Water is a very weak electrolyte, dissociates to a small extent, 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 is equal to the total concentration of water with sufficient accuracy, i.e. 1000/18 = 5.5 mol / dm 3.
In dilute aqueous solutions, the concentration of water changes little and can be considered a constant value. Then the expression for 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 value and is called ion product of water. In pure water at 25 ºС, the concentrations of hydrogen ions and hydroxide ions are equal and are
Solutions in which the concentrations of hydrogen ions and hydroxide ions are the same are called neutral solutions.
So, at 25 ºС
– neutral solution;
> - acidic solution;
< – щелочной раствор.
Instead of the concentrations of H + and OH ions it is more convenient to use their decimal logarithms, taken with the opposite sign; denoted by the symbols pH and pOH:
;
.
The decimal logarithm of the concentration of hydrogen ions, taken with the opposite sign, is called pH indicator(pH) .
Water ions in some cases can interact with the ions of the dissolved substance, which leads to a significant change in the composition of the solution and its pH.
table 2
Formulas for calculating the pH value (pH)
* Values of dissociation constants ( K) are listed in Appendix 3.
p K= -lg K;
HAN, acid; KtOH, base; KtAn - 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 pH (table 2).
2. If a weak acid or base is present in the solution, look in the reference book or in Appendix 3 p K this connection.
3. Determine the composition and concentration of the solution ( WITH).
4. Substitute the numerical values of the molar concentration ( WITH) and p K
into the calculation formula and calculate the pH of the solution.
Table 2 shows the formulas for calculating pH in solutions of strong and weak acids and bases, buffer solutions and solutions of salts undergoing hydrolysis.
If only a strong acid (HAn) 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 hydrogen ions (H +) in a given acid and is determined by 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, so when calculating pH in such solutions, the presence of weak acids is neglected and the calculation formula used for strong acids, (1), is used. The same reasoning is also true for the case when a mixture of strong and weak bases is present in the solution. pH calculations lead according to the formula (2).
If a mixture of strong acids or strong bases is present in the solution, then pH calculations are carried out according to the formulas for calculating the pH for strong acids (1) or bases (2), having previously summed up the concentrations of the components.
If the solution contains a strong acid and its salt, or a strong base and its salt, then pH depends only on the concentration of a strong acid or strong base and is determined by 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, then 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 (hydrolyzed by a cation) or a weak acid and a strong base (hydrolyzed by an anion), a weak acid and a weak base (hydrolyzed by cation and anion), then these salts, undergoing hydrolysis, change 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 concentration .
Solution. 1. In aqueous solution a salt formed by a weak base and a strong acid is hydrolyzed at the cation according to the equations:
In an aqueous solution, hydrogen ions (H +) remain in excess.
2. To calculate pH, we use the formula for calculating the pH value for a salt undergoing cation hydrolysis:
.
Dissociation constant of a weak base 
(R K = 4,74).
3. Substitute the numerical values into the formula and calculate the pH:
.
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.
Solution. 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. Hydrogen indicator will be determined by the sum of hydroxide ions. To do this, we summarize the concentrations of alkalis:
3. We substitute the calculated concentration into formula (2) to calculate the pH of strong bases:
Example 3 Calculate the pH of a buffer solution consisting of 0.10 M formic acid and 0.10 M sodium formate diluted 10 times.
Solution. 1. Formic acid HCOOH is a weak acid, in an aqueous solution it only partially dissociates into ions, in Appendix 3 we find formic acid 
:
2. Sodium formate HCOONa is a salt formed from a weak acid and a strong base; hydrolyzes by the anion, an excess of hydroxide ions appears in the solution:
3. To calculate pH, we use the formula for calculating the pH values of buffer solutions formed by a weak acid and its salt, according to formula (5)
Substitute the numerical values into the formula and get
4. The pH of buffer solutions does not change when diluted. If the solution is diluted 10 times, its pH will remain at 3.76.
Example 4 Calculate the pH value of a solution of acetic acid with a concentration of 0.01 M, the degree of dissociation of which is 4.2%.
Solution. Acetic acid is a weak electrolyte.
