 2– . Если насытить воду углекислым газом при атмосферном давлении, рН полученной «газировки» будет равен 3,7; такую кислотность имеет примерно 0,0007%-ный раствор соляной кислоты – желудочный сок намного кислее! Но даже если повысить давление CO 2 над раствором до 20 атм, значение pH не опускается ниже 3,3. Это значит, что газированную воду (в умеренных количествах, конечно) можно пить без вреда для здоровья, даже если она насыщена углекислым газом.

Certain pH values ​​are extremely important for the life of living organisms. Biochemical processes in them must occur at a strictly specified acidity. Biological catalysts - enzymes are able to work only within certain pH limits, and when they go beyond these limits, their activity can sharply decrease. For example, the activity of the enzyme pepsin, which catalyzes the hydrolysis of proteins and thus promotes the digestion of protein foods in the stomach, is maximum at pH values ​​of about 2. Therefore, for normal digestion it is necessary that gastric juice have fairly low pH values: normally 1.53–1. 67. With a gastric ulcer, the pH drops to an average of 1.48, and with a duodenal ulcer it can even reach 105. The exact pH value of gastric juice is determined by intragastric examination (pH probe). If a person has low acidity, the doctor may prescribe taking a weak solution of hydrochloric acid with food, and if there is increased acidity, take antacid agents, for example, magnesium or aluminum hydroxides. Interestingly, if you drink lemon juice, the acidity of gastric juice... will decrease! Indeed, a solution of citric acid will only dilute the stronger hydrochloric acid contained in gastric juice.

In the cells of the body the pH is about 7, in the extracellular fluid it is 7.4. Nerve endings that are outside cells are very sensitive to changes in pH. When mechanical or thermal damage occurs to tissues, cell walls are destroyed and their contents reach the nerve endings. As a result, the person feels pain. Scandinavian researcher Olaf Lindahl conducted the following experiment: using a special needle-free injector, a very thin stream of solution was injected through the skin of a person, which did not damage the cells, but acted on the nerve endings. It has been shown that it is hydrogen cations that cause pain, and as the pH of the solution decreases, the pain intensifies. Similarly, a solution of formic acid, which is injected under the skin by stinging insects or nettles, directly “acts on the nerves.” The different pH values ​​of tissues also explain why with some inflammations a person feels pain, and with others - not.

Interestingly, injecting clean water under the skin produced particularly severe pain. This phenomenon, strange at first glance, is explained as follows: when cells come into contact with clean water, as a result of osmotic pressure, they rupture and their contents affect the nerve endings.

The blood pH value must remain within very narrow limits; even a slight acidification (acidosis) or alkalization (alkalosis) can lead to the death of the organism. Acidosis is observed in diseases such as bronchitis, circulatory failure, lung tumors, pneumonia, diabetes, fever, kidney and intestinal damage. Alkolosis is observed with hyperventilation of the lungs (or with inhalation of pure oxygen), with anemia, CO poisoning, hysteria, brain tumor, excessive consumption of baking soda or alkaline mineral waters, and taking diuretic medications. Interestingly, the pH of arterial blood should normally be in the range of 7.37–7.45, and that of venous blood – 7.34–7.43. Various microorganisms are also very sensitive to the acidity of the environment. Thus, pathogenic microbes develop quickly in a slightly alkaline environment, while they cannot withstand an acidic environment. Therefore, for preserving (pickling, salting) products, as a rule, acidic solutions are used, adding vinegar or food acids to them. The correct selection of pH is also of great importance for chemical technological processes.

Maintaining the desired pH value and preventing it from noticeably deviating in one direction or another when conditions change is possible by using so-called buffer (from English buff - soften shocks) solutions. Such solutions are often a mixture of a weak acid and its salt or a weak base and its salt. Such solutions “resist”, within certain limits (called buffer capacity), attempts to change their pH. For example, if you try to slightly acidify a mixture of acetic acid and sodium acetate, then acetate ions will bind excess H + ions into slightly dissociated acetic acid, and the pH of the solution will hardly change (there are a lot of acetate ions in the buffer solution, since they are formed as a result of complete dissociation sodium acetate). On the other hand, if you introduce a little alkali into such a solution, the excess OH – ions will be neutralized by acetic acid while maintaining the pH value. Other buffer solutions act in a similar way, each of them maintaining a specific pH value. Solutions of acid salts of phosphoric acid and weak organic acids - oxalic, tartaric, citric, phthalic, etc. also have a buffering effect. The specific pH value of the buffer solution depends on the concentration of the buffer components. Thus, the acetate buffer allows you to maintain the pH of the solution in the range of 3.8–6.3; phosphate (mixture of KH 2 PO 4 and Na 2 HPO 4) - in the range of 4.8 - 7.0, borate (mixture of Na 2 B 4 O 7 and NaOH) - in the range of 9.2-11, etc.

