How does pH affect solubility
  • The total solubility changes with pH
  • The solubility of the unionized form (Su) of a molecule does not change with pH
  • The solubility of the ionized from (Si) of a molecule does change with pH

    • Solubility is positively related to the concentration of the ionized form
    • Ionized base (BH+) concentration increases as the pH decreases
    • Ionized acid (A-) concentration increases as pH increases
Total Solublity for a molecule with an ionizable form St = Su + Si [mg/L]
Total Solubility of a weak HA acid in aqueous solution St = Su(1 + 10(pH - pKa) )
Total Solubility of a weak base in aqueous solution St = Su(1 + 10(pKa-pH) )
When does the total solubility of a solution most resemble the solubility of the unionized form?
  • When the concentration of the ionized form is minimal the total solubility is described by the solubility of the unionized form
  • Acid ionization is minimal at low pH
  • Base ionization is minimal at high pH
When does the total solubility of a solution most resemble the solubility of the ionized form?
  • When the concentration of the ionized form is high the total solubility is described by the solubility of the ionized form
  • Acid ionization is high at high pH
  • Base ionization is high at low pH
When is the total solublity twice the solubility of the unionized form? When the pH = pKa, the concentration of ionized and unionized forms are equal Su = Si. Substituting this into the equation for total solubility gives St = Su + Su, therefor the total number of particles in solution is equal to Su x 2
What are the limits of total solubility
  • The practical limits of solubility for any pH vary by compound
  • The total amount of unionized molecules will always be the same, regardless of the pH and total solubility
  • If the theoretical number of particles soluble exceeds the actual solubility, the unionized form will be the maximum, with the ionized form being reduced
Factoid: pH of most medications causing hypersensitivity reactions pH ~ 9.0
Restrictions on parenteral delivery of Medications Requiring Cosolvents Mixing cosolvent products with aqueous solutions, in most cases, causes the drug product to fall out of solution. There are some exceptions, which are often not easily explained by the simple acid-base molecular models.
How do the acid-base characteristics affect the mixing of drugs Large (+300Daltons) organic molecules with opposite charges form salts and precipitate out.
Ions paired with Organic Acids in salts
  • Metal cations (Na, Ca, Mg, Zn etc)
  • Amines (diethanolamine, ethylenediamine)
  • Procaine
Ion paired with Organic Bases in salts
  • Hydrogen Halides (HCl, HBr, HI, etc)
  • Acetate
  • Citrate
  • Decanoate
  • Lactate
  • Maleate
  • Mesylate
  • Nitrate
  • Pamoate
  • Phosphate
  • Sulfate
  • Tartrate
Of Commonly available commercial buffers, which is appropriate for preparting a Silver Nitrate solution? Both phosphate and citrate react with silver, creating a low solubility salt. Silver acetate is reasonably soluble (1g/100mL) and the only acceptable buffer.
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