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Gravimetric Analysis

Gravimetric, electrogravimetric, forms of thermal analysis, are concerned with the process of producing and weighing a compound or element in as pure form as possible after some form of chemical treatment has been carried out on the substance to be examined. Traditional gravimetric determinations are carried out on the substance to be examined.

Traditional gravimetric determinations are concerned with the transformation of the element, ion or radical to be determined into a pure stable compound which is suitable for direct weighing or for conversion into another chemical form that can be readily quantified. The mass of the element, ion or radical in the original substance can then be readily calculated from the knowledge of the formula of the compound and the relative atomic masses of the constituent elements.

Traditional gravimetric procedures are essentially manual in character and labor intensive whereas electrogravimetric methods may be considered as partially instrumental and thermal methods as highly instrumental. It is appropriate to mention at this stage the reasons for the continuous use of gravimetric analysis despite the disadvantage that it is generally somewhat time-consuming.

The advantages offered by gravimetric analysis are:

  1. It is accurate and precise when modern analytical balances are used.
  2. Possible sources of errors are readily checked since filtrates can be tested for completeness of precipitation and precipitates may be examined for the presence of impurities.
  3. It is an absolute method, it involves direct measurement without any form of calibration being required.
  4. Determinations can be carried out with relatively inexpensive apparatus, the most expensive items are a muffle furnace and sometimes platinum crucibles.
Gravimetric analysis is a macroscopic method usually involving relatively large samples compared with many other quantitative analytical procedures. It is possible to achieve a very high level of accuracy, and even under normal laboratory conditions, it should be possible to obtain repeatability of results within 0.3-0.5%.

There are two main areas of application for gravimetric methods.
  1. Analysis of standards to be used for the testing and calibration of instrumental techniques.
  2. Analysis requiring high accuracy, although the time consuming nature of gravimetry limits this application to small numbers of determinations.
Besides this, gravimetric and electrogravimetric procedures provide a very broad training experience in laboratory procedures. Thermal analysis provides more and more insights into chemical and increasingly into biochemical, structures and reactions occurring under thermal conditions.

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Gravimetric Analysis Definition

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

Gravimetric analysis or gravimetric determination is a method of quantitative analysis in analytical chemistry in which any element or compound gets precipitated from its solution by using suitable reagent. The precipitate obtained from the reagent can be collected by filtration. The filtrate is washed and dried to remove moisture and other impurities and weighed. The mass of the precipitate gives the amount of analyte in the original sample and its chemical composition. The unknown compound involved in quantitative analysis is called as an analyte.

For successful gravimetric analysis, the desired substance must be completely precipitated and this precipitate must show low solubility. Gravimetric analysis also gets affected by common ion effect. The solubility of precipitate can be further decreased by decreasing the temperature of the solution by using an ice bath. The final product should have less impurity, should be filtered and dried as well so that the weight of moisture does not show any error in the final result.
Gravimetric process can be four types:
  1. Precipitation methods
  2. Volatilization methods
  3. Electrogravimetry
  4. Thermogravimetry

Mechanism of Precipitation

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The reagent used in gravimetric analysis for precipitation is called as precipitating agent. When it is added to the solution of the ion under analysis there is an initial induction period before nucleation occurs. The range of this induction period depends upon the reagent and reactant. 

Induction period is followed by nucleation in which small aggregates or clumps of atoms form and due to this clumps of particles crystals are formed in the filtrate. Each of these nuclei or aggregates of particles can be composed of just a few atoms. For example, if silver nitrate is slowly reacted with hydrochloric acid, silver chloride would form and silver ions which are present in excess compared to Cl- ions would congregate around them. 

With this silver ion, there are some nitrate ions also which further aggregate from the silver chloride nucleus. They are known as counter ions and have a tendency to aggregate around the center as these centers have a net positive charge due to the presence of excess of silver ions and negative charge is required to maintain electrical neutrality. Hence the presence of the layer of counterions around the primary adsorbed ions forms an electric double layer.

Gravimetric Determination

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For each compound or element, there is a certain type of procedure in gravimetric analysis and its determination. In general, we follow certain steps for this type of analysis.
  • First, the sample has to dissolve in solvent if it is not already insoluble.
  • For the proper precipitation and suppression of formation of other precipitates, the pH of the solution has to be checked. If there is any other compound or element present in the solution which can interfere with precipitation reaction, it must be removed. Such compounds are called as interference.
  • Once the precipitating reagent is added to the solution, it favors the formation of a "good" precipitate. For a good precipitation low concentration, extensive heating, or careful control of the pH is required. Strong heating is termed as digestion which can help reduce the amount of co precipitation. After the formation of precipitate, it has been allowed to "digest" and the solution is carefully filtered.
  • The filter must be comparable to the size of precipitate particles. Generally, a piece of ashless filter paper in a fluted funnel or a filter crucible is used for filtration. In place of filter paper, a crucible can be used whose bottom is made of some porous material, like sintered glass or porcelain. These crucibles are chemically inert and mechanically stable at high temperatures.
  • If there is no precipitation after adding more precipitating agent, it shows that the precipitation is completed.
  • Filtration collects precipitate on filter paper or on crucible hence the precipitate including the filter paper or crucible is heated. Due to this heating the moisture associated with precipitate is removed and it is converted to a stable form. For example, Calcium ion can be precipitated by using oxalate ion in the form of calcium oxalate (CaC2O4). This calcium oxalate can be further heated and form oxide (CaO). Hence due to the presence of CaO the results will be inaccurate.
  • For removing filter paper, it is carefully heated until the filter paper has burned away and only the precipitate is left.
  • After cooling of this precipitate, it is weighed in the crucible and the mass of analyte in the original sample is calculated.

