Ph 1 what environment. The acidity of the medium. The concept of the pH of the solution. The role of pH in chemistry and biology

Remember:

The neutralization reaction is the reaction between acid and alkali, as a result of which salt and water are formed;

Chemists understand clean water as chemically pure water that does not contain any impurities or dissolved salts, i.e., distilled water.

Medium acidity

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 is due to the dissociation of the water itself. And although we consider water a non-electrolyte, nevertheless, it can dissociate: H 2 O ^ H + + OH -. But this process occurs to a very insignificant degree: in 1 liter of water only 1 decays into ions. 10 -7 mol of molecules.

As a result of their dissociation, additional H + ions appear in acid solutions. In such solutions, H + ions are much larger than OH - ions formed upon slight dissociation of water; therefore, these solutions are called acidic (Fig. 11.1, left). It is customary to say that in such solutions the acidic environment. The more H + ions are contained in the solution, the greater the acidity of the medium.

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

In a solution of sodium chloride, the amount 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, in the center). In fact, this means that the solution does not contain either acid or alkali. A neutral medium is characteristic of solutions of certain salts (formed by alkali and strong acid) and many organic substances. Pure water also has a neutral environment.

Hydrogen indicator

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 acidic 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 the medium is acidic or alkaline, but it is necessary to quantitatively characterize it.

The acidity of the medium is quantitatively characterized by a hydrogen pH (pronounced pH) associated with the concentration

hydrogen ions. The pH value corresponds to a specific content of Hydrogen cations in 1 liter of solution. In pure water and in neutral solutions, 1 liter contains 1. 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 less. In accordance with this, the pH value also changes: in an acidic medium it ranges from 0 to 7, and in alkaline it ranges from 7 to 14. For the first time, the Danish chemist Peder Sørensen suggested using a hydrogen indicator.

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 math classes in grade 11. But the relationship between the content of ions in 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).

Fig. 11.2. PH value of various natural and artificial solutions

Søren Peder Lauritz Sørensen

Danish physical chemist and biochemist, president of the Danish Royal Society. He graduated from the University of Copenhagen. At 31, he became a professor at the Danish Polytechnic Institute. He headed the prestigious Physico-Chemical Laboratory at the Carlsberg Brewery in Copenhagen, where he made his main scientific discoveries. The main scientific activity is devoted to the theory of solutions: he introduced the concept of a hydrogen index (pH), studied the dependence of enzyme activity on the acidity of solutions. For 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 a scientist who introduced the concepts of “pH” and “pH-metry”.

Determination of acidity

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

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

Fig. 11.3. The universal indicator determines the approximate pH value

Fig. 11.4. To measure the pH of solutions use special instruments - pH meters: a - laboratory (stationary); b - portable

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

The use of a hydrogen indicator

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

Environmentalists regularly measure the pH of rainwater, river and lake water. A sharp increase in the acidity of natural waters can be a consequence of atmospheric pollution or the ingress of waste from industrial enterprises into water bodies (Fig. 11.5). Such changes entail the death of plants, fish and other inhabitants of water bodies.

A hydrogen indicator is very important for studying and observing processes occurring in living organisms, since numerous chemical reactions occur in the cells. In clinical diagnosis, determine the pH of blood plasma, urine, gastric juice, etc. (Fig. 11.6). The normal blood pH is from 7.35 to 7.45. Even a small change in the pH of a person’s blood causes serious diseases, and at pH \u003d 7.1 and below, irreversible changes begin that can lead to death.

Soil acidity is important for most plants, so agronomists analyze the soil in advance, determining their pH (Fig. 11.7). If the acidity is too high for a particular crop, the soil is calcified - chalk or lime is added.

In the food industry, using acid-base indicators, food quality control is carried out (Fig. 11.8). For example, the norm for milk is pH \u003d 6.8. Deviation from this value indicates either the presence of impurities or its souring.

