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Pharmaceutical Analytical Chemistry (PHCM223-SS16) Lecture 5

ACID- BASE EQUILIBRIUM-V “pH indicators”

PHCM223,SS16 Lecture 5, Dr. Rasha Hanafi 1

Dr. Rasha Hanafi

LEARNING OUTCOMES

By the end of this session the student should be able to:

1. Define indicators.

2. Predict range of color change of an indicator.

3. Select an Indicator for a certain titration.

4. Compare between the selection conditions of indicators in strong versus weak acids/ bases titrations

5. Select indicators for titrations of polyprotic acids.

6. Compare between difficulty of end point detection in different titrations.

7. Determine the exact concentrations a mixture of bases using a double indicator method (Warder titration).

PHCM223,SS16 Lecture 5, Dr. Rasha Hanafi 2

PHCM223,SS16 Lecture 5, Dr. Rasha Hanafi 3

Indicators are organic compounds with an extensive conjugated system that causes the absorption of colored light waves.

anthocyanins

A conjugated system consists of a series of connected p-orbitals with delocalized electrons. This has the effect of increasing stability and reducing the energy gap between the HOMO (highest obtained molecular orbital) and the LUMO (lowest unoccupied molecular orbital). This moves the absorption from the UV to the visible region giving it color.

REMINDER • The equivalence point in an acid-base titration is defined by the stoichiometry and not by

the pH (i.e. 50 ml 0.1M will neutralize 50 ml 0.1 M HCl, and will also neutralize 50 ml 0.1M CH3COOH).

• The equivalence point occurs when enough titrant has been added to react exactly with the acid or base being titrated.

PHCM223,SS16 Lecture 5, Dr. Rasha Hanafi 4

pH upon Weak acid vs. strong base Strong acid vs. strong base

0 mL added

[H+] = √(Ka [weak acid]) [H+] = [acid]

Some mL added

Henderson Hasselbach [H+] = [remaining acid]

End point pH of salts of weak acid 7

xss titrant [OH-]

• The pH value at the equivalence point is affected by the acid strength. i.e. the weaker the acid, the greater the pH value at the equivalence point.

• In approaching calculation of pH during titration curve should think first about the major species in solution and decide whether a reaction occurs that runs to completion.

• Run stoichiometry and equilibrium calculations a to get the pH.

WHAT ARE ACID –BASE INDICATORS?

• Indicator is usually a weak organic acid or base that has distinctly different colors in its non-ionized and ionized forms.

HIn(aq) H+(aq) + In-

(aq)

• 𝒌𝒊𝒏 =𝑯

+[𝑰𝒏

−]

[𝑯𝑰𝒏] 𝑯

+= 𝒌𝒊𝒏.

[𝑯𝑰𝒏]

[𝑰𝒏−] pH= pKin + log

[𝑰𝒏]

[𝑯𝑰𝒏−]

• If the indicator is in a sufficiently acidic medium, the equilibrium, according to Le Châtelier’s principle, shifts to the left and the predominant color of the indicator is that of the nonionized form (HIn).

• In a basic medium the equilibrium shifts to the right and the color of the solution will be due mainly to that of the conjugate base (In-).

• Not all indicators change color at the same pH, so the choice of indicator for a particular titration depends on the nature of the acid and base used in the titration (that is, whether they are strong or weak).

5 PHCM223,SS16 Lecture 5, Dr. Rasha Hanafi

Amount HIn Amount In- Color of indicator

10 1 of its acidic form

1 10 of its basic form

H+ OH-

1

10

[HIn]

][In

10

1

[HIn]

][In

6 PHCM223,SS16 Lecture 5, Dr. Rasha Hanafi

The end point of an indicator does not occur at a specific pH; rather, there is a range of pH within which the end point will occur.

A good indicator should have a pKin value that is close to the expected pH at the equivalence point.

For a typical acid-base indicator with dissociation constant Kin, the color changes over a range of pH values given by the pkin 1.

pH= pKin + log [𝑰𝒏]

[𝑯𝑰𝒏−]

CHOICE OF INDICATOR FOR A CERTAIN TITRATION The equivalence point also lies on the steep part of the titration curve, hence the end point of the indicator should also lie on this steep part.

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Strong base (analyte) versus strong acid Weak base (analyte) versus strong acid

Both phenolphthalein and methyl orange can be used to determine the end point fort the strong base titration. While for the weak acid only Methyl Red can be used.

For a strong acid–strong base titration, the choice of the indicator is easy due to the very large change in pH that occurs around the equivalence point.

In contrast, using the wrong indicator for a titration of a weak acid or a weak base can result in relatively large errors.

8 PHCM223,SS16 Lecture 5, Dr. Rasha Hanafi

CHOICE OF INDICATOR FOR A CERTAIN TITRATION

The [In−]/[HIn] ratio changes from 0.1 at a pH 1 unit below pKin to 10 at a pH 1 unit above pKin. Ex: pKin of PhPh is 9.5, the color change takes place in the range of 8.2 to 10. pKin of MR is 5, the color change takes place in the range of 4.95 to 6.

Most indicators change color over a pH range of about 2 pH units.

DOUBLE INDICATOR TITRATIONS

1. Methyl orange is used as indicator for titration of the strong acid (pH 7). 2. The weak acid is then determined using phenolphthalein as indicator (pH>7).

