A Gravimetric Analysis is a type of Chemical Analysis that focuses on discovering how much there is of a certain substance of interest in a worked sample, through the weight measured after a laboratory procedure or march.
The substance to be measured or calculated by means of a weighing is called Analyte . In Gravimetric Analysis, the amount of Analyte must be separated from the other components of the mixture or sample , as well as the solvent that has been involved in the separation mechanism.
Gravimetry Separation Methods
The separation methods are mainly of two types: Precipitation Methods and Volatilization Methods . Other methods are Electrodeposition, Solvent Extraction and Chromatography.
In Precipitation Methods , the Analyte is converted into a poorly soluble Precipitate that, after a treatment that leaves it pure and stable, is weighed. It is the most frequently used and with depth of concepts.
In Volatilization Methods , the Analyte or its decomposition products are volatilized at a suitable Temperature. The gas produced with this volatilization is collected and weighed or the analyte mass is determined indirectly by difference, such as the loss of mass in the sample.
Gravimetric Analysis by Precipitation
The Gravimetric Precipitation Analysis Method generally consists of seven well-defined stages:
1.- pH adjustment
2.- Addition of the Precipitating Reagent
6.- Drying, and in certain cases, calcination
7.- Heavy of the Pure Precipitate
A requirement that must be taken into account for this method to be effective is for the Precipitating Reagent to react specifically or selectively with the Analyte.
The Final Precipitate must comply with the following characteristics:
a.- Be easily filterable
b.- Be very slightly soluble, so that it is not distributed inside the solvent. Practically insoluble
c.- The Precipitate must not react with the components of the atmosphere, because it would be altered every second before the final weighing.
d.- It must have a known composition or formula after drying or calcining.
e.- It must precipitate quantitatively, that is, so that it can be calculated or measured.
f.- Enjoy high purity.
Formation and Properties of Precipitates
The diameter of the ions is a few tenths of Angstrom (1 Angstrom = 10 -8 cm); when they join, they form crystals, which must grow to a diameter greater than 10 -4 cm so that they can precipitate.
During the Growth Stage , the particles pass through a colloidal stage (diameter less than 10 -4 cm), in which they can still pass through a filter, and are not useful for determining a weighing. Already when they exceed this diameter and are more solid and stable, they are able to stay in a filter and begin to give confidence to the analysis.
The nucleation and growth are two processes can be distinguished to get to the formation of a precipitate. The average particle size of a precipitate is determined by the predominant process.
Larger particles are achieved when Growth predominates.
Types of Precipitates
The Precipitates, according to the size of their particles can be of 3 types: Colloidal Suspensions, Crystalline Precipitates, or Coagulated Precipitates.
The Colloidal Suspensions are commonly obtained. Colloidal particles pass through all types of filters. Luckily, by heating, stirring or adding an electrolyte you can get them to join each other forming agglomerates with amorphous mass, not a crystal, which settles and can seep.
The process of converting a colloidal suspension into a filterable solid is known as coagulation or flocculation .
The peptization is the process in which a coagulated colloid returns to its original state . To avoid it, an electrolyte is added to the wash water.
The crystalline precipitates , which are the most desirable precipitates but very few obtainable, more easily than the coagulated colloids filter. Unfortunately, very few substances form crystals when precipitating. The size of this type of particles can be improved using dilute solutions, slowly adding the precipitating reagent and stirring the solution well.
The Digestion of a Precipitate greatly helps the growth of the crystals . It consists of keeping the precipitate in contact with the solution, without stirring, at a temperature around 80 ° C.
The precipitates Coagulates obtained causing the particles to clump together.
To obtain better precipitates, it is recommended to slowly add the precipitating reagent, which must be diluted, stirring and in hot solution, in addition it should take digestion approximately one hour.
Precipitates can be contaminated by two processes: Coprecipitation and Post Precipitation.
The Coprecipitation is the process in which a substance that normally soluble is hauled along with the precipitate. It can occur due to occlusion or adsorption of impurity. In the Occlusion an impurity is enclosed within the crystal, because it grew around it. In the Adsorption the impurity is retained in the surface of the crystals.
The post – precipitation is the process in which an impurity is deposited after which the desired substance precipitated.
Examples of Gravimetric Analysis
- Nickel precipitates quantitatively in the form of Nickel Dimethylglyoxime.
- Sulfates from a sample are analyzed by precipitation of Barium Sulfate (BaSO 4 ).
- Magnesium present as Magnesium Oxide in a sample, precipitates as Ammonium Phosphate and Magnesium.
- Chloride analysis is achieved with a Silver Chloride Precipitate (AgCl).
- Aluminum is analyzed by precipitating with aqueous ammonia, forming Hydrated Aluminum Oxide (Al 2 O 3 xH 2 O).
- Iron is analyzed by precipitating as Hydrated Iron Oxide (Fe 2 O 3 xH 2 O).
- Tin is analyzed as a tin oxide precipitate (SnO 2 ).
- Lead is analyzed as a lead sulfate precipitate (PbSO 4 ).
- Copper is analyzed as a precipitate of Copper Thiocyanate (CuSCN).
- Zinc is analyzed as a precipitate of Zinc Pyrophosphate (Zn 2 P 2 O 7 ).