Fig. 11.5. The effect of the pH of water in reservoirs on the vital activity of plants in them

The pH value is important for the cosmetics that we use in everyday life. On average for human skin, pH \u003d 5.5. If the skin comes into contact with products whose acidity differs significantly from this value, this entails premature aging of the skin, its damage or inflammation. It was noticed that for laundries, which for a long time used ordinary laundry soap (pH \u003d 8-10) or washing soda (Na 2 CO 3, pH \u003d 12-13), the skin of the 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 skin's natural pH.

LABORATORY EXPERIENCE No. 1-3

Equipment: test tube rack, pipette.

Reagents: water, perchloric acid, solutions of NaCl, NaOH, 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;

Beware of contact with skin, eyes; If corrosive substance gets in, wash it off with plenty of water.

Determination of hydrogen ions and hydroxide ions in solutions. Estimation of the approximate pH of water, alkaline and acidic solutions

1. Pour 1-2 ml into five test tubes: 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 the universal indicator solution to each tube or lower the indicator paper. Determine the pH of the solutions by comparing the color of the indicator on a reference scale. Draw conclusions about the presence of Hydrogen cations or hydroxide ions in each tube. Draw up the dissociation equations of these compounds.

PH study of food and cosmetic products

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

Key idea

test questions

130. The presence of which ions in the solution is due to its acidity?

131. What ions are in excess in acidic 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) strongly alkaline?

Assignments for mastering the material

134. An aqueous solution of some substance has an alkaline environment. Which ions are more in this solution: H + or OH -?

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

136. In three test tubes are solutions of barium hydroxide, nitric acid and calcium nitrate. How to recognize these solutions with a single reagent?

137. From the above list, write 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 a pH of 5.6. What does this mean? What substance contained in air, when dissolved in water, determines such an acidity of the medium?

139. What medium (acid or alkaline): a) in a solution of shampoo (pH \u003d 5.5);

b) in the blood of a healthy person (pH \u003d 7.4); c) in human gastric juice (pH \u003d 1.5); d) in saliva (pH \u003d 7.0)?

140. The composition of coal used in thermal power plants contains compounds of Nitrogen and Sulfur. Emission of coal combustion products into the atmosphere leads to the formation of so-called acid rains containing small amounts of nitrate or sulfite acids. What pH values \u200b\u200bare typical for such rainwater: more than 7 or less than 7?

141. Does the pH of a strong acid solution depend on its concentration? Justify the 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 of acid is added to it with the amount of a substance: a) 0.5 mol; b) 1 mol;

c) 1.5 mol?

143. Colorless solutions of sodium sulfate, sodium hydroxide and sulfate acid are in three test tubes without labels. For all solutions, the pH value was measured: in the first tube - 2.3, in the second - 12.6, in the third - 6.9. Which tube contains which substance?

144. A student bought distilled water at a pharmacy. The pH meter showed that the pH of this water is 6.0. Then the student boiled this water for a long time, filled the container to the top with hot water and closed it with a lid. When the water cooled to room temperature, the pH meter determined a value of 7.0. After this, the pupil passed air through water with a tube, and the pH meter again showed 6.0. How can we explain the results of these pH measurements?

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

This is the material of the textbook.

Water is a weak electrolyte; it dissociates weakly by the equation

At 25 ° С in 1 liter of water it decomposes into ions of 10-7 mol H2O. 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. \u003e 10-7 mol / l; the concentration of OH– ions decreases, i.e. less than 10-7 mol / l. When alkali is added, the concentration of OH- ions increases:\u003e 10-7 mol / L, therefore, less than 10-7 mol / L.

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

In neutral water, pH \u003d 7. The pH values \u200b\u200band 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 time. In some reactions, the pH may change as a result of the binding or release of H + ions. To maintain a constant pH value, buffers 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 the 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 an alkali solution is added to the acetate buffer solution, OH– ions will be bound by undissociated acetic acid molecules CH3COOH:

Therefore, the pH of the solution in this case also will not change much.

