The reaction should be practically irreversible, i.e. end with full expenditure of starting materials and have great importance Equilibrium constants.

At the same time, no by-products should be formed as due to the interaction of the initial substances with environmental (for example, with oxygen or carbon dioxidecontained in the air) and as a result of several parallel reactions between them.

The reactions should accurately and quickly record the equivalence point.

2. The reaction between the substances of the working and the studied solutions should flow in a strict stoichiometric ratio corresponding to its chemical equation.

4. The reaction should proceed with sufficient speed, i.e. For a small time cut. The most optimal in this case is the time required for mixing one added type of titrant with the volume of the titratable solution, i.e. 1-3 seconds.

If the reaction is carried out slowly, it is difficult to accurately determine the onset of equivalence point. At the same time, the main advantage of titrimimetry is also lost - the speed of analysis and obtaining the result is rapidly.

If the chemical reaction does not satisfy at least one of the above requirements, it cannot be used in tutrimetric analysis. But if it cannot be replaced, then such a reaction is trying to "adapt" for use in tutrimetry. For example, many oxidative-reducing reactions under normal conditions are slow, are reversible and multi-directional, i.e. The starting materials in them are spent simultaneously in several directions. To eliminate these disadvantages, the conditions for conducting the reaction are changed. For example, it is carried out when heated or in the presence of catalysts (this makes it possible to significantly increase the reaction rate), as well as in a certain environment: sour, neutral or alkaline (it allows to eliminate reversibility and multi-directional).

It should, however, emphasize that such a device leads to the complication of the method. Application it is advisable to go only when a more convenient chemical reaction is unknown.

There are three main ways to titration: direct, reverse, indirect or deputy.

With direct titration Use the studied and one working solutions. In the process of determining a certain precisely measured volume of one of them, a second solution is added dropwise before the equivalence point occurs.

The law of equivalents in this case can be mathematically recorded as follows:

N 1 V 1 \u003d N 2 V 2

where V 1 and V 2 are the volumes of spent studied and working solutions, respectively; N 1 and N 2 - molar concentrations of chemical equivalents of the substances of the studied and working solutions, respectively.

The molar concentration of the chemical equivalent of the substance in the resulting solution is calculated by the formula:

With opposite titration Use the studied and two working solutions, one of which is auxiliary, and the second is used for titration.

In the process of analysis, to a certain precisely measured volume of the under study, a fixed amount of auxiliary working solution is added to the excessive volume of the auxiliary working solution. As a result of leakage chemical reaction The substance present in the resulting solution is fully spent. Not reacted by an excess of the auxiliary solution is then titrated by the second impeller before the equivalence point occurs, for example:

K 2 SO 3 + I 2 + H 2 O → K 2 SO 4 + 2Hi

The studied auxiliary

solution worm

I 2 + 2NA 2 S 2 O 3 → 2NAI + Na 2 S 4 O 6

Second worker

Thus, the substance present in the auxiliary working solution reacts both with the substance of the solution under study and with the substance of the second working solution. The law of equivalents in this case mathematically can be recorded as follows:

N 2 V 2 \u003d N 1 V 1 + N 3 V 3

where V 1, V 2, V 3 is the consumed volumes of the studied, auxiliary and second working solutions, respectively; N 1, N 2, N 3 is the molar concentrations of chemical equivalents of substances in the studied, auxiliary and in the second working solutions, respectively.

The molar concentration of the chemical equivalent is calculated by the formula:

Inverse titration In analytical practice, it may be called otherwise titration by the residue or with two titons.

It is used if the detectable substance reacts or react slowly with the substance of the second working solution, or in the reaction between them it is impossible to determine the equivalence point.

With indirect, or substituent, titration Also use the studied solution and two working solutions. During the analysis, a non-fixed deliberate excess of the first working solution is added to the precisely measured volume of the under study. As a result of the underlying reaction, the substance of the under study is fully consuming to form an equivalent amount of the corresponding reaction product, which is then titrated by the second working solution until the equivalence point occurs, for example:

K 2 Cr 2 O 7 + 6ki + 7h 2 SO 4 \u003d Cr 2 (SO 4) 3 + 3i 2 + 4k 2 SO 4 + 7H 2 O

the first worker is equivalent

solution solution Number of reaction product

2NA 2 S 2 O 3 + i 2 \u003d 2NAI + Na 2 S 4 O 6

second worker

Thus, we seem to replace the determined substance to another, which is subsequently exposed to the analysis.

