Before the electrodes are used for the first time, they must be calibrated. For this, there are special calibration solutions that are buffered at specific pH values. Buffering works in such a way that the ingress of a small amount of water when the electrode is immersed does not interfere with the calibration. The point of calibration is to adjust the electrode error associated with manufacture and use to specific values. In doing so, two errors should be considered: the zero point deviation and the "slope" of the error.

Both errors result in a common measurement error. Therefore, two points must be calibrated so that both measurement errors can be corrected.

Zero point error. The figure above shows the measurement curve and the reference curve. In this example, the measurement curve obviously deviates from the reference curve at pH 7, i.e. at the neutral point, we fix an obvious zero point error, which must be eliminated. The electrodes are first introduced into the pH 7 calibration solution. It is important that at least the glass membrane and the diaphragm are immersed in the solution. In our example, the measured value lies above the required one, therefore, deviates from the nominal value. The measured value is adjusted to the correct value on a potentiometer with variable resistance. This shifts the entire measurement curve in parallel by the zero point error so that it passes exactly through the neutral point. Thus, the measuring device is zero-point and ready for use.

To calibrate pH electrodes, a zero point is required first.

Slope error. After calibrating the zero point, we get the situation shown in the adjacent figure. Zero is accurately determined, but the measured value still has a significant error as the slope point has not yet been determined. A calibration solution is now selected with a pH value other than 7. For the most part buffer solutions are used in the pH range from 4 to 9. The electrode is immersed in the second buffer solution and the slope deviation from the nominal (standard) value is determined using a potentiometer. And only now the measurement curve coincides with the required curve; the device is calibrated.

If the zero point is set, the second must be set. relative magnitude - steepness

Influence of temperature. Changes in pH values \u200b\u200bare influenced by water temperature. However, it is not clear whether temperature compensation is necessary in our measuring instruments. The adjacent table shows the temperature dependence of pH values, with the instrument calibrated at 20 ° C. It should be noted that for the temperatures and pH values \u200b\u200bof interest to us, the measurement error due to temperature deviations is limited to the second decimal place. Therefore, this measurement error is of no practical importance for aquarists and temperature compensation is not required. Along with deviations of a purely measuring nature on the basis of different voltage on the electrodes, one should bear in mind the temperature deviations of the calibrated solutions, which are given in the table next to it.

We see here that these deviations are relatively small and amount to no more than ± 2%.

Deviation of measured pH values \u200b\u200bas a function of temperature

PH value
	4	5	6	7	8	9
0 ° C	3,78	4,85	5,93	7,00	8,07	9,15
5 ° C	3,84	4,89	5,95	7,00	8,05	9,11
10 ° C	3,89	4,93	5,96	7,00	8,04	9,07
15 ° C	3,95	4,97	5,98	7,00	8,02	9,03
20 ° C	4,00	5,00	6,00	7,00	8,00	9,00
25 ° C	4,05	5,03	6,02	7,00	7,98	8,97
30 ° C	4,10	5,07	6,03	7,00	7,97	8,93
35 ° C	4,15	5,10	6,05	7,00	7,95	8,90

Temperature dependence on buffer solutions

Temperature ° С	PH value	Deviation%	PH value	Deviation%	PH value	Deviation%
5	4,01	0,25	7,07	1,00	9,39	1,84
10	4,00	0,00	7,05	0,71	9,33	1,19
15	4,00	0,00	7,03	0,43	9,27	0,54
20	4,00	0,00	7,00	0,00	9,22	0,00
25	4,01	0,25	7,00	0,00	9,18	-0,43
30	4,01	0,25	6,97	-0,43	9,14	-0,87
35	4,02	0,50	6,96	-0,57	9,10	-1,30

The control. For control, it is recommended to immerse the electrodes again in a buffer solution at pH 7 and check if the values \u200b\u200bconverge. If the pH value of the electrode matches the meter, it can be used to measure water samples. If there are personal complaints about accuracy, the calibration should be repeated within the specified time frame. As a recommendation, you can suggest from one to two weeks. When calibrating pH electrodes, attention should also be paid to how quickly the pH on the instrument approaches the pH in the buffer solution.

