# Gravimetric Analysis

Gravimetric analysis is a type of lab technique used to determine the mass or concentration of a substance by measuring a change in mass. The chemical we are trying to quantify is also known as the analyte. In other words, it is a technique through which the amount of an analyte (the ion being analyzed) can be determined through the measurement of mass. An analyte, chemical species or component is a substance or chemical constituent that is of curiosity in an analytical procedure.

Gravimetric analyses depend on comparing the masses of two compounds which contain the analyte. It is thought to be the most accurate method of determining particulate mass concentration, as they are capable of sampling at the very lowest detection limits. A model is taken by drawing a calculated volume of air through a collection substrate, which is sent for further analysis. Gravimetric analysis is acclaimed as the most accurate method because it is the process of producing and weighing a compound or element in as pure form as possible after any type of chemical treatment has been carried out on the substances which have to be examined.

Gravimetric analysis can be applied in real life for many users say, for example, to monitor levels of lead in water for human consumption, which if not monitored could cause lead poisoning and death. Some more examples of daily usage are the nutritional information tables on foods printed on their packages because many of the components listed are analyzed in a lab and % composition measured by mass. One more application is determining the mineral content of your drinking water, and minerals such as lead as mentioned above, fluoride, mercury, calcium, etc.. Determining total suspended solids in water is yet another gravimetric application. One very valuable, as well as a very useful application, is making sure the gold content in your jewelry is true to the jewelry shop seller's description. Determining the amount of fat in milk can be done by gravimetric analysis. Even a patient being weighed at the doctor's office is a form of gravimetric analysis, as recorded weight changes over a period of time could be a sign of illness.

The identification of any element or compound can be done by using qualitative analysis or quantitative analysis. Gravimetric analysis is a Quantitative analysis and is based on the mass of a given solid while qualitative analysis is based on volume analysis.

The gravimetric analysis may be classified into four fundamental types:

1. Physical gravimetry or Volatilization Gravimetry also involves separating components of the mixture by examining them by heating them or chemically decomposing them
2. Thermogravimetry is a way of thermal analysis in which changes in physical and chemical properties of materials are calculated as a function of increasing temperature or as a function of time.
3. Precipitative gravimetric analysis, which uses a precipitation reaction to separate one or more parts of a solution by incorporating it into a solid and
4. Electrodeposition orElectrogravimetry is a method which is used to separate and quantify ions of a given substance, generally a metal.

These four methods differ in the preparation of the sample before weighing of the analyte. Physical gravimetry is one of the most common types used in environmental engineering. Of these methods, there are 2 common types involving changes in the phase of the analyte to separate it from the rest of a mixture, resulting in a change in mass.

The steps commonly followed in the gravimetric analysis are

(1) preparation of a solution with a known weight of the sample,
(2) separation of the desired constituent,
(3) weighing the isolated constituent, and
(4) computation of the total of the particular constituent in the sample from the realistic weight of the isolated substance.

The most commonly used method that is employed for isolating the desired constituent from a solution of a sample, is precipitation—that is, transformation into a substance which is not soluble in the solution. A reagent is added that forms an insoluble compound with the desired constituent but which will not precipitate other constituents of the sample. The precipitate obtained is separated by filtration, washed free of soluble impurities, dried in a desiccator or oven, or ignited to remove moisture, and then weighed.

Certain substances can be separated easily because of their property, which is easy convertibility into gaseous compounds. This can be seen in the determination of carbonate in mineral analysis. The sample is treated with an acid, and carbon dioxide is evolved as a gas. The gas is absorbed on a weighed quantity of a solid alkaline reagent, and the amount of carbon dioxide can be determined from the increases in weight of the absorbent.

The method of Electrode position is used in order to separate certain metals that can be plated out by passing an electric current through a solution of their salts. Copper in alloys can be determined using this method as long as the sample is free from another type of metals. To carry out an accurate gravimetric analysis, the following conditions must be met:

• 1. The ion being examined should be totally precipitated.

• 2. The precipitate must be a pure compound.

• 3. The precipitate must be easily filtered.

• Procedure for gravimetric analysis

• 1. Fill enough unknown into a weighing bottle with the lid on sideways and then dry in the oven. Cool it in a desiccator.

• 2. Using some indirect methods, weigh some mass, determined to 0.1 mg, of the unknown into a beaker.

• 3. Now, dissolve the unknown.

• 4. Add a precipitating agent to the solution

• 5. Test for complete precipitation, by adding one drop of the precipitating agent and the look for signs of the precipitate.

• Then, filter the solution using vacuum filtration. Using a rubber policeman (which is a hand-held flexible natural-rubber or plastic scraper. Dry and weigh the precipitate.

• 6. Use stoichiometry to determine the mass of the ion being analyzed.

• 7. Lastly, find percent by mass of analyte by dividing the mass of the analyte by the mass of the unknown.

• If Pb2+ had been used to precipitate the chloride, the computation would be required to be modified to report for the fact that each mole off PbCl2 comprises of two moles of chloride. The lead might not be a fine precipitating reagent, though, because PbCl2 is somewhat soluble and therefore, rather than in the precipitate, a small amount of chloride would remain in solution.

1. It is precise and accurate when using modern analytical balance.

2. The possible sources of error can be readily checked as filtrates can be tested for completeness of precipitation and precipitates might be scrutinized for the existence of impurities.

3. It is an absolute method; it only involves direct measurement without any form of calibration.

4. The determination could be carried out with relatively inexpensive apparatus; the most expensive items are platinum crucibles and muffle furnace.

5. Gravimetric analysis can also be used to determine the atomic masses of many elements up to the extent of six-figure accuracy.

6. Gravimetry provides only very little room for instrumental error and it also does not require a series of standards for calculating the unknown.