Kharkiv National Medical University Medical and bioorganic chemistry department

1. Kharkiv National Medical UniversityMedical and bioorganic chemistry department Lecture 4 Fundamentals of titrimetric analysis

2. Outline 1. History of development of titrimetric analysis 2. The main concepts of titrimetric analysis 3. Equipment for titrimetric analysis 4. Requirements that apply to the standard substance 5. Requirements that apply to the reaction in titration 6. Classification of titrimetric methods 7. Types of titrimetric determinations 8. Examples of calculations in titrimetric analysis

3. Actuality of the theme: Titrimetric (volumetric) analysis is one of the methods  of quantitative analysis in analytical chemistry which is widely used in medical-biological and sanitary-hygienic investigations for analysis of biological liquids, drinking and tap water and other objects. • This method is universal, it has sufficiently high accuracy, relatively simple and needn’t difficult measuring equipment.

4. Methods of quantitative analysis Chemical Physical Spectral analysis Luminescent analysis X-ray structural analysis NMR Gravimetric analysis Titrimetric analysis Gaseous analysis Physico-chemical (Based on measuring of optical, magnetic, thermic, electric properties) Electrochemical analysis(potentiometry) Optical (colorimetry, polarimetry) Chromatographic (thin layer, gas, liquid, ion-exchange chromatography)

5. History of development of titrimetry Mikhail Lomonosov is originator of quantitative analysis François Descroizilinvented burettes and pipettes, used volumetric flasks Robert Boyle studied neutralization of acids with bases in the presence of indicators K.F. Mohr summarize data and published (in 1853) first guidance on titrimetric methods. J. L. Gay-Lussac . known as an author of “The Law of Combining Volumes.”

6. Titrimetric analysis is based on the measurement of the volume of a solution with a precisely known concentrationof a reagent(the titrant) required for reaction with a given amount of a substance being determined (the analyte). Theoretical base of the titrimetric analysis is the law of equivalents. C(E)1 V1 = C(E)2V2 • C(E) – molar concentration of equivalent, mol/L C(E) = m/M(E)×V Substances react with each other in stoichiometric, or chemically equivalent proportions.

7. The main concepts of the volumetric analysis The operating solution(or titrant) – is asolution of accurately known molar concentration of equivalent(or normality). Titrant with known molar concentration of equivalent is called standard solution. Standard solutionis prepared by the dissolution of accurate quantity of pure chemical substance of known stoichiometric composition in definite volume of solvent. Standardized solution (solution with determined titer)is prepared with approximate concentration, close to expected, using not standard substanceandits concentration is determined (standardized) by suitablestandard substance.

8. The investigated solution (analyte) is a solution the molar concentration of equivalent and the titer of which should be determined. Titrationis  the process of determining unknown concentration by adding the small increments of standard solution untill the reaction is just complete. The equivalence point is a point in a titration at which the amount of titrant added is chemically equivalent to the amount of substance titrated i. e. The point at which the completion of the reaction occurs. End point - the point in a titration where we stop adding titrant. Indicator is a chemical substance, which indicates endpoint in the titration by changing its colour.

9. Equipment for titrimetric analysis Erlenmeyer flask or conical flask BALANCE Technical Analytical

10. Volumetric flasks Volumetric flask is a piece of laboratory glassware, a type of laboratory flask, calibrated to contain a precise volume at a particular temperature. Volumetric flasks are used for precise dilutions and preparation of standard solutions. .

11. Equipment for titrimetric analysis Burettes Pipettes Graduated cylinders

12. Three important precautions are needed when working with pipettes and volumetric flasks. • First,the volume delivered by a pipette or contained by a volumetric flask assumes that the glassware is clean. • Second,when filling a pipette or volumetric flask, set the liquid’s level exactly at the calibration mark. • Third, the liquid’s top surface is curved into a meniscus, the bottom of which should be exactly even with the glassware’s calibration mark.

13. Burette filling instruction • Always use a small funnel to fill a burette • To fill a burette, close the stopcock at the bottom. You may need to lift up the funnel slightly, to allow the solution to flow in freely • Fill the burette past the zero mark • Check the tip of the burette for an air bubble. To remove an air bubble you must lift up tip of burette and then open stopcock. If an air bubble is present during a titration, volume reading may be in error! • Take the funnel out of the burette so that drops of solution from the funnel will not fall into the burette.