In a solution of a weak acid, the concentration of ions is less than the concentration of the acid itself and is defined as a∙C.
To calculate pH, we use formula (3):
Example 5 To 80 cm 3 0.1 n solution of CH 3 COOH was added 20 cm 3 0.2
n CH 3 COONa solution. Calculate the pH of the resulting solution if K(CH 3 COOH) \u003d 1.75 ∙ 10 -5.
Solution. 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 = 100 cm 3, hence the concentrations of acid and salt will be equal:
3. We substitute the obtained values of the acid and salt concentrations
into the formula
.
Example 6 To 200 cm 3 0.1 N hydrochloric acid solution was added 200 cm 3 0.2 N potassium hydroxide solution, determine the pH of the resulting solution.
Solution. 1. Between hydrochloric acid(HCl) and potassium hydroxide (KOH) a neutralization reaction occurs, as a result of which potassium chloride (KCl) and water are formed:
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, in such a solution, KOH remains in excess with a concentration of 0.10 - 0.05 = 0.05 mol / dm 3. Since the KCl salt does not undergo hydrolysis and does not change the pH of the water, the potassium hydroxide present in excess in this solution will affect the pH value. KOH is a strong electrolyte, we use formula (2) to calculate pH:
135. How many grams of potassium hydroxide are contained in 10 dm 3 of a solution whose pH 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 how many times?
137. Indicate the reaction of the medium and find the concentration and 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 a monobasic acid (NAn) in a solution if: a) pH = 4, α = 0.01; b) pH = 3, α = 1%; c) pH = 6,
α = 0.001.
141. Calculate the pH of a 0.01 N solution of acetic acid, in which the degree of dissociation of the acid 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 a solution of acetic acid, the pH of which is 5.2?
144. Determine the molar concentration of a solution of formic acid (HCOOH), whose pH is 3.2 ( K HCOOH = 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 a solution of H 2 SO 4 with a mass fraction of acid 0.5% ( ρ = 1.00 g/cm3).
148. Calculate the pH of a potassium hydroxide solution if 2 dm 3 of the solution contains 1.12 g of KOH.
149. Calculate and pH of 0.5 M ammonium hydroxide solution. \u003d 1.76 10 -5.
150. Calculate the pH of the solution obtained by mixing 500 cm 3 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 NH 4 OH and NH 4 Cl solutions with mass fractions of 5.0%.
152. Calculate the ratio in which sodium acetate and acetic acid should be mixed in order to obtain a buffer solution with pH = 5.
153. In what aqueous solution is the degree of dissociation the greatest: a) 0.1 M CH 3 COOH; b) 0.1 M HCOOH; c) 0.1 M HCN?
154. Derive a formula for calculating pH: a) acetate buffer mixture; b) ammonia buffer mixture.
155. Calculate the molar concentration of an HCOOH solution having pH = 3.
156. How will the pH change if it is diluted twice 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 COOHa?
157*. A 0.1 N acetic acid solution was neutralized with a 0.1 N sodium hydroxide solution to 30% of its original concentration. Determine the pH of the resulting solution. 
158*. To 300 cm 3 0.2 M formic acid solution ( K\u003d 1.8 10 -4) added 50 cm 3 of 0.4 M NaOH solution. The pH was measured and then the solution was diluted 10 times. Calculate the pH of the dilute solution.
159*. To 500 cm 3 0.2 M solution of acetic acid ( K\u003d 1.8 ∙ 10 -5) added 100 cm 3 of 0.4 M NaOH solution. The pH was measured and then the solution was diluted 10 times. Calculate the pH of the dilute solution, write the chemical reaction equations.
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, he added 10 cm 3 of a 0.2% KOH solution ( p\u003d 1 g / cm 3) and the resulting volume was diluted 10 times. What is the pH value of the solution? ( K HCOOH = 1.8∙10 -4).
Hydrogen indicator (pH factor) is a measure of the activity of hydrogen ions in a solution, quantifying its acidity. When the pH is not at the optimal level, the plants begin to lose the ability to absorb some of the elements needed for healthy growth. For all plants there is a specific pH level that allows you to achieve maximum results when growing. Most plants prefer a slightly acidic growing medium (between 5.5-6.5).
