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It should be ensured that the amount of the indicator to be used is to be as small as possible.

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(1)

What are the points to consider when selecting indicators?

Indicators should have a range of color turns at a pH that is too close to or near the equivalence point in titration.

The turning range of the indicator to be selected must include the vertical region of the titration curve, not the horizontal region. Thus, the color change becomes more pronounced in the rapid pH changes at the equivalence point and the color change can be observed with the smaller amount of titrating.

It should be ensured that the amount of the indicator to be used is to be as small as possible.

It is convenient to use monochrome indicators to see the most comfortable turning.

Attention should be paid to the temperature of the environment in which the titration occurs.

In the calculations, the basic is that the substances react in equal numbers of grams (in equals).

eq

A

(eş

A

) = eq

B

(eş

B

)

N x V = N x V

(2)

0 10 20 30 40 50 60 70 80 0

2 4 6 8 10 12 14

0.1 M HCl + 0.1 M NaOH titration Indicator: phenolphytalein

pH

NaOH Volume (mL)

(3)

0 10 20 30 40 50 60 70 80 0

2 4 6 8 10 12 14

0.1 M CH

3

COOH + 0.1 M NaOH titration Indicator: phenolphytalein

pH

(4)

0 20 40 60 80 100 120 0

2 4 6 8 10 12 14

(0.1 M CH

3

COOH + 0.1 M HCl) + 0.1 M NaOH titration Indicator:

For HCl : methyl orange

For CH

3

COOH : Phenolphytalein pH

NaOH Volume (mL)

(5)

Titrations of very weak acid

1) H

3

BO

3

: After adding glycerol or mannitol, boric acid is complexed with it and one proton is titrated.

2) Ammonium salts: After adding formaldehyde, 4 protons are titrated.

3) H

3

PO

4

: For titration of third proton, AgNO

3

is added and Ag

3

PO

4

is precipitated and third proton is titrated.

4) Citric acid: After complexing with Cu, one proton is titrated.

5) Aminoacids: After adding formaldehyde, base grub is bloked. After that, carboxylic acid group is titrated.

6) Titrations in anhydrous media: In these titrations, CH

3

COOH, methanol and DMSO are

(6)

0 10 20 30 40 50 60 70 80 90 100 0

2 4 6 8 10 12 14

0.1 M H

3

PO

4

+ 0.1 M NaOH titration Indicator:

For first equivalent point (NaH

2

PO

4

is formed) : bromocresol green For second equivalent point (Na

2

HPO

4

is formed) : Phenolphytalein

For step by step titration of a polyprotic acid:

K x C  10

–8

K

1

/ K

2

 10

4

pH

NaOH Volume (mL)

(7)

0 20 40 60 80 100 120 140 0

2 4 6 8 10 12 14

0.1 M Na

2

CO

3

+ 0.1 M HCl titration Indicator:

For first equivalent point (NaHCO

3

is formed) : Phenolphytalein For second equivalent point (H

2

CO

3

is formed) : Methylorange

pH

(8)

• H

2

SO

4

is a strong acid with 2 protons and it gives two protons strongly. Thus, a single step is seen corresponding to 2 protons in the titration of H

2

SO

4

with a strong base.

• Only the 1st proton of the H

2

CO

3

can be titrated. For titratiton of 2nd proton, it is necessary to precipitate BaCO

3

with Ba

2+

ions. In this case, the released proton can be titrated.

• For H

2

C

2

O

4

, the ionization constants in two steps are very close to each other. Thus, when titrating with a base, a single step corresponding to 2 protons is seen.

• For H

2

SO

3

the first acidity is strong and can be easily titrated with a strong base. But 2nd

acidity is very weak and cannot be titrated. 2. The acid concentration must be 0.2M or

higher to titrate its acidity.

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