Acidity of the environment. The concept of solution pH. Hydrogen index of acidity (pH) Analytical volumetric method

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 changes color sequentially 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

Water is a weak electrolyte; it weakly dissociates according to the equation

At 25 °C, 10-7 mol H2O disintegrates into ions in 1 liter of water. The concentration of H+ and OH- ions (in mol/l) will be equal to

Pure water has a neutral reaction. When acid is added to it, the concentration of H+ ions increases, i.e. > 10-7 mol/l; the concentration of OH- ions decreases, i.e. less than 10-7 mol/l. When adding alkali, the concentration of OH- ions increases: > 10-7 mol/l, therefore less than 10-7 mol/l.

In practice, to express the acidity or alkalinity of a solution, instead of concentration, its negative decimal logarithm is used, which is called the pH value:

In neutral water, pH = 7. The pH values ​​and the corresponding concentrations of H+ and OH- ions are given in table. 4.

Buffer solutions

Many analytical reactions are carried out at a strictly defined pH value, which must be maintained throughout the reaction. During some reactions, the pH may change as a result of the binding or release of H+ ions. To maintain a constant pH value, buffer solutions are used.

Buffer solutions are most often mixtures of weak acids with salts of these acids or mixtures of weak bases with salts of the same bases. If, for example, a certain amount of a strong acid such as HCl is added to an acetate buffer solution consisting of acetic acid CH3COOH and sodium acetate CH3COONa, it will react with acetate ions to form slightly dissociating CH3COOH:

Thus, the H+ ions added to the solution will not remain free, but will be bound by CH3COO- ions, and therefore the pH of the solution will hardly change. When adding an alkali solution to an acetate buffer solution, OH- ions will be bound by undissociated molecules of acetic acid CH3COOH:

Consequently, the pH of the solution in this case will also remain almost unchanged.

Buffer solutions retain their buffering effect up to a certain limit, i.e. they have a certain buffer capacity. If there are more H+ or OH- ions in the solution than the buffer capacity of the solution allows, then the pH will change significantly, as in a non-buffered solution.

Typically, assay procedures specify which buffer solution should be used for a given assay and how it should be prepared. Buffer mixtures with precise pH values ​​are produced in ampoules for preparing a 500 ml solution.

pH = 1.00. Ingredients: 0.084 g glycol (aminoacetic acid NH2CH2COOH), 0.066 g sodium chloride NaCl and 2.228 g hydrochloric acid HCl.

pH = 2.00. Composition: 3.215 g citric acid C6H8O7-H2O, 1.224 g sodium hydroxide NaOH and 1.265 g hydrochloric acid HCl.

pH = 3.00. Composition: 4.235 g citric acid C6H8O7-H2O, 1.612 g sodium hydroxide NaOH and 1.088 g hydrochloric acid HCl.

pH = 4.00. Composition: 5.884 g citric acid C6H8O7-H2O, 2.240 g sodium hydroxide NaOH and 0.802 g hydrochloric acid HCl.

pH = 5.00. Composition: 10.128 g citric acid C6H8O7-H2O and 3.920 g sodium hydroxide NaOH.

pH = 6.00. Composition: 6.263 g citric acid C6H8O7-H2O and 3.160 g sodium hydroxide NaOH.

pH = 7.00. Composition: 1.761 g of potassium dihydrogen phosphate KH2PO4 and 3.6325 g of sodium hydrogen phosphate Na2HPO4-2H2O.

pH = 8.00. Composition: 3.464 g of boric acid H3BO3, 1.117 g of sodium hydroxide NaOH and 0.805 g of hydrochloric acid HCl.

pH = 9.00. Composition: 1.546 g of boric acid H3BO3, 1.864 g of potassium chloride, KCl and 0.426 g of sodium hydroxide NaOH.

pH = 10.00. Composition: 1.546 g of boric acid H3BO3, 1.864 g of potassium chloride KCl and 0.878 g of sodium hydroxide NaOH.

pH = 11.00. Composition: 2.225 g of sodium hydrogen phosphate Na2HPO4-2H2O and 0.068 g of sodium hydroxide NaOH.

pH = 12.00. Composition: 2.225 g of sodium hydrogen phosphate Na2HPO4-2H2O and 0.446 g of sodium hydroxide NaOH.

pH = 13.00. Composition: 1.864 g of potassium chloride KCl and 0.942 g of sodium hydroxide NaOH.

Deviations from the nominal pH value reach ±0.02 for solutions at pH from 1 to 10 and ±0.05 at pH from 11 to 13. This accuracy is quite sufficient for practical work.

To set up pH meters, standard buffer solutions with precise pH values ​​are used.

1. Acetate buffer solution with pH=4.62: 6.005 g of acetic acid CH3COOH and 8.204 g of sodium acetate CH3COONa in 1 liter of solution.

