Neutralization curves in acid base analytical titrations, indicators.

Neutralization curves in acid base analytical titrations, indicators,
strong acid strong base
weak acid strong bse
strong acid weak base
weak acid and weak base

1. NEUTRALIZATION CURVES Indicators used in acid base titrations

2. INDICATORS Definition Indicators may be defined as organic substances which show well marked changes of colour in certain intervals of pH. To explain the change in colour of indicators, three theories have been proposed which briefly stated as follows. 1. The physico -chemical theory attributes the colour to certain ions an increase in which causes the appearance of a new colour , and a decrease in which causes the disappearance of a colour or the appearance of a different colour . 2.The organic theory attributes the colour of indicator to certain groupings of the elements in a compound, and the change in colour to a change in molecular structure. 3. The colloidal theory assumes that indicators form colloidal solutions, the change in colour of which is dependent upon change in size of the colloidal particle.

3. The following rules should be observed in the use of indicators: 1. Use 3 drops of indicator test solution for a titration unless otherwise directed. 2. When a strong acid is titrated with a strong alkali, or a strong alkali with a strong acid methyl orange, methyl red, or phenolphthalein may be used. 3. When a weak acid is titrated with a strong alkali, use phenolphthalein as the indicator. 4. When a weak alkali is titrated with a strong acid, use methyl red as the indicator. 5. A weak alkali should never be titrated with a weak acid, or vice versa, since no indicator will give a sharp end point. 6. The appearance of a colour is more easily observable than is the disappearance. Therefore, always titrate where possible to the appearance of a colour.

4. These acid-base indicators are complex organic compound and can be classified as: 1.Azo dyes Methyl orange, Methyl red, Congo red, Chyrsoidine 2.Phthaleins Phenolphthalein, Eosin", Fluorescein * 3.Sulphonphthaleins : Bromo cresol green, Bromo cresol purple, Bromophenol blue, Bromothymol blue, cresol red, phenol red, Thymol blue 4.Triphenyl methane dyes : Crystal violet, Rosaniline (magenta) (*Adsorption indicator used in precipitation titration

5. Commonly used some Acid base indicators 1. Methyl orange ( dimethyl amino azobenzene sulphonic acid) Methyl orange may be taken as an example of an azo -indicator. It is available either as the free acid or as the sodium salt. It is the sodium salt 4-dimethyl amino azobenzene 4-sulphonic acid. In alkaline solution, in which it is yellow, it is present as the sodium salt of a sulphonic acid and the yellow colour being due to the - N=N - group. On addition of acids, there is a change in structure to a red quinonoid form. Preparation of Indicator solution Methyl orange 0.1 gm is dissolved in 80 ml of water and sufficient ethanol 95% is added to produce 100 ml. The pH range of methyl orange indicator is 2.9 – 4.0 and the colour change is red to yellow.

6. 2. Methyl red It is dark red powder or violet crystals. Soluble in ethanol practically insoluble in water. Preparation of Indicator solution Dissolve 50 mg of methyl red in a mixture of 1.86 ml of 0.1 ml sodium hydroxide and 50 ml of ethanol 95%. After solution is effected, add sufficient water to produce 100 ml. pH change 4.2 - 6.3 Colour change Red to yellow Methyl red indicator is generally used in the assay of weak bases, e.g. ammonia and amines. This useful indicator replaces methyl orange in the titration of ammonia and other weak bases, with which it gives a better end point.

7. 3. Congo Red It is Diphenyl di azobis-1-naphthylamine sulphonic acid pH range 3.0 - 5.0 Colour change Blue to red

8. 4. Phenolphthalein It is one of the simplest substances of the phthaleins group is prepared by heating phthalic anhydride with phenol in the presence of conc. H2SO4 as a dehydrating agent. Phenolphthalein is the typical indicator, most commonly used in acid base titration. It is a white crystalline powder, almost insoluble in water, but readily soluble in alcohol. It is well known that, phenolphthalein which is colourless in acid or neutral solution, becomes red in slightly alkaline solution, and the red colour fades in strongly alkaline solution.

9. PREPARATION OF INDICATOR SOLUTION ( i ) Phenolphthalein solution Dissolve 1 gm of phenolphthalein in 10 ml ethanol 95% and add sufficient quantity of ethiano 95% to produce 100 ml. (ii) Phenolphthalein solution Dilute Dissolve 0.1 gm of phenolphthalein in 80 ml of ethanol 95% and add sufficient water to produce 100 ml.

