Acid/Base Quick Q

[luken93]

Bromothymol blue changes colour from yellow in acid to blue in base between pH 6.0 and 7.6
If you only had this indicator and a sample of 1 mol L-1 NaOH, explain how you could determine the strength of different acids

I don't really understand what I'm meant to answer, do I compare the strength of acids compared to the NaOH, or am I meant to compare it with the colour?

Thanks

[pooshwaltzer]

NaOH is a weak acid. Apply BB to NaOH and measure the time of complete reaction in colour change from yellow to blue. Use this as a benchmark for subsequent solutions of varying acidity

[luken93]

NaOH is a weak acid. Apply BB to NaOH and measure the time of complete reaction in colour change from yellow to blue. Use this as a benchmark for subsequent solutions of varying acidity

aaaahhhh ok we haven't done anything to do with BB.
So basically what you're saying (just to reassure myself), you would measure the time it takes for NaOH to change from yellow to blue
Because you know that NaOH is a weak acid, if it is a quicker reaction, then it is a Stronger Acid? and if its slower, its a weaker acid? or is it the other way around?
I would think itd be my way because the faster the acid is able to ionise the solution, the stronger it would be?? but feel free to prove me wrong haha

[pooshwaltzer]

Don't forget that pH measures are an exponential log function when you account for the relative speed/rapidity of reaction times. Also, the test works for both acids as well as bases since, "Bromothymol blue changes colour from yellow in acid to blue in base between pH 6.0 and 7.6".

[Mao]

What? No. Completely wrong. NaOH is a strong base, not acidic, and usually the transition of indicator is almost immediate [or at most a second or two with stirring]. The transition is also gradual, so there's no exact way of measuring exactly how long a 'transition' will take. Off the top of my head, I don't think speed of the reaction is even related to acidic strength.

This is how I would use 1M NaOH and bromothymol blue, but this requires unit 4 chemistry knowledge:
1) prepare a known concentration of acid
2) titrate the acid with 1M NaOH, and stop the titration at the point when the yellow becomes green (pH = 6), record this
3) repeat steps 1) and 2) for various acids. Strong acids will generally take slightly more NaOH. Weaker acids will generally take less. However, I'd assume this difference to be within a couple of drops, considering we are working with a very high acidic pH (6) and a concentrated base (1M NaOH). This method will be subjected to high levels of errors, but you get that with those materials.

[luken93]

What? No. Completely wrong. NaOH is a strong base, not acidic, and usually the transition of indicator is almost immediate [or at most a second or two with stirring]. The transition is also gradual, so there's no exact way of measuring exactly how long a 'transition' will take. Off the top of my head, I don't think speed of the reaction is even related to acidic strength.

This is how I would use 1M NaOH and bromothymol blue, but this requires unit 4 chemistry knowledge:
1) prepare a known concentration of acid
2) titrate the acid with 1M NaOH, and stop the titration at the point when the yellow becomes green (pH = 6), record this
3) repeat steps 1) and 2) for various acids. Strong acids will generally take slightly more NaOH. Weaker acids will generally take less. However, I'd assume this difference to be within a couple of drops, considering we are working with a very high acidic pH (6) and a concentrated base (1M NaOH). This method will be subjected to high levels of errors, but you get that with those materials.

hmmmm thats strange then, because this is only for unit 2, and we aren't up to titrations yet.

[luken93]

Ok guys, I've come up with another theory, feel free to prove me wrong.
If BB is:
Yellow = Acidic
Green = Neutral
Blue = Basic

And NaOH is a base.

Could you add a predefined amount of the unknown acid, which would turn the BB solution yellow.
And then add drops (titrate) of the NaOH solution until it returns to green.
The more drops it takes, the more acidic it is (strong acid)
The less drops it takes, the less acidic it is (weak acid)

Oh wait, thats what you said Mao. Is this still correct how I said it? This is my understanding anyway

Thanks

[Mao]

Ok guys, I've come up with another theory, feel free to prove me wrong.
If BB is:
Yellow = Acidic
Green = Neutral
Blue = Basic

And NaOH is a base.

Could you add a predefined amount of the unknown acid, which would turn the BB solution yellow.
And then add drops (titrate) of the NaOH solution until it returns to green.
The more drops it takes, the more acidic it is (strong acid)
The less drops it takes, the less acidic it is (weak acid)

Oh wait, thats what you said Mao. Is this still correct how I said it? This is my understanding anyway

Thanks

That is kind of what I'm getting at, but there is a slight difference.

Weak AND strong acids will take the same amount of base to reach the neutral point. HOWEVER, weak acid approach points before neutrality faster. The key here is we see how fast they get to pH = 6 (not neutral, still slightly acidic). It is easier to make a weaker acid to pH = 6, but it is more difficult to make a strong acid pH = 6.

To put it another way, to make a weak acid go from pH 1 to pH 4 (for example), you don't need too much base. To make a strong acid from pH 1 to pH 4, you need a lot of base.
To make a weak acid go from pH 4 to pH 7, you need a fair bit of base. To make a strong acid go from pH 4 to pH 7, you only need a very small amount of base (a few drops).

Thus here, we take advantage that the colour change starts at pH 6, thus we stop adding base as soon as the indicator starts changing colour (still acidic). Usually we stop after the colour-change (neutral).