General Applications of Precipitation Titrations

To understand precipitation titration, it is essential to understand what titration and precipitation mean separately. Titration is a process that is used in quantitative chemical analysis when the concentration or purity of a compound has to be determined. The process uses two liquids called the titrant and the analyte.

The analyte is the substance that we are testing to identify the different components and their concentration in it. The titrant is the solution that is added to the analyte to trigger a chemical reaction that helps us identify different components in the analyte.

On the other hand, a precipitate is the solid by-product of a reaction between the titrant and a component of the analyte. Therefore, precipitation titration can be defined as the titration whose result will be the formation of a precipitate when the component we are testing for is present in the analyte.

The process called titration is used when one wants to determine or evaluate the concentration of any compound. This is done by using two liquids: the titrant and the analyte.

The analyte, in general chemistry, is referred to as the substance which is being analysed or measured. In a titration, it specifically refers to the solution of the compound with unknown impurities/concentrations in dissolved form. The titrant is the solution whose concentration is already known. The precipitate is the solid by-product of a precipitate reaction where the reaction between any two compounds leads to the production of a precipitate: a solid mass.

Precipitation titration – The volumetric analysis where a precipitate is formed is called precipitation titration, or when the analyte and titrant react to form a precipitate during titration.

Example :

molecular equation

Pb(NO₃)₂ (aq) + 2NaI (ag) → PbI₂ (s) + 2NaNO₃ (aq)

ionic equation

Pb²⁺ + 2I^– → PbI₂ (s)

net ionic equation

Na* and N are spectator ions.

The main principle of precipitation titration is that the mass of precipitate (titrant) is equal to the amount of precipitate created.

Amount of titrant added = amount of precipitated compound.

Types of precipitate titrations and their applications

There are three main types of precipitation titrations: Volhard’s method, Fajan’s method and Mohr’s method.

Volhard’s method

In this method of precipitation titrations discovered by Jacob Volhard from Germany, the anions such as halides, phosphate and chromate are determined in an acidic medium by silver ions. To avoid precipitation of iron ions as hydrated oxide, this titration must be formed in an acidic medium.

Here, when it reacts with excess silver nitrate, the chloride in the solution then is converted into silver chloride. This leftover silver nitrate then reacts with potassium thiocyanate solution. This excess thiocyanate is made to react with an indicator ferric ammonium sulphate which turns the solution red upon reaction, therefore, indicating the formation of a complex.

The medium should be acidic to avoid the formation of Fe(OH)₃.

Ag⁺ + Cl^– → white precipitate – AgCl

Ag⁺ + SCN^– → white precipitate – AgSCN

Fe³⁺ + SCN^– → reddish brown complex – [FeSCN]²⁺

This system is extremely sensitive and hence has good results.

The Volhard method is used to detect the concentration of halides in a given component. It is also key in detecting anions along with potassium and fluoride.

Fajan’s method

In this process of precipitate titration, the indicator absorption is used, dichlorofluorescein is used as an indicator and methyl chloride ions are absorbed when in excess onto the silver chloride surface.

The resulting precipitate is green in colour (AgCl), which turns pink. That results in a complex interaction between the indicator and agcl.

This was discovered by the chemist Kazimierz Fagan from America.

Reactions :

Ag⁺ + F^– → AgF

As fluorescein are weak acids, the pH of the solution should be slightly alkaline to keep the indicator in the anion form but also not extremely alkaline enough to convert Ag⁺ into AgOH.

Although we have Volhard’s method, Fajan’s method is used to determine the presence of halides in solutions that are neutral in nature. Hence, it is a very important method when it comes to detecting halides in real-life components that are usually neutral in nature.

Mohr’s method

In this process, titration is a direct method where silver nitrate is the titrant, and the analyte is the chloride ion. The indicator in this process is potassium chromate. The resulting precipitation is reddish-brown at the end of the silver-chromate ion reaction after the chlorine ion is dissolved.

This method was discovered by the chemist Karl Friedrich Mohr from Germany.

Reactions :

2 Ag⁺ + CrO₄^2- → Ag₂CrO₄

The medium should be neutral since if the alkaline level is high, silver is reactive to alkaline mediums.

The prominent usage of Mohr’s method is in detecting the presence of halides in water samples. Detection of halides in water samples is an important health safety measure, and Mohr’s method is an imperative need to analyse different water samples to check how safe they are.

Applications of precipitate titration in daily life

• The concept of precipitate titration is used to determine halide ions which are found in various natural water bodies such as lakes, rivers, etc. These halides are accumulated clusters of halogen which act as disinfectant chemicals in natural water bodies but are harmful if not limited in capacity.
• This also helps in the detection of salt levels in our day to day diet. This helps us analyse our daily salt consumption. Excess salt consumption leads to many diseases such as high blood pressure, etc. This process helps us regulate our diet.
• The method allows us to have a closer analysis of drugs and also achieve the right concentration.
• This is also used to detect the concentration levels of anions in any analyte.

Limitations of titration

• Not all halides can be titrated; hence many essential halides are missed through this process.
• Often multiple or co-precipitation results. Therefore, they are difficult to evaluate.
• The endpoint of titration graphs is very hard to determine.

The precipitation titration curves

The change of concentration in the titrant or analyte is tracked by a titration curve.

These curves are represented by

1. The change in concentration of reactants during titration.
2. The indicator may have an error during titration.
3. The condition is at the equivalence point.

The precipitation titration curves are a complex concept with various methods of calculations within the graph; hence, the determination of the endpoint is sometimes very cumbersome.

Using the plotted graph, one can determine and analyse the reaction in a very effective manner.

Conclusion

Precipitation titration is a very important method in qualitative chemical analysis. Compared to other qualitative analysis methods, the precipitation titration method has a more obvious change and ending point for identifying when to stop the titration. There are various types of precipitation titration depending on the component in the mixture that one wants to detect.