9 PHCM223,SS16 Lecture 5, Dr. Rasha Hanafi

Need of 2 indicators

Mixture of 2 monobasic acids

Ex: HCl + CH3COOH

HCl + H3BO3

Polyprotic acids

H3PO4

Condition: Difference in pKa of the acids >4

Ka(boric acid) = 5.5x10-10, Ka(acetic acid) = 1.8x10-5. Under this condition, H+ of HCl suppresses the ionization of the other weak acid, by common ion effect, so that the base neutralizes HCl first.

The pKin values of phenol red, bromophenol blue, and phenolphthalein are 7.4, 4.1, and 9.5, respectively. Which indicator is best suited for each acid–base titration?

• Titrating a solution of Ba(OH)2 with 0.10 M HCl

• Titrating a solution of trimethylamine with 0.15 M HNO3

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Test yourself

phenol red

bromophenol blue

Diprotic acids • For a diprotic acid, K2 is always smaller than K1. The acid can be titrated

stepwise with an alkali provided that the ratio of K1/K2 >104, so that well separated breaks in pH occur as titration progresses from one step to the other.

• If K1/K2 is less than 104, the inflections in the pH titration curves are smeared out and only one sharp break is observed corresponding to the neutralization of both hydrogen ions.

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0.1M solution of maleic acid titrated with 0.1M solution of strong base. pKa1=1.83, pKa2=6.07.

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At equal acid and conjugate base concentrations, pH=pKa.

There are three such points for phosphoric acid. These points are important in the prediction of the titration curves. They correspond to points where half of an equivalent of proton has been consumed by addition of strong base.

Thus, the point where pH=pKa1 is halfway to the first equivalence point. Where pH=pKa2 is halfway between the first and second equivalence points, etc.

The solution has maximum buffer capacity at these points.

TRIBASIC ACID: PHOSPHORIC ACID

The equivalence point is halfway between the two points with maximum buffer capacity

TRIBASIC ACID: PHOSPHORIC ACID

13

1. At the first equivalence point, K1/K2 = 1.2x105 stepwise titration is possible. pH= 1/2(pK1+pK2)=4.77. Methyl orange or bromocresol green are the most appropriate.

PHCM223,SS16 Lecture 5, Dr. Rasha Hanafi

2. At the 2nd equivalence point, pH= 1/2(pK2+pK3) = 9.7. The only suitable indicator is thymolphthalein (pKin = 10.0) . With Phenolphthalein (pKin = 9.2) the error would be high.

3. At the 3rd equivalence point, the 3rd hydrogen, being very weakly dissociated (pK3 = 12.3), no indicator can serve to show this point, that even after adding large excess of titrant, the acid remains mainly in the form of HPO4

2-.

For the second stage of dissociation, K2/K3 = 1.2x105 , so that separate titration of the 2nd hydrogen is possible. However, the 2nd end point is less sharp than the first one since the solution is well buffered by the HPO4

2- present.

WARDER TITRATION

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Soln. of 0.1M NaOH and 0.1M Na2CO3 titrated with 0.1M solution of strong acid. Carbonic acid dissociation constants: pKa1=6.37, pKa2=10.25.

NaOH- both solid and dissolved - easily reacts with atmospheric CO2 , it is usually contaminated with Na2CO3. It is not a problem to determine sum of hydroxide and carbonates concentration by titration with a strong acid. What are the main species in a solution of NaOH and Na2CO3 to be titrated with HCl? NaOH + HCl -----> NaCl + H2O ....................... (1) Sodium carbonate reacts with hydrochloric acid in a similar way but the reaction comes in 2 steps: Na2CO3 + HCl ----------->NaHCO3 + NaCl............. (2) NaHCO3 + HCl ----------->NaCl + H2O + CO2.......... (3) The pH for equivalence of reactions (1) and (2) at a pH value higher than 7. Reaction (3) takes place at a pH value lower than 7. So, using the 2 acid-base indicators, the end points for the reactions can be found. Phenolphthalein turns from pink to colorless as the pH value changes from 10 to 8. Methyl orange turns from yellow to orange when the pH value drops from about 4 to 3

• During titration of the mixture we first add phenolphthalein to the solution and titrate it till pink color disappears, noting the first end point volume (let's call it V1). Then we add methyl orange and titrate solution to the intermediate color of the indicator. Let's call the second end point volume V2.

• V1 is the volume of titrant that was necessary for completion of two reactions:

NaOH + HCl → NaCl + H2O

Na2CO3 + HCl → NaCl + NaHCO3

• V2-V1 is the hydrogen carbonate protonation:

NaHCO3 + HCl → NaCl + H2O + CO2

PHCM223,SS16 Lecture 5, Dr. Rasha Hanafi 15

WARDER TITRATION

REFERENCES

1. Chemistry, 10th ed., Raymond Chang, ISBN 978-0-07-017264-7, McGraw Hill. Chapter 14 and 16

2. Lecture 5 - PHCM223, Prof. Rasha Elnashar, GUC, SS15.

3. Home-made acid/ base indicator (https://www.youtube.com/watch?v=4phd0XRMKHU)

4. https://mylespower.co.uk/2012/04/06/homemade-ph-indicator/

5. http://www.foothill.edu/psme/larson/resources/1C-Lecture-and-Lab/Chapter-17-Part-3-Titration-Curves

6. https://saylordotorg.github.io/text_general-chemistry-principles-patterns-and-applications-v1.0/s20-05-acid-base-titrations.html

7. http://www.titrations.info/acid-base-titration-polyprotics-and-mixtures

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