Quick question on indicators

[virtualmaster999]

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Hi everyone!

Quick question. So In Destroyer number 246 for gen chem, you are given a compound, and you can tell it is an acidic salt. How do you determine which indicator to use though? The solution in the book shows the pH given for this, but how would you know the pH for a compound you havent memorized? Are you assuming to use the methyl red indicator b/c it has an acidic pH, but for a weaker acid?

 

[w4nt2baDDS]

I have the 2014 version so I cannot reference the exact problem.

Typically though, you need to try to categorize the compound given as either Strong Acid, Weak Acid, Neutral, Weak Base, Strong Base. If you can determine that the compound is acidic then by knowing if it is one of the 7 strong acids or not will allow you to quickly determine if its a weak or strong acid...

There may be some specific value cut offs... but for me.. I pretty much guesstimated the pH ranges for these as:

Strong Acids -- Below a pH of 3

Weak Acids -- 3.5-6.5

Neutral around 7

Weak Bases: 7.5-9.5

Strong Bases: 10+


Once you know the general pH region your compound you need to determine what color each indicator will be. If the pH of the compound is expected to fall in the middle range for an indicator then that indicator is typically considered the best and color of the indicator would most likely be a blend between its lower pH (protonated) form and higher pH (deprotonated) form.

Hope this helps answer your question.

 

[DAT Destroyer]

Hi everyone!

Quick question. So In Destroyer number 246 for gen chem, you are given a compound, and you can tell it is an acidic salt. How do you determine which indicator to use though? The solution in the book shows the pH given for this, but how would you know the pH for a compound you havent memorized? Are you assuming to use the methyl red indicator b/c it has an acidic pH, but for a weaker acid?

First of all, this is a very general question, but I will try to answer it the best I can. If you are titrating a strong acid with a strong base, the equivalence point will equal 7, thus select an indicator whose pKa is close to this value. If you are titrating a strong acid with a weak base such as HBr and NH3 the equivalence point will be around 5, thus select that indicator whose pKa is around 5. It you titrated a strong base with a weak acid such as NaOH and CH3COOH, the pH would be about 9 at the equivalence point, therefore look for an indicator whose pKa is around 9.

Dr. Romano

 

[virtualmaster999]

Thank you both! Sorry for not being specific enough, I didn't want to copy the exact question from Destroyer, so I tried to make it more general.