Objectives of studying the topic:
- Subject results: study of the concepts of "electrolytic dissociation", "degree of electrolytic dissociation", "electrolyte", development of knowledge about the hydrogen index, development of skills in working with substances based on compliance with safety rules;
- metasubject results: the formation of skills in conducting an experiment using digital equipment (obtaining experimental data), processing and presenting the results;
- personal results: the formation of skills in conducting educational research on the basis of a laboratory experiment.

Feasibility of using the "pH and temperature" project
1. Work on the project contributes to the formation of interest in the study of a difficult for a given age (13-14 years) theoretical topic "Theory of electrolytic dissociation". In this case, by determining the pH, the students establish the relationship between the degree of acid dissociation and the temperature of the solution. Working with a soda solution is of a propaedeutic nature in the 8th grade and allows you to return to the results of the project in the 9th grade (extracurricular activities), 11th grade (general course) in the study of salt hydrolysis.
2. Availability of reagents (citric acid, baking soda) and equipment (in the absence of digital pH sensors, you can use indicator paper) for research.
3. The reliability of the experimental technique ensures a smooth course of work, guaranteed against disruptions and methodological failures.
4. Safety of the experiment.

Instrumental section
Equipment:
1) digital pH sensor or laboratory pH meter, litmus paper or other indicator of acidity;
2) alcohol thermometer (from 0 to 50 0С) or digital temperature sensor;
3) citric acid (1 teaspoon);
4) baking soda (1 teaspoon);
5) distilled water (300 ml);
6) a container for a water bath (aluminum or enamel pot or bowl), solutions can be cooled with a stream of cold water or snow, and heated with hot water;
7) beakers with a ground-in lid with a capacity of 50-100 ml (3 pcs.).

Lesson number 1. Formulation of the problem
Lesson plan:
1. Discussion of the concepts "electrolytic dissociation", "degree of electrolytic dissociation", "electrolyte".
2. Statement of the problem. Planning an instrumental experiment.

Activity content
Teacher activity
1. Organizes a discussion of the concepts of "electrolytic dissociation", "degree of electrolytic dissociation", "electrolyte". Questions:
- What are the electrolytes?
- What is the degree of electrolytic dissociation?
- What is the form of writing the equation for the dissociation of strong (for example, sulfuric acid, aluminum sulfate) and weak electrolytes (for example, acetic acid)?
- How does the concentration of the solution affect the degree of dissociation?
The answer can be discussed using the example of dilute and concentrated solutions of acetic acid. If it is possible to determine the electrical conductivity, it is possible to demonstrate the different electrical conductivity of vinegar essence and table vinegar.

Perceive new information on the topic Development of ideas about the degree of dissociation, which are formed in chemistry lessons Cognitive

Assess the completeness of understanding the topic Ability to analyze understanding of the issue Regulatory

Teacher activity
2. Organizes the planning and preparation of the instrumental experiment:
- acquaintance with the information of the project "pH and temperature";
- discussion of the purpose of the project, hypothesis;
- organization of working groups (three groups);
- preparation of equipment

Actions taken Formable ways of activities Student activities
Perceive information about safety rules when working with acids (citric acid) Development of the concept of the need to comply with safety rules Cognitive
Clarify what remains unclear Ability to formulate a question on a topic Communicative
Evaluate the completeness of understanding of the methodology of working on the project Ability to analyze understanding of the issue Regulatory

Lesson number 2. Experimenting
Lesson plan:
1. Preparation for work of digital pH and temperature sensors.
2. Conducting a study of the dependence of pH on temperature:
1st group: measurement of pH of citric acid solution at 10 0C, 25 0C, 40 0C;
2nd group: pH measurement of baking soda solution at 10 0C, 25 0C, 40 0C;
3rd group: measuring the pH of distilled water at 10 0C, 25 0C, 40 0C.
3. Primary analysis of the results obtained. Filling out the GlobalLab project questionnaires.

