PH stability of emulsions and solutions

Dear Chemists,
i have an essential question regards maintaining stability in some kinds of formulations contain some selective materials like ascorbic acid or Alpha-Arbutin,
I used to add sodium citrate plus citric acid as buffer or sodium lactate plus lactic acid or disodium phosphate plus citric acid, is these 3 buffer systems are right to add? Regards concentrations I have no particular concentration but generally I add 0.5% of salt accompanied with adjustment to required PH by acid
my formulations are emulsions like creams and lotions

waiting your valuable comments 

Comments

  • PharmaPharma Member, Pharmacist
    A buffer is chosen based on target pH. The required amount depends on the entire formulation or rather the compound which leads to pH shifts.
    If you want to have just a one fits it all, your disodium phosphate plus citric acid has a very broad buffering range and might be your best bet. However, it's two polyprotic acids which may show incompatibilities in some formulations.
  • Pharma said:
    A buffer is chosen based on target pH. The required amount depends on the entire formulation or rather the compound which leads to pH shifts.
    If you want to have just a one fits it all, your disodium phosphate plus citric acid has a very broad buffering range and might be your best bet. However, it's two polyprotic acids which may show incompatibilities in some formulations.
    Could you explain more by example? If I want a Ph of 5 or 5.5; I am adding 0.5% sodium citrate and Then adjust PH by citric acid, is it right? Is there any references for such values?
    also can I add sodium metabisulfite in same formulation contain disodium phosphate?
  • PharmaPharma Member, Pharmacist
    A buffer is only suitable +/- 1 unit around the pKa of the employed acid or base, respectively. Citrate for a pH of 5-5.5 is suitable.
    There are buffer tables readily available on the internet.
    The procedure would usually be to use citric acid and then adjust with NaOH.
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