Many natural liquids have buffering properties. An example is ocean water, the buffering properties of which are largely due to dissolved carbon dioxide and bicarbonate ions HCO 3 -. The source of the latter, in addition to CO 2, are huge amounts of calcium carbonate in the form of shells, chalk and limestone sediments in the ocean. Interestingly, the photosynthetic activity of plankton, one of the main suppliers of oxygen to the atmosphere, leads to an increase in the pH of the environment. This happens in accordance with Le Chatelier’s principle as a result of a shift in equilibrium when absorbing dissolved carbon dioxide: 2H + + CO 3 2 – “ H + + HCO 3 – “ H 2 CO 3 “ H 2 O + CO 2. When CO 2 + H 2 O + hv ® 1/n(CH 2 O) n + O 2 is removed from the solution during photosynthesis, the equilibrium shifts to the right and the environment becomes more alkaline. In the cells of the body, the hydration of CO 2 is catalyzed by the enzyme carbonic anhydrase.

Cellular fluid and blood are also examples of natural buffer solutions. Thus, blood contains about 0.025 mol/l of carbon dioxide, and its content in men is approximately 5% higher than in women. The concentration of bicarbonate ions in the blood is approximately the same (there are also more of them in men).

When testing soil, pH is one of the most important characteristics. Different soils can have a pH from 4.5 to 10. The pH value, in particular, can be used to judge the nutrient content of the soil, as well as which plants can grow successfully in a given soil. For example, the growth of beans, lettuce, and black currants is hampered when the soil pH is below 6.0; cabbage – below 5.4; apple trees – below 5.0; potatoes – below 4.9. Acidic soils are generally less nutrient rich because they are less able to retain the metal cations needed by plants. For example, hydrogen ions entering the soil displace bound Ca 2+ ions from it. And aluminum ions displaced from clayey (aluminosilicate) rocks in high concentrations are toxic to agricultural crops.

To deoxidize acidic soils, liming is used - adding substances that gradually bind excess acid. Such a substance can be natural minerals - chalk, limestone, dolomite, as well as lime, slag from metallurgical plants. The amount of deoxidizer applied depends on the buffer capacity of the soil. For example, liming clay soil requires more deoxidizing substances than sandy soil.

Of great importance are measurements of the pH of rainwater, which can be quite acidic due to the presence of sulfuric and nitric acids in it. These acids are formed in the atmosphere from nitrogen and sulfur (IV) oxides, which are emitted with waste from numerous industries, transport, boiler houses and thermal power plants. It is known that acid rain with a low pH value (less than 5.6) destroys vegetation and the living world of water bodies. Therefore, the pH of rainwater is constantly monitored.

Ilya Leenson

The hydrogen index - pH - is a measure of the activity (in the case of dilute solutions, reflects the concentration) of hydrogen ions in a solution, quantitatively expressing its acidity, calculated as the negative (taken with the opposite sign) decimal logarithm of the activity of hydrogen ions, expressed in moles per liter.

pH = – log

This concept was introduced in 1909 by the Danish chemist Sørensen. The indicator is called pH, after the first letters of the Latin words potentia hydrogeni - the strength of hydrogen, or pondus hydrogenii - the weight of hydrogen.

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:

рОН = – log

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 autoprotolysis constant of water K w, which is otherwise called the ionic product of water:

K w = =10 –14 [mol 2 /l 2 ] (at 25°C)

pH + pH = 14

When the concentrations of both types of ions in a solution are the same, the solution is said to be neutral. 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 > the solution is said to be acidic, and when > it is alkaline.

pH determination

Several methods are widely used to determine the pH value of solutions.

1) The pH value can be approximately estimated using indicators, accurately measured with a pH meter, or determined analytically by performing acid-base titration.

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 (see Table 1, lesson 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 an acidic region to an alkaline one. Determining pH by the indicator method is difficult for cloudy or colored solutions.

2) The analytical volumetric method - acid-base titration - also gives accurate results for determining the total 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 total acidity of the solution is calculated.

The acidity of the environment is important for many chemical processes, and the possibility or outcome of a particular reaction often depends on the pH of the environment. To maintain a certain pH value in the reaction system during laboratory research or in production, buffer solutions are used, which allow maintaining an almost constant pH value when diluted or when small amounts of acid or alkali are added to the solution.

The pH value is widely used to characterize the acid-base properties of various biological media (Table 2).

