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Learning Goals:
Learning Goals:
Learn how the following play a role in isolating your product (the biodiesel synthesized in the previous week's experiment) from a reaction mixture.
Solubility
Density
Drying agents
Viscosity
Ecotoxicity wrap-up (radish seed germination experiment from last week)
Scientific Inquiry: How do scientists synthesize data to make arguments about scientific models?
Science as a Social Practice: How do scientists communicate their findings to each other?
In this experiment, you will isolate and purify the product from your biodiesel synthesis. You will also collect results from the ecotoxicity assay you set up last week. Lastly you will investigate the viscosity of your biodiesel along with that of several other different samples to show how the synthesis has changed the original vegetable oil sample and how your product compares to commercial diesel and biodiesel.
Synthesis of Biodiesel
Synthesis of Biodiesel
In the previous experiment, you set up your biodiesel synthesis by mixing vegetable oil and a solution of sodium hydroxide, NaOH, dissolved in methanol, CH3OH (as opposed to dissolving in water). Over the course of the week, the vegetable oil and methanol reacted to form biodiesel and glycerol, as shown in the following equation.
Biodiesel reaction equation
Biodiesel and glycerol do not dissolve into each other (they are immiscible). Instead, they settle into two separate layers, much like oil and water. This means that most of the glycerol can be extracted with a pipet. There will still be a very small amount of glycerol as well as other contaminants dissolved in the biodiesel, which is why you will further purify your sample with a technique called a brine wash.
You may notice that the sodium hydroxide is not listed as a reactant or a product. What is the role of this compound and how is it relevant to green chemistry?
A Model for Bioaccumulation: The Octanol-Water Partition Coefficient
A Model for Bioaccumulation: The Octanol-Water Partition Coefficient
One important factor to consider when studying the toxicity of a chemical is how well that chemical can dissolve into hydrophobic or fatty areas of cells. Chemicals that are water soluble can be removed through excretion. In contrast, chemicals that are "fat soluble" cannot be readily excreted and tend to accumulate in the body. One way to replicate this aspect of a biological environment is with a mixture of octanol and water.
When octanol and water are combined, they can be used to study whether a chemical will tend to dissolve into a fatty environment or a water based/polar environment. This behavior can be quantified using a value known as the octanol-water partition coefficient. In general, a partition coefficient quantifies the amount of a substance, a solute, that dissolves in one solvent compared to another solvent. The octanol-water partition coefficient specifically measures how much of a chemical dissolves into a layer of octanol compared to the amount that dissolves into a layer of water.
Equilibrium equation of a solute dissolved in a mixture of octanol and water
Partition coefficient equation for a solute dissolved in a mixture of octanol and water
If the octanol-water partition coefficient for a chemical is known or measured, that gives researchers some insight into how well that chemical may accumulate in the fatty areas of organisms. As you review your results from the radish seed experiment as well as those from the rest of your class, think about how the ability of a fuel to mix with water may or may not correlate with its toxicity.
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