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Topic: Indicator pH Calculation  (Read 7876 times)

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mhameer84

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Indicator pH Calculation
« on: April 17, 2004, 04:35:33 PM »
Need help with this question:
The dissociation constant of an acid base indicator is 6.31x10^-6. Its 'acid color' is blue, and 'alkaline color' is yellow. Calculate pKa value of this indicator, and state what color it will be WHEN intorduced into aqueous sodium carbonate of pH 5?

Offline Donaldson Tan

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Re:Indicator pH Calculation
« Reply #1 on: April 18, 2004, 02:10:20 AM »
pKa = -lg[ 6.31X10-6 ] = 5.20

pH 5 is in the acidic range, the colour of the indicator will be blue.

However, aq sodium carbonate is alkali in nature. I'm in doubt that its pH will ever be 5.
"Say you're in a [chemical] plant and there's a snake on the floor. What are you going to do? Call a consultant? Get a meeting together to talk about which color is the snake? Employees should do one thing: walk over there and you step on the friggin� snake." - Jean-Pierre Garnier, CEO of Glaxosmithkline, June 2006

mhameer84

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Re:Indicator pH Calculation
« Reply #2 on: April 18, 2004, 02:28:17 AM »
Hey thanx for the reply.

I believe this indicator is Methyl Red, which is a weak base. Therefore, shouldn't the dissociation constant be that of the weakbase. And so,

pKb = -log[6.32x10^(-6)]=5.20
pKa + pkb = 14
pKa = 14 - 5.2 = 8.8


Offline Donaldson Tan

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Re:Indicator pH Calculation
« Reply #3 on: April 19, 2004, 10:48:06 AM »
methyl orange indicator dont reflect blue and yellow. methyl orange is red (acid) or yellow (alkali) depending on the solution pH.

the indicator which your previous post describes suits the profile of bromothymol blue.
"Say you're in a [chemical] plant and there's a snake on the floor. What are you going to do? Call a consultant? Get a meeting together to talk about which color is the snake? Employees should do one thing: walk over there and you step on the friggin� snake." - Jean-Pierre Garnier, CEO of Glaxosmithkline, June 2006

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