10. UNIVERSAL INDICATORS (Multiple-Range Indicators) By mixing suitable indicators together changes in colour may be obtained over a considerable portion of the pH range. Such mixtures are usually called 'Universal indicators'. They are not suitable for quantitative titrations, but may be employed for the determination of the approximate pH of a solution by the colorimetric method. EXAMPLES 1. Methyl red - Thymol blue Universal Indicator Methyl red 0.125 gm Thymol blue 0.375 gm Ethyl alcohol 70 % upto 100 ml

11. 2. Kolthoff's Universal Indicator This Kolthoff's Universal Indicator is an alternative indicator to the above indicator, which is a mixture of several indicators. This universal indicator is prepared by dissolving the following indicator dyes in absolute ethanol and sodium hydroxide solution Composition Phenolphthalein 0.1 gm Methyl red 0.2 gm Methyl yellow 0.3 gm Bromothymol blue 0.4 gm Thymol blue 0.5 gm Absolute ethanol upto 500ml Sodium hydroxide solution qs

12. 3. Another recipe for a universal indicator has following composition: Methyl orange 0.05gm Methyl red 0.15gm Bromothymol blue 0.3gm Phenolphthalein 0.35gm Ethanol 66% upto 100 ml To use a Universal indicator, put 0.5 - 1 ml (10-20 drops) of the unknown solution in the depression of a tile for spot tests (or in a perception crucible) and add a drop at the indicator solution.

13. SCREENED INDICATOR The colour changes of a single indicator may also be improved by the addition of a pH sensitive dyestuff to produce the complement of one of the indicator colours . A screened indicator solution contains a suitable dye which does not change in colour in the pH range involved and is used purely to modify the acidic and alkaline colours of the indicator by light absorption with the object of giving a more easily distinguished endpoint at a definite pH.

14. EXAMPLES: 1. Screened methyl orange indicator This is a typical example for the screened indicator which is prepared by the addition of Xylene cyanol FF to methyl orange in the composition of Methyl orange 1.0 gm 2. Xylene cyanol Ff 1.4 gm 3. Ethyanol 50% V/V upto 500 ml Here the colour change from the alkaline to the acid side is Green → Grey + magenta. The middle (grey) stage being at pH = 3.8.

15. Screened Methyl red: (Methyl red with methylene blue) It has of course, the same range as unscreened methyl red, but its colour change is from red-violet (acid solution) to green (alkaline solution).

16. Theory of Indicator Acid-base Indicators

17. Theory of acid-base indicators: Two theories have been proposed to explain the change of colour of acid-base indicators with change in pH. Ostwald's theory Quinonoid theory

18. 1) Ostwald's theory According to this theory: The colour change is due to ionisation of the acid-base indicator. The unionised form has different colour than the ionised form. The ionisation of the indicator is largely affected in acids and bases as it is either a weak acid or a weak base. In case, the indicator is a weak acid, its ionisation is very much low in acids due to common H+ ions while it is fairly ionised in alkalies . Similarly if the indicator is a weak base, its ionisation is large in acids and low in alkalies due to common OH- ions.

19. Considering two important indicators phenolphthalein (a weak acid) and methyl orange (a weak base), Ostwald theory can be illustrated as follows: Phenolphthalein: It can be represented as HPh . It ionises in solution to a small extent as: HPh ↔ H + + Ph Colourles Pink

20. Applying law of mass action K= [H+][Ph- ]/[ HpH ] The undissociated molecules of phenolphthalein are colourless while Ph- ions are pink in colour . In presence of an acid the ionisation of HPh is practically negligible as the equilibrium shifts to left hand side due to high concentration of H+ ions. Thus, the solution would remain colourless . On addition of alkali, hydrogen ions are removed by OH- ions in the form of water molecules and the equilibrium shifts to right hand side. Thus, the concentration of Ph- ions increases in solution and they impart pink colour to the solution.

21. Colure change in solution

22. Methyl orange: It is a very weak base and can be represented as MeOH. It is ionized in solution to give Me+ and OH- ions. MeOH ↔ Me+ + OH- Yellow Red •In presence of an acid, OH- ions are removed in the form of water molecules and the above equilibrium shifts to right hand side. •Thus, sufficient Me+ ions are produced which impart red colour to the solution. On addition of alkali, the concentration of OH" ions increases in the solution and the equilibrium shifts to left hand side, i.e., the ionisation of MeOH is practically negligible. Thus, the solution acquires the colour of unionised methyl orange molecules, i.e., yellow.

