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Daniele Naviglio » 12.Precipitation titration


Precipitation titration

This kind of titration is based on precipitation reactions, i.e. separable solid compounds form during the course of the reaction. The titrating agent, therefore, has to react with the analyte to produce a compound of limited solubility.

Characteristics of precipitation reactions

  • they are fast and the stoichiometry is known and reproducibile, (no secondary reactions of interference);
  • they are complete or can be quantified depending on the amount of solubility product (in general a precipitation titration is considered complete when Kps < 10-8);
  • an indicator can be used to find the equivalence point or titration end point which, for this type of titration, corresponds to when precipitation of the analyte under analysis is complete.

Titrants and titrands

The most common applications for precipitation methods are with the halides (Cl-, Br-, l-), which precipitate as silver salts when we use silver nitrate (AgNO3) as the precipitating reagent. We can also determine the ions shown in the table below:

The methods of titration based on silver nitrate are defined argentometrici.


Titration curves

Titration curves for precipitation reactions are plotted in exactly the same way as those for strong acids or bases. The only difference is that we put the solubility product of the precipitate instead of the ionic product of water.

Titration curve for chloride ions (Cl-) with silver nitrate (AgNO3). Source: LaCellula.net

Titration curve for chloride ions (Cl-) with silver nitrate (AgNO3). Source: LaCellula.net


Solubility product

The solubility constant, or solubility product constant, is the equilibrium constant that describes, at constant pressure and temperature, the situation when a solid of limited solubility, which has released enough cations and anions in an aqueous solution (and is therefore an electrolyte) to reach saturation point, is in equilibrium with the ions themselves. If we take a mono-monovalent salt of very limited solubility in water AB, equilibrium is established between the solute and its ions A+ and B-, as expressed in the analytical equilibrium constant:

AB ↔A+ + B-

The equilibrium constant, which is also termed solubility product, can be written:

Kps = [A+] [B+]

With precipitation titration, as with acid-base titrations, the indicator needs to change colour within a fairly limited p[M] interval near the point of inflection of the curve to minimize error.

Indicators used in argentometric titration

  • Potenziometric
  • Amperometric
  • Chemical indicator

Chemical indicators

To find out the equivalence point in precipitation titration we can use:

Ionic species: these precipitate when they react with the same precipitating reactive used for the analyte, but give rise to an obvious colour change. Obviously the indicator should only precipitate when the original precipitation reaction is complete.

Adsorption indicators:  these are organic substances, usually fluorescent, (fluorescein, eosin), which undergo structural changes with a resultant colour change when they are adsorped onto the precipitate particles, as a result of secondary absorption.

 

The colour change needs to occur within a fairly limited interval of the p function of the reagent or analyte. It also needs to happen during the steep part of the titration curve.

Absorption indicators

Source: Facoltà di Farmacia – Università di Catania

Source: Facoltà di Farmacia - Università di Catania


Practical example


Practical example (cont.)


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