Teacher activity
1. Organizes workplaces for each group of students:
- explains how to cool solutions, and then gradually heat them and take measurements of temperature and pH;
- answers students' questions

Actions taken Formable ways of activities Student activities
They perceive information on the method of work Development of ideas about the work of digital sensors Cognitive
Clarify what remains unclear Ability to formulate a question on a topic Communicative
Evaluate the completeness of understanding of the work on the project Ability to analyze understanding of the issue Regulatory

Teacher activity
2. Organizes the work of students in groups. The teacher monitors the progress of work in groups, answers possible questions from students, monitors the completion of the table of research results on the blackboard

Actions taken Formable ways of activities Student activities
1. Connect digital sensors to PC.
2. Prepare solutions:
1st group - citric acid;
2nd group - baking soda;
3rd group - distilled water.
3. Cool the solutions and measure the pH at 10 ° C.
4. Gradually heat the solutions and measure the pH at 25 ° C and 40 ° C.
5. The measurement results are entered into a general table, which is drawn on the board (convenient for discussion) Formation of skills in instrumental research Cognitive
Working in groups Learning collaboration in groups Communicative
They work on a common problem, assessing the pace and completeness of the work done. Ability to analyze their actions and correct them based on the joint work of the whole class.

Teacher activity
3. Organizes the primary analysis of research results. Organizes the work of students to fill out the questionnaires of the GlobalLab project "pH and temperature"

Actions taken Formable ways of activities Student activities
Get acquainted with the results of the work of other groups Formation of ideas about the dependence of pH on temperature Cognitive
Asking questions to representatives of other groups Learning cooperation with classmates. Development oral speech Communicative
Analyze the results of work, fill out the project questionnaire Ability to analyze their actions and present the results of their work Regulatory

Lesson number 3. Analysis and presentation of the results obtained
Activity content
1. Presentation of the results: student performances.
2. Discussion of conclusions that are significant for project participants using digital pH sensors.

Teacher activity
1. Organizes student performances. Supports speakers. Concludes on the work on the project, thanks all participants

Actions taken Formable ways of activities Student activities
Present the results of their activities, listen to classmates' speeches Formation of ideas about the form of presentation of the project results Cognitive
Participate in the discussion of performances. Educational cooperation with classmates. Oral speech development Communicative
Analyze the results of their work, comment on the statements of classmates Ability to analyze the results of their own activities and the work of other people Regulatory

Teacher activity
2. Organizes a discussion of the question presented in the project “How will the pH of the solution behave if it is cooled or heated? Why are scientists trying to measure pH at the same temperature and what conclusion should be drawn from this for the participants of the GlobalLab project? "
Organizes a discussion of the results confirming or refuting the hypothesis of the project "When the temperature of the solutions changes, the dissociation constant of dissolved acids and alkalis and, consequently, the pH value"

Actions taken Formable ways of activities Student activities
Discuss the relationship between solution pH and temperature Development of ideas about the degree of electrolytic dissociation Cognitive
Express their thoughts on the project hypothesis and formulate a conclusion. Educational collaboration with classmates. Oral speech development Communicative
Evaluate the project hypothesis based on the results obtained Ability to evaluate the hypothesis based on the results already obtained and formulate a conclusion Regulatory

Potentiometry is one of the electrochemical methods of analysis based on determining the concentration of electrolytes by measuring the potential of an electrode immersed in a test solution.

Potential (from lat. potentia- force) is a concept that characterizes physical force fields (electric, magnetic, gravitational) and, in general, the field of vector physical quantities.

