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Choosing an indicator?

  1. Apr 14, 2007 #1
    1. The problem statement, all variables and given/known data

    The indicators listed:
    mtehyl orange 3.1-4.4
    bromocresol purple 5.2-6.8
    Phenolphthalein 8.3-10

    Choose one of the above indicators that would best indicate the endpoint of a titration of HNO2 with sodium hydroxide (NaOH). Explain?

    2. Relevant equations

    3. The attempt at a solution

    My reasoning is this: HNO2 is a weak acid and NaOH is a strong base...if you did a titration experiment with the HNO2 solution and the NaOH as the titrant, you would get an equivalence point or pH value above 7 making it more basic (I think it would be above 8 actually but I could be wrong) and this would make phenoplphthalein a good choice? Can anyone agree, disagree, help me here???

  2. jcsd
  3. Apr 14, 2007 #2


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    Fairly close; you could also do an equilibrium pH calculation and determine the equivalence point pH (which is usually a good estimation). You could probably find the value of the equilibrium constant for nitrous acid in some textbooks and any handbooks.
  4. Apr 14, 2007 #3
    the Ka for HN02 is 4.5x10-4

    But I am not given any M numbers or anything I am more or less thinking that we have to describe how we would pick the pH indicator? I am a bit lost in terms of where to go: should I come up with a reaction equation:

    HN02 + NaOH = NO2 + H20 + Na

    Then I would take the Ka and use that to solve :


    disregard Na and H20

    am I going in completely the wrong direction?

    but a bit lost from here???
  5. Apr 14, 2007 #4


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    Your qualitative understanding will give you the right idea. Titrating a strong acid might not matter much which indicator you choose. When your HNO2 becomes neutralized, you essentially have a solution of NaNO2. That is the conjugate base of HNO2. Therefore, the pH for the solution of NaNO2 can be expected slightly alkaline. I would choose phenolphthalein. Bromocresol purple might be fine/acceptable, but I would probably try an equilibrium constant calculation to be more certain if I wanted more reliable accuracy.
  6. Apr 14, 2007 #5
    I agree where would I start for the equilibrium concentration calculation? I understand by explaining but I want to understand by doing some math to get me to a number....does that make sense?

    Let me know
  7. Apr 14, 2007 #6


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    My method was a little crude; I arbitrarily assumed a 1 molar concentration of HNO2 for the equilibrium:
    HNO2 <==> H + NO2-

    and the reverse of this being:
    NO2- + H2O <==> HNO2 + OH-

    for which Kb = [OH][HNO2]/[NO2-]
    and we use the rule Kw = Ka*Kb
    and we already know Kw=1.0*10^(-14) and Ka=4.5*10^(-4)

    You could work very well with that if you want, or give me a chance to properly typeset the work and I might post that later. You really now just need to find [OH-] and determine pH from pH+pOH=14
  8. Apr 14, 2007 #7
    We were not giving any molar concentrations. I think its more of a reasoning out problem...but I think it's a very open ended question as I think it is meant to just think about what happens when you have a weak acid and a strong base...we just did a titration lab where we had acetic acid and NaOH...that would also be a weak acid and a strong base...so I was using that same sort of concept to organize a response...

    would the strong base (NaOH) dissociate completely and leaving an excess of OH- ions thus making the final pH more basic...I am just not sure of to include in the answer..I am not even sure that I am right? Didn't you mention something about it being alkaline?

    Thanks so much for this help already you have given me
  9. Apr 16, 2007 #8


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    You are not given any concentrations, but you may assume that during titrations you will work with REASONABLE concentrations. That means something between 0.001M to 1M. Most likely about 0.1M. Calculate endpoint pH for 0.001M and 1M solutions and you will see which indicator is the best.
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