Since the amount of the equivalent substance of the formed product and the amount of the equivalent substance in the resulting solution are equal to each other, the molar concentration of the chemical equivalent of the substance in the solution under study is calculated by the same formula as with direct titration.

Substitutional titration is used when the direct determination of the substance in the solution under study is impossible: there is no appropriate titrant, it is impossible to set the equivalence point, etc.

There are several ways to quantify acid or alkali in various substances. The most accessible under school is a titration method, which is usually carried out using normal solutions of acids or alkalis.

We will analyze an example of a quantitative determination of acid in vinegar using titration with a normal solution of caustic soda.

First prepare 100 ml of 1 N solution of NAON, for this you hang 4 g of clean for analysis or chemically pure caustic soda, place it in a measuring flask with a capacity of 100 ml and the result of distilled water up to the tag on the neck of the flask. Shake the flask with a solution several times. Well mixed solution Pour the burette to the burest almost toast, then descend it so that the concave part of the meniscus is on the point of zero division.

In the cup pour 20 ml of the vinegar test and add 5-7 drops of 1 percent solution in phenolphthalein alcohol. Stack a cup under the burest and drop the alkali solution, watching the color solution all the time. If the liquid in the cup when the solution of the caustic soda is infused and the painting disappears quickly, then continue adding alkali drops. When the pink color becomes stable and does not disappear for 1-2 minutes, the titration stop and calculate how many milliliters of the title fluid went to the neutralization of the fluid test.

Suppose you have spent 2.5 ml of a solution of caustic soda. Calculate quantity acetic acid In vinegar: 1 ml of 1 N, the solution of caustic soda contains 40 g: 1000 \u003d 0.04 g, and 2.5 ml - 0.1 g. What percentage of acetic acid is contained in the fluid test?

40 g Naone corresponds to 60 g sn 3 coxy

0.1 g - x

Such an amount of acetic acid is in 20 ml of vinegar, and 100 ml is five times more, that is, 0.75 g. Thus, the concentration of acetic acid is 0.75%.

Take another example: the determination of milk acidity.

In milk there is a lactic acid (CH 3 SNONSON). 1 l of a normal acid solution contains 90 g of anhydrous lactic acid. Take 100 ml of milk and Titrate 0.1 N solution of NAON.

Suppose you spent 18 ml of the NAON decinorormal solution.

Determine the presence of lactic acid (C 3 H 6 0 3) in 100 ml of milk.

In 1 l 0.1 N, the solution of caustic soda contains 4 g of caustic soda, and in 18 ml of the consumed solution - 0.072.

Knowing that 40 g of caustic natra is neutralized by 90 g of lactic acid, we establish that 0.162 g of lactic acid is neutralized with 0.072 g.

Such an amount of lactic acid is contained in 100 ml of milk, and in a liter - 1.620, every 0.09 g of lactic acid correspond to one degree of Turner, therefore, the test milk contains 1.62: 0.09 \u003d 18 degrees of Turner. Normal milk contains from 16 to 18 degrees. Milk is allowed for sale, having acidity not higher than 21 degrees. Thus, the tested milk has normal acidity and quite suitable for use.

When titration, follow the following rules:

1) The burette before the use must be carefully soaked with the "help of a special rash and rinse with distilled water.

2) First you need to fill in the burette above the zero division, and then slowly lower the liquid to zero division to remove the air from the rubber tube or the air crane.

3) Filling the burette with a liquid through a funnel and so that the liquid flows through the walls of the burette.

4) The burette must be in a strictly vertical position.

5) When counting the sections, the eyes should be on the same line with a meniscus.

6) fluid pouring should be carried out at the same speed. After the crane is closed, it is necessary to wait one or two minutes so that the fluid in the burest makes a normal position, after which the divisions can be countdown.

8) After the titration, the liquid from the burette is poured, the burette is washed and rinsed with distilled water.

If the caustic sterry or caustic potassium is contaminated or covered with carbon dioxide sodium or carbon dioxide, then before analyzing them should be washed in distilled water. They come as follows: take a piece of caustic soda, somewhat larger by weight than it is required for the preparation of a title solution (for example, for a decinorormal solution, not 4 g of caustic soda, and 5 g), and lowered it for some time to distilled

water. As soon as the upper layer is dissolved, the piece is removed and prepared by the decinorormal solution.

Weighing is best done in a purely washed and pre-weighed porcelain cup. Having found the total weight of the cups with caustic, subtract the weight of the cups and determine the presence of alkali.

Having read the titration questions, you can produce quantitative definition of acids and alkalis in the subjects of the fluids: in soil solution, milk, molasses, in various juices, water, etc.

