|Analytical Chemistry I
Gravimetric Methods of Analysis
Gravimetric Analysis Video Lecture
Gravimetric Data Calculations Video Lecture
Gravimetric methods of analysis are based on the measurement of mass. The two gravimetric methods are precipitation methods and volatilization methods. In precipitation methods the analyte is converted to an insoluble product, filtered, washed and heated. The mass of the resulting residue is determined. In volatilization methods the analyte is heated and the analyte or its decomposition product is collected. The resulting loss of mass is determined.
General form for calculations
The calculations for gravimetric analyses are fairly straight-forward.
Gravimetric calculations are based on the fundamental stoichiometric calculations. (Note: You may wish to review these calculations before continuing.) The basic form of the calculation is:
The gravimetric factor (GF) comes from a combination of the mole ratios and the formula weights used in the stoichiometric calculation.
For example, if you were looking for SO3 and your precipitate was BaSO4, the gravimetric factor would be:
The numbers, 80.064 and 233.391, are the formula weights of SO3 and BaSO4, respectively.
The main question is how to determine the mole ratio without knowing the entire reaction. This is actually quite easy. Simply balance the common element. Most of the time oxygen is not considered. In the above example, sulfur appears in both terms.
There is only one sulfur in each term and the sulfurs are balanced. In other words, the mole ratio is 1.
Consider the following GF:
The common element is silver, Ag.
However, there are two silver atoms represented in the upper term and only one in the lower term.
To "balance" the silver atoms, a 2 is placed in front of the substance in the lower term.
The calculation set-up for this gravimetric factor would be:
Precipitates can be classified according to the size of the particles. Colloidal suspensions are composed of particles ranging from 10-7 cm to 10-4 cm diameter. Particles in crystalline suspensions are greater than 0.1 mm diameter.
Large particles are desired since they are filtered much easier. In order to obtain these large precipitate particles we must understand the factors that affect the particle size and use them to our benefit.
The size of the precipitate particle is affected by four factors.
- Precipitate solubility
- Reactant concentrations
- Rate reactants are mixed
Precipitation reactions are generally slow and supersaturation is very likely. When supersaturation is HIGH, a large number of small particles is formed. When supersaturation is LOW, a small number of large particles is formed. This results in a crystalline suspension.
Therefore, the goal in precipitation reactions is to minimize supersaturation conditions.
Variables That Minimize Supersaturation
The filterability of precipitates may be improved by encouraging the growth of large precipitate particles. "Digestion" refers to this process and generally means to allow the precipitate to form in hot, unstirred solutions.
Sometimes a precipitation reaction will only form very small particles and result in a colloidal suspension. Quantitatively filtering colloidal suspensions is difficult because of the small particle diameter. However, a colloidal suspension may be converted into a filterable solid. The process of converting a colloidal suspension to a filterable solid is called coagulation. The reverse process is called peptization.
Coagulation can be encouraged by either precipitating from hot, stirred solutions containing an electrolyte or by letting the precipitate stand for an hour or more in its mother liquor. (Digestion) The mother liquor is the hot solution from which the precipitate is formed.
Sometimes soluble compounds are removed from solution during the formation of precipitates. This is called co-precipitation. There are four types of co-precipitation.
- Surface adsorption (equilibrium process)
- Mixed crystal formation (equilibrium process)
- Occlusion (mechanical process)
- Entrapment (mechanical process)
After the precipitate is filtered, it is heated until its mass becomes constant. The primary purpose of this is to drive off any solvent and any other volatile species that may be present. The temperature and time required to produce a suitable product varies from precipitate to precipitate.
Some precipitates are also ignited. This process decomposes the solid, whose exact composition may not be known, and forms a new compound of known composition. This new compound is called the weighing form.
Gravimetric methods are slow. However, they are more efficient since they require less operator time and no calibration. The results are calculated directly from the experimental data and requires only the gravimetric factor and molar masses
Equipment used for gravimetric analyses are simple, reliable, relatively inexpensive and easy to maintain.
The sensitivity and accuracy of many analytical methods are limited by the devices used for measurements. For gravimetric analyses the analytical balance is the limiting measurement device. However, other factors must also be considered, such as solubility losses, coprecipitation errors, and mechanical losses of the precipitate.
Gravimetric methods are the method of choice, particularly if the analyte concentration in the sample is larger that 1%. Other analytical methods should be considered for analyte concentrations less than 0.1%.
Reagents used for gravimetric methods tend to form precipitates with groups of ions. In this sense they are selective. Usually they do not form precipitates with a specific ion. This means that ions within a group interferes with the determination of any other ion in the group unless a preliminary separation is performed.
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