acid/base indicators

[natureboy]

What kinds of indicator(s)/their results as it pertains to acid/basic solutions should we know for the MCAT?

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[Elastase]

I took this past Augusts test...and it had a question on specific indicator... One of those, pick one and move on...I bet only 25% ppl got that right...cause no one in their right mind would spend time memorizing indicators..

In short..

I doubt there will be specific indicators in this next test.

 

[OSUdoc08]

What kinds of indicator(s)/their results as it pertains to acid/basic solutions should we know for the MCAT?

litmus

 

[natureboy]

I took this past Augusts test...and it had a question on specific indicator... One of those, pick one and move on...I bet only 25% ppl got that right...cause no one in their right mind would spend time memorizing indicators..

In short..

I doubt there will be specific indicators in this next test.

I remember reading about people complaining about phenolphthalein on last year's april or august mcat. I didn't really think of it until I saw a litmus question on AAMC 8...

 

[PHECT]

I remember reading about people complaining about phenolphthalein on last year's april or august mcat. I didn't really think of it until I saw a litmus question on AAMC 8...

Yeah, there was a question on my test about indicators.. I just made my best guess and went on.. I think that they were asking Bromthymol Blue. But, I'm not sure-- it was a discrete though.

 

[Shredder]

Even though litmus was asked on 8, you didn't have to know anything about it since the colors under different situations were given. I think time would be better spent elsewhere besides indicators and color changes, as long as you know a good indicator should change near the equivalence point, where mol acid=mol added base, and the point where the color changes is the end point I think, and ideally these points should coincide I believe.

 

[tank you]

on aamc 7, the last question in PS asked about when indicator changes color. the answer was that the indicator changes color over a specific pH range, and the range depends on the point at which HIn is converted to In-, which depends on pKa of indicator.

 

[laya533]

Can any one (specially Shrike) explain about the indicators and the color change in visible light?(complamentary color and so on?)thanks alot

 

[researchprof]

There is no need to memorize the colors of acid-base indicators and their ranges, except moist litmus which is red in acid and blue in base.

Also it is good to know that indicators change color when they are protonated or deprotonated (HIn -->/<--- H+ + In-).

Using the Hend--Hasselbach equation, we realize that an appropriate indicator should change color within pH = pKa +/-1. This is so because we want the ratio of conjugate base to acid (or the reverse) to be in a ratio of 1 to 10. If u know the pH at which an acid-base titration reaches end-point, you look for an indicator with the appropriate pKa based on the above equation.

 

[KiTmAn]

so, r u sayin that when they ask u a question on the mcat given the pH of lets say CH3COOH and NaOH titration to be about lets say 10 ....u use that pH to find hte pKa which would be high because Ka low...correct?

 

[BaylorGuy]

indicators only work within +/- 1 pH unit of what you are looking for. Example, lets say a titration of acetic acid and sodium hydroxide is performed. the equivalence point is approximately 9. Well, you dont have any way of measuring the pH except indicators, thus you must choose the best indicator that will change color near the equivalence point (WHY? because you need to know the mLs in order to figure out the molarity...etc, etc.)

Indicators work best within a standard deviation of 1 pH unit. THus the best indicator for the above titration is one in which the pKa is between 8 and 10....when you see the color change it means that you're close enough to the equivalence point and can thus stop titrating.