The acidity of the reaction medium is of particular importance for biochemical reactions occurring in living systems. The concentration of hydrogen ions in a solution often affects the physicochemical properties and biological activity of proteins and nucleic acids, therefore, for the normal functioning of the body, maintaining acid-base homeostasis is a task of exceptional importance. Dynamic maintenance of the optimal pH of biological fluids is achieved through the action of buffer systems.

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, is convenient and highly accurate, allows you to measure the pH of opaque and colored solutions and therefore widely used.

Using a pH meter, the concentration of hydrogen ions (pH) is measured in solutions, drinking water, food products and raw materials, environmental objects and production systems for continuous monitoring of technological processes, including in aggressive environments.

A pH meter is indispensable for hardware monitoring of pH solutions for the separation of uranium and plutonium, when the requirements for the correctness of equipment readings without calibration are extremely high.

The device can be used in stationary and mobile laboratories, including field laboratories, as well as clinical diagnostic, forensic, research, and production laboratories, including the meat, dairy and baking industries.

Recently, pH meters are also widely used in aquarium farms, monitoring water quality in domestic conditions, agriculture (especially in hydroponics), and also for monitoring health diagnostics.

Table 2. pH values ​​for some biological systems and other solutions

The hydrogen index - pH - is a measure of the activity (in the case of dilute solutions, reflects the concentration) of hydrogen ions in a solution, quantitatively expressing its acidity, calculated as the negative (taken with the opposite sign) decimal logarithm of the activity of hydrogen ions, expressed in moles per liter.

pH = – log

This concept was introduced in 1909 by the Danish chemist Sørensen. The indicator is called pH, after the first letters of the Latin words potentia hydrogeni - the strength of hydrogen, or pondus hydrogenii - the weight of hydrogen.

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:

рОН = – log

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 autoprotolysis constant of water K w, which is otherwise called the ionic product of water:

K w = =10 –14 [mol 2 /l 2 ] (at 25°C)

pH + pH = 14

When the concentrations of both types of ions in a solution are the same, the solution is said to be neutral. 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 > the solution is said to be acidic, and when > it is alkaline.

pH determination

Several methods are widely used to determine the pH value of solutions.

1) The pH value can be approximately estimated using indicators, accurately measured with a pH meter, or determined analytically by performing acid-base titration.

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 (see Table 1, lesson 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 an acidic region to an alkaline one. Determining pH by the indicator method is difficult for cloudy or colored solutions.

2) The analytical volumetric method - acid-base titration - also gives accurate results for determining the total 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 total acidity of the solution is calculated.

The acidity of the environment is important for many chemical processes, and the possibility or outcome of a particular reaction often depends on the pH of the environment. To maintain a certain pH value in the reaction system during laboratory research or in production, buffer solutions are used, which allow maintaining an almost constant pH value when diluted or when small amounts of acid or alkali are added to the solution.

The pH value is widely used to characterize the acid-base properties of various biological media (Table 2).

The acidity of the reaction medium is of particular importance for biochemical reactions occurring in living systems. The concentration of hydrogen ions in a solution often affects the physicochemical properties and biological activity of proteins and nucleic acids, therefore, for the normal functioning of the body, maintaining acid-base homeostasis is a task of exceptional importance. Dynamic maintenance of the optimal pH of biological fluids is achieved through the action of buffer systems.

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, is convenient and highly accurate, allows you to measure the pH of opaque and colored solutions and therefore widely used.

Using a pH meter, the concentration of hydrogen ions (pH) is measured in solutions, drinking water, food products and raw materials, environmental objects and production systems for continuous monitoring of technological processes, including in aggressive environments.

A pH meter is indispensable for hardware monitoring of pH solutions for the separation of uranium and plutonium, when the requirements for the correctness of equipment readings without calibration are extremely high.

The device can be used in stationary and mobile laboratories, including field laboratories, as well as clinical diagnostic, forensic, research, and production laboratories, including the meat, dairy and baking industries.

Recently, pH meters are also widely used in aquarium farms, monitoring water quality in domestic conditions, agriculture (especially in hydroponics), and also for monitoring health diagnostics.

Table 2. pH values ​​for some biological systems and other solutions

pH value (pH factor) is a measure of the activity of hydrogen ions in a solution, quantitatively expressing its acidity. When pH is not at optimal levels, plants begin to lose the ability to absorb some of the elements they need for healthy growth. All plants have a specific pH level that allows them to achieve maximum results when growing. Most plants prefer a slightly acidic growing environment (between 5.5-6.5).

Hydrogen index in formulas

In very dilute solutions, the pH is equivalent to the concentration of hydrogen ions. Equal in magnitude and opposite in sign to the decimal logarithm of the activity of hydrogen ions, expressed in moles per liter:

pH = -lg

Under standard conditions, the pH value ranges from 0 to 14. In pure water, at a neutral pH, the concentration of H + is equal to the concentration of OH - and is 1·10 -7 mol per liter. The maximum possible pH value is defined as the sum of pH and pOH and is equal to 14.

