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First year student needs help with this question! Solubilities etc

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  • First year student needs help with this question! Solubilities etc

    If you have plotted a graph of solubility of a drug against concentration of surfactant with given values, how do you find the 'intrinsic solubility' of the drug from the graph? And how do you find the critical micellar concentration of the surfactant from the graph? So confused!

  • #2
    Re: First year student needs help with this question! Solubilities etc

    Presumably by extrapolating concentration of surfactant back to zero.
    We only used to do solubility products in the old days.
    johnep

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    • #3
      Re: First year student needs help with this question! Solubilities etc

      If you plot any physical property like solubility/surface tension/osmotic pressure against concentration, you'll see a change in the slope of the graph. The concentration at which this change occurs is called the critical micelle concentration (CMC), and is the point at which micelle formation first occurs.

      Intrinsic solubility is simply the fundamental solubility of the chemical or drug when completely unionised. For a weak acid, it's measured in acidic solns and for weak bases its measured in alkaline solns.
      If you know solubility (S) at a particular pH for a drug, and you know its pKa, then you can work out intrinsic solubility (So) using the equation:

      pH = pKa + log [S-So/So] for acidic drugs, or

      pH = pKa + log [So/S-So] for basic drugs.
      http://i620.photobucket.com/albums/t...snroses2-1.jpg

      ”We are real. We are not glam sh*t or anything else. We are Guns N’ Roses.”

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      • #4
        Re: First year student needs help with this question! Solubilities etc

        Thanks for the explanation, even I can understand it.
        johnep

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        • #5
          Re: First year student needs help with this question! Solubilities etc

          Originally posted by Nikolai View Post
          If you plot any physical property like solubility/surface tension/osmotic pressure against concentration, you'll see a change in the slope of the graph. The concentration at which this change occurs is called the critical micelle concentration (CMC), and is the point at which micelle formation first occurs.

          Intrinsic solubility is simply the fundamental solubility of the chemical or drug when completely unionised. For a weak acid, it's measured in acidic solns and for weak bases its measured in alkaline solns.
          If you know solubility (S) at a particular pH for a drug, and you know its pKa, then you can work out intrinsic solubility (So) using the equation:

          pH = pKa + log [S-So/So] for acidic drugs, or

          pH = pKa + log [So/S-So] for basic drugs.
          Thank you! But, what if you are not given the solubility at a praticular ph or the pka? This question doesn't give me that information, only the values of the solubility at certain concentrations of the surfactant. I was asked to draw the graph then work out the intrinsic solubility of the drug in water. Would I just use pH 7? What about the pKa?
          Thanks again!

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          • #6
            Re: First year student needs help with this question! Solubilities etc

            Originally posted by shenzys View Post
            Thank you! But, what if you are not given the solubility at a praticular ph or the pka? This question doesn't give me that information, only the values of the solubility at certain concentrations of the surfactant. I was asked to draw the graph then work out the intrinsic solubility of the drug in water. Would I just use pH 7? What about the pKa?
            Thanks again!
            Right here's what i think: the pka value you would use is the drug's pka. The solubility value would then probably be the one at ph7 considering it's in water.
            http://i620.photobucket.com/albums/t...snroses2-1.jpg

            ”We are real. We are not glam sh*t or anything else. We are Guns N’ Roses.”

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