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Ionic Strength and Activity

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  • Outcomes:

    1. Calculate the ionic strength of a solution.
    2. Describe the relationship between ionic strength and activity.
    3. Quantify the impact of ionic strength on the equilibrium concentration of an analyte.

    Background: Barium enema is a diagnostic medical procedure in which the inside of the large intestine is coated with an aqueous slurry of insoluble BaSO4 to facilitate X-ray imaging. However, barium is a heavy metal that’s absorbed in the gastrointestinal tract; levels as low as ~7 x 10-5 M have been shown to result in a risk of heart attack and stroke. Should you worry about having this procedure done?

    Pre-Class Assignment:

    Show your work for each of the questions below.

    1. Calculate the molar concentration of Ba2+ in a saturated solution of BaSO4 in deionized water. The Ksp for BaSO4 is 1.1 x 10-10.
    2. Using your recollection of Le Châtelier’s Principle, predict how dissolving BaSO4 in a 0.1 M NaCl solution will impact the molar concentration of Ba2+.
























    In-Class Assignment:

    Answer the following questions regarding BaSO4 dissolved in a sodium chloride solution. Assume the NaCl is 10x more concentrated than dissolved BaSO4.

    1. As a group, use your whiteboard to illustrate and describe how any ions might arrange themselves in solution. What ions will be in close proximity to Ba2+? What ions will be in close proximity to SO42-? Be sure to consider the relative number and size of each ion.
    2. Consider the edge of the “ionic atmosphere” drawn around Ba2+ and SO42- in question 1. How does the apparent charge of Ba2+ and SO42- at this point compare to the actual charge of each ion?
    3. Based on your answer to question 2, predict how the ionic atmospheres around Ba2+ and SO42- will impact their attractive force toward one another.
    4. As a group, decide how to complete this statement:

    Dissolving BaSO4 in a sodium chloride solution will cause its solubility to


    relative to water.



    1. In reality, the barium slurry isn’t prepared in deionized water or even a simple sodium chloride solution. It’s made in a physiological saline solution that contains:
      • 0.137 M NaCl
      • 0.0027 M KCl
      • 0.0809 M Na2HPO(dissociates into Na+and HPO42- ions)
      • 0.0176 M KH2PO(dissociates into K+and H2PO4- ions).

      Calculate the ionic strength of this saline solution.











    1. What are the activity coefficients for Ba2+ and SO42- in this saline solution? (αBa2+ = 500 pm; αSO42- = 400 pm)









    1. Based on the ionic strength of the saline solution, determine the actual molar concentration of Ba2+ in the slurry.









    1. Describe how your answer to question 5 compares to what you calculated in the pre-class assignment.






    1. Based on the Ba2+ concentration at which health concerns may arise (~7 x 10-5 M), decide as a group whether you should be concerned about getting a barium enema.









    Thinking ahead…

    1. You arrive at the doctor’s office to receive a barium enema, but the nurse accidentally administers the slurry as a swallow. How will the environment in your stomach (pH = 2.5) impact the solubility of BaSO4?

      HINT: Consider the acid/base equilibrium processes that the dissolution products of BaSO4 may be involved in.








    Post-Lesson Activity

    In one sentence, summarize the relationship between ionic strength and activity.




    In one sentence, summarize the relationship between ionic strength and solubility.






    You may be asked to respond to one of the following:

    1. Something I learned today…
    2. Something I already knew…
    3. Something I’m still confused about…


    For supplemental reading on this topic, see section 6.9: Activity Effects in Analytical Chemistry 2.1.

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