Mini-Encyclopedia of Papermaking Wet-End
Part Two: Definitions and Concepts
The colloid titration method is one way to estimate the net charge density of surfaces, polyelectrolytes, and the charge demand of colloidal materials in an aqueous mixture. What is actually measured is the capacity of the mixture to adsorb a polyelectrolyte of opposite net charge. In a typical case the mixture is first treated with a known excess of highly cationic polymer such as poly-DADMAC. Then it is filtered or centrifuged to remove solids. A small amount of indicator dye (usually toluidine blue-O) is added to a known volume or filtrate or supernatant solution. The blue solution is back-titrated with an anionic polyelectrolyte such as potassium polyvinylsulfate (PVSK) to a purple-pink endpoint. Complexation between the dye and the negatively charged polymer causes the color-change. Though the method can be a quick and reliable way to estimate charge demand of process water samples and fibrous slurries from paper mills, the results tend to be off-set from related methods based on electrokinetic measurements.
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