NONAQUEOUS TITRATION

INDICATOR ERRORS

Advantages of Non Aqueous Solvent over Aqueous Solvent 1. Organic acids and bases that are insoluble in water are soluble in non-aqueous solvent. 2. Organic acid, which is of comparable strength to water, can be titrated easily in non-aqueous solvent. Bases also follow the same rules. 3. A non-aqueous solvent may help two or more acids in mixture. The individual acid can give separate end point in different solvent. 4. By the proper choice of the solvents or indicator, the biological ingredients of a substance whether acidic or basic can be selectively titrated. 5. Non aqueous titrations are simple and accurate

Merits of Non Aqueous over Aqueous Titrations:

The substances, which are either weakly acidic or too weakly basics to give sharp end point in aqueous solutions, can easily be titrated with accuracy is non-aqueous solvent.

(1879 - 1947) (1874 –1936)

Thomas Martin LOWRY

Johannes Nicolaus BRØNSTED

1923

The acid is an H+ donor (ex.: HCl, HNO3, HCO3-,

H2PO4-).

The base is an H+ acceptor (ex.: NH3, CO3

2-, OH-).

Brønsted–Lowry acids are H+ donors.

Any material that has H+ can potentially be a Brønsted–Lowry acid.

Ex. When HCl dissolves in water, the HCl is

the acid because HCl transfers an H+ to H2O, forming H3O

+ ions. Water acts as base, accepting H+.

HCl + H2O → Cl– + H3O+

Brønsted–Lowry bases are H+ acceptors.

Any material that has atoms with lone pairs can potentially be a Brønsted–Lowry base.

Ex.: When NH3 dissolves in water, the NH3 is

the base because NH3 accepts an H+ from H2O, forming OH–. Water acts as acid, donating H+.

NH3 + H2O NH4+ + OH–

The ability of substances to act as acids or bases will very much depend on the choice of solvent system.

HCl and HNO3 are strong acids because they are good proton donors and essentially donate all their protons to H2O. Water in this case plays role of a strong base.

If we place acetic acid in water the dissociation reaction does not proceed to a significant extent because water is a weak base in this case and cannot accept many protons from acetic acid.

COOCHОHOHCOOHCH 3323

123

22

1 baseacidbaseacidClОHOHHCl

If we place acetic acid in a solvent, such as ammonia, that is a stronger base than water, then the reaction proceeds to a greater extent.

13

24

23

13

основаниекислотаоснованиекислота

COOCHNHNHCOOHCH

All other things being equal, the strength of a weak acid increases if we place it in a solvent

that is more basic than water, and the strength of a weak base increases if we place it in a

solvent that is more acidic than water.

Acids and salts with acidic properties (with Ка < 10-7) can be directly titrated in basic solvents such as liquid ammonia, pyridine etc.

Bases and salts with basic properties (with Ка < 10-7) can be directly titrated in acidic solvents such as glacial acetic acid, formic acid etc.

Amidopyrine – is a very weak organic base with Ka = 10-9, which cannot be

titrated in an aqueous solution. When a weak base, such as amidopyrine, is dissolved in acetic acid, the acetic acid exerts its levelling effect and enhances the basic properties of the amidopyrine. It is possible, therefore, to titrate a solution of a weak base in acetic acid with perchloric acid, and obtain a sharp endpoint when attempts to carry out the titration in aqueous solution are unsuccessful.

Examples

solvent

COOHCH3

Potassium acetate can be viewed as a weak base with Ka = 10-9, which cannot be titrated in an aqueous solution. It can be titrated in acetic acid with perchloric acid.

COOKCH 3solvent

COOHCH3

Salicylic acid is a rather strong acid with Ka = 10-4. It is slightly soluble in water and can be determined in aqueous solution. Salicylic acid is dissolved in ethylic alcohol and then titrated.

solvent

OHHC 52

Nicotinamide (pKb=11,7) can be directly titrated with perchloric acid HClO4 in the following solvents:

A. NH3 liquid; B. H2O; C. C2H5OH; D. CH3COOH; E. HCOOH.

TEST

Hexamethylenetetramine (pKb=8,9) cannot be directly titrated with perchloric acid HClO4 in the following solvents :

A. CH3COOH; B. NH3 liquid; C. H2O; D. C2H5OH; E. pyridine.

Ammonium chloride (pKb(NH3)=4,75) can be directly titrated with KOH in the following solvents :

A. NH3 liquid; B. H2O; C. C2H5OH; D. pyridine; E. CH3COOH.

When performing a titration, the aim is to burette in just enough so that the equivalence point is reached (the point at which all the substrate in the conical flask has been neutralised/reacted completely with the titrant according to their mole ratio). Usually, this equivalence point is marked by a colour change of the mixture. We call the exact point at which this colour change occurs, the "end point". The end point is basically a very good estimate at the equivalence point (although it isn't always exactly the equivalence point) so we use this to help us determine when the substance in the flask and titrant have reached the point where they have reacted in exact stoichometric ratios.

INDICATOR ERROR is a slight difference between the change in indicator color and the actual equivalence point of the titration.

INDICATOR ERRORS

INDICATOR ERRORS can be of four types:

1. Hydrogen error – H+ - error

2. Hydroxide error – OH- - error

3. Acidic error– HA- error

4. Basic error – MeOH - error

NaOH

Equivalence point рН = 7

рТ m/о = 4

In the case of methyl orange (pT = 4) if we end titration once the color change is complete, we will not reach the equivalence point, we will stop to titrate before the equivalence point. In this case Н+-error appears, because a part of strong acid remains in a titration flask unestimated.

Let’s focus on the titration of HCl with NaOH in the presence of methyl orange

1

HCl рН = 1

HCl

NaOH

рТ ph = 9

Let’s focus on the titration of HCl with NaOH in the presence of phenolphtaleine

Equivalence point рН = 7

In the case of phenolphthalein (pT = 9) if we end titration once the color change is complete, we will stop to titrate after the equivalence point. In this case OН--error appears, because acid will be overestimated, we introduced too much NaOH (titrant) from the burette (overshoot of the equivalent point).

2

рН = 1

CH3COOH

NaOH

рТ m/о = 4

Equivalence point рН = 8.73

Let’s focus on the titration of CH3COOH with NaOH in the presence of methyl orange

In the case of methyl orange (pT = 4) if we end titration once the color change is complete, we will not reach the equivalence point, we will stop to titrate before the equivalence point. In such a case НA-error appears, because a part of weak acid remains in a titration flask unestimated.

3

рН = 2,8

NH3

HCl

Equivalence point рН = 5.27

рТph= 9

Let’s focus on the titration of NH3 with HCl in the presence of phenolphthalein

In the case of phenolphthalein (pT = 9) if we end titration once the color change is complete, we will not reach the equivalence point, we will stop to titrate before the equivalence point. In such a case MeOH-error appears, because a part of weak base remains in a titration flask unestimated.

4

рН = 11

NaOH

Equivalence point рН = 7

рТ m/о = 4

What is the indicator error in this case?

Let’s focus on the titration of NaOH with HCl in the presence of methyl orange

5

HCl

рН = 13

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