The method of potentiometric measurement of the concentration of ions in a solution is based on measuring the difference in the electrical potentials of two special electrodes placed in the test solution, and one auxiliary electrode has a constant potential during the measurement.

Potential Ean individual electrode is determined by the Nernst equation (W. Nernst - German physicist and chemist, 1869 - 1941) through its standard (normal) potential E 0 and ion activity a + that take part in the electrode process

E \u003d E 0 + 2,3 lg a + , (4.1)

where E 0 - the component of the interfacial potential difference, which is determined by the properties of the electrode and does not depend on the concentration of ions in the solution; R- universal gas constant; n- valence of the ion; T -absolute temperature; F – faraday number (M.Faraday - English physicist of the nineteenth century).

The Nernst equation, derived for a narrow class of electrochemical systems metal - a solution of cations of the same metal, is valid in a much wider range.

The potentiometric method is most widely used to determine the activity of hydrogen ions, which characterizes the acidic or alkaline properties of a solution.

The appearance of hydrogen ions in solution is caused by dissociation (from lat. dissociatio- separation) of a part of water molecules decaying into hydrogen and hydroxyl ions:

H 2 O↔
+
. (4.2)

According to the law of mass action, the constant TOthe equilibrium of the water dissociation reaction is K=
.
/
.

The concentration of undissociated molecules in water is so high (55.5 M) that it can be considered constant, therefore, equation (5.2) is simplified:
= 55,5 =
.
where
- a constant called the ionic product of water,
\u003d 1.0 ∙ 10 -14 at a temperature of 22 o C.

During the dissociation of water molecules, hydrogen and hydroxyl ions are formed in equal amounts, therefore, their concentrations are the same (neutral solution). Based on the equality of the concentrations and the known value of the ionic product of water, we have

[H +] \u003d
=
= 1∙10 -7 . (4.3)

For a more convenient expression of the concentration of hydrogen ions, chemist Zerensen (P.Sarensen is a Danish physicist and biochemist) introduced the concept of pH (p is the initial letter of the Danish word Potenz is the degree, H is the chemical symbol for hydrogen).

Hydrogen indicator pH is a value characterizing the concentration (activity) of hydrogen ions in solutions. It is numerically equal to the decimal logarithm of the concentration of hydrogen ions
taken with the opposite sign, i.e.

pH = - lg
. (4.4)

Aqueous solutions can have a pH in the range from 1 to 15. In neutral solutions at a temperature of 22 ° C pH \u003d 7, in acidic pH< 7, в щелочных рН > 7.

When the temperature of the controlled solution changes, the electrode potential of the glass electrode changes due to the presence of the coefficient S = 2,3∙in equation (4.1). As a result, different values \u200b\u200bof the emf of the electrode system correspond to the same pH value at different solution temperatures.

The dependence of the emf of the electrode system on pH at different temperatures is a bunch of straight lines (Fig. 4.1), intersecting at one point. This point corresponds to the pH value of the solution at which the emf of the electrode system does not depend on temperature, it is called isopotential (from the Greek  - equal, the same and ... potential) point. Isopotential point coordinates ( E AND and pH I) are the most important characteristics of the electrode system. Taking into account the temperature, the static characteristic (4.1) takes the form

240 μmol / min

0.002 μmol

Molar activity indicates how many substrate molecules are converted by one enzyme molecule in 1 minute (molar activity is sometimes referred to as "number of revolutions"). 2.5 shows the molar activity of some enzymes.

Table 2.5. Molar activity of some enzymes

	L kgi vn osg.
Carbonic anhydrase C		(3-Galactosidase
L5-3-ketosteroid isomerase		Phosphoglucomutase
Superoxide dismutase		Cyccinate dehydrogenase
Catalase		Bifunctional
(3-Amylase
Fumaraza

The so called bifunctional enzyme has the lowest molar activity among the known ones. However, this does not mean that its physiological role is also low (for more details about this enzyme, see Fig. 9.31).

