Surface charge density sample problem

In case of materials showing some degree of solubility the problem how to subtract the blank" is not trivial. Schulthess and Sparks proposed a back titration method to determine the surface charge density of such materials [45]. In this method a series of solutions of different pH is prepared and the solid is added to each solution. After certain time necessary to reach equilibrium a sample of supernatant is taken from each suspension and it is titrated back to the original pH value (before addition of solid). This method was designed to distinguish between surface charging and uptake/release of protons by soluble species. The volume of titrant is then substituted to Eq. (3.1) to obtain (Tq as discussed above for the pH and cip results, and the results obtained by back titration method are listed as pH and cip in Table 3.1. 

Multicommutation [40] provides flow analysis technique with flexibility and controllability. As a fluid delivery device, the electroosmotic pump has proved to be suitable for flow analysis with its large flow range (10 pL/min to 5.0 mL/min), stable flow rate (<4%, 4 h), pulseless driving, simplified apparatus, convenient control operation, and proper back pressure (<1.1 MPa) [41,42], etc. However, electrolyte solutions cannot be introduced into the porous core of the pump because of the influence on the surface charge density of the porous core and the electroosmotic flow (EOF). This problem can be resolved by aspirating sample and reagent solutions into a holding coil, as performed in SIA. 

The measurement of the electrical properties of highly resistive polymers presents a number of problems. The surface conductivity is often greater than the bulk value and is increased by absorbed impurities. The latter is also affected by impurities. The presence of localized-defect states in the energy gap means that contact with metals is accompanied by charge transfer to the polymer in the absence of an applied field. The triboelectric charge appears to be relatively insensitive to sample purity or preparation and even to polymer structure. This indicates that the important defects are structural, with a density that is almost sample independent. The work function of the metal affects the charge transfer, but large experimental uncertainty renders precise functional fits unreliable. 

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