Chemistry Research Question: Comparing blanks and samples with same pH?

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Aug 7, 2021, 4:04:20 AMAug 7
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[[Mod. note --
1. I have manually re-wrapped over-long lines.
2. This post isn't really within the scope of this newsgroup, but
alas the logical newsgroup for this material, /sci.chem/, has a poor
signal/noise ratio (lots of irrelevant posts). So... let's talk
about chemistry a bit!
-- jt]]

Goal:
-Need to generate a pH vs. Fluorescent Intensity calibration curve
using a spectrofluorometer for a particular fluorescent dye with
an analyte (metal ion).

Method:
-Generating pairs of samples at varying pH (9, 9.5, 10, etc) with one
sample being a blank and the other containing the analyte.
1. First mix the dye with ethanol and HNO3 in a beaker to dissolve.
2. Next, 2.0 mL of this dye solution is added to two cuvettes as a pair.
This is repeated for varying pHs by adding base.
3. Finally, 0.1 mL of the analyte suspended in 3% or 0.3% nitric acid
(required to solubilize) is added to one cuvette (test sample), and 3%
or 0.3% nitric acid is added to the other cuvette (blank sample). This
changes the pH yet again, so it's measured in the cuvette. Of course,
the cuvette pH is always lower than that of the beaker since the analyte
is in acidic solution.

Problem:
-The analyte can only be managed in a separate facility, so I cannot add
it to the beaker solution. It must be added to the cuvette in the final
step.
This creates the issue: I cannot adjust the pH in the cuvette given
its small volume, so whatever pH it ends up being after addition of the
analyte, I have to accept. This prevents me from adjusting the final pH
to my desired outcome.
-This is made even more difficult with the equivalence point, preventing
me from reaching pHs of ~10 and 11 since the addition of the analyte in
acid results in a drastic pH decrease and even 0.1 mL is proportionally
large compared to 2.0 mL of the dye solution..


Attempts to resolve:
-Tried using the analyte suspended in 0.3% acid solution rather than 3%
acid solution, but this doesn't fix the problem, only reduces the decrease
in pH.

Ideally, whatever the pH of my beaker solution is, I could just add that
to the cuvettes that contains the analyte and the dye solution. Is this
really the only way? I feel I have been using my method for so long that
I do not see another way.

Thanks for reading
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