Hydrogen indicator in formulas
In very dilute solutions, the pH is equivalent to the concentration of hydrogen ions. Equal in modulus 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 lies in the range from 0 to 14. In pure water, at 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 concentration of hydrogen ions = 10 −15 mol/l, pH = 15, at a concentration of hydroxide ions of 10 mol/l pOH = −1.
It's important to understand! The pH scale is logarithmic, which means that each unit of change equals a tenfold change in the concentration of hydrogen ions. In other words, a pH 6 solution is ten times more acidic than a pH 7 solution, and a pH 5 solution will be ten times more acidic than a pH 6 solution and a hundred times more acidic than a pH 7 solution. This is means that when you are adjusting the pH of your nutrient solution and you need to change the pH by two points (e.g. 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 the pH value
Several methods are widely used to determine the pH value of solutions. The pH value can be approximated using indicators, accurately measured with a pH meter, or determined analytically by performing an acid-base titration.
Acid-base indicators
For a rough estimate of 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 famous 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 acidity range, usually 1-2 units.
Universal indicator
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 consistently changes color from red through yellow, green, blue to purple when moving from an acidic region to a basic one.
Solutions of such mixtures - "universal indicators" are usually impregnated with strips of "indicator paper", with which you can quickly (with an accuracy of pH units, or even tenths of pH) determine the acidity of the studied aqueous solutions. For a more accurate determination, the color of the indicator paper obtained by applying a drop of solution is immediately compared with the reference color scale, the form of which is shown in the images.
Determination of 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), other combinations of indicators are also used. So, for example, ours has a working range and a scale from 4.0 to 8.0, which makes such a test more accurate than 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 with universal indicators. The method is convenient and highly accurate, especially after calibration of 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
Analytical volumetric method - acid-base titration - also gives 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, chemical reaction. The equivalence point - the moment when the titrant is exactly enough to completely complete the reaction - is fixed using an indicator. Further, 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 as the 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 the different dissociation of hydrogen ions (H+) and is not an experimental error. The temperature effect cannot be compensated by the electronics of the pH meter.
Adjusting the pH of the Nutrient Solution
Acidification of the nutrient solution
The nutrient solution usually needs to be acidified. The absorption of ions by plants causes a gradual alkalinization of the solution. Any solution having a pH of 7 or higher will most often need to be adjusted to the optimum pH. Various acids can be used to acidify the nutrient solution. Most often, sulfuric or phosphoric acid is used. A better solution for hydroponic solutions are buffer additives such as and. These products not only bring the pH values to the optimum, but also stabilize the values for a long period.
When adjusting the pH with both acids and alkalis, rubber gloves should be worn to avoid burns to the skin. An experienced chemist skillfully handles concentrated sulfuric acid, he adds acid to water drop by drop. But as a beginner hydroponist, it's probably best to ask an experienced chemist to prepare a 25% sulfuric acid solution. While the acid is being added, the solution is stirred and its pH is determined. Having learned the approximate amount of sulfuric acid, in the future it can be added from a graduated cylinder.
Sulfuric acid must be added in small portions so as not to acidify the solution too much, which then has to be alkalized again. For an inexperienced worker, acidification and alkalization can go on indefinitely. In addition to wasting time and reagents, such regulation unbalances the nutrient solution due to the accumulation of ions that the plants do not need.
Alkalinization of the nutrient solution
Too acidic solutions are alkalized with sodium hydroxide (sodium hydroxide). As its name suggests, it is caustic so rubber gloves should be worn. It is recommended to purchase caustic sodium in the form of pills. In household chemical stores, caustic sodium can be purchased as a pipe cleaner, such as Mole. Dissolve one pill in 0.5 liters of water and gradually pour the alkaline solution into the nutrient solution with constant stirring, checking its pH frequently. No mathematical calculations can calculate how much acid or alkali needs to be added in this or that case.
