Earth Science Today, Lab #2, Partitioning (60 minutes)

Materials:  paper cups, beakers, sand, clay (or cornstarch), graduated cylinders, food coloring, brain.
DO THIS FIRST:  Put 5 drops of red food coloring in 50 ml water and mix.  Add 20 ml of clay (or cornstarch) and stir until mixed.  Let this sit for 30 minutes without disturbance while you continue the lab.

Goal:  Measure the partitioning between sand and water and between clay and water (we may use cornstarch as a substitute for clay).

We will use red food coloring to represent some element or pollutant partitioned between each of these pairs.

This experiment should be quantitative, meaning that you need to get a real number.  In addition, you need to consider units.  We will use drops of red food coloring per 50ml as units.  Note that this does not mean you have to use this concentration or amount in your experiment!

Because this experiment is quantitative, you need to have some way of determining how much red food coloring is in the water and in the sand or clay.  HINT:  Most quantitative chemical work involves comparing what you want to analyze against a standard solution, or a series of standard solutions whose concentrations you know (because, for example, you made the standard mixtures).  Comparisons can be visual and still be considered quantitative. For example, you could make a series of standards solutions with different numbers of drops of food coloring, and compare these to your unknown solution.

HINT:  get organized first, figure out what you want to do.
HINT: your standard solutions need to be in the same range of compositions as your experiments.  The concentration of food coloring can't be so low as to be undetectable with your eye, nor so high as to make variations in concentration undetectable.  (e.g. you may want to have standard solutions whose concentrations range from 1 to 5 drops per 50ml water).  Also, you probably want only about 20 ml or thereabouts of standards solution, so that you don't need so much of the unknown solution to compare against.
HINT:  you can figure out the amount of food coloring in the clay or sand by difference between what you started with and what is left in the water.
HINT:  you can calculate the amount of food coloring by determining the concentration, then scaling to an amount of water.  This lab involves understanding how to determine concentration and what it means mathematically.  For example, you don't have to have 50ml of water to have a concentration of 4 drops per 50 ml.  I want you to try to figure this out on your own, but if it doesn't make sense or you aren't sure, be sure to talk to me about it because if you don't understand what concentration is, you can't understand these experiments.

In your report: be sure to explain your experimental procedures carefully, your experimental results, and how you interpret those results, about 1.5 pages.  In addition, your report should include your measurement and calculation of the following partition coefficients (again make sure you understand what concentration is!).

Partitioning for sand/water (drops of red coloring per 50 ml sand/drops of red coloring per 50ml water)

Partitioning for clay/water (drops of red coloring per 50 ml clay/drops of red coloring per 50ml water)

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