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Home > News & Articles > Precipitation Titration: Definition, Types, Application, Indicators & Examples
Updated on 27th July, 2023 , 6 min read
Titration is a widely used analytical technique that involves the controlled addition of a reagent to a solution to determine the concentration of an unknown analyte. Precipitation titration is one of the classical titration methods, based on the formation of a precipitate during the reaction between the analyte and the titrant. This method is particularly useful for the determination of various ions and compounds in solution, and it offers a reliable and cost-effective approach to quantitative analysis.
Precipitation titration is a quantitative analytical method used to determine the concentration of an analyte by titrating it with a standardized titrant. During this titration, a precipitate forms due to a reaction between the analyte and titrant, and the endpoint is determined visually or by using specific indicators.
Let's consider an example of precipitation titration to determine the concentration of chloride ions in a given solution. In this titration, we will use a known concentration of silver nitrate solution as the titrant. The chemical reactions involved are as follows:
Ag⁺(aq) + Cl⁻(aq) → AgCl(s).
During the titration, a white precipitate of AgCl will form and settle at the bottom of the flask. The amount of silver ions used to reach the equivalence point is equivalent to the amount of chloride ions originally present in the solution.
To calculate the number of moles of chloride ions (or silver ions), we can use the formula: n = cV, where n represents the number of moles, c is the concentration, and V is the volume in dm³ (liters) of the added solution.
To determine the volume of the added solution or the molar concentration of the ions, the corresponding values of either of the ions should be known. By accurately measuring these values, we can calculate the concentration of chloride ions in the original solution using the stoichiometry of the reaction.
The principle of precipitation titration is based on the concept of a stoichiometric reaction between the analyte and the titrant, resulting in the formation of an insoluble precipitate. The reaction is driven to completion, ensuring that all of the analytes have reacted with the titrant. The endpoint is reached when the stoichiometric ratio is met, and any excess of the titrant leads to the appearance of the precipitate.
To carry out a successful precipitation titration, several key requirements must be met:
Argentometric titration, also known as silver nitrate titration, is a common type of precipitation titration that involves the use of silver nitrate (AgNO3) as the titrant. It is often used for the determination of halide ions, such as chloride, bromide, and iodide.
Titration There are several methods of precipitation titration. Three widely used ones are:
Volhard's Method, named after the German Chemist Jacob Volhard who developed it in 1874, is an analytical technique used for the determination of halide ions (F, Cl, Br, I) and certain anions like phosphate and chromate in an acidic medium. The method relies on the reaction between the analyte and silver ions (Ag⁺) in an acidic solution. It is crucial to perform this titration in an acidic medium to avoid the precipitation of iron ions as hydrated oxide, which could interfere with the results. Iron ions are employed as indicators in Volhard's Method. The titration proceeds by titrating the first analyte (halide ion solution or any other anionic solution) with a measured excess of silver nitrate (AgNO₃).
Reaction - If the analyte contains chloride anions, the reaction can be represented as follows:
Cl⁻ + Ag⁺ → AgCl + Ag⁺ (in excess)
After reacting with the silver ions, the unreacted or excess silver ions are titrated with a standardized solution of potassium thiocyanate (KSCN), using iron ions (Fe³⁺) as an indicator. The endpoint is reached when the formation of a red-colored complex, FeSCN²⁺, indicates the presence of excess thiocyanate ions.
Reaction - The reaction involved can be shown as follows:
Ag⁺ + SCN⁻ → AgSCN
As thiocyanate ions are in excess in the titration mixture, a red color appears due to the formation of the FeSCN²⁺ complex.
Fajan's Method, named after American chemist Kazimierz Fajan, is another precipitation titration technique. It is also referred to as the "indicator adsorption method" because in this method, excess chloride ions are adsorbed on the surface of silver chloride (AgCl). In Fajan's Method, dichlorofluorescein is used as an indicator. The endpoint is determined by observing the color change of the suspension from green (AgCl and indicator) to pink (complex of AgCl and indicator).
