Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Titration of NaOH and HCl need help

  1. Jan 31, 2005 #1
    How to answer the question: why the pink colour (from 2-3 phenolphthalein indicator) obtained at the end point of the titration in this experiment gradually disappears over time? :confused:
    Is it because of that the water will reverse back to the form of H+ and OH-?
     
    Last edited: Jan 31, 2005
  2. jcsd
  3. Jan 31, 2005 #2

    cronxeh

    User Avatar
    Gold Member

    Equilibrium shifts towards the side where pH is acidic.
     
  4. Jan 31, 2005 #3
    can you tell me in more detail?
    you mean HCl + NaOH --> NaCl + H2O? or BaCO3 + HCl --> BaCl2 + H2O + CO2??
     
    Last edited: Jan 31, 2005
  5. Feb 1, 2005 #4

    chem_tr

    User Avatar
    Science Advisor
    Gold Member

    Hello, I think phenolphthalein begins to decompose with increasing hydrolysis over time. The quinoid form is pinky-purple, but with increasing hydrolysis, acidic products develop and as cronxeh said, the pH is lowered. It is very likely that the carbon dioxide in the medium causes the pH drop, as it gives carbonic acid with water.
     
  6. Feb 1, 2005 #5

    cronxeh

    User Avatar
    Gold Member

    [​IMG]


    It depends on which way the reaction went - either increasing in pH (from 8.2 to 10 - becoming red colour) or decreasing in ph (becoming acidic and dropping from 8.2 down - colorless)

    So in your case you are asked (probably) to add phenolphtalein into HCl, and then gradually make drops of NaOH while stirring it.

    What happens is that phenolphtalein is colorless at first (becomes the environment is acidic - HCl), and stirring the mixture while neutralizing it (adding NaOH) insures that you dont get fooled by forming pink bubbles at individual spots

    What happens next is out of my realm but I'm going to imagine here, so bear with me:
    phenolphtalein + HCl --> HC2H3O2 (phenolphtalein added to HCl, the pH is less than 8.2 - its colorless)

    You titrating (neutralizing):
    HC2H3O2 + NaOH --> Na(C2H3O2) + H2O (the pH raises around 8.2 to 10 depending on how vigorously you titrate)

    Now during this titration reaction the electrons are free to move between phenolphtalein-HCl juncture and if you stop titrating the color will disappear and pH would drop into acidic range

    AFAIK if you titrate fully and reaction is complete the color will stay pink/red depending how much NaOH u added
     
  7. Mar 26, 2007 #6
    yes it is due to the water which slowly reduses the color:smile: :smile:
     
  8. Mar 26, 2007 #7

    chemisttree

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Phenolthalein does NOT decompose at its endpoint pH.
    Phenolthalein does NOT re-equilibrate to produce an acidic product at its endpoint pH.

    Slightly basic solutions DO absorb CO2 from the air and produce the weak acid HCOOOH. The absorption happens fairly slowly and a faint pink endpoint will gradually fade. Adding more NaOH after the faint endpoint is reached (producing a darker endpoint) will cause the pink solution to persist much longer but also results in an inaccurate titration and should be avoided. When I'm grading a student's lab technique score, these dark endpoints are a guaranteed low score!
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?