[Sammy5124]

I believe the goal is to know exactly how much of a soluble substance to add to neutralize an acid, and the problem is that methyl orange only detects between pH 3.1 (red) -4.4 (yellow), which is not within the neutral range, so it can't possibly help you know how much of a base you need to add to neutralize an acid.

I have a book that says that when making sodium sulfate crystals, when mixing sodium hydroxide with sulfuric acid, once the indicator turns orange (which should be around pH 3.8 ), the titration is complete, which doesn't make sense because at pH 3.8 the acid is not neutralized.

It says once the indicator has turned orange, do the experiment again without the indicator, using the same amounts that you used to get the indicator to turn orange, then evaporate the solution (by boiling) until the point where crystals form on cooling - but once again orange on methyl orange is not neutral so why is it being used? Shouldn't the book suggest an indicator that indicates when the solution is neutral?

[Archer]

Can you draw a graph of pH vs titre for a strong acid and a strong base?

At pH 3.8 calculate how much acid would be present in your solution?

[Borek]

Also, end point pH is not necessarily neutral. It is almost neutral for strong acids/strong bases titrations.

http://www.titrations.info/acid-base-titration-equivalence-point-calculation

http://www.titrations.info/acid-base-titration-end-point-detection

[Kate]

I believe the goal is to know exactly how much of a soluble substance to add to neutralize an acid, and the problem is that methyl orange only detects between pH 3.1 (red) -4.4 (yellow), which is not within the neutral range, so it can't possibly help you know how much of a base you need to add to neutralize an acid.

The equivalence point in a titration of a strong acid with a weak base will be below 7.

Not all titrations will have equivalence points in the neutral range. That only applies to titrations of strong acids with strong bases, because the resulting species will be "neutral" in terms of acid and base properties.

I have a book that says that when making sodium sulfate crystals, when mixing sodium hydroxide with sulfuric acid, once the indicator turns orange (which should be around pH 3.8 ), the titration is complete, which doesn't make sense because at pH 3.8 the acid is not neutralized.

I think they're probably talking about the first neutralization, in which the initial H₂SO₄ has been converted to HSO4^-. HSO4- is amphoteric, so the pH of the solution definitely won't be neutral.

I checked the K_a of HSO4- and it's 1.2*10^-2. So in equilibrium there will be a considerable amount of H+ ions present.

[Borek]

I have a book that says that when making sodium sulfate crystals, when mixing sodium hydroxide with sulfuric acid, once the indicator turns orange (which should be around pH 3.8 ), the titration is complete, which doesn't make sense because at pH 3.8 the acid is not neutralized.

I think they're probably talking about the first neutralization, in which the initial H₂SO₄ has been converted to HSO4^-. HSO4- is amphoteric, so the pH of the solution definitely won't be neutral.

I checked the K_a of HSO4- and it's 1.2*10^-2. So in equilibrium there will be a considerable amount of H+ ions present.

At pH 3.8 only around 2% of HSO₄^- stays in the solution, everything else is in the form of SO₄^2-.

When methyl orange changes color there is a slight excess of the acid present, around 1%. You will never hit exactly pH 7.00 with neutralization, and it doesn't even make sense to try - it is a waste of time. You want to get as close to the stoichiometric mixture as reasonably possible, but without spending hours on fine tuning. If it really matters, you can get rid of the excess acid/base during crystallization.

[Sammy5124]

So basically:

1) sodium sulfate is not neutral
2) When methyl orange turns orange, the majority of the original acid is gone
3) if methyl orange turned yellow, not only would all the original acid be gone but there would be extra base

Is this correct?

[Borek]

1) sodium sulfate is not neutral

Yes, but very close to neutral.

2) When methyl orange turns orange, the majority of the original acid is gone

What is titrated, what is the titrant? If you mean titration of base with the acid, the moment solution turns orange all the base is gone.

3) if methyl orange turned yellow, not only would all the original acid be gone but there would be extra base

No. If you start with the acid, and you add bases, the moment solution turns yellow at least 98% of the acid is titrated.

[Sammy5124]

The reason I said that is because I was thinking of adding base to acid, instead of adding acid to base, because I never really understood why acid is added to base in titrations instead of doing it the other way around.

[Borek]

There is no such rule. Sometimes we titrate with acid, sometimes we titrate with base.

Titrating with acid is safer for the burette, as acid won't attack stopcock the way base can.