Reaction - The reaction involved can be represented as follows:
Ag⁺ + Cl⁻ → AgCl
Ag⁺ + AgCl + Indicator → AgCl-Ag⁺ Indicator
Mohr's Method, named after German chemist Karl Friedrich Mohr, is a direct titration method used for the determination of chloride ions in a solution. In this method, silver nitrate (AgNO₃) is used as the titrant, and the analyte is a chloride ion solution. Potassium chromate is employed as the indicator. At the endpoint, when all chloride ions are consumed by silver ions, a reddish-brown-colored precipitate of silver chromate (Ag₂CrO₄) is formed.
Reaction - The reaction involved can be written as follows:
AgNO₃ + Cl⁻ → AgCl + NO₃⁻ (in the presence of NaCl)
2Ag⁺ + CrO₄²⁻ → Ag₂CrO₄ (Reddish-brown precipitate)
Adsorption indicators are substances that form complexes or adsorb on the precipitate surface, causing a visible change at the endpoint of the titration. They play a crucial role in precipitation titrations as they help in detecting the endpoint accurately.
Volhard’s Method | Mohr’s Method |
It is an indirect method of titration. | It is a direct method of titration. |
In this method, a red precipitate of ferric thiocyanate is formed which indicates the end point of the titration. Fe+3 + SCN- 🡪 FeSCN+2 (Red colored compound) | In this method, a red precipitate of silver chromate is formed, which indicates the end point. 2Ag+ + CrO4-2 🡪 Ag2CrO4 (Reddish Brown ppt) |
Conditions for titration should be acidic. Otherwise, iron ions form hydroxide ions. | Conditions for titration should be neutral to alkaline. |
By this method, we can determine the concentration of halides. | By this method, titration of iodide and cyanate is not possible. |
This titration is carried out below 20℃ | This titration can be carried out at room temperature. |
The mechanism of adsorption involves the formation of a surface complex between the indicator and the precipitate. This complex imparts a distinct color to the precipitate, making it easily recognizable.
Choosing a suitable adsorption indicator is essential for the success of precipitation titrations. Some factors to consider are:
Precipitation titration finds applications in various fields, including:
Despite its usefulness, precipitation titration has some limitations:
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By - Nikita Parmar 2023-08-25 09:48:44 , 11 min readPrecipitation titration is a type of titration in analytical chemistry used to determine the concentration of an analyte by forming and measuring the amount of precipitate that forms in a chemical reaction.
In precipitation titration, a titrant of known concentration is added gradually to the analyte solution. When the titrant reacts with the analyte, a sparingly soluble precipitate is formed, which is then separated, collected, and quantified to determine the concentration of the analyte.
Precipitation titration is widely used to determine the concentration of various ions, such as chloride, sulfate, bromide, and carbonate, in water samples, pharmaceuticals, and other chemical substances.
The endpoint in precipitation titration is the point at which the reaction between the analyte and titrant is complete. It is marked by the formation of a visible precipitate, which indicates that the exact stoichiometric amount of titrant has been added.
Precipitation titration is relatively simple and requires minimal equipment. It can be performed without the need for expensive instrumentation and is suitable for analyzing a wide range of analytes.
Indicators are not commonly used in precipitation titration since the endpoint is visually observed as the appearance of the precipitate. However, a pH indicator may be used to detect the completion of the reaction in some cases.
Precipitation titration can be affected by factors like co-precipitation, the solubility of the precipitate, and the presence of interfering substances, which may lead to inaccurate results.
The concentration of the analyte can be calculated using the stoichiometry of the reaction and the volume and concentration of the titrant added to reach the endpoint.
Always wear appropriate personal protective equipment (PPE) when handling chemicals. Follow proper laboratory procedures and disposal guidelines to ensure safety.
Precipitation titration involves the formation of a visible precipitate, which sets it apart from other titration methods like acid-base titration or complexometric titration that rely on color changes or formation of complexes for endpoint detection.
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