Indicators

[plzNOCarribbean]

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So I did a search an fond a thread by jamesq that answered most of the indicator ? but I am still unsure of some things.

So, when a passage shows a titration curve and gives a list of indicators, you want to select an indicator that's pKa will be in the PH range of the equivalence point, right?

So, if we are given a graph, we can just use the graph and draw a horizontal line to the corresponding pH, and then pick an indicator whose pka value PLUS OR MINUS 1 PH unit falls within the pH of the equivalence point? does this always work? what if we are not given a table? is it safe to assume that when titrating a:

Strong acid with Strong base = pick indicator with pKa = 7
Strong acid with Weak base = pick indicator with pka < 7
Weak acid with Strong base = pick indicator with pka > 7

is this how you would solve it? Or would you use the equation for estimating the pH at the equivalence point if you are unsure? i think the equation is:

pH = Pka acid + PH titration /2 ; obviously you need to be given the pKa or the Ka of the acid component in the solution and the concentration of the titrant to solve the problem this way. Is this the way you guys solve it?

I have seen some examples where the pH values are somewhat above/below the 7 range (say 8.85-9) so I just want to understand and make sure that this is the equation to use when a graph is not provided for a rough estimate of the pH of the solution at the equivalence point. The graph estimation method and the above equation are the only two ways of solving these kinds of problems, or did i miss something?

and for polyprotic acids, is the equivalence point always the second proton, or will the question have to specify which proton they want removed?

 

[whiteshadodw]

So I did a search an fond a thread by jamesq that answered most of the indicator ? but I am still unsure of some things.

So, when a passage shows a titration curve and gives a list of indicators, you want to select an indicator that's pKa will be in the PH range of the equivalence point, right?

So, if we are given a graph, we can just use the graph and draw a horizontal line to the corresponding pH, and then pick an indicator whose pka value PLUS OR MINUS 1 PH unit falls within the pH of the equivalence point? does this always work? what if we are not given a table? is it safe to assume that when titrating a:

Strong acid with Strong base = pick indicator with pKa = 7
Strong acid with Weak base = pick indicator with pka < 7
Weak acid with Strong base = pick indicator with pka > 7

is this how you would solve it? Or would you use the equation for estimating the pH at the equivalence point if you are unsure? i think the equation is:

pH = Pka acid + PH titration /2 ; obviously you need to be given the pKa or the Ka of the acid component in the solution and the concentration of the titrant to solve the problem this way. Is this the way you guys solve it?

I have seen some examples where the pH values are somewhat above/below the 7 range (say 8.85-9) so I just want to understand and make sure that this is the equation to use when a graph is not provided for a rough estimate of the pH of the solution at the equivalence point. The graph estimation method and the above equation are the only two ways of solving these kinds of problems, or did i miss something?

and for polyprotic acids, is the equivalence point always the second proton, or will the question have to specify which proton they want removed?

for polyprotic acids, each proton has a separate and distinct pKa. almost always pKa1 (first proton) < pKa2 (second proton) (< pKa 3...... so on and so forth)

i'm pretty sure TBR said to pick an indicator with pKa = pH equivalence + 1, but then later in some passage they said indicator pKa = pH eq + 2. honestly, they get too picky sometimes. i'd assume the passage will hint or straight up tell you how to pick an indicator.