Gravimetric analysis
Gravimetric analysis is a quantitative analytical technique focused on determining the amount of a specific analyte in a sample by converting it into a known substance that can be accurately weighed. The process typically begins with a precipitation reaction, where a precipitating agent is added to a solution containing the analyte, forming a solid precipitate. Subsequent steps include digestion to enhance precipitate size for easier filtration, followed by filtration to separate the solid from the liquid, and washing to improve purity. After drying or igniting the precipitate, it is cooled in a desiccator to prevent moisture absorption before weighing.
The weight of the precipitate, along with the weight of the original sample, allows for calculations to determine the analyte's concentration. Key criteria for successful gravimetric analysis include achieving complete precipitation, ensuring the precipitate is pure and stable, and facilitating easy filtration. This method can be exemplified by determining the concentration of chloride ions in seawater, where silver nitrate is used to precipitate silver chloride, enabling quantification of chloride content. Gravimetric analysis is valued for its accuracy and reliability in various fields, including environmental monitoring and quality control.
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Gravimetric analysis
Gravimetric analysis is a type of quantitative analysis involving the conversion of an analyte, the component of a sample that is the subject of the analysis, into a substance with a known composition—such as a pure compound or element—that can be weighed. Gravimetric analysis usually begins with a precipitation reaction, a process that separates a solid from the liquid part of a solution. A number of steps, including digestion, filtration, washing, drying/ignition, and cooling, typically follow the precipitation reaction. To conclude the analysis, the precipitate is weighed, and calculations are made to determine the amount of the analyte in the original sample.
Process
The most common method for isolating an analyte from a sample is precipitation. If the sample containing the analyte is not already in a solution, the sample should be weighed and then added to a flask containing a solvent, or a dissolving agent, to form a solution. Following is a breakdown of the next steps.
Precipitation
Precipitation is the process that separates a solid, called a precipitate, from the liquid part of a solution. In this case, a substance known to react with the analyte is added to the solution to serve as a precipitating agent. This leads to the formation of a precipitate with a known chemical composition. For gravimetric analysis to be accurate, the analyte must be precipitated completely. This is accomplished by adding the precipitating agent drop by drop until no more precipitate settles out of the solution.
Accurate gravimetric analysis requires the analyte to be as pure as possible. To get a purer result, it may be necessary to perform the precipitation a second time. In such cases, the precipitate from the first reaction is dissolved in an appropriate solvent and allowed to precipitate again.
Digestion
The process of digestion causes particles of the precipitate to increase in size, making them easier to filter. During digestion, the solution containing the precipitate is heated, usually on a hot plate, until it reaches a temperature just below its boiling point. The high temperature allows smaller particles of the precipitate to dissolve and larger particles to grow.
Digestion also helps to reduce adsorption of unwanted materials to the precipitate. Adsorption is when a thin film of molecules sticks to the surface of a solid or liquid. By reducing the overall number of particles of precipitate, digestion reduces the surface area of the precipitate to which molecules of other substances can adhere.
To check for complete precipitation after digestion, a drop of the precipitating agent may be added to the solution. If additional precipitate does not appear, then precipitation is complete.
Filtration
After complete precipitation of the analyte has occurred, the precipitate is separated from the reaction solution through filtration. The solution is poured over filter paper or into a crucible containing an appropriate filtering medium. The flask containing the precipitate is rinsed repeatedly with an electrolyte solution to ensure that all of the precipitate is transferred to the filter paper or crucible.
Washing
Following filtration, the precipitate is washed. The precipitate may be washed with water or with an electrolyte solution. Washing the precipitate improves the purity of the analyte. It is important to make the analyte as pure as possible.
In nearly every gravimetric analysis, some coprecipitation occurs. Coprecipitation is when a material other than the desired precipitate settles out of the solution during the reaction. The best way to minimize the effects of coprecipitation is to thoroughly wash the precipitate.
Drying/Ignition
After the precipitate has been washed, it must be dried. Some precipitates require low-temperature drying, while others require high-temperature drying, or ignition. For low-temperature drying, the precipitate is placed in a drying oven. High-temperature drying is typically used for precipitates that have been filtered on paper. This process burns away the paper and its residue.
Cooling
After drying, the precipitate is placed in a desiccator for cooling. A desiccator is a vessel that contains a drying agent, or desiccant. Here, the precipitate cools off without absorbing moisture from the surrounding atmosphere.
Weighing and Calculating
Once the precipitate is cool, it can be weighed. Using the weight of the precipitate and the weight of the original sample, it is possible to calculate the amount of the analyte in the original sample.
Criteria for a Successful Analysis
The steps in the process for conducting gravimetric analysis reveal several criteria necessary for success. These criteria include the following:
The precipitate containing the analyte must have a known chemical composition.
The precipitation process must occur completely.
The resulting precipitate must be a pure compound or element.
The particles of the precipitate must be easily filterable.
The precipitate must remain stable after drying/ignition and cooling.
Gravimetric Analysis Example
As previously mentioned, gravimetric analysis may be used to determine the amount of an analyte in a sample of a substance. For example, gravimetric analysis can be used to find the concentration of chloride ions, negatively charged chlorine ions (Cl-), in a sample of seawater. When silver nitrate (AgNO3) is added to a seawater solution, the positively charged silver ions (Ag+) combine with negatively charged chloride ions (Cl-) to form a solid precipitate called silver chloride (AgCl). After the precipitate has been digested, filtered, washed, dried, and cooled, it can be weighed. Then, using the weight of the silver chloride and the weight of the original sample of seawater, the percentage of the seawater composed of chloride ions can be calculated.
Bibliography
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