Precipitation titration

Precipitation titration is quite a unique titrimetric analysis method. As its name implies, it is based on the formation of an insoluble chemical compound called a precipitate. How is this insoluble chemical compound formed, and how does it serve the purpose of titration, all of this interesting information is compiled for you in this article.  So, without any further delay, let’s start reading!

What is precipitation titration – Definition

Titration refers to determining the amount of an analyte by reacting it with a standard solution of known concentration called a titrant. In precipitation titration, a chemical reaction takes place between the analyte and the titrant to produce an insoluble or sparingly soluble colorful chemical compound called a precipitate. The precipitate settles at the bottom of the titration flask and marks the endpoint of the titration as the analyte is consumed completely.

The precipitate is usually formed by the combination of metal ions with halide ions. Therefore, precipitation titration is predominantly used for determining the unknown concentration of halogens and metal ions in a sample solution. It can be performed using three different methods, namely:

• Volhard’s method
• Fajan’s method
• Mohr’s method

How to perform precipitation titration – General procedure

The following step-by-step guide can be used to determine the unknown concentration of chloride (Cl^–) ions in an NaCl sample using a standard solution of silver nitrate (AgNO₃).

Step I: Preparation of 0.1 M standard AgNO₃ solution

n =

mass = moles x molar mass

molar mass of AgNO₃ = 169.87 g/mol

mass = 0.01 x 169.87 = 1.70 grams

As per the above calculations, 1.70 grams of silver nitrate (AgNO₃) solid is accurately weighed using an analytical mass balance. It is completely dissolved in distilled water and transferred to a 100 mL volumetric glass flask. The solution is raised up to the mark with distilled water. This solution is then filled in a 50 mL burette, as shown below.

Step II: A specific volume of analyte solution is pipetted out in the titration flask

A specific volume, such as 10 mL of analyte solution, is pipetted out in the titration flask. A small volume of an acidic buffer is also added.

Step III: The analyte solution is titrated with AgNO₃ from the burette

Dropwise addition of AgNO₃ from the burette leads to the following chemical reaction in the titration flask.

A net ionic equation for the above chemical reaction is:

Solubility product: The solubility product (K_sp) of a precipitate is defined as the product of the ionic concentrations at a given temperature.

K_sp for AgCl can be calculated as follows:

A white AgCl precipitate is formed when the product of its ionic concentrations exceeds its solubility product. This precipitate settles down at the bottom of the titration flask. The disappearance of all the analytes from the titration flask marks the endpoint of the titration. The final burette reading is then recorded. It is subtracted from the initial burette reading to determine the volume of AgNO₃ that is used to precipitate out all the Cl^– ions from the sample solution. It is known as the titre volume. This titre volume and the net ionic equation above can be used to determine the unknown chloride concentration, as shown below.

Step IV: The unknown Cl^– concentration is determined using stoichiometry.  

3 different types of precipitation titration

1. Volhard’s method of precipitation titration

Introduced by: Jacob Volhard

Basic principle: In Volhard’s method, a standard solution of silver (Ag) ions is used to determine an unknown anion concentration in an acidic media (as we did in the titration example above). Ferric ions are used as an indicator. The formation of a colored complex marks the endpoint of the titration. Precipitation titration involving AgNO₃ is also known as Argentometric titration.

Analytes: This type of precipitation titration is mainly used for determining the unknown concentration of halides (X^–), chromate (CrO₄^2-), and phosphate (PO₄^3-) ions.

• The analyte solution is titrated with an excess of AgNO₃.
• Unreacted AgNO₃ in the titration flask is again titrated with potassium thiocyanate (KSCN) in the presence of ferric ammonium sulfate or ferric alum (NH₄Fe(SO₄)) used as an indicator.
• As all the AgNO₃ gets consumed, a red-colored complex is formed in the presence of excess thiocyanate ions.
• The volume of KSCN used helps determine the amount of AgNO₃ initially present in the titration flask.
• Once the amount of AgNO₃ left unreacted is determined, it can then be used to calculate the amount of AgNO₃ that reacted with chloride ions. Ultimately giving us the concentration of Cl^– ions present in the sample.