Buffer solutions retain their buffer action 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 to a large extent, as in a non-buffer solution.

Typically, analysis methods indicate which buffer solution should be used when performing this analysis and how to prepare it. Buffer mixtures with an exact pH value are released in ampoules to prepare 500 ml of solution.

pH \u003d 1.00.  Composition: 0.084 g of glycocol (amino acetic acid NH2CH2COOH), 0.066 g of sodium chloride NaCl and 2.228 g of hydrochloric acid HCl.

pH \u003d 2.00.  Composition: 3.215 g of citric acid C6H8O7-H2O, 1.224 g of sodium hydroxide NaOH and 1.265 g of hydrochloric acid HCl.

pH \u003d 3.00.  Composition: 4.235 g of citric acid C6H8O7-H2O, 1.612 g of sodium hydroxide NaOH and 1.088 g of hydrochloric acid HCl.

pH \u003d 4.00.  Composition: 5.884 g of citric acid C6H8O7-H2O, 2.240 g of sodium hydroxide NaOH and 0.802 g of hydrochloric acid HCl.

pH \u003d 5.00.  Composition: 10.128 g of citric acid C6H8O7-H2O and 3.920 g of sodium hydroxide NaOH.

pH \u003d 6.00.  Composition: 6.263 g of citric acid C6H8O7-H2O and 3.160 g of sodium hydroxide NaOH.

pH \u003d 7.00.  Composition: 1.761 g of potassium dihydrogen phosphate KH2PO4 and 3.6325 g of sodium hydrogen phosphate Na2HPO4-2H2O.

pH \u003d 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 \u003d 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 \u003d 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 \u003d 11.00.  Composition: 2.225 g of sodium hydrogen phosphate Na2HPO4-2H2O and 0.068 g of sodium hydroxide NaOH.

pH \u003d 12.00.  Composition: 2.225 g of sodium hydrogen phosphate Na2HPO4-2H2O and 0.446 g of sodium hydroxide NaOH.

pH \u003d 13.00.  Composition: 1.864 g of potassium chloride KCl and 0.942 g of sodium hydroxide NaOH.

Deviations from the nominal pH reach ± \u200b\u200b0.02 for solutions at pH from 1 to 10 and ± 0.05 at pH from 11 to 13. Such accuracy is quite sufficient for practical work.

Standard pH buffers are used to adjust pH meters.

1. Acetate buffer solution with pH \u003d 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 \u003d 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 \u003d 9.22:  3.81 g of sodium tetraborate Na2B4O7-10H2O in 1 liter of solution.

4. Phosphate buffer solution with pH \u003d 11.00:  4.450 g of sodium hydrogen phosphate Na2HPO4-2H2O and 0.136 g of sodium hydroxide NaOH in 1 liter of solution.

For the preparation of buffer solutions for agrochemical and biochemical analysis with pH values \u200b\u200bfrom 1.1 to 12.9 with an interval of 0.1, 7 basic stock solutions are used.

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

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

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

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

Solution 5.  12.367 g of boric acid H3BO3 are dissolved in water, 100 ml of 1N are added. NaOH solution and diluted with water to 1 l in a volumetric flask. By mixing this solution with 0.1 N. HCl solution prepare buffer solutions with a pH of from 7.8 to 8.9; mixing with 0.1 N. NaOH solution prepare buffer solutions with a pH of from 9.3 to 11.0.

Solution 6.  Prepare exactly 0.1 n. HCl solution;

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

In some solutions, mold deposits form during storage, to prevent this, a few drops of thymol are added to the solution as a preservative. To prepare a buffer solution of the required pH, these solutions are mixed in a certain ratio (Table 5). Volume is measured using a burette with a capacity of 100.0 ml. All pH values \u200b\u200bof the buffer solutions in the table are shown at a temperature of 20 ° C.

