Environmental chemistry One of the ways to destabilize a colloid is to add alum.
ID: 1055528 • Letter: E
Question
Environmental chemistry One of the ways to destabilize a colloid is to add alum. How does alum act to destabilize a colloid, and how is it more effective than NaCl? Why is it not practical to titrate a colloid to the point of zero charge? As alum is added to a colloid, the turbidity initially decreases, indicating an improvement in the situation, but then the turbidity increases again. Eventually the alum forms a sweep floc to deal with the colloid. Explain what is happening at each point along the way. What are the advantages and disadvantages ot destabilizing a colloid using alum rather than using synthetic polyelectrolytes? Under what conditions would you use each method?Explanation / Answer
a)
Colloids are stable because of their surface charge. In order to destabilize the particles, we must neutralize this charge by adding alum. Since most colloids found in water are negatively charged, neutralization can take place by addition of an ion of opposite charge to the colloid.
Alum is more effective than NaCl, because it is a trivalent ion can reduce the charge as much as 1,500 to 2,500 moles of a monovalent ion.
b) It is not practical to titrate a collide at to the point of zero charge. At point of zero charge, the colloidal system exhibits zero zeta potential, minimum stability, maximum solubility of the solid phase, maximum viscosity of the dispersion.
c)
When alum is added to a water containing alkalinity, the following reaction occurs.
Al2(SO4)3.14H2O + 6HCO3 -----à 2Al(OH)3 + 6 CO2 + 14 H2O + 3 SO42-
If the chemical reactions in coagulating water are going to take place, the chemical must be mixed with the water. Mixing as it is called is the process whereby the chemicals are quickly and uniformly dispersed in the water. During coagulation the chemical reactions that take place in rapid mixing form precipitates, such as aluminum hydroxide. Turbidity initially decreases. The precipitates formed in these processes must be brought into contact with one another so that they can agglomerate and form larger particles, called flocs. This contacting process is called flocculation
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