If you want to grow several crops in one pallet, you need to select them so that not only their optimal pH, but also the needs for other growth factors coincide. For example, yellow daffodils and chrysanthemums need a pH of 6.8 but a different humidity regime so they cannot be grown on the same pallet. If you give daffodils as much moisture as chrysanthemums, the daffodil bulbs will rot. In experiments, rhubarb reached its maximum development at pH 6.5, but could grow even at pH 3.5. Oats, which prefer a pH around 6, produce good yields even at pH 4 if the amount of nitrogen in the nutrient solution is greatly increased. Potatoes grow over a fairly wide pH range, but grow best at a pH of 5.5. Below this pH, high yields of tubers are also obtained, but they acquire a sour taste. For maximum yields High Quality, you need to accurately adjust the pH of nutrient solutions.
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 and dissolved salts, that is, 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, there are also H + and OH - ions. This is due to the dissociation of the water itself. And although we consider water to be a non-electrolyte, nevertheless it can dissociate: H 2 O ^ H + + OH -. But this process occurs to a very small extent: in 1 liter of water, only 1 decomposes into ions. 10 -7 mol molecules.
In acid solutions, as a result of their dissociation, additional H+ ions appear. In such solutions, there are much more H + ions than OH - ions formed during a slight dissociation of water, therefore these solutions are called acidic (Fig. 11.1, left). It is customary to say that in such solutions an acidic environment. The more H+ ions are contained in the solution, the greater the acidity of the medium.
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 medium 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 an environment 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 matter. Pure water also has a neutral environment.
Hydrogen indicator
If we compare the taste of kefir and lemon juice, then we can safely say that lemon juice is much more acidic, that is, the acidity of these solutions is different. You already know that pure water also contains H+ ions, but the water does not taste sour. This is due to the too low concentration of H+ ions. Often it is not enough to say that the environment 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. 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 less in alkaline solutions. In accordance with this, the value of the pH value also changes: in an acidic environment, it ranges from 0 to 7, and in alkaline environments, from 7 to 14. For the first time, the Danish chemist Peder Sørensen suggested 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 math lessons in grade 11. But the relationship between the content of ions in a 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 Lauritz Sørensen
Danish physical chemist and biochemist, President of the Royal Danish Society. Graduated from the University of Copenhagen. At 31, he became a professor at the Danish Polytechnic Institute. He headed the prestigious physical and chemical laboratory at the Carlsberg brewery in Copenhagen, where he made his main scientific discoveries. Main scientific activity devoted to the theory of solutions: he introduced the concept of hydrogen index (pH), studied the dependence of enzyme activity on the acidity of solutions. Behind scientific achievements Sørensen is 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 the acidity of the medium
To determine the acidity of a solution in laboratories, a universal indicator is most often used (Fig. 11.3). By its color, one can 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 measurement of pH, there are special devices - 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 the progress of chemical reactions. For example, if hydrochloric acid is added to a solution of sodium hydroxide, then 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, the solutions of the reactants and reaction products are colorless. If, however, the electrode of a pH meter is placed in the initial alkali solution, then the complete neutralization of the alkali with acid can be judged by the pH value of the resulting solution.
The use of the pH indicator
Determining the acidity of solutions is of great practical importance in many areas of science, industry and other areas of human life.
Environmentalists regularly measure the pH of rainwater, rivers and lakes. A sharp increase in the acidity of natural waters may be the result of air pollution or waste entering water bodies. industrial enterprises(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 the 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 analyze soils in advance, determining their pH (Fig. 11.7). If the acidity is too high for a particular crop, the soil is limed - chalk or lime is added.
IN Food Industry with the help of acid-base indicators, food quality control is carried out (Fig. 11.8). For example, the normal pH for milk is 6.8. A deviation from this value indicates either the presence of 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 of cosmetic products 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, then this leads to premature aging of the skin, its damage or inflammation. It was noticed that laundresses who used regular laundry soap (pH = 8-10) or washing soda (Na 2 CO 3 , pH = 12-13) for washing for a long time, the skin of the hands became very dry and cracked. Therefore, it is very important to use various cosmetic products (gels, creams, shampoos, etc.) with a pH that is close to the natural pH of the skin.