2. Phosphate buffer solution with pH=6.88: 4.450 g of sodium hydrogen phosphate Na2HPO4-2H2O and 3.400 g of potassium dihydrogen phosphate KH2PO4 in 1 liter of solution.

3. Borate buffer solution with pH=9.22: 3.81 g of sodium tetraborate Na2B4O7-10H2O in 1 liter of solution.

4. Phosphate buffer solution with pH=11.00: 4.450 g of sodium hydrogen phosphate Na2HPO4-2H2O and 0.136 g of sodium hydroxide NaOH in 1 liter of solution.

To prepare buffer solutions for agrochemical and biochemical analysis with pH values ​​from 1.1 to 12.9 with an interval of 0.1, 7 basic stock solutions are used.

Solution 1. Dissolve 11.866 g of sodium hydrogen phosphate Na2HPO4-2H2O in water and dilute in a volumetric flask with water to 1 liter (solution concentration 1/15 M).

Solution 2. Dissolve 9.073 potassium dihydrogen phosphate KH2PO4 in 1 liter of water in a volumetric flask (concentration 1/15 M).

Solution 3. Dissolve 7.507 g of glycol (aminoacetic acid) NH2CH2COOH and 5.84 g of sodium chloride NaCl in 1 liter of water in a volumetric flask. From this solution by mixing with 0.1 N. buffer solutions with pH from 1.1 to 3.5 are prepared with HCl solution; mixing with 0.1 N. NaOH solution is used to prepare solutions with pH from 8.6 to 12.9.

Solution 4. Dissolve 21.014 g of citric acid C6H8O7-H2O in water, add 200 ml of 1 N to the solution. NaOH solution and dilute to 1 liter with water in a volumetric flask. By mixing this solution with 0.1 N. buffer solutions with pH from 1.1 to 4.9 are prepared using HCl solution; mixing with 0.1 N. Buffer solutions with pH from 5.0 to 6.6 are prepared using NaOH solution.

Solution 5. Dissolve 12.367 g of boric acid H3BO3 in water, add 100 ml of 1 N. NaOH solution and dilute with water to 1 liter in a volumetric flask. By mixing this solution with 0.1 N. buffer solutions with pH from 7.8 to 8.9 are prepared with HCl solution; mixing with 0.1 N. Buffer solutions with pH from 9.3 to 11.0 are prepared using NaOH solution.

Solution 6. Prepare exactly 0.1 N. HCl solution;

Solution 7. Prepare exactly 0.1 N. NaOH solution; To prepare the solution, distilled water is boiled for 2 hours to remove CO2. During storage, the solution is protected from the ingress of CO2 from the air with a calcium chloride tube.

In some solutions, mold develops during storage; to prevent this, add a few drops of thymol to the solution as a preservative. To prepare a buffer solution of the required pH, mix the indicated solutions in a certain ratio (Table 5). The volume is measured using a burette with a capacity of 100.0 ml. All pH values ​​of buffer solutions in the table are given at a temperature of 20 °C.

Chemical grade reagents are used to prepare initial solutions. Sodium hydrogen phosphate Na2HPO4-2H2O is pre-crystallized twice. During the second recrystallization, the solution temperature should not exceed 90 °C. The resulting preparation is slightly moistened and dried in a thermostat at 36 °C for two days. Potassium dihydrogen phosphate KH2PO4 is also recrystallized twice and dried at 110-120 °C. Sodium chloride NaCl is recrystallized twice and dried at 120 °C. Citric acid C6H8O7-H2O is recrystallized twice. During the second recrystallization, the solution temperature should not exceed 60 °C. Boric acid H3BO3 is recrystallized twice from boiling water and dried at a temperature not exceeding 80 °C.

The pH value is influenced by the temperature of the buffer solution. In table Figure 6 shows pH deviations depending on the temperature of standard buffer solutions.

To create a given pH in the analyzed solution during complexometric titrations, buffer solutions of the following composition are used.

pH = 1. Hydrochloric acid, 0.1 N. solution.

pH = 2. A mixture of glycol NH2-CH2-COOH and its hydrochloric acid salt NH2-CH2-COOH-HCl. Solid glycol (0.2-0.3 g) is added to 100 ml of hydrochloric acid salt solution.

pH = 4-6.5. Acetate mixture 1 N. sodium acetate solution and 1 N. acetic acid solution. The solutions are mixed before use in equal volumes.

pH = 5. A mixture of a solution of 27.22 g of crystalline sodium acetate and 60 ml of 1 N. HCl solution is diluted to 1 liter with water.

pH = 5.5. Acetate mixture. Dissolve 540 g of sodium acetate in water and dilute to 1 liter. To the resulting solution add 500 ml of 1 N. acetic acid solution.

pH = 6.5-8. Triethanolamine and its hydrochloric acid salt. Mix 1 M solution of triethanolamine N(C2H4OH)3 and 1 M solution of HCl in equal volumes before use.

pH = 8.5-9.0. Ammonia-acetate mixture. To 500 ml of concentrated ammonia add 300 ml of glacial acetic acid and dilute with water to 1 liter.

pH = 9. Borate mixture. Mix 100 ml of 0.3 M boric acid solution with 45 ml of 0.5 N. caustic soda solution.

pH = 8-11. Ammonia is ammonium chloride. Mix 1 N. NH4OH solution and 1 N. NH4Cl solution in equal volumes before use.

pH = 10. To 570 ml of concentrated ammonia solution, add 70 g of ammonium chloride and dilute with water to 1 liter.

pH = 11-13. Caustic soda, 0.1 N. solution.

