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

2. You must know definition of some useful terms:
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. Titration setup: the titrant drops from the burette into the analyte solution in the flask. An indicator present then changes color permanently at the endpoint.

4. 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 titrant reacts 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;
precipitation titrations, in which the analyte and titrant react to form a precipitate..

5. Titrant - the reagent added to a solution containing the analyte and whose volume is the signal.
A reagent, called the titrant, of known concentration (a standard solution) and volume is used to react with a solution of the analyte, whose concentration is not known.
equivalence point - the point in a titration where stoichiometrically equivalent amounts of analyte and titrant react.

6. End point - the point in a titration where we stop adding titrant.
Indicator - a colored compound whose change in color signals the end point of a titration.
Titration error - the determinate error in a titration due to the difference between the end point and the equivalence point.
A primary standard is a standard that is accurate enough that it is not calibrated by or subordinate to other standards. A primary standard in chemistry is a reliable, readily quantified substance.

7. Secondary reagent - a reagent whose purity must be established relative to a primary reagent
A burette (also buret) is a vertical cylindrical piece of laboratory glassware with a volumetric graduation on its full length and a precision tap, or stopcock, on the bottom (or calibrated glass tube).
Even the thickness of the lines printed on the burette matters; the bottom of the meniscus of the liquid should be touching the top of the line you wish to measure from.

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

9. Burette
A burette is a device used in analytical chemistry for the dispensing of variable, measured amounts of a chemical solution. A volumetric burette delivers measured volumes of liquid. Piston burettes are similar to syringes, but with precision bore and plunger. Piston burettes may be manually operated or may be motorized.

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

11. chemical dropper
A pipette, pipet, pipettor or chemical dropper is a laboratory tool commonly used in chemistry, biology and medicine to transport a measured volume of liquid, often as a media dispenser.

12. Laboratory flasks
There are several types of laboratory flasks, all of which have different functions within the laboratory. Flasks, because of their use, can be divided into:
Reaction flasks
Multiple neck flasks
Schlenk flask
Distillation flasks
Reagent flasks
Volumetric flask

13. Volumetric flask
A volumetric flask (measuring flask or graduated 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.

14. Graduated cylinder
A graduated cylinder, measuring cylinder or mixing cylinder is a piece of laboratory equipment used to measure the volume of a liquid. Graduated cylinders are generally more accurate and precise than laboratory flasks and beakers

15. Funnels (laboratory)
Laboratory funnels are funnels that have been made for use in the chemical laboratory. There are many different kinds of funnels that have been adapted for these specialized applications. Filter funnels, thistle funnels (shaped like thistle flowers), and dropping funnels have stopcocks which allow the fluids to be added to a flask slowly. For solids, a powder funnel

16. 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.

17. 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.

18. 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.

19. 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

20. 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.

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

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

23. 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.

24. 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

25. 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 Acidimetric An acid-base titration in which an acid is titrated with a standard solution of an alkali (a base) is called Alkalimetric

26. 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

27. 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.

28. 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.

29. 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

30. oxidation -- loss of electron(s)
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)

31. 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

32. 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 → Inred colour A  colour B

33. 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

34. 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.

35. Complexometric titration
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

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

37. 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 scale ACID NEUTRAL BASE (ALKALINE)

38. Techniques of titrimetric analysis.
Washing up and drying ware
Preparation of standard solutions
Sample preparation
Titration:
- Measurement of volumes
- An indicator choice
Calculations

39. 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.

40. Methods to determine the end point
visual indicators:
Colour change: In some reactions, the solution changes colour without any added indicator. This is often seen in redox titrations, for instance, when the different oxidation states of the product and reactant produce different colours.
Precipitation: If the reaction forms a solid, then a precipitate will form during the titration. A classic example is the reaction between Ag+ and Cl- to form the very insoluble salt AgCl. This usually makes it difficult to determine the endpoint precisely. As a result, precipitation titrations often have to be done as "back" titrations (see below).
Physical and chemical methods with the subsequent analysis of curves of the titration showing changes which occur in the course of titration (change of physical and chemical parametres standard solutions)

41. Types of titrimetric determinations.
Titration can be:
direct titration
back-titration (on residue)
substitute-titration (displacement titration)
revertive titration

42. Requirements to reactions in direct titration
direct titration – titrant add to an analyte solution and react with determined substrance
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 by 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
А + Т = product

43. This titration is used, when: Тresidue + Тpadding = product2
Back titration. 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

44. This titration is used, when:
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 substance
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

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

47. Calculate concentration of primary standard

48. Establishment of secondary standard concentration
А) a measured volume of another primary standard solution
where:
CN2 and V2 are concentration and volume of secondary standard solution
CN1 and V1 are concentration and volume of primary standard solution

49. B) a weighed quantity of a primary standard
where:
CN and V are concentration and volume of secondary standard solution
m and Em are mass and equivalent weight of primary standard

50. 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 - ?