LABORATORY EXPERIMENTS No. 1-3
Equipment: stand with test tubes, pipette.
Reagents: water, hydrochloric acid, NaCl, NaOH solutions, table vinegar, universal indicator (solution or indicator paper), food and cosmetic products (e.g. 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 the skin, in the eyes; wash it off if it comes into contact with big amount 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 - perchloric acid, No. 3 - sodium chloride solution, No. 4 - sodium hydroxide solution and No. 5 - table vinegar.
2. Add 2-3 drops of universal indicator solution to each tube, or omit indicator paper. Determine the pH of solutions by comparing the color of the indicator against a reference scale. Draw conclusions about the presence of Hydrogen cations or hydroxide ions in each test tube. Write the dissociation equations for these compounds.
pH testing 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 matter per 0.5-1 ml of water). Determine the pH of the 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) slightly acidic; c) slightly alkaline; d) strongly acidic; e) strongly alkaline?
Tasks 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. Two test tubes contain solutions of nitrate acid and potassium nitrate. What indicators can be used to determine which 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 above list, write out separately the formulas of substances whose solutions have an environment: 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 such an acidity of the environment?
139. What medium (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 (рН = 1.5); d) in saliva (pH = 7.0)?
140. The composition of coal used in thermal power plants contains Nitrogen and Sulfur compounds. The emission 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 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 acid is added to it with the amount of the substance: a) 0.5 mol; b) 1 mol;
c) 1.5 mol?
143. In three test tubes without inscriptions there are colorless solutions of sodium sulfate, sodium hydroxide and sulfate acid. 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 in a pharmacy. The pH meter showed that the pH value of this water is 6.0. Then the student boiled this water for a long time, filled the container to the top hot water and closed the lid. When the water cooled to room temperature, the pH meter read 7.0. After that, the student passed air through the water with a tube, 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 indicator, pH(lat. pondus hydrogenii- "weight of hydrogen", pronounced "pash") is a measure of the activity (in highly dilute solutions, equivalent to the concentration) of hydrogen ions in a solution, which quantitatively expresses its acidity. Equal in modulus and opposite in sign to the decimal logarithm of the activity of hydrogen ions, which is expressed in moles per liter:
History of pH.
concept pH introduced by the Danish chemist Sorensen in 1909. The indicator is called pH (in first letters Latin words potentia hydrogeni is the strength of hydrogen, or pondus hydrogeni is the weight of hydrogen). In chemistry, the combination pX usually denote a value that is equal to lg X, but with a letter H in this case denote the concentration of hydrogen ions ( H+), or rather, the thermodynamic activity of hydronium ions.
Equations relating pH and pOH.
pH value output.
In pure water at 25 °C, the concentration of hydrogen ions ([ H+]) and hydroxide ions ([ Oh− ]) are the same and equal to 10 −7 mol/l, this clearly follows from the definition of the ionic product of water, equal to [ H+] · [ Oh− ] and is equal to 10 −14 mol²/l² (at 25 °C).
If the concentrations of two types of ions in a solution are the same, then it is said that the solution has a neutral reaction. When an acid is added to water, the concentration of hydrogen ions increases, and the concentration of hydroxide ions decreases; when a base is added, on the contrary, the content of hydroxide ions increases, and the concentration of hydrogen ions decreases. When [ H+] > [Oh− ] it is said that the solution is acidic, and when [ Oh − ] > [H+] - alkaline.
To make it more convenient to represent, to get rid of the negative exponent, instead of the concentrations of hydrogen ions, their decimal logarithm is used, which is taken with the opposite sign, which is the hydrogen exponent - pH.
Basicity index of a solution pOH.
Slightly less popular is the reverse pH value - solution basicity index, pOH, which is equal to the decimal logarithm (negative) of the concentration in the solution of ions Oh − :
as in any aqueous solution at 25 ° C, then at this temperature:
pH values in solutions of different acidity.
