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BUKHARA STATE MEDICAL INSTITUTE NAMED AFTER ABU ALI IBN SINA MEDICAL CHEMISTRY DEPARTMENT TOPIC: TITRIMETRIC ANALYSIS. NEUTRALIZATION METHOD. LABORATORY WORK. PLAN - WHAT IS THE TITRIMETRY MEHTOD
- TYPES OF TITREMETRIC ANALYSE
- ACID-BASE TITRATION
- OXIDYMETRY
- INDICATORS
The purpose of the less - general: to be able to analyze the principles of titrimetric research methods
- - Specific: analyze the quantitative content of substances in acid solutions and bases using acid-base titration methods.
Students should know - The law of equivalents; the basic concepts of volumetric analysis, the requirements for reactions used in volumetric analysis, the classification of volumetric analysis methods, the requirements that apply to the standard substance, the measuring utensils that are used in volumetric analysis, the titration technique and the determination of the equivalence point.
Students should be able to - determine molecular mass equivalent matters
- write the law of equivalent for any interacting compounds
- count mass of compounds
- prepare exact volume of working solution with the given molar concentration of equivalents
- determine mass concentration of compound.
PRACTICAL SKILLS - - determine the molecular mass of equivalents;
- - to write down the law of equivalents for any pair of interacting substances;
- - Calculate the mass of the substance to prepare a certain volume of solution with a given concentration of equivalent;
- - Calculate the molar equivalent concentration by mass of the substance in a certain volume;
- - calculate the molar concentration of the equivalent and the titre of the solution under study based on the results of titration;
- - Determine the purity of the drug, i.e. the mass fraction of the main substance in the initial mass or in the solution, based on the results of titration
Motivation characteristic of the topic - Volumetric analysis methods are widely used in biomedical and sanitary-hygienic research for the analysis of biological fluids, drinking and waste water, food products, etc. This is a topic that a future physician will need to address many scientific and practical issues
Classification of methods of titrimetric (volumetric) analysis
Neutralization methods
Methods of sedimentary titration
Alcalimetry
Acidymetry
Complexonometry
Use of complexons in medicine and during the preparation of medicines
Oxidymetry
Permanganatometry
Yodometry
Definition of CPC and BOD
Using the method during laboratory and clinical analyses
- Basic concepts of volumetric analysis
- 1) Working solution (titrant) - a solution for which its molar concentration of equivalent (C(e)) and titre (T) are known exactly.
- 2) Solution under study - solution with molar concentration of equivalent and titre to be determined.
- 3) Titration - a process in which the quantity of a substance under study is determined by the amount of standard reagent consumed.
- Titration is usually performed by adding a known concentration solution (titrant) until the reaction with the substance to be determined is complete, to determine C(e) and the titre of the latter.
Basic concepts of volumetric analysis - 4) Equivalence point - the moment of titration, when the number of molt-equivalents of the working solution becomes equal to the number of molt-equivalents of the studied solution; this moment is fixed by the change in color of the added indicator and the volume of titrant is noted.
- 5) Acid-main indicators.
- Indicators for acid-base titration are weak acids or bases. Indicators that have only one form painted are called monochrome, as opposed to two-colored, where both forms are painted in different colors.
or
.
Law of equivalents - The volumes of solutions of substances reacting among themselves are inversely proportional to their molar concentrations of equivalents.
For example, for the reaction - 2NaOH + H2C2O4 → 2H2O + Na2C2O4
- the law of equivalents is written:
V(NaOH) × Ce(NaOH) = V(H2C2O4)×C(1/2H2C2O4). - Example - titration of HCl solution, C(HCl) = 0.1 mol/l with NaOH solution, C(NaOH) = 0.1 mol/l.
-
Acid neutralized (%)
|
C (H+)
|
pH
|
0
|
10-1
|
1
|
90,0
|
10-2
|
2
|
99,0
|
10-3
|
3
|
99,9
|
10-4
|
4
|
100
|
10-7
|
7
|
Added excess NaOH
| | |
0,1
|
10-10
|
10
|
1,0
|
10-11
|
11
|
10,0
|
10-12
|
12
| Titration of weak acid with a strong base - An example is titration of acetic acid with NaOH solution.
- CH3COOH+NaOH → CH3COONa + H2O.
- In this case, the neutralization of weak acid by a strong base pH of the solution gradually increases. The titration jump for this case is small and less abrupt (about three pH units). As a result of titration, salt is formed, which is subjected to anionic hydrolysis. At the moment of equivalence the CH3COONa salt is obtained. This salt is hydrolysed:
- CH3COOO- +H2O= CH3COOH+OH-.
