Precipitation Titrations

Precipitation titrations are a type of titration that involves the formation of a precipitate during the titration process. In a precipitation titration, a solution of a known concentration (the titrant) is added to a solution of the substance being studied (the analyte). The titrant reacts with the analyte to produce a precipitate, which is a solid that forms from the reaction and separates from the solution. The point at which enough titrant has been added to react with all of the analyte is called the end point of the titration.

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Principle of Precipitation titrations

In a precipitation titration, we have two solutions. One solution has a substance we want to measure (let’s call it Substance A), and the other solution can react with Substance A to form a solid (we’ll call this Substance B). We slowly add Substance B to Substance A’s solution. As we add more of Substance B, it reacts with Substance A to form a solid that falls out of the solution. This is the ‘precipitate’.

We keep adding Substance B until there’s no more Substance A left to react with. We know we’ve reached this point (the ‘end point’) because an indicator changes color or a meter shows a change. The amount of Substance B we added tells us how much of Substance A was in the original solution.

Precipitation titrations are commonly used in the analysis of halides and pseudohalides, and are particularly useful when the analyte and titrant form a precipitate that is highly insoluble in water. They are also used in the analysis of certain metal ions and organic compounds.

Types of precipitation titrations

Precipitation titrations can be classified as follows.

Mohr’s method
Volhard’s method
Modified Volhard’s method
Fajans method

Mohr’s method

Mohr’s method is a type of precipitation titration used to determine the concentration of chloride ions in a solution. It was developed by the German chemist Karl Friedrich Mohr. The titration under this method is carried out by using following steps.
The titration uses silver nitrate (AgNO3) as the titrant and a solution of chloride ion as the analyte.
A small quantity of potassium chromate (K2CrO4) is used as the indicator.
The titrant is titrated against a supply of chloride ions such that silver chloride (AgCl) and silver chromate (Ag2CrO4) precipitate.
Silver nitrate is titrated against chloride ions to produce a white precipitate (AgCl) according to the reaction,

AgNO3(aq)+Cl−(aq) → AgCl(s)+NO3⁻(aq)

When all chloride ions in a given sample have been converted to AgCl, additional AgNO3 reacts with yellow K2CrO4 (indicator) to produce a reddish-brown precipitate of Ag2CrO.
The endpoint is reached when the reddish-brown color remains stable and does not disappear with stirring.

One important thing to note is that Mohr’s method can only be carried out between a pH of 7 and 10. In acidic environments, chromate (CrO₄^2-) is converted to chromic acid which does not react with excess silver ions.

Precipitation Titrations source: wikimedia

Volhard’s method

Volhard’s method is a type of precipitation titration used to determine the concentration of chloride ions in a solution. It was developed by the German chemist Jacob Volhard. The titration under this method is carried out by using following steps.

The titration uses silver nitrate (AgNO3) as the titrant and a solution of chloride ion as the analyte.

A small quantity of ferric alum is used as the indicator.
The titrant is titrated against a supply of chloride ions such that silver chloride (AgCl) and silver thiocyanate (AgSCN) precipitate.
Silver nitrate is titrated against chloride ions to produce a white precipitate (AgCl) according to the reaction

AgNO3(aq) + Cl⁻(aq) → AgCl(s) + NO3⁻(aq)

When all chloride ions in a given sample have been converted to AgCl, additional AgNO3 reacts with ammonium thiocyanate (NH4SCN) to produce a white precipitate of AgSCN.
The endpoint is reached when the deep red color of the ferric thiocyanate complex [FeSCN]2+ remains stable and does not disappear with stirring.

One important thing to note is that Volhard’s method can only be carried out in an acidic solution. In basic environments, Fe3+ will hydrolyse.

Modified Volhard’s method

The Modified Volhard’s method is a type of back titration used to determine the concentration of halide ions in a solution.

The sample is first treated with an excess standard solution of silver nitrate (AgNO3) to precipitate all the halide ions as silver halide.
The excess silver ions are then titrated with a standard solution of ammonium thiocyanate (NH4SCN).
Iron (III) is used as an indicator. The end point of the titration is detected by the formation of a red-colored complex due to the reaction of iron (III) with thiocyanate.

This method is a modification of the original Volhard’s method and is used when the original method is not applicable due to the presence of substances that interfere with the precipitation of silver halide.

Fajans method

Fajan’s method, also known as the adsorption indicator method, is a type of precipitation titration used to determine the concentration of anions, such as chloride (Cl⁻) and bromide (Br⁻), in a solution.

A known amount of silver nitrate (AgNO3) solution is added to the sample containing the anions.
The silver ions (Ag+) in the solution react with the anions to form a precipitate.
An indicator, such as dichlorofluorescein, is added to the solution. The indicator is adsorbed onto the surface of the precipitate, causing a visible color change.
The titration continues until the color change indicates that all the anions have reacted with the silver ions.
The volume of silver nitrate solution used gives the concentration of the anions in the original sample.

This method is based on the principle of adsorption, where the indicator ion is adsorbed onto the surface of the precipitate. This leads to a visible color change at the endpoint of the titration.

Estimation of sodium chloride

The estimation of sodium chloride (NaCl) by precipitation titration is typically done using the Mohr’s method. Here’s a step-by-step process.

Preparation of the Sample: A known quantity of the sodium chloride sample is dissolved in distilled water.
Addition of the Indicator: A few drops of the indicator, such as potassium chromate, are added to the sample solution1. The indicator forms a complex with silver ions, which changes color when all the silver ions have reacted with the chloride ions.
Titration: The silver nitrate solution is slowly added to the sample solution until the endpoint is reached. The endpoint is determined by observing the color change of the indicator from yellow to brick-red1. At the endpoint, all the chloride ions in the sample have reacted with the silver nitrate.
Calculation of Results: The amount of silver nitrate solution added to the sample solution is used to calculate the amount of sodium chloride present in the sample, using the stoichiometry of the reaction.

The chemical reaction between sodium chloride and silver nitrate can be expressed as follows.

AgNO3 + NaCl → AgCl + NaNO3

Where,

AgNO3 represents the silver nitrate, which acts as the titrant.
NaCl represents the sodium chloride in the sample.
AgCl represents the precipitate of silver chloride that forms.
NaNO3 is the sodium nitrate formed as a byproduct.

This method is widely used in various industries, including food and beverage, pharmaceuticals, and environmental monitoring, for the accurate estimation of sodium chloride.

Applications of precipitation titrations

Precipitation titrations have a wide range of applications, particularly in the field of analytical chemistry. Here are some of the key applications,

Halide Ions: Precipitation titration is used to determine the presence of halide ions in solutions.
Salt Content: It is applied to measure the salt content in various food items, water, and beverages.
Drug Analysis: Many drugs such as carbromal, KCl infusion, NaCl infusion, etc. can be analyzed by precipitation titration.
Sulphur, Thiocyanate, Dichromate: It is used for the analysis of sulphur, thiocyanate, dichromate, and similar compounds.
Argentometric Titration: This is a form of precipitation titration that uses silver ions. Titrations involving silver ions are called argentometric titrations. The precipitation of silver salts occurs at rapid reaction rates.

Summary

We discussed precipitation titrations, a type of volumetric analysis where a precipitate forms during the titration process. We explored different methods including Mohr’s, Volhard’s, Modified Volhard’s, and Fajan’s methods, each with unique procedures and applications. These methods are used to determine concentrations of various ions in solutions. We also discussed the principle of precipitation titrations, which involves the reaction between an analyte and a titrant to form a precipitate. Lastly, we looked at the applications of precipitation titrations in fields like food analysis, drug analysis, and more.