Aluminium chloride, flocculation

The temperature also affects the process of coagulation.8 With the chlorides of potassium, sodium, lithium and ammonium the velocity of flocculation varies inversely as the temperature 9 with the chlorides of barium, strontium, calcium, magnesium and cadmium the velocity varies directly as the temperature with aluminium chloride it is independent of the temperature. Heating thus stabilises the sol towards univalent cations but diminishes the stability towards bivalent ions.10 The... 

Figure 7.31 Diagrammatic drawing of flocculation and controlled flocculation in a sulfamerazine suspension the effect of the addition of sodium dioctyl sulfosuccinate (docusate sodium) and aluminium chloride is shown and two interpretations of the results are outlined.

The attractive forces between suspension particles are considered to be exclusively London-van der Waals interactions (except where interparticle bridging by long polymeric chains occurs). The repulsive forces, as discussed in Chapter 8, comprise both electrostatic repulsion and entropic and enthalpic forces. In aqueous systems the hydrophobic dispersed phase is coated with hydrophilic surfactant or polymer. As adsorption of surfactant or polymer (or, of course, both) at the solid-liquid interface alters the negative charge on the suspension particles, the adsorbed layer may not necessarily confer a repulsive effect. Ionic surfactants may neutralize the charge of the particles and result in their flocculation. The addition of electrolyte such as aluminium chloride can further complicate interpretation of results electrolyte can alter the charge on the suspension particles by specific adsorption, and can affect the solution properties of the surfactants and polymers in the formulation. Some aspects of the application of DLVO theory to pharmaceutical suspensions and the use of computer programmes to calculate interaction curves are discussed by Schneider et al. [4]. 

Typically, the electrodes are of lead dioxide on a titanium substrate in the form of horizontal perforated plates, usually 5-40 mm apart, depending on the conductivity of the liquid. A potential difference of 5-10 V may be applied to give current densities of the order of 100 A/m2. Frequently, the conductivity of the suspension itself is adequate, though it may be necessary to add ionic materials, such as sodium chloride or sulphuric acid. Electrode fouling can usually be prevented by periodically reversing the polarity of the electrodes. Occasionally, consumable iron or aluminium anodes may be used because the ions released into the suspension may then assist flocculation of the suspended solids. 

As an example the following data obtained by Gann he. cit.) on the flocculation of an aluminium hydroxide sol with potassium chloride (60 millimol. KOI per litre) may be cited. 

If treatment with ammonia gave a flocculent precipitate (ferric or aluminium hydroxide), the weighed zinc oxide is dissolved in dilute hydrochloric acid, the solution treated with ammonium chloride and excess of ammonia, and the precipitate collected, washed, again dissolved in hydrochloric acid and reprecipitated with ammonium chloride and ammonia the precipitate thus obtained is collected, washed, calcined and weighed. The weight of the ferric oxide and alumina thus obtained is deducted from that of the zinc oxide previously found. 

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