Weak acid + weak base titration (1 Viewer)

[jessi90]

Is it possible to do this? If so, what would happen during titration? I understand that the pH change aroun the equivalence point is not very large so its harder to determine the equivalence point accurately. I have a prac assessment next week and need help badly



potassium hydrogen phthalate + ammonia

 

[uberturtle]

Yes it is possible, it's just harder to see.
You have to select the right indicator for this titration


potassium hydrogen phthalate also absorbs moisture from the air so care when you use it for your standard

 

[dan.121212]

if uve got a task on this titration, by googling titration and potassium hydrogen phthalate there are websites that might help you. This seems really helpful:

http://www2.truman.edu/~blamp/chem222/manual/pdf/khp.pdf

 

[midifile]

You can do this titration, but it would be pretty pointless as there is no indicator that would show the equivilance point as there are no indicators which change at exactly 7. Thats why you need a strong acid and/or base, as it means there is the pH changes dramatically with one extra drop, so indicators which dont necessarily change at 7, but somewhere over this range

 

[dan.121212]

bromothymol blue would work as an indicator, wouldn't it?

 

[Shedow]

In a weak acid - weak base titration, pH doesn't rise sharply at the equivalence point. Therefore it's impossible to get an accurate result no matter what indicator you use.

 

[rooeys2]

bad choice of chemicals though, weak acid/weak base good luck with that yea i think any indicator would work but you might fall asleep to reaise the changes

 

[Undermyskin]

You'd better use data logger for this titration plus you should take into account the ionisation degree (right term?) of the weak acid. As you can see that the graph gets flatter and flatter as if approaching an asymptote. It's very hard to determine the 'point of infexion' if not hardly possible.

 

[minijumbuk]

ATTENTION:

They would NEVER ask you to do a weak acid/base titration. The reaction is an equilibrium, and so the reaction never stops shifting from left to right.

Don't even bother researching into it.

 

[xiao1985]

ATTENTION:

They would NEVER ask you to do a weak acid/base titration. The reaction is an equilibrium, and so the reaction never stops shifting from left to right.

Don't even bother researching into it.

Quite true.

If you ever need to find the conc. of a weak acid, use a strong base to titrate against it. Vice versa with weak base.

 

[Azreil]

ATTENTION:

They would NEVER ask you to do a weak acid/base titration. The reaction is an equilibrium, and so the reaction never stops shifting from left to right.

Don't even bother researching into it.

QFT

If there is a graph that looks like a weak base/weak acid titration graph [fairly flat with no sharp downturn] and they ask you which indicator would be suitable, the answer is none.

A lot of people in my class got caught out with this in the home studies we were set.

 

[Tiffanys]

QFT

If there is a graph that looks like a weak base/weak acid titration graph [fairly flat with no sharp downturn] and they ask you which indicator would be suitable, the answer is none.

Why? If you had two colour solutions of a weak base and a weak acid, you would have to make visual observations of a colour change so wouldn't it be logical to use an indicator with a range around neutral, eg litmus? Otherwise you couldn't tell when the titration has reach its equivalence pt...

 

[xiao1985]

Re: above: For two reasons:

a) the end point does not necessarily lie at pH 7

b) colour change of indicator is too slow to determine the end point.

The bottom line is, you would never NEED to do a weak acid w/ a weak base. If you need to determine the concentration of a weak acid, use strong based instead, and vice versa.

 

[Undermyskin]

Q.E.D.

For a weak acid, unless the ionization degree of it and that of the base is somewhat close, then the ability to have its equivalence point on 7 is possible.

For a graph like wA/wB titration, you cannot find its point of inflexion. If you don't believe, just try to find that of the e^x or simply some rectangular hyperbola (or maybe I haven't known yet).

 

[Tiffanys]

What does QED mean? I guess it doesn't matter, but would I ever have to explain ANYTHING about Weak acid/ Weak base titrations?

 

[Undermyskin]

QED= Quod erat demonstrandum. Roughly translated as 'it is demonstrated, proved'

According to Conquering Chem, you must be able to account for any chemistry phenomenon but using your mandatory Chem knowledge and skills. In many books, they have questions about the impraticality of wA/wB titration in lab.

 

[Tim035]

Since weak acids and weak bases have low ionization percentages and thus higher associated pka values the pH of the final solution can be calculated using
pH = pka + Log (A-)/(HA) where HA represents the concentration of weak acid in its non ionized state and A- is the concentration of acid in its ionized state.

Now if we add a weak base into the equation, this is going to shift the equilibrium and thus the percentage of dissociated acid at equilibrium. The weak base will favour formation of HA (weak acid complex), hence the concentration of ionized acid (A-) will decrease by an amount equal to the concentration of ionized base and the weak acid complex(HA) will gain an amount equal to the concentration of ionized base added.
Remember adding the base will also cause mutual dilution of both the acid and bases concentration so this will need to be found prior to finding the final concentration of HA and A-. From this data a titration graph can then be plotted.




















If none of this made senese, don't worry it's first year uni fundamental chemistry. The point was to illustrate that your teacher would have to be a very cruel person to make you do a weak acid vs weak base as a practical experiment for yr 12 chemistry.

 

[xiao1985]

Re: above, hmmm sounds fair, though overly complicated for the purpose of titration.