- Due to the hydrolysis of salt, OH- ions accumulate at the equivalence point, so the solution is alkaline.
Titration of a weak base with strong acid - An example is titration of ammonium hydroxide solution with hydrochloric acid: NH3-H2O + HCl NH4Cl + H2O
- At the moment of equivalence the salt of NH4Cl is obtained, which is subjected to cationic hydrolysis:
- NH4+ + H2O NH3-H2O + H+.
- The indicator can be litmus, methyl orange or methyl red, as they change color at pH less than 7, in acidic environment (see Fig. 3). As strong acid is added, the pH of the solution decreases. The titration jump is in the area with pH = 4.0-6.25. The equivalence point corresponds to pH = 5.13 and does not coincide with the neutralization point, because as a result of titration salt is formed, which is subjected to hydrolysis by cation.
Oxidation-recovery methods (oxidymetry). - Breathing and metabolism, rotting and fermentation, photosynthesis and nervous activity of living organisms are associated with redox reactions.
- Methods of redox titration are based on redox reactions. Currently, many methods of redox titration have been developed and classified according to the applied working solution (titrant). The greatest use has been made of the methods:
- a) Permanganatometry
- b) Iodometry.
- Solutions of oxidizing agents (e.g. potassium permanganate KMnO4) and reducing agents (e.g. iron sulfate (II), FeSO4), etc. are used as working solutions.
There are many known redox reactions, but only those reactions that are suitable for chemical analysis: - 1) Flow to the end (not reversible);
- 2) They are fast and stoichiometric;
- 3) Form products of a certain chemical composition;
- 4) Allows you to accurately fix the equivalence point;
- 5) Allow not to react with by-products present in the studied solution
Permanganatometry - Permanganatometry can be used in clinical analysis to determine uric acid, calcium, potassium, oxidative enzyme catalase in blood.
- In hygienic practice, oxidimetry is used in the analysis of water, which determines the chemical oxygen consumption (COD) and biochemical oxygen consumption (BOD).
- COD is a measure of the total pollution of water by organic and inorganic reducers reacting with a strong oxidizer. It is usually expressed in molars of oxygen equivalent used for the oxidation reaction of impurities with excess bihromate:
- Cr2O72- + 14H+ +6ē → 2Cr3+ + 7H2O
Potassium permanganate oxidation can be carried out in acidic, neutral and alkaline environments. - Potassium permanganate oxidation can be carried out in acidic, neutral and alkaline environments.
- In an acidic environment, MnO4- ions are reduced to colorless Mn2+ ions:
- MnO4- +5e + 8H+ → Mn2+ + 4H2O.
- In an acidic environment, MnO4---ions are reddish-violet-colored and reduced to almost colorless Mn2+ ions, which makes it possible to set the equivalence point (end of titration).
Iodometry - Iodometry is a volumetric analysis method based on reactions:
- I20 + 2ē → 2I-
- 2I- - 2ē → I20
- Both oxidizing agents and reducing agents can be determined by iodometry. In laboratory and clinical analyses, iodometry determines the content of blood sugar, peroxidase enzyme, ascorbic acid, in sanitary-hygienic practice - the content of active chlorine in chlorine lime, residual chlorine in drinking and domestic water; in pharmacy and forensic studies - the content of potent and poisonous drugs: formalin, analgine, corazol, etc
Complexonometry - Titrimetric methods based on the formation of stable intra-complex compounds between titrant-complexon and metal ions are used in complexometric analysis to determine metals.
- The method of complexonometry has a number of advantages:
- 1) High sensitivity (up to 10-3 mol/l);
- 2) Reactions are fast;
- 3) Highly selective, which has ensured wide application of the method in chemical analysis;
- 4) It's quick and easy.
