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I got some good information about this topic yesterday, and I understood what I was told, but I have come up with a specific way of phrasing the problem I'm having. It is slightly different than before, and I'm looking for someone to tell me if what I'm doing makes any sense or not, rather than just rote learning facts like 'methyl orange indicator is a weak acid/base'. I want to be able to use this process for any weakly acidic/basic indicator I come across.

(Note: Methyl orange is taken to be orange when it has a moderate pH (HIn, not very acidic or basic, and red between a pH of 4.4 and 3.1 - i.e. when it is In- or In+ depending on the scenario below)

Assume methyl orange, an indicator, is a weak acid. It dissociates as follows in aqueous solution;

HIn is orange, In- is red

We know that if an acid is added to this system at equilibrium, the concentration of hydrogen ions will increase. Using Le Chatlier's principle; to oppose this stress, the reaction shifts to the right, and the colour of the initial indicator (in this case methyl orange, so orange) will dominate and become more intense.

The solution will not go red, like it would if a base was added and proton concentration was decreased, as the equilibrium would shift in the other direction.

The problem is, that if you add an acid to this system in real life, a colour change occurs at a pH of 4.4, from orange to red. This means that methyl orange indicator cannot be an acid. Right?

If you instead assume that methyl orange is a weak base, it dissociates like so in aqueous solution;

OHIn is orange, In+ is red

So here, if you add an acid, the concentration of hydroxide ions will decrease (due to bonding between the hydrogen ions in the acid and these hydroxide ions to form water), so the equilibrium shifts to the right, to compensate.

This means that the red colour will dominate.

This result is in line with observation, as methyl indicator will change colour from orange to red at a pH of 3.1-4.4, which is acidic.

Does my reasoning therefore demonstrate that methyl indicator must be a weak base?

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    $\begingroup$ It's too broad, you could read many books acid/base properties of organic compounds $\endgroup$ – Mithoron Dec 15 '15 at 22:04
  • $\begingroup$ Ok. but surely for indicators where their properties (in terms of acid/base) are very relevant, there is some answer on this? For some reason phenolphthalein is defined as a 'weak acid' but Methyl Red/Orange can't be defined? How am I supposed to know how they dissociate in solution? $\endgroup$ – Malnurturedㅤㅤㅤㅤ Dec 15 '15 at 22:06
  • $\begingroup$ I've seen mechanisms for at least two first and imo it would be good to know them as they are interesting. $\endgroup$ – Mithoron Dec 15 '15 at 22:12
  • $\begingroup$ Methyl orange is weak base, their properties are well known. And if you want to know them you can check pKa or pKb+ values. You should edit your question - narrow it down. $\endgroup$ – Mithoron Dec 15 '15 at 22:19
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    $\begingroup$ I don't think it is too broad. Did you have a look at any of the structures of the indicators you have mentioned? If you find controversial statements, include them in the post so that they can be cleared up. And since you have not done it yet, I invite you to take the tour. $\endgroup$ – Martin - マーチン Dec 16 '15 at 5:07
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Methyl Orange is the common name for Sodium 4-[(4-dimethylamino)phenyldiazenyl]benzenesulfonate. So the compound is a salt or a conjugate base. It is the sodium salt for the indicator since that form is more soluble than the acid form.

enter image description here

The pKa is 3.47 in aqueous solution, so at pH values of 3.1 or lower the molecule is protonated.

enter image description here

At pH values greater than 4.4 all the molecules in solution are deprotonated as the sulfonate anion.

enter image description here

Linked two images from ChemWiki.


The confusion is caused by rather poor wording in a question. enter image description here

I'd prefer the wording to be:

...
In aqueous solution methyl orange indicator behaves like a weak acid. Letting HX represent the protonated form of the indicator, it dissociates as follows:

It is splitting hairs, but methyl orange is the name of the sodium salt, not the protonated form of the indicator.

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  • $\begingroup$ So... if it is a conjugate base that means it dissociates like a base would in aqueous solution? (XOH ---> X+ and OH-)? I apologise for not using the correct terminology - I'm just beginning to understand what is involved in acid/base titrations $\endgroup$ – Malnurturedㅤㅤㅤㅤ Dec 15 '15 at 23:19
  • $\begingroup$ No, when the acid form loses a proton, then the anion is a conjugate base - it needs a proton to become neutrally charged. But the anion isn't an anion of "methyl orange" per se since methyl orange itself is a salt. // So this is analogous to acetic acid and sodium acetate. In solution acetic acid is the protonated acid form, and acetate is the deprotonated anion form. The acetate anion is a conjugate base. $\endgroup$ – MaxW Dec 15 '15 at 23:31
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    $\begingroup$ It doesn't get protonated on sulfonic group, which is least basic, but on N atom connected to the same ring as sulfonic group. This way most stable cation is created. $\endgroup$ – Mithoron Dec 15 '15 at 23:35
  • $\begingroup$ So, the acid is the one used as the indicator? I'm essentially trying to find out the properties of the indicator itself - not the conjugate base - so that I can know how it dissociates in aqueous solution. If indeed it is an acid, how can I explain what I've done above when I assume methyl orange to be an acid, then use Le Chatlier's Principle to effectively show that not to be true? Did I make a mistake? $\endgroup$ – Malnurturedㅤㅤㅤㅤ Dec 15 '15 at 23:38
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    $\begingroup$ @Malnurturedㅤㅤㅤㅤ Gee, it's the salt, which has basic anion $\endgroup$ – Mithoron Dec 15 '15 at 23:46

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