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.
Thus, when (Q - S)/S is large, the precipitate tends to be colloidal, when (Q - S)/S is small, a crystalline solid is more likely.
| High relative supersaturation | many small crystals |
| (high surface area) | |
| Low relative supersaturation | fewer, larger crystals |
| (low surface area) |
Obviously, then, we want to keep Q low and S high during precipitation. Several steps are commonly taken to maintain favorable conditions for precipitation or theexperimental control of particle size
1. Precipitate from dilute solution. This keeps Q low.
2. Add dilute precipitating reagents slowly, with effective stirring, this also keeps Q low. Stirring prevents local excesses of the reagent.
3. Precipitate from hot solution. This increase S. the solubility should not be too great or the precipitation will not be quantitative (with less than 1 part per thousand remaining). The bulk of the precipitation may be performed in the hot solution, and then the solution may be cooled to make the precipitation quantitative.
4. Precipitate at as low a pH as is possible to maintain quantitative precipitation. As we have seen, many precipitates are more soluble in acid medium, and this slows the rate of precipitation. They are more soluble because the anion of the precipitate combines with protons in the solution.
Applications of Gravimetric Methods
Gravimetric methods have been developed for most inorganic anions and cations, as well as for such neutral species as water, sulfur dioxide, carbon dioxide, and iudine. A variety of organic substances can also be easily determined gravimetrically.
Examples include lactose in milk products, phenolphthalein in laxatives, nicotine in pesticides, cholesterol in cereals, and benzaldehyde in almond extracts. Indeed, gravimetric methods are among the most widely applicable of all analytical procedures.
Colloidal Precipitates
We said avoid colloid suspension, but if we do several process it can be make filterable.
Individual colloidal particles are so small that they are not retained by ordinary filters. Moreover, Brownian motion prevents their settling out of solution under the influence of gravity. Fortunately, however, we can coagulate, or agglomerate, the individual particles of most colloids to give a filterable, amorphous mass that will settle out of solution.
Structure of Colloids
Colloidal suspensions are stable because all of the particles of the colloid are either positively or negatively charged. Colloidal particles are very small and have a very large surface-to-mass ratio, which promotes surface adsorption. (The process by which ions are retained on the surface of a solid is known as adsorption).
As a precipitate forms, the ions are arranged in a fixed pattern. In AgCl, for example, there will be alternating Ag+ and Cl- ions on the
surface. While there are localized (+) and (-) charges on the surface, the net surface charge is zero.
The adsorption creates a primary layer that is strongly adsorbed and is an integral part of the crystal. It will attract ions of the opposite charge in a counter layer (counter –ion layer)or secondary layer so the particle will have an overall neutralcharge. There will be solvent molecules interspersed between the layers. Normally, the counter layer completely neutralizes the primary layer and is close to it, so the particles will collect together to form larger sized particles; that is, they will coagulate. However, if the secondary layer is loosely bound, the primary surface charge will tend to repel like particles, maintaining a colloidal state.