For the preparation of the initial solutions, reagents of qualification hc are used. Sodium hydrogen phosphate Na2HPO4-2H2O is previously recrystallized twice. During the second recrystallization, the temperature of the solution should not exceed 90 ° C. The resulting preparation is slightly moistened and dried in an thermostat at 36 ° C for two days. Potassium dihydrogen phosphate KH2PO4 is also recrystallized twice and dried at 110-120 ° C. Sodium chloride NaCl was recrystallized twice and dried at 120 ° C. C6H8O7-H2O citric acid is recrystallized twice. In the second recrystallization, the temperature of the solution should not be higher than 60 ° C. Boric acid H3BO3 is recrystallized twice from boiling water and dried at a temperature not exceeding 80 ° C.

The pH is affected by the temperature of the buffer solution. In the table. Figure 6 shows the deviations of pH 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 \u003d 1.  Hydrochloric acid, 0.1 N. solution.

pH \u003d 2.  A mixture of glycocol NH2-CH2-COOH and its hydrochloric acid salt NH2-CH2-COOH-HCl. Solid glycocol (0.2-0.3 g) is added to 100 ml of hydrochloric acid salt solution.

pH \u003d 4-6.5.  Acetate mixture 1 N. a solution of sodium acetate and 1 N. acetic acid solution. The solutions are mixed before use in equal volumes.

pH \u003d 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 \u003d 5.5.  Acetate mixture. 540 g of sodium acetate are dissolved in water and diluted to 1 liter. To the resulting solution was added 500 ml of 1 N. acetic acid solution.

pH \u003d 6.5-8.  Triethanolamine and its hydrochloric acid salt. Mix 1 M solution of triethanolamine N (C2H4OH) 3 and 1 M HCl solution in equal volumes before use.

pH \u003d 8.5-9.0.  Ammonia acetate mixture. 300 ml of glacial acetic acid are added to 500 ml of concentrated ammonia and diluted with water to 1 liter.

pH \u003d 9.  Borate mixture. 100 ml of a 0.3 M solution of boric acid are mixed with 45 ml of 0.5 N. caustic soda solution.

pH \u003d 8-11.  Ammonia is ammonium chloride. Mix 1 N. NH4OH solution and 1 N. NH4Cl solution in equal volumes before use.

pH \u003d 10.  To 570 ml of concentrated ammonia solution, 70 g of ammonium chloride are added and diluted with water to 1 liter.

pH \u003d 11-13.  Caustic soda, 0.1 N solution.

In the complexometric determination of the total hardness of water, gray-brown buffer tablets are used, prepared together with the indicator (black Eriochrom 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 dissolution of the tablets, the solution turns red; it is titrated with a 0.02 M EDTA solution until a stable green color is obtained. 1 ml of a 0.02 M EDTA solution corresponds to 0.02 eq / L of water hardness. Issued in the GDR.

PH measurement

To determine the pH of solutions use special reagents - indicators, as well as instruments - 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 indicator universal paper, you can determine the pH more accurately (in the range of 0.2-0.3 pH units). In the table. 7 and 8 show data on reactive and universal indicator papers.

The color transition of the universal indicator paper is given in table. 8 and 9. The resulting intermediate colors are compared with the attached comparison scale and the pH of the test solution is found from it. Indicator papers can be used to determine the pH of aqueous solutions with a low salt concentration and in the absence of strong oxidizing agents. Having determined the pH using universal indicator paper with an interval of pH \u003d 1.0-11.0 or 0-12, refine the result using paper "Rifan" with a narrower pH range.

Electrometric pH measurement.  This method is convenient for measuring the pH of color solutions in which it is practically impossible. For measurements, instruments are used - pH meters with a glass electrode, which is usually replaced by a hydrogen electrode. Very rarely, an antimony or chinhydron 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. The determination of pH with a glass electrode is based on a change in the emf an element reversible with respect to hydrogen ions.