 In conclusion, Volhard’s method of precipitation titration is actually a back titration method, quite comparable to Iodometry.

2. Fajan’s method of precipitation titration

Introduced by: Kazimierz Fajan  

Basic principle: Unknown concentration of halide ions are determined using a standard AgNO₃ solution in the presence of an adsorption indicator such as eosin, fluorescein, dichlorofluorescein, rhodamine, etc. 

Analytes: Fajan’s method of precipitation titration is mainly used for determining the unknown concentration of chloride (Cl^–), bromide (Br^–), iodide (I^–), and thiocyanate (SCN^–) ions. 

• It is a direct titration method.
• An unknown concentration of halide ions such as Cl^– is titrated with AgNO₃ using an adsorption indicator.
• The adsorption indicator, such as fluorescein, use its special functional groups to get chemically adsorbed on the surface of the precipitate formed (AgCl in this case).
• Fluorescein is a weak acid that partially dissociates in an aqueous solution to yield fluoresceinate anions that give a yellowish-green color to the titration mixture.

• The negatively charged fluoresceinate ions are initially repelled by AgCl colloids at the start of the titration.
• As the equivalence point is reached, the AgCl colloidal particles exhibit a positive charge overall due to an excess of Ag⁺ ions on their surface.
• The fluoresceinate ions are thus attracted and adsorbed on the AgCl colloids at the equivalence point.
• The green AgCl suspension changes color from green to pink as a result of this adsorption.
• This color change finally marks the endpoint of the titration.

The Fajan’s method of precipitation titration is a modified version of Volhard’s method that is most popularly used in recent times.

However, the oldest precipitation titration method is Mohr’s method.

3. Mohr’s method of precipitation titration

Introduced by: Karl Friedrich Mohr

Basic principle: Unknown concentration of halide ions is determined using a standard AgNO₃ solution in the presence of potassium chromate (K₂CrO₄) as an indicator. 

• It is also a direct titration method.
• The unknown halide ion concentration is determined using a standard AgNO₃ solution.
• The titration is performed under neutral or slightly alkaline pH conditions (pH = 6.5 -9) that is maintained using buffer solutions.
• A few drops of K₂CrO₄ indicator are added.
• The titration endpoint is marked as a brick-red precipitate of silver chromate (Ag₂CrO₄) is formed.

Precipitation titration curve

The chemical changes taking place while a precipitation titration can be recorded by plotting a titration curve. The titration curve is plotted between analyte concentration versus the volume of titrant (AgNO₃) added from the burette.

The titration curve shown below represents how p[Cl^–] changes with the increase in AgNO₃ volume. p [Cl^–] = -log [Cl^–] where [Cl^–] is the concentration of chloride ions. The precipitation titration curve tells us that as the volume of AgNO₃ added increases, p[Cl^–] increases, i.e., [Cl^–] decreases in the titration flask. [Cl^–] decreases as more and more chloride ions are precipitated out as AgCl.

An equivalence point is reached at the mid of the titration curve. All the Cl^– ions get precipitated out of the solution as AgCl at the equivalence point, so the value of p[Cl^–] increases sharply against a small change in AgNO₃.

Further increase in AgNO₃ volume leads to a constant p[Cl^–] as there are no more Cl^– ions to be precipitated.

What are the uses of precipitation titration

• Precipitation titration finds several valuable applications in the food industry. It is used for quantitative analysis of salts in food and beverage items.
• It is used for drug analysis in the pharmaceutical industry, such as for studying NaCl and KCl infusion in medicines and therapeutic agents.
• It is also used for determining the salt concentration of metal alloys and in environmental samples such as water.

Learn more about the uses and applications of titrations here.

You may also like the following:

• Complexometric titration.
• Gravimetric analysis.

References

1. C.Harris, Daniel. 2010. Quantitative Chemical Analysis (W.H Freeman and Company ).

2. Goyal, Kamya, Navdeep Singh, Shammy Jindal, Rajwinder Kaur, Anju Goyal, and Rajendra Awasthi. 2022. Advanced Techniques of Analytical Chemistry.