Gravimetric Analysis

Gravimetric analysis is a technique for determining the amount of an analyte (the ion under investigation) by measuring its mass. The mass of two substances containing the analyte is compared in gravimetric studies. Gravimetric analysis works on the idea of determining the mass of an ion in a pure compound and then using that information to calculate the mass percent of that same ion in a known quantity of an impure compound. Precipitation, volatilization, electro-analytical, and various physical methods are the four basic forms of this method of analysis.

Requirements :

Certain requirements must be met in order for the analysis to be accurate:

• The ion to be studied must be fully precipitated.
• It must be a pure chemical that forms the precipitate.
• Filtration of the precipitate must be simple.

Steps Involved :

• In a weight container with the lid cracked open, place an unknown. To get rid of the water, dry the bottle and sample in the oven. In a desiccator, cool the sample.
• In a beaker, indirectly weigh an unknown mass.
• To make a solution, dissolve the unknown.
• To the solution, add a precipitating agent. You may want to heat the solution because this reduces filtering loss by increasing the particle size of the precipitate. Digestion is the process of heating the solution.
• Filter the solution using vacuum filtration.
• Dry and weigh the precipitate you’ve gathered.
• To find the mass of the ion of interest, use stoichiometry based on the balanced chemical equation. Divide the mass of the analyte by the mass of the unknown to get the mass percent.

Types of Gravimetric Analysis :

1. Precipitation Method : Precipitation Gravimetry splits one or more portions of a solution by integrating it into a strong solution using a precipitation reaction.

1. Volatilization Method : Volatilization By heating or chemically decomposing the sample, we can separate components of our mixture using gravimetry.

1. Electrogravimetry : Electrogravimetry is a technique for separating and quantifying ions in a substance, most commonly a metal.

1. Thermogravimetry : Thermogravimetric analysis is a type of thermal analysis that measures changes in physical and chemical properties of materials as a function of temperature or time.

Procedure :

• If it isn’t already in solution, the sample gets dissolved.

• It is possible to modify the pH of the solution (so that the proper precipitate is formed, or to suppress the formation of other precipitates). If species that interfere with the analyte (by generating precipitates under the same circumstances as the analyte) are present, the sample may need to be treated with a different reagent.

• The precipitating reagent is added to the solution at a concentration that promotes the production of a “good” precipitate . Low concentrations, extensive heating (commonly referred to as “digestion”), or careful pH control may all be required. Coprecipitation can be minimized with proper digestion.

• The solution is thoroughly filtered after the precipitate has formed and had time to “digest.” The precipitate is collected using the filter; smaller particles are more difficult to filter.

The filter could be a piece of ashless filter paper in a fluted funnel or a filter crucible, depending on the technique. Filter paper is useful because it does not normally need to be cleaned before use; however, some solutions (such as concentrated acid or base) can chemically attack filter paper, causing it to tear during the filtration of large volumes of solution.

A crucible with a porous bottom, such as sintered glass, porcelain, or metal, is an option. Even at high temperatures, these are chemically inert and mechanically stable. They must, however, be thoroughly cleaned to avoid contamination or cross-infection (cross-contamination). To capture small particles, crucibles are frequently employed with a glass or asbestos fibre mat.

The solution should be analyzed after it has been filtered to ensure that the analyte has been completely precipitated. By adding a few drops of the precipitating reagent, this is easily accomplished; if a precipitate forms, the precipitation is incomplete.

• After filtration, the precipitate is burned, or charred, which includes the filter paper or crucible. The following is accomplished as a result of this:

The moisture that remains is eliminated (drying).

Second, the precipitate is transformed into a chemically more stable state. Calcium ions, for example, might be precipitated with oxalate ions to make calcium oxalate (CaC2O4), which could then be heated to form the oxide (CaO). The empirical formula of the weighed precipitate must be known, as well as the purity of the precipitate; if two forms exist, the findings will be erroneous.

The precipitate cannot be accurately weighed while still on the filter paper, nor can it be totally removed from the filter paper to be weighed. In a crucible, carefully heat the precipitate until the filter paper burns away, leaving only the precipitate. (As the name implies, “ashless” paper is employed to ensure that the precipitate is free of ash.)

• The precipitate is weighed after it has been allowed to cool (ideally in a desiccator to prevent it from absorbing moisture) (in the crucible). The beginning mass of the empty crucible is deducted from the final mass of the crucible containing the sample to compute the analyte’s final mass. Because the precipitate composition is known, calculating the mass of the analyte in the original sample is simple.

Advantages :

• When employing a current analytical balance, it is precise and accurate.
• Filters can be tested for completeness of precipitation, and precipitates can be examined for the presence of contaminants, thus possible causes of mistake can be quickly identified.
• It is an absolute method, requiring simple direct measurement with no calibration.
• The test might be done with relatively inexpensive equipment; platinum crucibles and a muffle furnace are the most expensive items.
• Gravimetric analysis can also be used to calculate the atomic masses of numerous elements with six-figure precision.
• Gravimetry allows for relatively little instrumental error and does not necessitate a sequence of standards to calculate the unknown.

Disadvantages :

• The main downside of this procedure is that it takes a long time to complete.
• In today’s world, chemists prefer other procedures to this one.
• In general, gravimetric analysis can only analyse a single element or a small group of elements at a time.
• Techniques are frequently complicated, and a little blunder in one can spell disaster for an investigation (colloid formation in precipitation gravimetry, for example).
• Gravimetric analysis is based on mass measurement.

Conclusion : 

Gravimetric analysis is a set of laboratory procedures for quantitative analysis based on the mass of an analyte.

Gravimetric analysis is a technique for determining the amount of an ion in a solution by dissolving a known amount of a compound containing the ion in a solvent and then separating the ion from its constituent. The ion is then weighed after being precipitated or evaporated out of solution. Precipitation gravimetry is a type of gravimetric analysis.

Volatilization gravimetry is another type of gravimetric analysis. Compounds in a combination are separated using this approach, which involves heating them to chemically breakdown the material.

Volatile substances evaporate and are lost (or collected), resulting in a quantifiable reduction in the solid or liquid sample’s mass.