LECTURE 2

1. LECTURE 2 Titration method ass. prof. I. R. Bekus

2. Titrimetric analysis- is a method of quantitative analysis used to determine unknown concentration of known substance. • You must know definition of some useful terms: • Titrant or Standard solution – a solution of accurately known concentration. • Titration – the process of determining unknown concentration by adding the small increments of standard solution until the reaction is just complete.

3. Overview of Titrimetry: Titrimetric methods are classified into four groups based on the type of reaction involved. These groups are acid–base titrations, in which an acidic or basic titrantreacts with an analyte that is a base or an acid; complexometric titrations involving a metal–ligand complexation reaction; redox titrations, where the titrant is an oxidizing or reducing agent; and precipitation titrations, in which the analyte and titrant react to form a precipitate..

4. Equipment for Measuring Volume Analytical chemists use a variety of glassware to measure volume: beaker; graduated cylinder;volumetric flask; pipette;dropping pipette.

5. (a) (b) (c) (d) Common types of pipettes and syringes: (a) transfer pipette; (b) measuring pipette;(c) digital pipette; (d) syringe.

6. Burette– kind of laboratory glass for exact measurement of volume of solution used. Burette is graduated and has a burette tap or stopcock at one extreme end to control the flow of titrant. Equivalence point. The point in a titration at which the amount of titrant added is chemically equivalent to the amount of substance titration. End point. The point at which the completion of a reaction is practically observed. When using an indicator, the end point occurs when enough titrant has been added to change the color of the indicator.

7. 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. • Sird, the liquid’s top surface is curved into a meniscus, the bottom of which should be exactly even with the glassware’s calibration mark.

8. 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.

9. 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

10. After filling burette, a known volume of the unknown concentration solution should be taken with the pipette and placed into the conical flask, along with a small amount of the indicator. Slowly release known solution from the burette into the conical flask, while swirling the mixture.

11. The solution should be let out of the burette until the indicator changes colour and value on the burette should be recorded.

12. Types of Titration • Neutralisation (Acid-Base) titration • Precipitation titration • Reduction-Oxidation (Redox) titration • Complexometric titration

13. Acid-Base Titration • As the second step in this investigation you are now going to compare two solutions (an acid and a base) using a method called "titration". • In the first procedure you are simply going to add an acid solution to a basic solution. Each solution will be of a different "strength", or concentration, or amount, and you will simply observe the relative results. • In the second procedure you are going carry out a number of titrations in which an acid solution is carefully added to a basic solution. In each case you have to find the "end point", which is the point at which you have added just enough of the acid solution to exactly neutralize all the base that was in the original solution. • The properties of the acid solution are standardized, and fully known. So, by finding the exact amount of acid that neutralizes a known solution of base, it is possible to carry out a calculation and find out the molecular weight of the base.

14. These titrations are based on the neutralization reaction that occurs between an acid and a base, when mixed in solution. A neutralization reaction in aqueous solution is a reaction of an acid and a hydroxide base to produce a salt and water

15. An acid-base titration is the determination of the concentration of an acid or base by exactly neutralizing the acid/base with an acid or base of known concentration. This allows for quantitative analysis of the concentration of a unknown acid or base solution.An acid-base titration in which a base is titrated with a standard solution of an acid is called AcidimetricAn acid-base titration in which an acid is titrated with a standard solution of an alkali (a base) is called Alkalimetric

16. Precipitation Titration Precipitation Titration it 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 termed argentometric methods. Argentometry, where the titrant is a standard AgNO3 solution 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. The titrant needs to be stored in a dark (brown) container. 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) The most often used Mohr method

17. Mohr method Mohr titration is used for determination of halide in a solution. Potassium chromate can serve as an indicator for the determination of chloride, and bromide ions by reacting with silver ion to form a brick-red silver chromate (Ag2CrO4) precipitate in the equivalence-point region.

18. Mohr titration has to be performed at a neutral or weak basic solution of pH 7-9 (or 6-10), because silver hydroxide forms at high pH, while the chromate forms H2CrO4 at low pH, reducing the concentration of chromate ions and delaying the formation of the precipitate.If Ag+ solution is add to a Cl- solution containing of small quantity of CrO4-, then AgCl will firstly precipitated, while Ag2CrO4 has not yet, and concentration Ag+ increases progressively until solubility product of the ions reach the value of Ksp Ag2CrO4 (2,0·10-12) to form brick-red precipitate.Before titration small amount of sodium or potassium chromate is added to the solution, making it’s slightly yellow colour. During titration, as long as chlorides are present, concentration of Ag+ is too low for silver chromate formation. Near equivalence point concentration of silver cations rapidly grows, allowing precipitation of brick-red silver chromate which signals end point.