51. BUFFER SOLUTIONS
Buffers are solutions which can resist changes in pH by addition of acid or alkali.

52. Buffers are mainly of two types: (а) mixtures of weak acids with their salt with а strong base (b) mixtures of weak bases with their salt with а strong acid. А few examples are given below: Н2СО3 / NаНСО3 (Bicarbonate buffer;carbonic acid and sodium bicarbonate) СН3СООН / СН3СОО Na (Acetate buffer; acetic acid and sodium acetate) Na2HPO4/ NaH2PO4 (Phosphate buffer)

53. BASE - a substance that provides OH- ions in water
Acid–Base Concept
The Arrhenius theory
ACID - a substance that provides H+ ions in water
BASE - a substance that provides OH- ions in water

54. 2) The Brønsted-Lowry Theory
All Brønsted–Lowry bases have one or more lone pairs of electrons:

55. 3) The Lewis Acids and Base theory
LEWIS ACID An electron-pair acceptor
LEWIS BASE An electron-pair donor

56. Acids
There are many acids present in our everyday lives.
Lemon juice contains citric acid, and vinegar contains ethanoic acid.
Some strong acids are hydrochloric acid, sulphuric acid and nitric acid.
Some weak acids are ethanoic acid, citric acid and carbonic acid.

57. Alkalis Many everyday substances are alkalis. They feel soapy.
They are corrosive.
When the oxides of some metals dissolve in water they make an alkali solution.
Alkalis react with acids and neutralise them.

58. Alkalis
Alkalis are present in many cleaning substances in use in our homes.
Kitchen cleaners are alkaline because they contain ammonia or sodium hydroxide, which attack grease.
Calcium hydroxide and sodium hydroxide are strong alkalis.
The most recognisable and common weak alkali is ammonia.

60. An acid (НА) reacts with а base (Н2О) to form the conjugate base of the acid (А-) and the conjugate acid of the base (H3O+). Accordingly, the acetate ion (СН3СОО- ) is the conjugate base of acetic acid (СН3СООН), and the ammonium ion (NH4+ ) is the conjugate acid of ammonia (NН3). The acid-base reaction is frequntly abbreviated НА = Н+ + А- with the participation of H2O implied.

61. The relationship between the pH of а solution and the concentrations of an acid and its conjugate base is easily derived.
[НА]
[Н+]= K
[А-]
Taking the negative log of each term
рН = - log К + log
рН = pК + log
This relationship known as the Henderson-Hasselbalch equation.

62. Factors Affecting pH of а Buffer
The pH of а buffer solution is determined by two factors:
1. The value of pK: The lower the value of pK, the lower is the pH of the solution.
2. The ratio of salt to acid concentrations: Actual concentrations of salt and acid in а buffer solution may be varied widely, with по change in рН, so long as the ratio of the concentrations remains the same.

63. Buffer Capacity
On the other hand, the buffer capacity is determined by the actual concentrations of salt and acid present, as well as by their ratio. Buffering capacity is the number of grams of strong acid or alkali which is necessary for а change in pH of one unit of one litre of buffer solution.
The buffering capacity of а buffer is, definеd аs the ability of the buffer to resist changes in pH when an acid or base is added.

64. Buffers Act
When hydrochloric acid is added to the acetate buffer, the salt reacts with the acid forming the weak acid, acetic acid and its salt. Similarly when а base is added, the acid reacts with it forming salt and water. Thus, changes in the pH are minimised.
СН3СООН + NaOH = СН3COONa + Н2О
СН3СООNа + HCI = СН3СООН + NaCI
The buffer capacity is determined by the absolute concentration of the salt and acid. But the рН of the buffer is dependent on the relative proportion of the salt and acid (see the Henderson - Hasselbalch's equation). When the ratio between salt and acid is 10:1, the pH will be one unit higher than the pKa. When the ratio between salt and acid is 1:10, the pH will be one unit lower than the pKa.

65. Mechanisms for Regulation of pH
1.   Buffers of body fluids,
2.   Respiratory system,
3. Renal excretion.
These mechanisms are interrelated.
Acidic solutions have a high H+ concentration. Base solutions have a low H+ concentration. The pH scale is used to indicate the acidity or alkalinity of a solution. Pure water with an equal number of hydrogen and hydroxide ions has a pH of 7.

66. Thank you for attention