The potential of the glass surface in contact with the acid solution depends on the pH of the solution. This property of glass is used in glass electrodes - pH indicators. The glass electrode is usually in the form 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 not more than 0.01 mm. A buffer solution of known pH is poured into the glass electrode and placed in the test solution.

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

The pH meter instrument is always checked and adjusted using the buffer solution whose pH is close to the pH of the test solution. For example, to measure pH in the range from 2 to 6, a Zerensen buffer solution with pH \u003d 3 or 4 is prepared or a standard buffer solution with pH \u003d 4.62 is used.

In laboratory practice, a pH meter LPU-01 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. A laboratory special LPS-02 pH meter is also used; pH meter type PL-U1 and portable pH meter-millivoltmeter PPM-03M1.

An industrial precision converter is a pH meter type pH-261, which is designed to measure the pH of solutions and pulps. In the field, a pH meter pH-47M is used to measure the pH of aqueous solutions; for pH measurements of salt soil extracts - pH meter PLP-64; for milk and dairy products, use a pH meter pH-222-2. Work on pH meters is carried out according to the instructions attached to each device.

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

pН \u003d - log

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

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

rON \u003d - lg

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 also called the ionic product of water:

To w \u003d · \u003d 10 –14 [mol 2 / l 2] (at 25 ° C)

pH + pOH \u003d 14

When the concentrations of both types of ions in a solution are the same, they say that the solution has a neutral reaction. When acid is added to water, the concentration of hydrogen ions increases, and the concentration of hydroxide ions decreases accordingly, when adding a base, on the contrary, the content of hydroxide ions increases, and the concentration of hydrogen ions decreases. When\u003e they say that the solution is acidic, and when\u003e - alkaline.

PH determination

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

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

For a rough estimate of the concentration of hydrogen ions, acid-base indicators are widely used - organic dyes, the color of which depends on the pH of the medium. The most famous indicators include litmus, phenolphthalein, methyl orange (methyl orange) and others. Indicators can exist in two differently colored forms - either in acidic or in basic. The color change of each indicator occurs in its range of acidity, usually 1–2 units (see Table 1, session 2).

To extend the working range of pH measurement, the 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 changing from acidic to alkaline. Determination of pH by the indicator method is difficult for turbid 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 mixed, a chemical reaction proceeds. The equivalence point - the moment when the titrant is precisely enough to completely complete the reaction - is fixed using the indicator. Further, knowing the concentration and volume of the added titrant solution, the total acidity of the solution is calculated.

The acidity of the medium is important for many chemical processes, and the possibility of occurrence or the result of a reaction often depends on the pH of the medium. To maintain a certain pH value in the reaction system during laboratory tests or in production, buffer solutions are used that can maintain a practically constant pH value when diluted or when small amounts of acid or alkali are added to the solution.

The hydrogen pH 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 an acid-base homeostasis is an extremely important task. Dynamic maintenance of optimal pH of biological fluids is achieved due to the action of buffer systems.

3) The use of a special device - a pH meter - allows you to measure pH over a wider range and more accurately (up to 0.01 pH units) than with indicators, is convenient and highly accurate, allows you to measure the pH of opaque and color solutions and therefore widely is used.

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

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

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

Recently, pH meters have also been widely used in aquarium farms, monitoring water quality in domestic conditions, agriculture (especially in hydroponics), and also for monitoring the diagnosis of health conditions.

Table 2. pH values \u200b\u200bfor some biological systems and other solutions

System (solution)
Duodenum
Gastric juice
Human blood
Muscle
Pancreatic juice
Protoplasm of cells
Small intestine
Sea water
Chicken Egg Protein
Orange juice
Tomato juice

Water is a very weak electrolyte, slightly dissociates, forming hydrogen ions (H +) and hydroxide ions (OH -),

This process corresponds to the dissociation constant:

.

Since the degree of dissociation of water is very small, the equilibrium concentration of undissociated water molecules with sufficient accuracy is equal to the total concentration of water, i.e. 1000/18 \u003d 5.5 mol / dm 3.
  In dilute aqueous solutions, the concentration of water varies little and can be considered a constant value. Then the expression of the dissociation constant of water is transformed as follows:

.