Dependence of the rate of the enzymatic reaction on temperature, pH and incubation time

The dependence of the reaction rate on temperature.The rate of enzymatic reactions, like any other, depends on the temperature: as the temperature rises for every 10 ° C, the rate approximately doubles (Van't Gough rule). However, for enzymatic reactions, this rule is valid only at low temperatures - up to 50-60 ° С. At higher temperatures, the denaturation of the enzyme is accelerated, which means a decrease in its amount; the reaction rate also decreases accordingly (Fig. 2.17, d). At 80-90 ° C, most of the enzymes are denatured almost instantly. It is recommended to quantify enzymes at 25 ° C.

The dependence of the reaction rate on pH.A change in pH leads to a change in the degree of ionization of ionogenic groups in the active center, and this affects the affinity of the substrate for the active center and the catalytic mechanism. In addition, a change in protein ionization (not only in the region of the active center) causes conformational changes in the enzyme molecule. The bell-shaped shape of the curve (Fig. 2.17, e) means that there is some optimal state of enzyme ionization, which provides the best connection with the substrate and catalysis of the reaction. The optimum pH for most enzymes lies in the range from 6 to 8. However, there are exceptions: for example, pepsin is most active at pH 2. Enzymes are quantified at the optimum pH for this enzyme.

Time dependence of the reaction rate.As the incubation time increases, the reaction rate decreases (Fig. 2.17, f). This can happen

due to a decrease in the concentration of the substrate, an increase in the rate of the reverse reaction (as a result of the accumulation of the product of the direct reaction), inhibition of the enzyme by the product of the reaction, and denaturation of the enzyme. In enzyme quantification and kinetic studies, the initial reaction rate (the rate immediately after the start of the reaction) is measured. The time during which the rate with an admissible approximation can be considered initial is selected experimentally for each enzyme and for the given conditions, based on the graph shown in Fig. 2.17, that is, the straight line section of the graph, starting from the zero time mark, corresponds to the time interval during which the reaction rate is equal to or close to the initial rate (in the figure, this interval is marked with a dashed line).

ENZYME INHIBITORS

Enzyme inhibitors are substances that reduce their activity. Of greatest interest are inhibitors interacting with the active center of the enzyme. Such inhibitors are most often structural analogs of the substrate and, therefore, are complementary to the active center of the enzyme. Therefore, they inhibit the activity of only one enzyme or a group of enzymes with a very similar structure of the active center. There are competitive and noncompetitive inhibitors, reversible and irreversible inhibitors.

Malonic acid HOO C -CH2-COOH is a structural analog of succinic acid; therefore, it can bind to the active site of succinate dehydrogenase (see above). However, the dehydrogenation of malonic acid is impossible. If the reaction mixture contains both succinic and malonic acids, then the following processes occur:

E + S J ± E S «2 E + P

Some enzyme molecules turn out to be occupied by inhibitor (I) and do not participate in the substrate transformation reaction: therefore, the rate of product formation decreases. If the concentration of the substrate is increased, then the proportion of the ES complex increases, and the EI complex decreases: the substrate and inhibitor compete for the active site of the enzyme. This is an example of competitive inhibition. At a sufficiently high concentration of the substrate, the entire enzyme will be in the form of an ES complex and the reaction rate will be maximal, despite the presence of the inhibitor.

Some inhibitors form a complex not with a free enzyme, but with an enzyme-substrate complex:

IN in this case, an increase in the substrate concentration does not diminish the inhibitor effect; such inhibitors are called noncompetitive.

