Methyl Orange

Modifying the groups present in the molecule can have an effect on the light absorbed, and so on the colour you see. You can take advantage of this in indicators.Methyl orange is an azo dye which exists in two forms depending on the pH:

Note: You may find other structures for the red form (I use a variation elsewhere in this site!) with different arrangements of the bonds (although always with the hydrogen attached to that same nitrogen). The truth is that there is delocalisation over much of the structure, and no simple picture will show it properly. This case is discussed in detail on a page in the analysis section of the site aboutUV-visible spectroscopy. This is quite difficult stuff, and if you are coming at this from scratch you will have to explore at least one other page before you can make sense of what is on that page. There is a link to help you to do that.Don't start this lightly!

As the hydrogen ion is lost or gained there is a shift in the exact nature of the delocalisation in the molecule, and that causes a shift in the wavelength of light absorbed. Obviously that means that you see a different colour.When you add acid to methyl orange, a hydrogen ion attaches to give the red form. Methyl orange is red in acidic solutions (in fact solutions of pH less than 3.1).If you add an alkali, hydrogen ions are removed and you get the yellow form. Methyl orange is yellow at pH's greater than 4.4.In between, at some point there will be equal amounts of the red and yellow forms and so methyl orange looks orange.

Methyl orangeMethyl orange is one of the indicators commonly used in titrations. In an alkaline solution, methyl orange is yellow and the structure is:

Now, you might think that when you add an acid, the hydrogen ion would be picked up by the negatively charged oxygen. That's the obvious place for it to go. Not so!In fact, the hydrogen ion attaches to one of the nitrogens in the nitrogen-nitrogen double bond to give a structure which might be drawn like this:

Note: You may find other structures for this with different arrangements of the bonds (although always with the hydrogen attached to that same nitrogen). The truth is that there is delocalisation over the entire structure, and no simple picture will show it properly. Don't worry about this exact structure - it is just to show a real case where the colour of a compound is drastically changed by the presence or absence of a hydrogen ion.

You have the same sort of equilibrium between the two forms of methyl orange as in the litmus case - but the colours are different.

You should be able to work out for yourself why the colour changes when you add an acid or an alkali. The explanation is identical to the litmus case - all that differs are the colours.

Chromatesanddichromatesare salts of chromic and dichromic acid. Salts have an intense yellow or orange color, respectively. When solid potassium dichromate (K2Cr2O7) is dissolved in water the resulting solution is orange. The dichromate ion in aqueous solution is in equilibrium with the chromate ion, and this can be shown with the following equation:

This is a dynamic equilibrium and as such is sensitive to the acidity or basicity of the solution. Shifting the equilibrium with pH changes is a classic example of Le Chateliers principle at work.

Le Chatelier's principle states that if a chemical dynamic equilibrium is disturbed by changing the conditions (concentration, temperature, volume or pressure), the position of equilibrium moves to counteract the imposed change. So if more reactant is added, the equilibrium shifts to the right in order to consume that extra reactant, which results in more product; also if the product is removed from the system, the equilibrium shifts to the right completely increasing the yield.

Yellow chromate and orange dichromate are in equilibrium with each other in aqueous solution. The more acidic the solution, the more the equilibrium is shifted to the left towards the dichromate ion. As hydrochloric acid is added to the chromate solution, the yellow color turns to orange. Increasing the hydrogen ion concentration is shifting the equilibrium to the left in accordance with Le Chatelier's principle, where we expect the reaction to try remove some of the H+we have added by reacting with theCrO42-, and yielding moreCr2O72-which we observe as color change.

When sodium hydroxide is added to the dichromate solution, the orange color turns back to yellow, hydroxide ions react with hydrogen ions forming water, driving the equilibrium to the right (OH-removes H+ions by neutralizing them and the system acts to counteract the change) and further shifting the color. We can observe that the addition of hydroxide ions promotes the conversion of dichromate to chromate.

Acids and bases are added to a system so as to shift the position of a chemical equilibrium. The ions have different colors, so that changes are detected visually. Yellow chromate ion turns orange by addition of acid, while the orange dichromate in reaction with bases turns yellow. The equilibrium depends on the acidity of the solution, so the color in this case is pH dependent.

Successive addition of sodium hydroxide and hydrochloric acid causes alternative changes in solution color, during which the color intensity fades due to dilution.

The addition of concentrated acids, such as sulfuric acid into chromate/dichromate solution causes further shifting of the equilibrium, and more intense colors, turning the solution to carmine-red.