23. 2) ​Quinonoid theory • According to quinonoid theory, an acid-base indicators exist in two tautomeric forms having different structures which are in equilibrium. • One form is termed benzenoid form and the other quinonoid form.

24. • The two forms have different colors. The color change is due to the interconversation of one tautomeric form into other. One form mainly exists in acidic medium and the other in alkaline medium. • Thus, during titration the medium changes from acidic to alkaline or vice-versa. The change in pH converts one tautomeric form into other and thus, the colour change occurs.

25. Phenolphthalein has benziod form in acidic medium and thus, it is colourless while it has quinonoid form in alkaline medium which has pink colour.

26. Methyl orange has quinonoid form in acidic solution and benzenoid form in alkaline solution. The color of benzenoid form is yellow while that of quinoniod form is red .

27. TYPES OF ACID BASE TITRATIONS Aqueous acid base titration It is the titration between acids and bases dissolved in water. Types of acid-base titrations: 1)strong acid – strong base titration 2)weak acid – strong base titration 3)strong acid – weak base titration 4) weak acid - weak base

28. strong acid – strong base titration Strong acid + strong base ---->salt + H 2 O Reaction occurs in stochiometric form Each molecule of acid reacts with the corresponding molecule At the end point no molecule of acid and base exist Hence endpoint is precise and sharp. Example: Acid: Hcl , H2SO4, HNO3, HBr , HclO4, H3PO4 Base: NaOH , MgOH2, Al2OH3

29. Example: Hcl + NaOH ---> NaCl + H2O Ph at the endpoint is neutral 7 So indicatorthat change colour in this pH is used Indicators: phenophthalein 8.2- 10 : bromocresol green 3.8-5.4

30. 2)Titration with weak acid by strong base 0.1 M acetic acid with 0.1 M NaOH Titration starts at high pH because acetic acid is weak acid pH changes slowly at first and rapidly increases at equivalence point ( rapid change occurs at pH 7-11) As pH range is shorter the choice of indicator is more critical

31. indicators Phenophthalein -- 8.2 - 10 Methylred --2.8 - 4.6 Methyl orange --4.2 -6.3 Bromocresolgreen -- 3.8 -5.4 CH 3 COOH + NaOH --> CH3COONa + H2O CH2COONa salt solution (basic). so equivalence point is present on the basic pH

32. 100 ml of 0.1 N CH3COOH solution is titrated with 0.1 N NaOH We know [H + ][CH 3 COO - ] -------------------- = Kacid CH 3 COOH From law of mass action it is constant. Initially CH 3 COOH ---> H + + CH 3 COO - [H + ][CH 3 COO - ] Kacid = -------------------- CH 3 COOH [CH 3 COO - ] = [H + ] [H + ] 2 Kacid = -------------------- CH 3 COOH

33. Rearrangement and taking negative log pH = 1/2 PKa - 1/2 log conc of acid PKa = - log of Ka = - log 1.86 x 10 -5 = 4.73 pH = 1/2 4.73 - 1/2 log 0.1 = 2.87

34. pH of intermediate points pH = PKa - log [acid]/[salt] Consider if 50% acetic acid is titrated with 50 ml of NaOH Then pH = PKa - log [50]/[50] = 4.73 Consider if 0.1 ml of acetic acid is remaining Then, pH = PKa - log [0.1]/[99.9] = 7.73 pH at equivalance point H20 + CH 3 COO - ---> CH 3 COOH + OH - Partially hydrolyses becoz acetic acid is weaker acid . strong base does not hydrolysis. Applying law of mass action, [CH 3 COOH][ OH - ] ------------------------ = K [H20] [CH 3 COO - ] pH = 7+ 1/2 PKa + 1/2 log conc of salt

35. 3)Titration of weak base with strong acid NH4OH + HCL ---> NH4Cl + H2O pH declines slowly at first and falls abruptly from pH 7 to 3. Indicators: methyl red, Methyl orange Bromophenol blue Bromocresolgreen Equivalence point: 5.3 pH Initial pH = 14 - 1/2 Pkbase + 1/2 log [base] Intermediate pH = 14 - Pkbase + log [base]/[salt] Equivalence point pH: 7-1/2 Pkbase - 1/2 log [salt]

36. 4)Titration of weak acid with weak base CH3COOH + NH4OH --> CH3COONH4 + H2O Change of pH is gradual No sharp end point So mixed indicator is used. Indicator: neutral red - methylene blue pH: 7+ 1/2 Pkacid -1/2 Pkbase = 7+ 2.37-2.38 = 6.99