When analyzing alkalis in the burest, pour the normal solution of acids and use an aqueous solution of methyl orange or methyl red, as an indicator.

When titrating a liquid containing a large percentage of acid or alkalis, use one-, two- or three-corneal solutions for liquids with a small concentration of acid or alkali. Use deco- and substigormal solutions.

As an exercise, solve a number of experimental tasks.

1. Take 20 ml of corrupt vinegar and determine the percentage of acetic acid in it. Titration Conduct 0.5 N with a solution of caustic potassium.
2. Determine the concentration of alkali eating solution (NAON, KOV, BA (OH) 2), available in the school laboratory. To do this, take 25 ml of solution and titrate with 1 N hydrochloric acid solution. As an indicator, use a 0.5% methyl orange solution. It should be pouring into the test liquid, not more than five drops. When changing the color not disappearing for 1-2 minutes, the titration stop and calculate.

After studying the techniques of surround analysis, you can conduct analytical work with organic and inorganic substances.

Acquired skills will help you further independently carry out analytical work in factory laboratories over various substances, as well as in the laboratories of agricultural institutions for the analysis of soil, fertilizers, food products etc.

The tyrimetric method of analysis is based on measuring the volume of the solution of a known concentration that has joined the reaction with the test substance. To analyze pona ...

From MasterWeb.

The titrimetric analysis method (titration) allows for a volumetric quantitative analysis and is widely used in chemistry. Its main advantage is a variety of ways and methods, so that it can be used to solve various analytical tasks.

Principle of Analysis

The titricimetric analysis method is based on measuring the volume of the solution of a known concentration (titrant), which has joined the reaction with the test substance.

For analysis, there will be special equipment, namely, the burette is a thin glass tube with applied graduation. The upper end of this tube is open, and on the bottom there is a locking crane. The fallated buree with the help of a funnel is filled with a titrant to zero mark. Analysis is carried out to the final point of titration (KTT), adding a small amount of solution from the burette to the studied substance. The final titration point is identified by changing the color of the indicator or any physico-chemical properties.

The final result is calculated according to the spent volume of the titrant and is expressed in the titer (T) - mass of the substance per 1 ml of solution (g / ml).

Justification of the process

The tyrimetric method of quantitative analysis gives accurate results, since substances react with each other in equivalent quantities. This means that the product of their volume and quantity is identical to each other: C1V1 \u003d C2V2. From this equation, it is easy to find an unknown value C2 if the remaining parameters are specified independently (C1, V2) and are installed during the analysis (V1).

Detection of the final titration point

Since timely fixation of the end of titration is the most important part of the analysis, you need to properly choose its ways. The use of colored or fluorescent indicators is considered to be the most convenient, but instrumental methods can also be used - potentiometry, amperometry, photometry.

The final choice of the method of detection of KTT depends on the desired accuracy and selectivity of the definition, as well as its speed and the possibility of automation. It is especially important for muddy and painted solutions, as well as aggressive media.

Titing Reaction Requirements

In order for the titrimetric method of analysis to give a correct result, it is necessary to correctly choose the reaction that will lie in it. The requirements for it are as follows:

• stoichiometricity;
• high speed flow;
• high equilibrium constant;
• the presence of a reliable method for fixing the experimental end of titration.

Suitable reactions can belong to any type.

Types of analysis

The classification of methods of titrimetric analysis is based on the type of reaction. This feature distinguishes the following titration methods:

• acid-basic;
• redox reductive;
• complexometric;
• precipitative.

At the heart of each species lies its type of reaction, specific titons are selected, depending on which subgroups of methods are distinguished in the analysis.

Acid-basic titration

Tutrimetric method of analysis using hydroxide reaction with hydroxide ion (H3O + + O. \u003d H2O) is called acid-main. If the known substance in the solution forms a proton, which is typical for acids, the method refers to a subgroup of acidimetry. Here, a stable hydrochloric acid HCl is usually used as titrant.

If the titrant forms hydroxide ion, the method is called alkalimetry. Used substances - alkali, for example, NaOH, or salts obtained by the interaction of a strong base with a weak acid, like Na2Co3.

Indicators are used colored. As they are weak organic compounds - acids and bases, which differ in the structure and painting of protonated and not protonated forms. Most often in acid-based titration, a single-color phenolphthalein indicator is used (a clear solution in an alkaline medium becomes raspberry) and a two-color methyl orange (the red substance becomes yellow in an acidic environment).

Their widespread use is associated with high light absorption, due to which their coloring is well noticeable with the naked eye, and the contrast and narrow color transition area.