As an example, let's consider the case of complexonometric determination of ions - Mg2+, which are titrated with a solution of trillon B (Na2H2L) in the presence of an ammonium buffer (pH = 8-10) and a metal chrome indicator - black T red chrome (eryo T or H3Ind) or murexide. Trilon B, forms strong soluble intra-complex salts. At the same time, the metal replaces hydrogen atoms of carboxylic groups of SNU and binds to nitrogen atoms by a coordination bond: - As an example, let's consider the case of complexonometric determination of ions - Mg2+, which are titrated with a solution of trillon B (Na2H2L) in the presence of an ammonium buffer (pH = 8-10) and a metal chrome indicator - black T red chrome (eryo T or H3Ind) or murexide. Trilon B, forms strong soluble intra-complex salts. At the same time, the metal replaces hydrogen atoms of carboxylic groups of SNU and binds to nitrogen atoms by a coordination bond:
- The equation shows that the equivalent of metal salt always equals half of the molecular mass, because two hydrogen ions are displaced during the reaction. Before the beginning of titration with the addition of black T eryochrome, a part of Mg2+ cations in the analyzed solution interacts with it, resulting in the formation of a chelate complex that stains the solution in pink-red:
- Mg2+ + H3Ind → MgNInd + 2H+
- During the titration process, the main reaction between Mg2+ and trilon B takes place:
- Mg2+ + Na2N2L → Na2Mg + 2H+
- Near the equivalence point, when all free Mg2+ ions are titrated, a substitution reaction occurs between the chelate complex
- MgNInd and trilon B:
- MgNInd + Na2H2L → H3Ind + Na2Mg
- At this point of titration, the color of the solution changes dramatically as the released indicator turns the solution blue. Therefore, in the presence of this indicator it is possible to determine with high accuracy the content of Mg2+ ions in the studied solution.
Working solutions (standard and standardized) - (a) A standard solution is a reagent solution with a known concentration. Usually prepared according to the exact attachment of the standard substance or using fixationals, which are ampoules, the content of which is enough to dilute the specified amount of water. The standard solution for titrimetric analysis should have the following properties.
- 1. Its concentration should not be changed during long-term storage (for several months and years) without restandardization.
- 2. to react quickly with the substance to be determined.
- 3. The reaction between the reagent and the substance to be detected must be sufficiently complete.
Requirements applicable to the standard substance - 1. The substance must be easily obtained in its pure form, its composition must be well known and exactly match its chemical formula.
- 2. The substance must be stable when stored both as a dry and as a solution.
- 3. The substance must have a sufficiently large molecular weight, as the weighing error is reduced during the preparation of the solution.
- 4. The substance should dissolve relatively easily in water.
- 5. The substance must react with the solution under investigation according to a certain chemical equation and with a relatively fast speed.
The essence of the technique. - The essence of the technique.
- The method of acid-base titration (method of neutralization) is one of the methods of volume (titrimetric) analysis. Since solid NaOH does not meet the requirements for standard (initial) substances (hygroscopic, has impurities of sodium carbonate, etc.), standardization of the ready working solution of sodium hydroxide by standard solution of oxalic acid is carried out. Standardization is the determination of the exact value of the molar concentration of the equivalent and titre of the solution. Titrations are conducted in the presence of phenolphthalein indicator. Titration is a reactionary process:
- 2NaOH + H2C2O4 → Na2C2O4 + H2O
LABORATORY WORK
Methodology of the experiment - 1. rinse the burette with the studied alkali solution and then fill it with the same solution, setting the lower meniscus to zero.
- 2. With a pipette we select 5 cm3 of standard solution of oxalic acid and transfer it into a flask for titration. Add two drops of indicator.
- Titrate with NaOH solution, pouring it out of the burette and constantly mixing the contents of the flask. Titration leads until the appearance of weak pink coloring.
RECOMMENDED LITERATURE - I. The main one:
- 1. Z. Országhová, I. Žitňanová, Textbook of medical chemistry, Comenius University Bratislava” 2018.
- 2. D. Shriver, M. Weller, T. Overton, J.Rourke, F.Armstrong. Inorganic chemistry 6th edition. “W.H. Freeman and Company”.2014
- 3. N.A. Tyukavkina, Y.I.Baukov. A guide to laboratory exercises in bioorganic chemistry. "Medicine" M., 1985.
- II. Extra:
- 4. Baynes, J., Dominiczak, M.H Biochemistry. 4th Ed., Saunders 2015, 658 pp..
- 5. A.Ryles va b. Basics of organic chemistry. "Enlightenment, M., 1983.
- 6. Z.B. Hauptman va b. Organic chemistry. "Enlightenment, M., 1983.
- 7. J. Roberts, M. Casserio. Basics of organic chemistry. "Peace", M., 1978.
- 8. J.Dostal. T.Essentials of Medical Chemistry and Biochemistry. “Brno”.2014
- 9.D. McMonale. B. Chemistry: An Illustrated Guide to Science. “Chelsea house”.2006
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