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.

It is important to understand! The pH scale is logarithmic, meaning that each unit of change equals a tenfold change in the concentration of hydrogen ions. In other words, a solution with pH 6 is ten times more acidic than a solution with pH 7, and a solution with pH 5 will be ten times more acidic than a solution with pH 6 and a hundred times more acidic than a solution with pH 7. This is means that when you are adjusting the pH of your nutrient solution and you need to change the pH by two points (for example from 7.5 to 5.5) you must use ten times more pH adjuster than if you only changed the pH by one point (from 7.5 to 6.5 ).

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.

Acid-base indicators

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.

Universal indicator

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 region to the basic one.

Solutions of such mixtures - “universal indicators” - are usually impregnated with strips of “indicator paper”, with the help of which you can quickly (with an accuracy of pH units, or even tenths of pH) determine the acidity of the aqueous solutions under study. For a more accurate determination, the color of the indicator paper obtained when applying a drop of solution is immediately compared with the reference color scale, the appearance of which is presented in the images.

Determining pH by the indicator method is difficult for cloudy or colored solutions.

Given the fact that the optimal pH values ​​for nutrient solutions in hydroponics have a very narrow range (usually from 5.5 to 6.5), I also use other combinations of indicators. For example, ours has a working range and scale from 4.0 to 8.0, which makes such a test more accurate in comparison with universal indicator paper.

pH meter

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 universal indicators. The method is convenient and highly accurate, especially after calibrating the indicator electrode in the selected pH range. Allows you to measure the pH of opaque and colored solutions and is therefore widely used.

Analytical volumetric method

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.

Effect of temperature on pH values

The pH value can change over a wide range with temperature changes. Thus, a 0.001 molar solution of NaOH at 20°C has pH=11.73, and at 30°C pH=10.83. The effect of temperature on pH values ​​is explained by different dissociation of hydrogen ions (H +) and is not an experimental error. The temperature effect cannot be compensated for by the electronics of the pH meter.

Adjusting the pH of the Nutrient Solution

Acidification of the nutrient solution

The nutrient solution usually has to be acidified. The absorption of ions by plants causes a gradual alkalization of the solution. Any solution that has a pH of 7 or higher will most often need to be adjusted to its optimal pH. Various acids can be used to acidify the nutrient solution. The most commonly used are sulfuric or phosphoric acid. A better solution for hydroponic solutions are buffer additives such as and. These products not only bring pH values ​​to optimal levels, but also stabilize values ​​for a long period.

When adjusting pH with both acids and alkalis, rubber gloves should be worn to avoid causing skin burns. An experienced chemist skillfully handles concentrated sulfuric acid; he adds the acid drop by drop to water. But for beginner hydroponists, it is perhaps better to contact an experienced chemist and ask him to prepare a 25% solution of sulfuric acid. While adding acid, the solution is stirred and its pH is determined. Once you know the approximate amount of sulfuric acid, you can then add it from a graduated cylinder.

Sulfuric acid must be added in small portions so as not to acidify the solution too much, which will then have to be alkalized again. For an inexperienced worker, acidification and alkalization can continue indefinitely. In addition to wasting time and reagents, such regulation unbalances the nutrient solution due to the accumulation of ions unnecessary for plants.

Alkalizing the Nutrient Solution

Solutions that are too acidic are made alkaline with sodium hydroxide (sodium hydroxide). As its name suggests, it is a caustic substance, so rubber gloves must be used. It is recommended to purchase sodium hydroxide in pill form. In household chemical stores, sodium hydroxide can be purchased as a pipe cleaner, such as "Mole". Dissolve one pill in 0.5 liters of water and gradually add the alkaline solution to the nutrient solution with constant stirring, often checking its pH. No amount of mathematical calculations can determine how much acid or alkali should be added in a given case.

If you want to grow several crops in one tray, you need to select them so that not only their optimal pH coincides, but also their needs for other growth factors. For example, yellow daffodils and chrysanthemums require a pH of 6.8 but different humidity levels, so they cannot be grown in the same tray. If you give daffodils the same amount of moisture as chrysanthemums, the daffodil bulbs will rot. In experiments, rhubarb reached maximum development at pH 6.5, but could grow even at pH 3.5. Oats, which prefer a pH of about 6, produce good yields at a pH of 4 if the dose of nitrogen in the nutrient solution is greatly increased. Potatoes grow over a fairly wide pH range, but they grow best at a pH of 5.5. Below this pH, high tuber yields are also obtained, but they take on a sour taste. To obtain maximum high-quality yields, the pH of nutrient solutions must be accurately adjusted.