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Mixing of chemical indicators

  1. Aug 2, 2011 #1
    As an experiment I was going to investigate the use of a spectrophotometre as a pH meter by using different chemical indicators which should change colour depending upon the pH of the substance. I know that indicators such as bromthymol blue only experience a distict change in colour over a pH range of about 2 (e.g. between a pH of 6 and 8) and also that this colour change varies between different indicators (e.g. that bromcresol green changes colour between a pH's of 4 and 6). What I want to know is will mixing these two indicators allow the spectrophotometre to detect a change in pH accross a larger spectrum (e.g. using this mixture of indicators will I be able to accurately calculate the pH of substances within the range of 4-8) or will the mixture not work as an indicator at all?


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  2. jcsd
  3. Aug 2, 2011 #2
    Things to consider -

    1.) When you say you intend to use a spectrophotometer, what exactly do you intend to do with it? Do you know the absorption spectra of each compound as a function of pH and know what intervals/characteristic wavelengths - if any - will give you accurate readings without too much interference?

    2.) Have you looked into other pH indicators, or combinations thereof? Are you familiar with the so-called "universal indicator?"

    3.) What sort of system do you want to measure the pH of using this assay? Will the components of this system interfere with your spectrophotometric assay? If it is noticeably turbid or chemically reactive, a colorimetric assay might not be ideal.
  4. Aug 2, 2011 #3


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    Your experiment is possible in principle, although as Mike H suggests, you might want to consider using universal indicator. Some other things you might also need to consider:

    1) An indicator is itself a weak acid .. the color change(s) are induced by protonation/deprotonation reactions. Therefore, if you have very low concentrations of acid or base, your indicator can interfere with the equilibria that you are trying to measure.

    2) All forms of your reactant and product species need to be colorless (ideal), or need to have well-known spectra that can be accounted for (less than ideal and potentially quite tricky).

    3) You need to make sure that no precipitates are formed, for the reasons pointed out by Mike H.
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