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Precipitation Titrations

Lecture 7

Lecturer: Amal Abu-Mostafa

Lecture 7

Lecturer: Amal Abu-Mostafa

Clinical Analytical ChemistryCLS 231

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Session Objectives:

Titration Calculations

Precipitation Titrations

Argentometric titration

Methods of Argentometric titration

Mohr’s Method

Fajan’s Method

Volhard’s Method

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Titration Calculations

• Terms and Definitions: Blank Titration: Titration procedure is carried out

without analyte (e.g., a distilled water sample). It is used to correct titration error.

Back titration: A titration in which a (known) excess reagent is added to a solution to react with the analyte. The excess reagent remaining after its reaction with the analyte, is determined by a titration.

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Precipitation TitrationsA reaction in which the analyte and titrant form an insoluble precipitate. We call this type of titration a precipitation titration.

The basic requirements are:

The reaction must be sufficiently rapid and complete, lead to a product of reproducible composition and of low solubility.

And a method must exist to locate the end point.

Some difficulties in meeting these requirements must be noted. (Precipitation reactions are generally)

Slow

Involving periods of digestion, cooling, filtration etc.

This tends to limit the reactions that are available for titration.

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Due to these difficulties, precipitation titrations are not so popular in present–day routine analysis.

The major areas of application include the determination of silver or halides by the precipitation of the silver salts (argentometric titrations) and the determination of sulphate by precipitation as barium sulphate.

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Argentometric titration:

Titrations involving silver are termed argentometric, from the Latin name for silver, argentum.

The major precipitation reaction used is that of silver with a range of anions. These anions include:

• Chloride

• Bromide

• Iodide

• Thiocyanate SCN-

The reaction rates for the silver salt precipitation is rapid.

The reaction ratio is 1:1 and silver salts formed are generally quite insoluble.

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METHODS OF ARGENTOMETRIC TITRATIONS:

Argentometric methods involving precipitation titrimetry:

Mohr’s Method

Fajan’s Method

Volhard’s Method

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Mohr’s Method:

The Mohr method uses potassium chromate (chromate ions) as an indicator in the titration of chloride ions (analyte) with a silver nitrate standard solution (titrant).

After all the chloride has been precipitated as white silver chloride, the first excess of titrant results in the formation of a silver chromate precipitate,

which signals the end point (1). The reactions are:

Ag +(aq) + Cl - (aq) → AgCl(s) Ksp = 1.8 x 10–10

End point determination by brick red color precipitate, Ag2CrO4(s):

2 Ag +(aq) + CrO4 2-(aq) → Ag2CrO4(s) Ksp = 1.2 x 10–12

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Mohr’s method:

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Mohr’s Method:

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Mohr’s Method:

This method is used in neutral solutions because in acid solution, silver chromate gets dissolved.2CrO4

2- + 2H+ = Cr2O72- + H2O

And in alkali solution, it makes a precipitate of silver hydroxide.

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Fajan’s Method

This method uses an adsorption indicator such of fluorescein (Dichlorofluorescein) and eosin.

The indicator adsorb onto the surface of the silver salt precipitate at the endpoint.

The adsorption process causes a change in the color of the indicator.

Common Fajans adsorption indicators are weakly acidic organic compounds and in alkaline conditions will exist as the conjugate base, In- (or Ind-).

It is this form of the indicator which interacts with the precipitate.

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The mechanism of indicators action:

The best–known adsorption indicator is fluorescein, which is used to indicate the equivalence point in the titration of Cl- with Ag+ . Fluorescein is a weak acid, which partially dissociates in water to form fluoresceinate anion.

The fluoresceinate anion has a yellow–green colour in solution.

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When Cl- is titrated with Ag+ in the presence of fluorescein, the negatively charged fluoresceinate anions are initially repelled by the negatively charged AgCl colloidal particles, with their primary adsorption layer of Cl- ions.

Thus the fluorescein remains in a yellow–green solution prior to the equivalence point.

At the equivalence point, the colloidal AgCl particles undergo an abrupt change from a negative charge to a positive charge by virtue of Ag+ ions adsorbed in the primary adsorption layer.

The fluoresceinate ions are strongly adsorbed in the counter–ion layer of the AgCl colloids, giving these particles a red colour and providing an end point colour change from yellow–green to red or pink.

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Fajan’s Method

The mechanism of indicators action:

AgNO3 + NaCl = AgCl + NaNO3

HInd H+ + Ind-

Adsorption indicator whose color when adsorbed to the precipitate is different from that when it is in solution

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Fajan’s Method

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Fajan’s Method

Common adsorption indicators:dichlorofluoroscein (by definition of chlorides)

eosine (by definition of bromides and iodides)

Color:

Indicator Solution Surface of precipitate

fluoroscein greenish yellow pink

eosine yellowish-red redish - violet

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Volhard’s Method

• The Volhard’s method was first described by the Jacob Volhard, a German Chemist, in 1874.

• This is an indirect titration procedure, (back titration) used for determining the anions that precipitate with silver.

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Volhard’s Method

This method uses a back titration with potassium

thiocyanate and is suitable for the determination of

chlorides, bromides and iodides in acidic solutions.

A known excess of silver nitrate solution is added to the

sample and the excess is back titrated with standard

thiocyanate solution. The titration uses iron III as the

indicator.

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STEPS OF VOLHARD’S METHOD:

Analyte is treated with the measured excess of Silver nitrate:

X- + Ag+ AgX ↓ + Ag+ (excess)

• Removing AgCl(s) by filtration / washing

• Adding Fe3+ into filtrate as indicator (i.e., the left Ag+)

The unreacted Silver ions are titrated with a standard solution of Thiocyanate ion, using Fe(III) as indicator:

Ag+ + SCN- AgSCN

(when all Ag+ has been consumed, SCN– reacts with Fe3+)

The first slight excess of Thiocyanate ion gives Red colour of FeSCN2+ complex at the end point.

Fe3+ + SCN- FeSCN2+

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Total mol Ag+ = (mol Ag+ consumed by Cl–) + (mol Ag+ consumed by SCN–)

CONDITIONS FOR VOLHARD’S METHOD:

The solution must be acidic, with a concentration of about 1 M in nitric to ensure the complex formed is stable, and to prevent the precipitation of Iron(III) as hydrated oxide.

The indicator concentration should not be more than 0.2M.

In case of I-, indicator should not add until all the I- is precipitated with Ag+, since it would be oxidized by the Fe(III).

2Fe3+ + 2I- 2Fe2+ + I2

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CONDITIONS FOR VOLHARD’S METHOD:

The AgX ↓precipitate must be filtered off, before titrating with SCN- to prevent any error, for example in the case of chloride ion, AgCl will react with the titrant (SCN- ) and cause a diffuse end point.

AgCl + SCN- AgSCN + Cl-

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Use tartrazine as indicator instead of Iron(III) .

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Thank you