Zwitterionic additives provide a means of altering the EOF without increasing conductivity. The magnitude of the EOF in a bare silica capillary increased by as much as 69% upon addition of 500 mM of zwitterion to the running buffer. The EOF enhancement increases linearly with the zwitterion concentration. With zwitterionic additives of the form +NH3-(CH2)n-COO-, the magnitude of the EOF increase is directly related to the number of methylene groups, (n), which ranges from n = 1 to 7. The endgroups on the zwitterions also affect the EOF enhancement. The effect of Z1-methyl (+N(CH3)3CH2CH2CH2SO3-) on EOF was not a function of either the buffer cation or pH. The EOF enhancement is a function of the dielectric increment of the additive and the nature of the amine functionality.
pH buffers are used extensively in research and industry making them an important chemistry topic for students to learn. This qualitative study uses the phenomenographic method and a resource theoretical framework to provide the first insights into how students approach conceptual buffer problems. Three scaffolded buffer question sets were designed to promote in-depth conceptual responses during a think aloud interview followed by retrospective reporting. Open-coding for activated resources led to three levels of resource activation: Surface Features, Building Connections, and Interconnected. Layered resource graphs provide a visual representation of a diverse array of activated resources, how resources are connected, and which question type promoted particular activations. Some resources such as Accept or Donate H + were consistently activated in all three questions whereas other resources such as pH relative to pK a were productive only in particular contexts, thereby highlighting the contextual dependence of resource productivity. Challenges were observed in productively activating crucial resources such as Accept or Donate H + and in maintaining activations over time even within the same scaffolded question. Specific suggestions are provided on making connections between resources to promote students to a higher level of resource activation and success with buffer problems. Future research should probe the types of activities that can promote productive resource activations and connections.
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