The constant equal to the product of the concentration of H + and OH - ions is a constant and is called ionic product of water. In pure water at 25 ºС, the concentrations of hydrogen ions and hydroxide ions are equal and amount to

Solutions in which the concentrations of hydrogen ions and hydroxide ions are the same are called neutral solutions.

So, at 25 ºС

  - neutral solution;

  \u003e - acidic solution;

< – щелочной раствор.

Instead of H + and OH - it is more convenient to use their decimal logarithms taken with the opposite sign; are indicated by pH and pOH:

;

.

The decimal logarithm of the concentration of hydrogen ions, taken with the opposite sign, is called hydrogen indicator(pH) .

Water ions in some cases can interact with ions of a dissolved substance, which leads to a significant change in the composition of the solution and its pH.

table 2

Hydrogen Index Formulas (pH)

* Values \u200b\u200bof dissociation constants ( K) are indicated in Appendix 3.

p K  \u003d - lg K;

HAn is an acid; KtOH is the base; KtAn is the salt.

When calculating the pH of aqueous solutions, it is necessary:

1. Determine the nature of the substances that make up the solutions, and select a formula for calculating the pH (table 2).

2. If a weak acid or base is present in the solution, find by reference or in Appendix 3 p K  of this compound.

3. Determine the composition and concentration of the solution ( FROM).

4. Substitute the numerical values \u200b\u200bof the molar concentration ( FROM) and p K
  into the calculation formula and calculate the pH of the solution.

Table 2 shows the pH calculation formulas in solutions of strong and weak acids and bases, buffer solutions and solutions of salts subjected to hydrolysis.

If only strong acid (HAn) is present in the solution, which is a strong electrolyte and almost completely dissociates into ions   then the hydrogen index (pH) will depend on the concentration of hydrogen ions (H +) in a given acid and is determined by the formula (1).

If only a strong base is present in the solution, which is a strong electrolyte and almost completely dissociates into ions, then the pH (pH) will depend on the concentration of hydroxide ions (OH -) in the solution and is determined by formula (2).

If only a weak acid or only a weak base is present in the solution, then the pH of such solutions is determined by formulas (3), (4).

If a mixture of strong and weak acids is present in the solution, then the ionization of the weak acid is practically suppressed by the strong acid, therefore, when calculating the pH in such solutions, the presence of weak acids is neglected and the calculation formula used for strong acids is used (1). The same reasoning is true for the case when a mixture of strong and weak bases is present in the solution. PH Calculations lead by the formula (2).

If a solution contains a mixture of strong acids or strong bases, then pH calculations are carried out according to the pH calculation formulas for strong acids (1) or bases (2), after summing up the concentrations of the components.

If the solution contains a strong acid and its salt or a strong base and its salt, then the pH depends only on the concentration of a strong acid or a strong base and is determined by the formulas (1) or (2).

If a weak acid and its salt (for example, CH 3 COOH and CH 3 COONa; HCN and KCN) or a weak base and its salt (for example, NH 4 OH and NH 4 Cl) are present in the solution, this mixture is buffer solution  and pH is determined by formulas (5), (6).

If there is a salt in the solution formed by a strong acid and a weak base (hydrolyzes according to a cation) or a weak acid and a strong base (hydrolyzes by an anion), a weak acid and a weak base (hydrolyzes by a cation and anion), then these salts undergo hydrolysis pH value, and the calculation is carried out according to formulas (7), (8), (9).

Example 1  Calculate the pH of an aqueous solution of NH 4 Br salt with a concentration.

Decision.  1. In an aqueous solution, a salt formed by a weak base and a strong acid is hydrolyzed according to the cation according to the equations:

In an aqueous solution, hydrogen ions (H +) remain in excess.

