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  • pH Question and selecting a buffer for spectrofluorometry

    From M P@21:1/5 to All on Wed May 5 16:38:43 2021
    I'm working on detecting a particular metal ion using fluorescent dyes in conjunction with absorbance and emission spectroscopy, and I need to test the chosen dye over a range of pH values (i.e. 10-14). I first create the dye solution in a beaker and
    adjust its pH using NaOH (lets say one dye solution at 12.5 and one at 13.0), then I add 1.9 mL of this to a cuvette along with 0.1 mL of a metal ion in acid solution. However, the cuvette's pH always drops to the same value (typically 12.3) once I add
    my 0.1 mL metal ion in acid solution. I'm assuming this is because pH is logarithmic, to reiterate the proportions are 1.9 mL of the dye solution to 0.1 mL of the metal ion in acid solution.

    Any suggestions on resolving this? I cannot titrate a solution in a small, 2.5 mL cuvette. I've considered using a buffer from what I've read but I do not know how to select one, or if they affect absorbance/emission data.

    I do not have a background in chemistry but I've been on this project and am now hitting a brick wall. Any advice or guidance is much appreciated.

    Thanks

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  • From Martin Brown@21:1/5 to M P on Thu May 6 21:41:07 2021
    On 06/05/2021 00:38, M P wrote:
    I'm working on detecting a particular metal ion using fluorescent
    dyes in conjunction with absorbance and emission spectroscopy, and I
    need to test the chosen dye over a range of pH values (i.e. 10-14). I
    first create the dye solution in a beaker and adjust its pH using
    NaOH (lets say one dye solution at 12.5 and one at 13.0), then I add
    1.9 mL of this to a cuvette along with 0.1 mL of a metal ion in acid solution. However, the cuvette's pH always drops to the same value
    (typically 12.3) once I add my 0.1 mL metal ion in acid solution. I'm assuming this is because pH is logarithmic, to reiterate the
    proportions are 1.9 mL of the dye solution to 0.1 mL of the metal ion
    in acid solution.

    Why don't you do it the other way round?

    Make the dye and metal ion solution in bulk first and then add N-n drops
    of 1M NaOH and n drops of 1M HCl to each sample aliquot in a cuvette.
    Then measure the pH of each test specimen after stirring.

    Any suggestions on resolving this? I cannot titrate a solution in a
    small, 2.5 mL cuvette. I've considered using a buffer from what I've
    read but I do not know how to select one, or if they affect absorbance/emission data.

    Alkaline buffers tend to be something like glycine/NaCl/NaOH eg

    https://www.thomassci.com/scientific-supplies/Ph-Buffer-13

    You would have to test if it affects your choice of fluoro dye(s).

    I do not have a background in chemistry but I've been on this project
    and am now hitting a brick wall. Any advice or guidance is much
    appreciated.

    AA spectroscopy or ICPMS might be an alternative option for metals.

    --
    Regards,
    Martin Brown

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