19. Reduction-Oxidation (Redox) Titration A redox titration is based on an oxidation-reduction reaction between analyte and titrant. In this experiment you will use a standard solution of potassium permanganate (KMnO4) to determine the of iron (as Fe2+) in an unknown solution. Permanganate ion reduces to a manganese (II) ion in the acidic solution. This reaction requires 5 electrons and 8 hydrogen ions: MnO4-+ 8H+ + 5 e- = Mn2+ + 4H2O Only one electron is necessary to reduce Fe (III) to Fe (II) Fe3+ + e- = Fe2+ Therefore, 1 mole of MnO4-(the oxidizing agent) reacts with 5 moles of Fe2+ (the reducing agent) to form 5 moles of Fe3+ and 1 mole of Mn2+. Thus, in net ionic form: MnO4- + 5Fe2+ + 8H+ = 5Fe3+ + Mn2+ + 4H2O

20. Reactions in which electrons are transferred from one species to another are known as redox reactions, or oxidation-reduction reactions. 2 Na + Cl2 2 NaCl A redox reaction is made up of two reactions: reduction -- gain of electron(s) oxidation -- loss of electron(s)

21. Writing Redox Equations In a redox reaction, the number of electrons lost by the species being oxidized must balance the number of electrons gained by the species being reduced. In a balanced redox reaction equation: * the number of atoms of each element must be balanced * the total charge on the ions on the left hand side of the equation will equal the total charge on the ions on the right hand side of the equation

22. In the redox titrations, we need a chemical species that can change colour in the potential range corresponding to the sharp change at the end point. A chemical substance, which changes colour when the potential of the solution reaches a definite value, is termed as an oxidation-reduction or redox indicator.Inox + ne → Inredcolour A  colour B

23. 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. The permanganate ion, MnO4-, gets reduced to Mn2+ ion in acidic medium and to MnO2 in neutral and alkaline media. Titrations involving potassium permanganate are usually carried out in acidic medium. Since MnO4– is intense purple while Mn2+ is colour less, the reaction mixture at equivalence point is colour less and even a single drop of the permanganate would impart sufficient pink colour to the solution acting as self indicator. The reducing agent in the titration to be discussed is oxalic acid here. The composition of it is H2C2O4·2H2O. In spite of being a dehydrate it is a good primary standard as its composition is unchanged during storage or weighing. This redox reaction can be split apart in two parts- one showing the oxidation and the other reduction

24. This titration is carried out in warm conditions (temperature about 60 C). The reaction at room temperature is slow because of the equilibrium nature of this reaction. CO2 is highly soluble in water and thus heating removes all dissolved carbon dioxide out of the solution. While noting the burette readings, it should be taken into account that the solution is so intensely coloured that the lower meniscus of the solution may not be clear. Thus for permanganate titrations the upper meniscus in the burette is noted.

25. Erio - T indicator or Eriochrome Black-T indicator is used in this titration. 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 multi-dentate ligands. A ligand is a substance that binds with a metal ion to form a complex ion. Multidentate 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. Complexometric titration

26. The picture on the left shows the color of the indicator before titration.

27. This color change from wine red to violet to blue is due to the compact nature of the complex. The statement "the compact nature of the complex" means when the indicator is added to the hard water, the indicator Erio-T forms a complex with the Ca+2 ions that is pink in color. As EDTA is added to the solution, the EDTA forms a complex with the Ca+2 leaving the indicator Erio-T uncomplexed, which is blue in color.pH scaleACIDNEUTRAL BASE (ALKALINE)0-----------------------------7--------------------------------14

28. Acid - Base indicators Acid - Base indicators (also known as pH indicators) are substances which change colour with pH. They are usually weak acids or bases, which when dissolved in water dissociate slightly and form ions. The acid and its conjugate base have different colours. At low pH values the concentration of H3O+ is high and so the equilibrium position lies to the left. The equilibrium solution has the colour A. At high pH values, the concentration of H3O+ is low - the equilibrium position thus lies to the right and the equilibrium solution has colour B.

29. Most commonly used indicators in acid-base titration are:

31. N1V1=N2V2 From the total volume of known solution needed to react the end point, the concentration of the unknown solution can be calculated. • N1 – normality of solution with known concentration • V1 – volume of solution with known concentration • N2 – normality of solution with unknown concentration • V2 – volume of solution with unknown concentration Example: Problem. 30 ml of 0.10N NaOH neutralised 25.0 ml of hydrochloric acid. Determine the concentration of the acid. Solution. N1-normality of NaOH = 0,1 mol-equiv/l V1 - volume of NaOH = 30 ml V2 - volume of HCl = 25 ml N2 - normality of HCl - ?

32. Thank you for attention