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.

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 the experimental 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.

26)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 neutral charge. 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.

Practical Treatment of Colloidal Precipitates

Colloids are best precipitated from hot. stirred solutions containing sufficient electrolyte to ensure coagulation. The filterability of a coagulated colloid frequently improves if it is allowed to stand for an hour or more in contact with the hot solution from which it was formed. This process is known as digestion.

Digestion is a process in which a precipitate is heated for an hour or more in the solution from which it was formed (the mother liquor).

Coagulation of Colloids

Coagulation of a colloidal suspension can often be brought

1.by short period of heating to decreases the number of adsorbed ions and thus the thickness, of the double layer.

2.Increase the electrolyte concentration of the solution.

If we add ionic compound to a colloidal suspension. The concentration of counter – ions increases in the vicinity of each particle. The net effect of adding an electrolyte is thus a shrinkage of the counter-ion layer.

Peptization of Colloids

Peptization is the process by which a coagulated colloid reverts to its original dispersed state. Peptization is the reverse of coagulation (the precipitate reverts to a colloidal state and is lost). It is avoided by washing with an electrolyte that can be volatized by heating.

Crystalline Precipitates

Crystalline precipitates are generally more easily filtered and purified than are coagulated colloids. In addition, the size of individual crystalline particles, and thus their filterability, can be controlled to a degree.

Mechanism of Precipitate Formation

The effect of relative supersaturation on particle size can be explained if we assume that precipitates form in two ways, by: nucleation and by particle growth. The particles size of a freshly formed precipitate is determined by the mechanism of predominates.

After the addition of the precipitating agent to the solution of the ion under analysis there is an initial induction period before nucleation occurs. This induction period may range from a very short time period to one which is relatively long, ranging from almost

instantaneous to several minutes.