14. When you burette is filled, with no air bubbles, you must level of the liquid to exactly the zero mark. Read the bottom of the meniscus. Be sure your eye is at the level of meniscus, not above or below

15. Typical instrumentation for performing an automatic titration.

16. A standard substance should correspond the following requirements: • The substance should be chemically pure, it should be easily obtained in pure form. It should not be hygroscopic, oxidized by air or affected by CO2. Its composition is to be well known and to meet its chemical formula. 2) The substance should be stable during the storage being in the dry form as well as in the form of solution. 3) A substance should have a high molecular weightso that weighing errors are minimum or negligible. 4) The substance should be readily solved in water. 5) The substance should react with the investigated solution according to the strict chemical ratio. The reactionrate should be quite high.

17. Few reagents meet all these ideal perfectly, for example: • H2C2O4×2H2O, Na2CO3or Na2S2O3, Na2B4O7×10H2O or EDTA. • The following substance can not be taken to prepare standard solution. • Acid : HCl, HNO3, H2SO4etc. • Base : NaOH because it absorbs the air moisture and CO2. • Salt: KMnO4because it decomposed in air & light. • FeSO4because it oxidized to ferric sulphate from air.

18. Requirements which must be fulfilled for the reaction in titration: • 1) The reaction should be very fast and practically irreversible. • 2) The reaction must be simple and well defined i.e., stoichiometric. • 3) There should be no complicating side reactions. • 4) Reaction should proceed quantitatively, i.e. reaction constant should be high. • 5) There should be the method of the equivalence point determination for the reaction.

19. Classification of titirimetric (volumetric) methods

20. Acidimetry and Alkalimetry • Alkalimetry is a method of determination of strong and weak acids, acid salts and organic compounds with acidic properties. As operating solution can be taken 0,1 МNaOH; KOH and as standard substances for standardization - Oxalic acidН2С2O4.2H2O; Succinic acid Н2C4Н4O4. Acidimetry is a method of determination of strong and weak bases, basic salts and organic compounds with basic properties. As operating solution can be taken0,1 МHCl; H2SO4 and as standard substances for standardization Na2B4O7 . 10H2O; Na2CO3 As indicator methyl orange and phenolphtalein are used in these methods of acid-base titration.

21. Indicators in acid-base titration • Indicators used in acid-base titration are usually weak organic acids or bases, which change colour with pH. Both two forms of the indicator (acidHIndand conjugate baseInd-) differ in color. The color of the solution depends on these forms’ concentration ratio. Кind. = [H+][Ind-]/[HInd] pH = pK + lg[Ind-]/[HInd]

22. To the indicator is specified the following requirements: 1) Indicator should have high absorbance to see its coloration with eyes even at the small amount of indicator. 2) Changing of coloration must be contrast. 3) Range of changing color must be as narrow at it’s possible. 4) A small amount of an indicator is needed. If a large amount of indicator is used, the indicator will effect the final pH, lowering the accuracy of the experiment. 5) The indicator should also have a pKa value near the pH of the titration's end point. 

23. pH ranges for the most widely used indicators

25. HCl + NaOH NaCl + H2O Strong Acid-Base Titration Curve Atitration curve is a plot of pH vs. the amount of titrant added.

26. Weak Acid-Strong Base Titration Curve C3H8COOH + NaOH ↔ C3H8COONa + H2O

27. Weak Base-Strong Acid Titration Curve NH4OH + HCl ↔ NH4Cl + H2O

28. Permanganatometry • Potassium permanganate is a very strong oxidizing agent and is employed in the estimation of reducing agents like ferrous salts, oxalic acid, arsenious oxide, etc. • Titrations involving potassium permanganate are usually carried out in acidic medium. • MnO4- + 8H+ + 5e- = Mn2+ + 4H2O purple colorless - Red-Ox titration method used for reducing analytes determination • Operating solution – Potassium permanganate KMnO4 • Indicator is not needed Permanganate acts as self indicator.

