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Preparation of murexide indicator for Ca measurement

  1. Apr 21, 2017 #1
    I want to prepare murexide indicator for Ca measurement using EDTA titrimetric method. I am following the procedure set by the American Public Health Association (APHA), however, I only have murexide diluted with K2SO4 (1:250). I want to know if I can use this as a substitute, and if yes, how.

    The following is the procedure described by APHA:

    Murexide (ammonium purpurate) indicator: This indicator changes from pink to purple at
    the end point. Prepare by dissolving 150 mg dye in 100 g absolute ethylene glycol. Water
    solutions of the dye are not stable for longer than 1 d. A ground mixture of dye powder and
    sodium chloride (NaCl) provides a stable form of the indicator. Prepare by mixing 200 mg
    murexide with 100 g solid NaCl and grinding the mixture to 40 to 50 mesh. Titrate immediately
    after adding indicator because it is unstable under alkaline conditions. Facilitate end-point
    recognition by preparing a color comparison blank containing 2.0 mL NaOH solution, 0.2 g solid
    indicator mixture (or 1 to 2 drops if a solution is used), and sufficient standard EDTA titrant
    (0.05 to 0.10 mL) to produce an unchanging color.

  2. jcsd
  3. Apr 21, 2017 #2


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    Staff: Mentor

    I don't see why the mixture you have can't be substituted for the suggested NaCl/murexide powder. Even the mass ratio is in the same ballpark.
  4. Apr 23, 2017 #3
    Thanks a lot!
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