Gravimetric Analysis of a Metal Carbonate

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For the determination and identification of a Group 1 metal carbonate compound by gravimetric analysis, an unknown metal carbonate is weighed and dissolved in water. Then the solution of calcium chloride is added to the solution, so calcium carbonate is precipitated. 

The obtained precipitate CaCO3 is filtered, dried, and weighed. Carbonate ions came from unknown metal carbonate, hence the moles of calcium carbonate must be equal to the moles of given unknown metal carbonate which are added to the original solution. 

The mass of the unknown carbonate divided by the moles of calcium carbonate gives the weight of the formula. From the mass of obtained calcium carbonate, we can calculate the precipitated moles of CaCO3 and molar mass of the unknown metal carbonate.

Reaction involve in analysis

M2CO3(s) $\to$ 2M+(aq) + CO32-
Ca2+(aq) + CO32-(aq)
$\to$ CaCO3(s)

Hence the overall reaction is

CaCl2(aq) + M2CO3(aq) $\to$ CaCO3(s) + 2MCl(aq)

Method of gravimetric analysis of metal carbonate
  1. Measure the mass of empty and dry crucible.
  2. Take unknown metal carbonate in the crucible and measure the weight.
  3. Heat this crucible with metal carbonate slowly on the Bunsen burner for 2-3 minutes and then set the crucible to cool on a heat-resistant pad.
  4. Weigh the crucible on an analytical balance. Repeat the same steps around 6 and 7 times until the mass of the crucible and unknown carbonate no longer decreases.
  5. Add the crucible residue to a 500-mL beaker.
  6. Add 200 ml of distilled water to the beaker and stir to dissolve the residue.
  7. Then add about 125 ml of the 0.2 M CaCl2 solution to the same beaker and stir.
  8. Once precipitate formed, let the precipitate settle for 5 minutes.
  9. Take a piece of filter paper, weigh it on the analytical balance and record the mass of the filter paper.
  10. Decant the solution using a stirring rod for removing excess of water.
  11. After removing the excess water, rinse the flask with distilled water and allow the funnel to drain.
  12. Take the filter paper put it on a watch glass. Place the watch glass with filter paper in a drying oven set at 383– 393 K.
  13. Cool, weigh the filter paper and the solid CaCO3 on an analytical balance and record the mass.
  14. Repeat the same steps for 5–27 times, until the mass readings do not change by more than 0.005g.

Gravimetric Analysis of a Chloride Salt

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Just like estimation of metal carbonates, metal chloride can also be estimated by using gravimetric analysis.

Chloride ion can be quantitatively precipitated from its solution by the addition of silver ion and forms white precipitate of silver chloride:

Ag+(aq) + Cl-(aq) AgCl(s)

Due to less solubility of silver chloride (0.0001 g in 100 ml or H2O at 293K), the addition of silver nitrate solution to the aqueous solution containing chloride ion precipitates AgCl. 

The precipitate can be collected after filtration on a filter paper, dried, and mass. The mass of the AgCl obtained gives the amount of chloride present in the given sample. 

Since in the balanced chemical reaction of silver ion with chloride ion, 1 mole of chloride ions reacts with 1 mole of silver ions to form 1 mole of silver chloride. 

grams AgCl 
Moles Cl- = moles AgCl = __________________
Molar mass AgCl

Mass of Cl in sample = (moles Cl-) (molar mass of Cl) 

(Mass of Cl in sample) x (100) 
%Cl in sample = ______________________
Mass of sample in gram
  • Take the sample from the paper into a weighing bottle and weigh it.
  • Transfer the sample from the weighing bottle to a clean beaker and weigh the empty bottle.
  • Dissolved sample with distilled water and 1 ml of 6 MHNO3. Stir the solutions with glass rods until the sample has dissolved. During stirring add 30 ml of 0.125 M AgNO3 solution in drop wise manner. Place a lid over the beaker.
  • To protect from light, cover the beakers and watch glasses with foil.
  • Warm the solutions gently on hot plate and keep it warm for approximately 30 minutes.
  • Make the paper wet with distilled water and transfer the precipitate. Now wash the obtained precipitate with water.
  • Wash same precipitate with acetone. Remove the filter paper and store it.
  • When the precipitate of AgCl becomes dry, mass the filter papers plus AgCl and calculate the mass of AgCl.

Thermal Gravimetric Analysis

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Thermogravimetric Analysis (TGA) is the technique to measure the amount and rate of change in the weight of a material as a function of temperature or time in a controlled atmosphere. 

By using this analysis, one can predict the thermal stability of given sample and it is used to determine the composition of given sample. This technique is used for those substances which can exhibit weight loss or gain due to certain reactions like decomposition, oxidation, and dehydration.

Thermogravimetric analysis gives information about the composition of multi component Systems and their thermal Stability. This technique is useful to measure the oxidative Stability of Materials and in the estimation of lifetime of a product. It shows the decomposition Kinetics of Materials and the effect of reactive or corrosive atmospheres on Materials. By using this technique, we can calculate the moisture and volatiles content of materials.

Thermogravimetric Analyzer

The thermogravimetric analyzer consists a container which is in the form of a crucible for holding the sample, a furnace for heating the given sample at a high temperature, and an appropriate balance which can continuously monitor the sample.

The modern equipment of thermogravimetric analyzer is far more sophisticated and provides much greater accuracy, precision and speed and sensitivity. 

Out of all these characteristics, the main advantage of modern thermogravimetric analyzer instrumentation is of greater speed, hence it generates much more data with quicker interpretation and, consequently, betters quality control and faster scientific progress.
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