IN in some cases, the inhibitor can undergo chemical transformation by the action of an enzyme. For example,n-nitrophenyl acetate is hydrolyzed by the proteolytic enzyme chymotrypsin; hydrolysis occurs in two stages (Fig. 2.18).

a O2 N-
E- O- C- CH, + H, O - E- OH + HO- C- CH3 + H0O

Figure: 2.18. Hydrolysis of l-nitrophenyl acetate with chymotrypsin

First, the acetyl residue is attached to the hydroxyl group of the serine residue in the active center of the enzyme (reaction a), and then hydrolysis of the acetyl enzyme occurs (reaction b). The first stage is fast, and the second is very slow, therefore, even at low concentrations of i-nitrophenyl acetate, a significant part of the enzyme molecules are in the acetylated form, and the rate of hydrolysis of the natural substrate (peptides) decreases. Such inhibitors are called pseudosubstrates or poor substrates.

Sometimes the chemical transformation of the inhibitor in the active center leads to the formation of an intermediate product, which is very tightly, irreversibly bound to the enzyme: this phenomenon is called suicidal catalysis. For example, 3-chloroacetol phosphate irreversibly inhibits triose phosphate isomerase. This inhibitor is a structural analogue of dioxyacetone phosphate: it is dechlorinated and irreversibly attached to the glutamic acid residue in the active center of the fer

	cop (Fig.2.19).
				CH2 - O P O 3 H2

	C Th 2

Figure: 2.19. Irreversible inhibition of triose phosphate isomerase

Inhibitors can be not only analogs of substrates, but also analogs of coenzymes that can take the place of a real coenzyme, but cannot perform its function.

The interaction of an enzyme with an inhibitor is often as specific as the interaction with a substrate or coenzyme. Based on this

the use of inhibitors to selectively suppress the activity of an enzyme in a complex enzyme system or in the body. In particular, many medicinal substances are inhibitors of certain enzymes.

There are inhibitors that are less selective. For example, n-chloromercuribenzoate is a specific reagent for sulfhydryl groups in proteins (Fig. 2.20). Therefore, i-chloromercuribenzoate inhibits all enzymes that have SH groups involved in catalysis.

Cys- SH + Cl- Hg-

COOH ™ Cys- S- Hg- (^ j\u003e - COOH

Figure: 2.20. Reaction of l-chloromercuribenzoate with sulfhydryl groups of proteins

Another example is the inhibition by diisopropyl fluorophosphate of peptide hydrolases and esterases with serine in the active site. The inhibitor is irreversibly attached to the serine residue (Fig. 2.21).

H3C - C H - C H 3

Figure: 2.21. Inhibition of serine enzymes by diisopropyl fluorophosphate

Serine residues outside the active site remain unaffected; therefore, the enzyme itself catalyzes the reaction that destroys it. Diisopropyl fluorophosphate is a representative of the group of organophosphorus compounds with extremely high toxicity. The toxic effect is due precisely to the inhibition of enzymes, and primarily acetylcholinesterase (see Chapter 22).

Penicillin, one of the best known and most widely used drugs, is used to treat a number of infectious diseases. Penicillin irreversibly inhibits the bacterial enzyme glycopeptide transferase. This enzyme is involved in the synthesis of the bacterial wall, and therefore bacterial reproduction is impossible in the presence of penicillin. Glycopeptide transferase contains a serine residue in the active site (serine peptide hydrolase). In the penicillin molecule there is an amide bond, which is similar in properties to a peptide bond (Fig. 2.22). As a result of the cleavage of this bond, catalyzed by the enzyme, the penicillin residue is irreversibly bound to the enzyme.

Inhibitors are very effective tools for studying the structure of the active site of enzymes and the mechanism of catalysis. Inhibitors, irreversible

Hydrogen exponent, pH (lat. pondus Hydrogenii - "weight of hydrogen", pronounced "Pe ash") Is a measure of activity (in highly dilute solutions is equivalent to concentration) of hydrogen ions in a solution, which quantitatively expresses its acidity. Equal in magnitude 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 valueintroduced by the Danish chemist Sørensen in 1909. The indicator is called pH (by the first letters of Latin words potentia hydrogeni - the power of hydrogen, or pondus hydrogeni Is the weight of hydrogen). In chemistry by combining pX usually denote a value that is equal to lg X, and the letter H in this case denote the concentration of hydrogen ions ( H +), or rather, the thermodynamic activity of hydronium ions.