When complexometrically determining the total hardness of water, gray-brown buffer tablets are used, prepared together with an indicator (eriochrome black T). To a water sample (100 ml), it is enough to add a few drops of sodium sulfide solution (to mask heavy metals), two buffer tablets and 1 ml of concentrated ammonia. After the tablets dissolve, the solution turns red; it is titrated with a 0.02 M EDTA solution to a stable green color. 1 ml of 0.02 M EDTA solution corresponds to 0.02 eq/l water hardness. Produced in the GDR.

pH measurement

To determine the pH of solutions, special reagents are used - indicators, as well as devices - pH meters (electrometric determination of pH).

Indicator determination of pH. Most often in analytical practice, the pH of solutions is determined approximately using reactive indicator paper (in the range of 0.5-2.0 pH units). Using universal indicator paper, you can determine pH more accurately (in the range of 0.2-0.3 pH units). In table 7 and 8 show data on reactive and universal indicator papers.

The color transition of universal indicator paper is given in table. 8 and 9. The resulting intermediate colors are compared with the attached comparison scale and the pH values ​​of the test solution are found from it. Indicator papers can be used to determine the pH of aqueous solutions with low salt concentrations and in the absence of strong oxidizing agents. Having determined the pH using universal indicator paper with a pH range of 1.0-11.0 or 0-12, the result is clarified using Rifan paper with a narrower pH range.

Electrometric pH measurement. This method is useful for measuring the pH of colored solutions, in which it is practically impossible. For measurements, instruments are used - pH meters with a glass electrode, which usually replaces the hydrogen electrode. Very rarely, an antimony or quinhydrone electrode is used for this purpose.

Glass electrodes are used to determine the pH of solutions containing heavy metals, oxidizing agents and reducing agents, as well as colloidal solutions and emulsions. Determination of pH with a glass electrode is based on the change in emf. an element that is reversible with respect to hydrogen ions.

The potential of the glass surface in contact with an acid solution depends on the pH of the solution. This property of glass is used in glass electrodes - pH indicators. A glass electrode usually has the shape of a test tube, the bottom of which is made in the form of a thin-walled glass plate or in the form of a ball with a wall thickness of no more than 0.01 mm. A buffer solution with a known pH is poured into a glass electrode and placed into the test solution.

A calomel electrode is used as a reference electrode. This electrode is a vessel containing mercury at the bottom, connected to a circuit by platinum wire. Above the mercury there is calomel paste with KCl crystals, and on top are saturated solutions of KCl and calomel (Hg2Cl2). Contact of the electrode with the test solution occurs through a thin asbestos fiber. The calomel reference electrode can be used for pH measurements at temperatures not exceeding 60 °C; The pH of solutions containing fluorides cannot be measured.

The pH meter is always checked and adjusted using a buffer solution whose pH is close to the pH of the solution being tested. For example, to measure pH in the range from 2 to 6, prepare a Serensen buffer solution with pH = 3 or 4, or use a standard buffer solution with pH = 4.62.

In laboratory practice, the LPU-01 pH meter is used to measure pH, which is designed to determine the pH of solutions in the range from -2 to 14 with a range of 4 pH units: -2-2; 2-4; 6-10; 10-14. The sensitivity of the device is 0.01 pH. They also use a special laboratory pH meter LPS-02; pH meter type PL-U1 and portable pH meter-millivoltmeter PPM-03M1.

An industrial converter of increased accuracy is a pH meter type pH-261, which is intended for measuring the pH of solutions and pulps. In field conditions, a pH-47M pH meter is used to measure the pH of aqueous solutions; for measuring the pH of salt soil extracts - pH meter PLP-64; For milk and dairy products, a pH meter pH-222-2 is used. Work with pH meters is carried out according to the instructions included with each device.

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 high 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 should 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

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 during 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 are contained in the solution, the more acidic the medium is.

In alkali solutions, as a result of dissociation, on the contrary, OH - ions predominate, and H + cations are almost absent due to the 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 changes: in an acidic environment it ranges from 0 to 7, and in an alkaline environment it ranges from 7 to 14. The Danish chemist Peder Sørensen first proposed using the pH value.

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 indicator

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 a soil analysis 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 the 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?

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