2. To calculate the pH, we use the formula for calculating the hydrogen index for a salt undergoing cation hydrolysis:

.

Weak base dissociation constant
  (R K = 4,74).

3. Substitute the numerical values \u200b\u200bin the formula and calculate the hydrogen index:

.

Example 2  Calculate the pH of an aqueous solution consisting of a mixture of sodium hydroxide,   mol / dm 3 and potassium hydroxide,   mol / dm 3.

Decision.1. Sodium hydroxide (NaOH) and potassium hydroxide (KOH) are strong bases that almost completely dissociate in aqueous solutions into metal cations and hydroxide ions:

2. The hydrogen index will be determined by the sum of hydroxide ions. To do this, we summarize the concentration of alkalis:

3. We substitute the calculated concentration in formula (2) for calculating the pH of strong bases:

Example 3  Calculate the pH of the buffer solution consisting of a 0.10 M solution of formic acid and a 0.10 M solution of sodium formate diluted 10 times.

Decision.1. Formic acid HCOOH - a weak acid, in an aqueous solution only partially dissociates into ions, in Appendix 3 we find formic acid :

2. Sodium formate HCOONa - salt formed by weak acid and strong base; is hydrolyzed by anion, an excess of hydroxide ions appears in the solution:

3. To calculate the pH, we use the formula for calculating the hydrogen parameters of buffer solutions formed by weak acid and its salt, according to the formula (5)

Substitute the numerical values \u200b\u200bin the formula and get

4. The hydrogen index of the buffer solutions does not change upon dilution. If the solution is diluted 10 times, its pH will remain equal to 3.76.

Example 4  Calculate the pH of a solution of acetic acid with a concentration of 0.01 M, the degree of dissociation of which is 4.2%.

Decision.  Acetic acid is a weak electrolyte.

In a weak acid solution, the concentration of ions is less than the concentration of the acid itself and is defined as a∙C.

To calculate the pH, we use the formula (3):

Example 5  To 80 cm 3 0.1 n CH 3 COOH solution was added 20 cm 3 0.2
  n solution of CH 3 COONa. Calculate the pH of the resulting solution if K(CH 3 COOH) \u003d 1.75 ∙ 10 –5.

Decision.1. If the solution contains a weak acid (CH 3 COOH) and its salt (CH 3 COONa), then this is a buffer solution. We calculate the pH of the buffer solution of this composition according to the formula (5):

2. The volume of the solution obtained after draining the initial solutions is 80 + 20 \u003d 100 cm 3, hence the concentration of acid and salt will be equal to:

3. The obtained values \u200b\u200bof acid and salt concentrations are substituted
  into the formula

.

Example 6  To 200 cm 3 of a 0.1 N hydrochloric acid solution was added 200 cm 3 of a 0.2 N potassium hydroxide solution, to determine the pH of the resulting solution.

Decision.1. A neutralization reaction takes place between hydrochloric acid (HCl) and potassium hydroxide (KOH), resulting in the formation of potassium chloride (KCl) and water:

HCl + KOH → KCl + H 2 O.

2. Determine the concentration of acid and base:

According to the reaction, HCl and KOH react as 1: 1; therefore, KOH with a concentration of 0.10-0.05 \u003d 0.05 mol / dm 3 remains in excess in such a solution. Since the KCl salt does not undergo hydrolysis and does not change the pH of water, the potassium hydroxide in excess in this solution will influence the pH value. KOH is a strong electrolyte, to calculate the pH we use the formula (2):

135. How many grams of potassium hydroxide is contained in a 10 dm 3 solution, the hydrogen index of which is 11?

136. The hydrogen index (pH) of one solution is 2 and the other is 6. In 1 dm 3 of which solution is the concentration of hydrogen ions greater and by how many times?

137. Indicate the reaction of the medium and find the concentration of ions in solutions for which the pH is: a) 1.6; b) 10.5.

