Precipitates have often been placed in one of three different categories based on their macroscopic appearance. A precipitate can be colloidal, crystalline, or curdy gelatinous. The kind of precipitate given by a specific salt has no correlation to its degree of insolubility but is determined by the particle size distribution of the precipitate, and its affinity toward the water molecules and ions of the surrounding solution.
Identification tests are most often designed to give fast results. In general this means that test solution and reagent concentrations are adjusted so that a relatively high degree of supersaturation of the desired salt is achieved. Under such conditions, chances are that the precipitation will occur immediately, but it also means that the precipitate formed will consist of a high number of very small particles. Often particles are so small that a colloidal suspension is achieved. Colloids are particles with diameters in the range of 1 ^m down to 0.001 ^m. They are so small that they form suspensions so stable that they are sometimes called colloidal solutions. Particles of a size below the interval tend to form true solutions, and particles of a size above the interval tend to form less stable real suspensions.
Colloids are drawn together by van der Waals forces, but these are balanced by the repulsive power of an electrical double layer that builds up on their surfaces. The ratio of these forces largely influences the appearances of such suspensions. In the core of an ionic particle, cations and anions are well ordered in stochiometric units. The salt forming anion or cation, whichever is in excess in the solution, will be absorbed on the surface of the colloid. This anion or cation, called the primary absorbed ion, will give the colloids a positive or negative charge, causing opposites to repel each other. This hinders aggregation of the individual particles into larger structures. But the electrical charge also attracts ions of the opposite charge to the immediate vicinity of the particle surface, building up an electrical double layer. If this ion, called the counter-ion, is loosely attracted, giving a diffuse double layer, its presence will not neutralize the charge of the primary absorbed ion, creating repulsion between the particles. But if for some reason, the counter-ion is very closely attracted to the primary absorbed ion layer, the result is a narrower double layer, causing the particle charge to be neutralized. Such particles have a tendency to aggregate into a more assembled precipitate, which appears curdy if the salt in question has a limited affinity toward water and gelatinous if it has a high affinity toward water.
If, for example, chloride is precipitated with an excess of silver nitrate, colloidal silver chloride is formed having silver as the primary absorbed ion and nitrate as the counter-ion. This is often represented by the formulas below. The number of dots between the primary absorbed ion and the counter-ion symbolizes the proximity of the two.
The process of reducing the distance between the primary absorbed ion and the counter-ion, and thereby settling the colloidal precipitate is called floccu-lation or coagulation. This process can be achieved by heating the colloidal suspension or by raising its ionic strength by adding an inert ion. The process of heating is sometimes referred to as digestion and is also used with crystalline precipitates, but then the aim is to facilitate crystal growth and not coagulation. However, since the aim in both cases is to change a small particle size precipitate into a more settled one that is easier to handle, names are used somewhat interchangeably.
The fact that flocculation is the result of high ionic strength is of some importance in relation to identification tests, since ionic strength is influenced by sample preparation steps, the nature of the counter-ion of the substance to be examined, and the test solution contaminants. The concentration that is necessary for a given ion to cause flocculation depends on the nature of the ion, especially its valance. The higher the valance, the stronger its floc-culation ability.
Some salts have a tendency to form a precipitate consisting of fewer but larger particles, even though precipitation is initiated from a highly supersaturated solution. Or, they may form a colloidal precipitate, which rapidly changes in particle size distribution. Such a precipitate will appear crystalline, and particles will, unlike in a colloidal suspension, quickly settle. The factors determining the particle size distribution of a precipitate are discussed in some detail in Chapter 4 "Precipitation in Limit Tests."
It is of some importance for the operator performing qualitative tests to have a basic understanding of the various types of precipitates, since a certain appearance of the obtained precipitate is sometime an explicit requirement of the test. Even if no specific type of precipitate is obligatory, some insight into their differences can prove valuable in the case of analytical problems in performing a test.
One obvious way of investigating a suspected analytical problem is to perform a positive control, using a test on a substance of known identity. If a known sample of the substance to be examined is at hand, it would be the obvious choice. In cases where this is not obtainable, other compounds containing the substance to be examined can be used, as for example another salt of the wanted cation. It should then be remembered, however, that precipitate appearance can deviate from what one would have obtained using the right substance, owing to effects given by the other part of the molecule. One example is a precipitate formed as directed in 3.32 Sulfates might deviate when made on the sulfate salt of a large organic cation rather than on sodium sulfate. Another example is that differences in ionic strength in test solutions of different substance cause varying degrees of coagulation of otherwise similar precipitates.
To introduce the appearance of a colloidal, curdy, gelatinous, or crystalline to an inexperienced operator, one could use a few precipitates of established nature. Performing the first part of test (a) in 3.16. Chlorides on sodium chloride, without shaking the test tube, will yield a colloidal precipitate, which is easily coagulated to a curdy precipitate, for example by shaking, heating, or adding an inert ion. Performing test (a) for 3.32. Sulfates on sodium sulfate will yield a colloidal precipitate that can be digested by heating to a crystalline precipitate. A good example of a gelatinous precipitate is achieved by performing the first part of the test 3.35. Zinc on zinc chloride, since the zinc hydroxide is gelatinous, as are many other metal hydroxides.
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