Equations linking pH and pOH.

Display of pH value.

In pure water at 25 ° C, the concentration of hydrogen ions ([ H +]) and hydroxide ions ([ OH -]) are the same and equal 10 −7 mol / l, this clearly follows from the definition of the ionic product of water, equal to [ H +] · [ OH -] and equals 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 acid is added to water, the concentration of hydrogen ions increases, and the concentration of hydroxide ions decreases, when adding a base, 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 turns out to be 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.

The basicity index of the pOH solution.

The reverse is slightly less popular pH value - solution basicity index, pOH, which is equal to the decimal logarithm (negative) of the concentration in the ion solution OH − :

as in any aqueous solution at 25 ° C, which means at this temperature:

PH values \u200b\u200bin solutions of different acidity.

• Contrary to popular belief pH it can change except for the interval 0 - 14, it can also go beyond these limits. For example, at a concentration of hydrogen ions [ H +] \u003d 10 −15 mol / l, pH \u003d 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 such a temperature pH + pOH \u003d 14.

Because in acidic solutions [ H +]\u003e 10 −7, which means that in acidic solutions pH < 7, соответственно, у щелочных растворов pH > 7 , pH neutral solutions equals 7. At higher temperatures, the constant of electrolytic dissociation of water increases, which means that the ionic product of water increases, then neutral will be pH \u003d 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 roughly estimated using indicators, accurately measured using pH-meter or determine analytically, carrying out acid-base titration.

1. To roughly estimate the concentration of hydrogen ions, one often uses acid-base indicators- organic substances-dyes, the color of which depends on pH Wednesday. The most popular indicators: litmus, phenolphthalein, methyl orange (methyl orange), etc. Indicators can be in 2 differently colored forms - either in acidic or basic. The color change of all indicators occurs in their acidity range, often 1-2 units.
2. To increase the working measurement interval pH apply universal indicatorwhich is a mixture of several indicators. The universal indicator sequentially changes color from red through yellow, green, blue to violet when passing from an acidic region to an alkaline one. Definitions pH the indicator method is difficult for turbid or colored solutions.
3. Application of a special device - pH-meter - makes it possible to measure pH in a wider range and more accurately (up to 0.01 units pH) than using indicators. Ionometric determination method pH based on the measurement of the electromotive force of the 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 the calibration of the indicator electrode in the selected range pHwhich gives to measure pH opaque and colored solutions and is therefore often used.
4. 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 investigated. When they are mixed, a chemical reaction occurs. The equivalence point - the moment when the titrant is exactly enough for the complete completion of 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.
5. pH:

0.001 mol / L HCl at 20 ° C has pH \u003d 3, at 30 ° C pH \u003d 3,

0.001 mol / L NaOH at 20 ° C has pH \u003d 11.73, at 30 ° C pH \u003d 10.83,

Effect of temperature on values pH explained by different dissociation of hydrogen ions (H +) and is not an experimental error. The temperature effect cannot be compensated for electronically pH-meter.

The role of pH in chemistry and biology.

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

Hydrogen exponent pH often used to characterize the acid-base properties of various biological media.

For biochemical reactions, the acidity of the reaction medium taking place 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, for the normal functioning of the body, maintaining acid-base homeostasis is a task of exceptional importance. Dynamic maintenance of optimal pH biological fluids are achieved under the action of the body's buffer systems.

In the human body, the pH value is different in different organs.

Some meanings pH.
Substance
Electrolyte in lead-acid batteries
Gastric juice
Lemon juice (5% citric acid solution)
Food vinegar
Coca Cola
Apple juice
Healthy human skin
Acid rain
Drinking water
Pure water at 25 ° C
Sea water
Hand soap (fat)
Ammonia
Bleach (bleach)
Concentrated alkali solutions