138. Calculate the pH of solutions in which the concentration is (mol / dm 3): a) 2.0 ∙ 10 –7; b) 8.1 ∙ 10 –3; c) 2.7 ∙ 10 –10.

139. Calculate the pH of solutions in which the concentration of ions is (mol / dm 3): a) 4.6 ∙ 10 –4; b) 8.1 ∙ 10 –6; c) 9.3 ∙ 10 –9.

140. Calculate the molar concentration of monobasic acid (HAn) in solution if: a) pH \u003d 4, α \u003d 0.01; b) pH \u003d 3, α \u003d 1%; c) pH \u003d 6,
  α \u003d 0.001.

141. Calculate the pH of 0.01 n acetic acid solution in which the degree of acid dissociation is 0.042.

142. Calculate the pH of the following solutions of weak electrolytes:
  a) 0.02 M NH 4 OH; b) 0.1 M HCN; c) 0.05 n HCOOH; d) 0.01 M CH 3 COOH.

143. What is the concentration of acetic acid solution, whose pH is 5.2?

144. Determine the molar concentration of formic acid (HCOOH) solution, whose pH is 3.2 ( K UNSOO \u003d 1.76 ∙ 10 –4).

145. Find the degree of dissociation (%) and 0.1 M solution of CH 3 COOH if the dissociation constant of acetic acid is 1.75 ∙ 10 –5.

146. Calculate the pH of 0.01 M and 0.05 n solutions of H 2 SO 4.

147. Calculate the pH of the solution of H 2 SO 4 with a mass fraction of acid of 0.5% ( ρ   \u003d 1.00 g / cm 3).

148. Calculate the pH of the potassium hydroxide solution if 2 dm 3 of the solution contains 1.12 g of KOH.

149. Calculate the pH of a 0.5 M solution of ammonium hydroxide. \u003d 1.76 ∙ 10 –5.

150. Calculate the pH of the solution obtained by mixing 500 cm 3 of 0.02 M CH 3 COOH with an equal volume of 0.2 M CH 3 COOK.

151. Determine the pH of the buffer mixture containing equal volumes of solutions of NH 4 OH and NH 4 Cl with mass fractions of 5.0%.

152. Calculate the ratio of sodium acetate and acetic acid to obtain a buffer solution with pH \u003d 5.

153. In which aqueous solution is the degree of dissociation the greatest: a) 0.1 M CH 3 COOH; b) 0.1 M UNS; c) 0.1 M HCN?

154. Derive the formula for calculating the pH: a) acetate buffer mixture; b) ammonia buffer mixture.

155. Calculate the molar concentration of the HCOOH solution having a pH \u003d 3.

156. How will the pH change if diluted in half with water: a) 0.2 M HCl solution; b) 0.2 M solution of CH 3 COOH; c) a solution containing 0.1 M CH 3 COOH and 0.1 M CH 3 COONa?

157 *. A 0.1 N acetic acid solution was neutralized with a 0.1 N sodium hydroxide solution at 30% of its initial concentration. Determine the pH of the resulting solution.

158 *. To 300 cm 3 0.2 M solution of formic acid ( K  \u003d 1.8 ∙ 10 –4) 50 cm 3 of a 0.4 M NaOH solution were added. The pH was measured and then the solution was diluted 10 times. Calculate the pH of the diluted solution.

159 *. To 500 cm 3 0.2 M solution of acetic acid ( K  \u003d 1.8 ∙ 10 –5) 100 cm 3 of a 0.4 M NaOH solution was added. The pH was measured and then the solution was diluted 10 times. Calculate the pH of the diluted solution, write the equations of the chemical reaction.

160 *. To maintain the required pH value, the chemist prepared a solution: to 200 cm 3 of a 0.4 M solution of formic acid added 10 cm 3 of a 0.2% KOH solution ( p  \u003d 1 g / cm 3) and the resulting volume was diluted 10 times. With what pH is the solution obtained? ( K  HCOOH \u003d 1.8 ∙ 10 –4).