Precipitation Titrations

Clinical Analytical Chemistry CLS 231 Precipitation Titrations Lecture 7 Lecturer: Amal Abu-Mostafa

Session Objectives: • Titration Calculations • Precipitation Titrations • Argentometric titration • Methods of Argentometrictitration • Mohr’s Method • Fajan’sMethod • Volhard’s Method

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.

Precipitation Titrations • A 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.

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.

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.

METHODS OF ARGENTOMETRIC TITRATIONS: Argentometric methods involving precipitation titrimetry: • Mohr’s Method • Fajan’s Method • Volhard’s Method

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) + CrO42-(aq) → Ag2CrO4(s) Ksp = 1.2 x 10–12

Mohr’s method:

Mohr’s Method:

Mohr’s Method: • This method is used in neutralsolutions because in acid solution, silver chromate gets dissolved. • 2CrO42- + 2H+ = Cr2O72- + H2O • And in alkali solution, it makes a precipitate of silver hydroxide.

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 precipitateat 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.

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.

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 colourand providing an end point colour change from yellow–green to red or pink.

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

Fajan’s Method

Fajan’s Method • Common adsorption indicators: • dichlorofluoroscein (by definition of chlorides) • eosine(by definition ofbromidesandiodides) • Color:

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.

Volhard’s Method • This method uses a back titration with potassium thiocyanateand is suitable for the determination of chlorides, bromides and iodides in acidic solutions. • A knownexcess 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.

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 colourof FeSCN2+ complex at the end point. Fe3+ + SCN- FeSCN2+

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

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- • OR Use tartrazine as indicator instead of Iron(III) .

Thank you

Precipitation Titrations