29. Iodimetry - Red-Ox titration method used for reducing as well as oxidizing agents determination - Reactions for the method: I2 + 2e  2I- 2I- - 2e  I2 Starch can be used as indicator; It can be used in laboratory-clinical analysis to determine content of glucose in blood, ascorbic acid, peroxidase and in hygienic practice to determineactive chlorine in drinking and utility water; in pharmacy and forensic investigations - content of superpotent and poisonous drugs: formalin, analgene, etc

30. Complexometry Complexometric titration (sometimes chelatometry) is a form of volumetric analysis in which the formation of a colored complex is used to indicate the end point of a titration. Complexometric titrations are particularly useful for the determination of a mixture of different metal ions in solution. An indicator capable of producing an unambiguous color change is usually used to detect the end-point of the titration. Common indicators are organicdyes such asFast Sulphon Black, Eriochrome Black T, Eriochrome Red B, Patton Reeder, orMurexide. Operating solutions are:Hg(NO3)2,EDTA, B Trilon

31. EDTA is a versatile chelating agent. A chelating agent is a substance whose molecules can form several bonds to a single metal ion. Chelating agents are polydentate ligands. A ligand is a substance that binds with a metal ion to form a complex ion. Polydentate ligands are many clawed, holding onto the metal ion to form a very stable complex. EDTA can form four or six bonds with a metal ion.

32. Precipitation Titration Precipitation titration is a volumetric titration method where the reaction between the titrant and sample solution yield precipitate (low solubility, usually ionic compounds) The most important precipitating reagent is silver nitrate. Titrimetric methods based upon silver nitrate are sometimes termedargentometric methods. It is the most common precipitation titrimetric method, because • silver precipitates are usually highly insoluble • many species form steichiometric precipitates with Ag+ (e.g. Cl-, Br-, I-, F-, CN-, SCN-, CrO42-, PO43- etc.) • these precipitates are formed quickly Titrant is a standardized AgNO3 solution. Argentometry is most often used for determination of chloride ions, but it can be used for other halides (bromide, iodine). There are 3 techniques of end point determination:method of Mohr (indicator: potassium chromate), method of Volgard (indicator: ferric salt), method of Fajans (indicator: fluorescein).

33. Types of titrimetric determinations Titration can be: • direct titration • back-titration (on residue) • substitute-titration (displacement titration) • revertive titration Direct titration – titrant is added to an analyte solution and it reacts with determined substance. А + Т = product

34. Requirements to reactions in direct titration • reaction involving the titrant and analyte must be of known stoichiometry, quantitatively • the titration reaction must occur rapidly • a suitable method must be available for determining the end point with an acceptable level of accuracy • Reactions should proceed at room temperature • Titration should not be accompanied by collateral reactions which deform the results of the analysis • Reactions should be specific • a suitable indicator is available

35. Back titrationis a titration in which a reagent is added to a solution containing the analyte, and the excess reagent remaining after its reaction with the analyte is determined by a titration. This titration is used, when: • the titration reaction is too slow, • a suitable indicator is not available, • there is no useful direct titration reaction • the standard solution lacks of stability (fugitive) А + Тexcess = product1 + Тresidue Тresidue + Тpadding = product2

36. Displacement titration.A titration in which the analyte displaces a species, usually from a complex, and the amount of the displaced species is determined by a titration This titration is used, when: • the analytes are unstable substances • It is impossible to indicate the equivalent (end) point in direct reaction • Analyte doesn’t react with titrant • Reaction involving the titrant and analyte mustn’t be of known stoichiometry, quantitatively

37. А + Т1(paddingcompounds) = А1(substituent) А1(substituent) + Т = product • CrCl2 + FeCl3 = CrCl3 + FeCl2 • analyte substitute • 5FeCl2 + KMnO4 + HCl = 5FeCl3 + KCl + MnCl2 + 4H2O

38. Revertive titrationis aprocess when standard solution is titrated by solution of investigated substance. А(in burette) + Т(in flask) = product This titration is used, when: the titration reaction is too slow, a suitable indicator is not available, Side-effects are observed

39. Example: 8 ml of NaOH solution were used to titrate 10 ml of Н2С2О42Н2О with С(1/2Н2С2О42Н2О) = 0,05 mol/l. Calculate C (NaOH) and T (NaOH). Solution: According to the law of equivalents С(E)(Н2С2О4 . 2Н2О)×V(Н2С2О4 . 2Н2О)= CE(NaOH)×V(NaOH)then CE(NaOH) = =

40. Example: Problem. 30 ml of 0.10N NaOH neutralised 25.0 ml of hydrochloric acid. Determine the concentration of the acid. Solution. C1-normality of NaOH = 0,1 mol/l V1 - volume of NaOH = 30 ml V2 - volume of HCl = 25 ml C2 - normality of HCl - ? C1 ×V1 = C2 ×V2 → C2 = C1 ×V1/ V2 = 0,1 ×30/25 = 0,12N

41. Thank you for your attention!