indicators

[jamesq]

Whenever a question asks which is the best indicator to use, what is a quick method of determining this? Thanks

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[BloodySurgeon]

Whenever a question asks which is the best indicator to use, what is a quick method of determining this? Thanks

pka of the indicator equals the pH at equivalence pt

 

[jamesq]

pka of the indicator equals the pH at equivalence pt

Thank you, but what if u are not given the pka of the indicator, conceptually how could you figure it out

 

[Chemist0157]

On a couple of tests, they provide a graph of the titration and a table of indicators with pH ranges. For those, just look at the graph for Veq and match it with the indicator that has that point in its range.

 

[Kaustikos]

Thank you, but what if u are not given the pka of the indicator, conceptually how could you figure it out

Whichever gives a color change closest to the equivalence point? The point of an indicator is to react with the solution when it can.
Take for example a titration of unknown HCL with known NaOH. You would want to use an indicator that would most closely approximate the equivalence point by changing color at that point/volume of NaOH added and not stray too far away from that point.

 

[TawMus]

Whenever a question asks which is the best indicator to use, what is a quick method of determining this? Thanks

The whole concept of indicators is the fact that they undergo a color change at a certain pKa. pKa = pH at the equivalence point. So the ideal indicator's pKa will = the pH of the solution.

 

[jamesq]

The whole concept of indicators is the fact that they undergo a color change at a certain pKa. pKa = pH at the equivalence point. So the ideal indicator's pKa will = the pH of the solution.

do you mean the half equivalence point, or the equivalence point?

 

[bluemonkey]

do you mean the half equivalence point, or the equivalence point?

Keep in mind that indicators themselves are weak acids/bases. So when the titrated system has reached it's equivalence point, the indicatior is at its half-equivalence point (if you have picked the right indicator). Via HH, pH=pKa+log[In-/HIn]. When the system's pH rises above the indicators half-eq point, the In- form predominates. When the system's pH is below the indicators half-eq point, the HIn predominates.

 

[coolchix321]

if you titrate NaOH with HCl (add HCl to uuknown concentration of NaOH)... what is the pka of a good indicator (qualitatively)? How do you determine this exactly?
AND
What is the indicator like when titrating HCl with a base?
Do you want the indicator to have a slightly lower pkb than HCl? Why?
Explain....
Thank you

 

[coolchix321]

Whichever gives a color change closest to the equivalence point? The point of an indicator is to react with the solution when it can.
Take for example a titration of unknown HCL with known NaOH. You would want to use an indicator that would most closely approximate the equivalence point by changing color at that point/volume of NaOH added and not stray too far away from that point.

So the indicator would have to have a pkb slightly lower than HCL?
Why or why not??

 

[wanderer]

If you are given the color change range for the indicator you want the entire range to be within the rapid rise/drop portion of the titration curve. For a strong acid/base indicator choice is not that important because there is a very large drop/rise.