Redox titration

Oxidative and reduction Tutrimeric analysis is a quantitative analysis method based on a change in the ratio of concentrations of oxidized and reduced forms: AOX1 + BRED2 \u003d ARED1 + BOX2.

The method is divisible to the following subgroups:

• permanganateometry (Trant - KMNO4);
• iodometry (I2);
• dichromatometry (K2Cr2O7);
• bromatometry (KBRO3);
• iodatometry (KIO3);
• cerimetry (CE (SO4) 2);
• vanadatometry (NH4VO3);
• titanometry (TiCl3);
• chromometry (CRCL2);
• ascorbineometry (C6N8In).

In some cases, the role of the indicator can play a reactant involved in the reaction and changing its painting with the acquisition of an oxidized or reduced form. But also use specific indicators, for example:

• when determining iodine, starch is used, which forms a dark blue connection with i3-ions;
• when titrating the trivalent iron, thiocionate ions that form complexes painted in bright red color are used.

In addition, there are special redox indicators - organic compounds having different color oxidized and reduced forms.

Comprehensive titration

If briefly, the titrimetric analysis method, called complexometric, is based on the interaction of two substances to form a complex: M + L \u003d ML. If mercury salts are used, for example, Hg (NO3) 2, the method is called mercurimetry, if ethylenediaminetetraacetic acid (EDTA) - complexonometry. In particular, with the help of the last method, a tyrimetric method of water analysis is carried out, namely, its hardness.

In composure meters, transparent metal indices are used, acquiring painting in the formation of complexes with metal ions. For example, when titrating the salts of trivalent iron, EDTA as an indicator uses transparent sulfosalicylic acid. It stains the solution in red when the complex is formed with iron.

However, more often, metal indices have their own color, which change depending on the concentration of metal ion. Material acids are used as such indicators, forming fairly stable complexes with metals, which at the same time are rapidly destroyed when exposed to EDTA with a contrast change of color.

Precipitating titration

The titrimetric analysis method, which is based on the reaction of the interaction of two substances to form a solid compound falling into a precipitate (M + x \u003d MX ↓) is precipitating. It has a limited value, since the precipitation processes are usually inconspicuously and nonstociometric. But sometimes it is still used and has two subgroups. If silver salts are used in the method, for example, AGNO3, it is called an argentometry, if mercury salts, Hg2 (NO3) 2, then mercurometry.

To detect the final titration point, the following ways are used:

• mOLA method, in which the indicator is a chromate ion that form a red-brick precipitate with silver;
• fOLGARDE METHOD, based on titration of silver ions with potassium thiocyanate in the presence of trivalent iron, forming a red complex in an acidic complex;
• faience method, providing for titration with adsorption indicators;
• the Gay-Loursak method, in which KTT is determined by the enlightenment or turbidity of the solution.

The last method has been practically not used lately.

Titing methods

Titing is classified not only by the underlying reaction, but also by the method of execution. On this basis, the following types are distinguished:

• direct;
• reverse;
• titration of deputy.

The first case is used only in the conditions of the ideal reaction flow. Titrant is added directly to the determinable substance. So with the help of EDTA, magnesium, calcium, copper, iron and about 25 more metals are determined. But in other cases more complex ways are used more often.

Inverse titration

The ideal reaction is possible to choose not always. Most often, it slowly proceeds, or it is difficult to choose a method for fixing the end point of titration, or volatile compounds are formed among the products, which is why the determined substance is partially lost. You can overcome these disadvantages using the inverse titration method. To do this, a large amount of titrant is adhered to the determined substance so that the reaction passes to the end, and then determine how much the solution remained unreacted. For this, the remnants of the titrant from the first reaction (T1) are titrated by another solution (T2), and its number is determined by the difference in the products of volumes and concentrations in two reactions: ST1VT1-CT2VT2.

The use of a tyrimetric analysis method in the inverse method is based on the determination of manganese dioxide. Its interaction with iron sulfate flows very slowly, so the salt is taken in excess and the reaction is accelerated by heating. The unreacted amount of iron ion is titrated by dichromate potassium.

Titration of the substituent

The titration of the substituent is used in the case of non-optometric or slow reactions. Its essence is that for the determined substance, a stoichiometric reaction with auxiliary compound is selected, after which the product of interaction is subject to titration.

This is exactly what is done when determining the dichromate. Potassium iodide is added to it, as a result of which the amount of iodine equivalent to the determined substance is released, which is then titrated sodium thiosulfate.