- Contrary to popular belief, pH can vary except for the interval 0 - 14, it can also go beyond these limits. For example, at a concentration of hydrogen ions [ H+] = 10 −15 mol/l, pH= 15, at a concentration of hydroxide ions of 10 mol / l pOH = −1 .
Because at 25 °C (standard conditions) [ H+] [Oh − ] = 10 −14 , it is clear that at this temperature pH + pOH = 14.
Because in acidic solutions [ H+] > 10 −7 , which means that for acidic solutions pH < 7, соответственно, у щелочных растворов pH > 7 , pH neutral solutions is 7. At higher temperatures, the electrolytic dissociation constant of water increases, which means that the ion product of water increases, then it will be neutral pH= 7 (which corresponds to simultaneously increased concentrations as H+, and Oh−); with decreasing temperature, on the contrary, neutral pH increases.
Methods for determining the pH value.
There are several methods for determining the value pH solutions. The pH value is approximately estimated using indicators, accurately measured using pH-meter or determined analytically by conducting acid-base titration.
- For a rough estimate of the concentration of hydrogen ions, one often uses acid-base indicators- organic dyes, the color of which depends on pH environment. The most popular indicators are: litmus, phenolphthalein, methyl orange (methyl orange), etc. Indicators can be in 2 differently colored forms - either acidic or basic. The color change of all indicators occurs in their acidity range, often 1-2 units.
- To increase the working measurement interval pH apply universal indicator, which is a mixture of several indicators. The universal indicator consistently changes color from red through yellow, green, blue to purple when moving from an acidic to an alkaline region. Definitions pH indicator method is difficult for cloudy or colored solutions.
- The use of a special device - pH-meter - makes it possible to measure pH over a wider range and more accurately (up to 0.01 units pH) than with indicators. Ionometric method of determination pH is based on the measurement of the EMF of a galvanic circuit with a millivoltmeter-ionometer, which includes a glass electrode, the potential of which depends on the concentration of ions H+ in the surrounding solution. The method has high accuracy and convenience, especially after calibration of the indicator electrode in the selected range pH, which makes it possible to measure pH opaque and colored solutions and is therefore often used.
- Analytical volumetric method — acid-base titration- also gives accurate results for determining the acidity of solutions. A solution of known concentration (titrant) is added dropwise to the solution to be tested. When they are mixed, a chemical reaction occurs. The equivalence point - the moment when the titrant is exactly enough to complete the reaction - is fixed using an indicator. After that, if the concentration and volume of the added titrant solution are known, the acidity of the solution is determined.
- pH:
0.001 mol/L HCl at 20 °C has pH=3, at 30 °C pH=3,
0.001 mol/L NaOH at 20 °C has pH=11.73, at 30 °C pH=10.83,
Influence of temperature on values pH explain the different dissociation of hydrogen ions (H +) and is not an experimental error. Temperature effect cannot be compensated electronically pH-meter.
The role of pH in chemistry and biology.
The acidity of the environment is important for most chemical processes, and the possibility of occurrence or the result of a particular reaction often depends on pH environment. To maintain a certain value pH in the reaction system during laboratory research or buffer solutions are used in production, allowing to save almost constant value pH when diluted or when small amounts of acid or alkali are added to the solution.
Hydrogen indicator pH often used to characterize the acid-base properties of various biological media.
For biochemical reactions, the acidity of the reaction medium occurring in living systems is of great importance. The concentration of hydrogen ions in a solution often affects the physicochemical properties and biological activity of proteins and nucleic acids, therefore, maintaining acid-base homeostasis is a task of exceptional importance for the normal functioning of the body. Dynamic maintenance of optimal pH biological fluids is achieved under the action of buffer systems of the body.
In the human body in different organs, the pH value is different.
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Some Meanings pH. |
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Substance |
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electrolyte in lead batteries |
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Gastric juice |
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Lemon juice (5% rr lemon acids) |
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food vinegar |
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Coca Cola |
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Apple juice |
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Skin of a healthy person |
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Acid rain |
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Drinking water |
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Pure water at 25°C |
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Sea water |
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Soap (fatty) for hands |
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Ammonia |
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Bleach (bleach) |
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Concentrated alkali solutions |
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