Thus, the titrimetric analysis allows you to determine the quantitative content of a wide circle of substances. Knowing their properties and features of the reaction flow, you can choose the optimal method and method of titration, which will give the result with a high degree of accuracy.

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TITRATION

TITRATIONThe method used in volume analysis to determine the concentration of the compound in the solution. Measurement of the amount required to complete the reaction with another compound. The solution of the known concentration is added in measured quantities from the burette (graded glass tube) into the measured volume of the liquid of an unknown concentration, until the reaction is completed (this will show the indicator or electrochemical device). The added volume allows you to calculate an unknown concentration.

Scientific and Technical Encyclopedic Dictionary.

Watch what is "titration" in other dictionaries:

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Books

• Analytical chemistry. Oxidation and recovery titration. Tutorial for SPO, Podkaytov A.L. Category: Miscellaneous Series: Vocational Publisher: Yrayt., Manufacturer: Yurait.,
• Analytical chemistry. Oxidation and recovery titration. Tutorial for universities, Podkaytov A.L. . Tutorial Lights the theory of the theory of oxidative-reducing titration methods, the relationship of the theoretical foundations of methods with their practical application. Much attention Daided ... Category: Miscellaneous Series: Russian Universities Publisher: Yrayt.Manufacturer:

1. direct titration. With direct titration, the titrant is directly added to the titratum substance. This method is applicable only when fulfilling all the requirements listed above.

2. inverse titration (With excess), used with a slowly occurring reaction. If the reaction rate is small, or it is not possible to select an indicator, or are observed side effects, for example, the loss of the determined substance due to volatility, you can use the receipt of the inverse titration: Add to the determined substance, a deliberate excess of titrant T 1, bring the reaction to the end, and then find the amount of unreacted titranage by the titration of its other reagent T 2 with a concentration from 2. It is obvious that the amount of titrant T 1 is spent on the determined substance, equal to the difference with T1 V T 1 - C T 2 V T 2.

Very important question is methods for expressing the concentration of the solution.

Molar Solutions - Mol / L

1M R-R - 1 liter is 1 g / mol of substances

Normal solutions (the solution must contain a given number of equivalent masses in a 1 liter).

Chemical equivalent is called the amount of substance equivalent to one g -Atoma hydrogen.

Titre -t. Working Titre

T \u003d M V-BA / 1000 g / ml T \u003d 49/1000 \u003d 0.049

The titer on the working substance must be transferred to the titer on the determined substance, using the factor of recalculation.

T onp \u003d t slave · f

Example: NaOH + HCl \u003d Na Cl + H 2 O F \u003d M NaOH / M HCl

The main equations in tiptimetric analysis

All calculations in the titrimetric method of analysis are based on the use of the law of equivalents: substances react among themselves in equivalent quantities.

N 1 ∙ V 1 \u003d n x ∙ v x,

where N 1 is the normality of titrant, V 1 is the amount of solution that was poured from a burette for a chemical reaction, N x V x is the characteristic of the desired substance

N x \u003d n 1 ∙ v 1 / v x,

ω \u003d (T ∙ V x / a) 100%

a - mood of the analyzed substance.

During the titration, the exact volume of the standard solution, which went to the titration of the determined substance. The calculation is based on the equality of the amounts of equivalents of a standard solution and the determined substance. The number of equivalents of the standard solution is calculated using different ways of expressing concentrations: a molar concentration, molar concentration of the equivalent, the titer of the working solution, the titer of the working solution on the determined substance.

Example: To determine the concentration of acetic acid, 20 ml of the analyzed solution was taken. The titration of this solution went15 ml of 0.1 M NaOH solution. Calculate the concentration of the analyzed solution of acetic acid.

The calculation of the concentration of acetic acid C (CH 3 of the Soam) in the analyzed solution is based on the equality of the number of equivalents of acetic acid contained in 20 ml of its solution, the amount of equivalents of sodium hydroxide in 15 ml of 0.1 m standard NaOH solution.

n (CH 3 COOH) \u003d N (NaOH).

The number of equivalents of sodium hydroxide is calculated as

n (NaOH) \u003d (C (NaOH) / 1000) V (NaOH).

Similarly, it is possible to submit the number of equivalents of acetic acid:

n (CH 3 COOH) \u003d (C (CH 3 COOH) / 1000) V (CH 3 COOH).

Hence the concentration of acetic acid is calculated by the equation:

c CH3COOH \u003d [(C NaOH V NaOH] / V CH3COOH \u003d (0.1 15) / 20 \u003d 0.075 mol / l.